Landracing Forum
Tech Information => Technical Discussion => Topic started by: Saltfever on January 09, 2012, 07:42:00 PM
-
I have had a concern about the roll cage base-plate thickness ever since the rule book changed from 1/8 thick material to 1/4” thickness a couple of years ago. I think an accident analysis was the reason for the change but since analysis is never shared in public the technical reason is unknown. Although, one might guess that a roll bar tube punched through the 1/8” base plate. I totally support SCTA’s decision to want a safer cage attachment. However, without access to FEA or the SCTA analysis I am concerned about the “improvement” on thin structures.
Is there anyone here that could FEA model a 1.625” or 1.750” roll cage upright tube loading the rule-book-base-plate welded to unibody body sheet metal with maybe two thicknesses (0.035” and 0.065”)? For load would 15-20Gs on a 3,800lb car be reasonable?
-
Comment regarding welding thick base plates to thin unibody structures.
The debate about how you weld a 1/4 inch thick plate to perhaps 18 - 20 gauge sheet metal got me to thinking and my first thought was to put a sheet steel doubler on the sheet metal structure that is larger than the base plate of the roll bar. Say for example you have a 6" square 1/4 thick plate, put it on top of a 7" square piece of 16 gauge that is stitch welded and plug welded to the sheet metal structure. That way the 1/4" plate has a much more difficult time trying to cookie cutter through the body structure as it has to shear 2 pieces of thin sheet metal rather than a single layer of sheet steel. There might be some concern about the strength of attachment of the doubler plate, so options include photographs taken during fabrication showing the matrix of plug welds hidden under the 1/4 inch plate, or use backup bolts to provide redundant attachment.
After that thought had passed I realized that the rule does not specify the edge shape on the 1/4" plate. I see no reason why the 1/4" base plate could not be full 1/4" thickness for the required surface area with a wide bevel at perhaps 30 degrees beyond that so the actual weld occurs between similar thickness materials.
This latter method would avoid the issue of concern about the strength of attachment of the thin doubler plates, and give more inches of perimeter to the base plate and a more ductile edge which might give under extreme impact rather than trying to cut a disk of sheet metal out of the body work.
Just some food for thought!
Larry
-
Not having witnessed the failures around LSR, I can only comment on SCCA cars.
The trend has been to go from 1/4" plates down to .080 min thickness (last GCR I had '09).
The issues I have seen are not the roll structure penetrating the foot, it is the foot tearing out the flooring under it. It is my understanding that the lesser material will deform and stay attached to the sheet steel without tearing.
IMHO, the materials used are secondary to the design and attachment of cage. The last car I caged, Miata open top had 20 points of attachment to the car. Of course, it was to help stiffen the chassis so it picked up the front suspension points and the rear sub frame mounts. Also, because it is a small car the cage mounted not on the floor, but on top of the rocker panel structures. All the attachment points (footings) were .120 plate and about 36 sq/in.
somewhere I have a video of a Mustang going on its lid and you can see the cage feet punch through the floor.
I will try to model up a few plates and upload them later.
John
-
Edit . . . the rule does not specify the edge shape on the 1/4" plate. I see no reason why the 1/4" base plate could not be full 1/4" thickness for the required surface area with a wide bevel at perhaps 30 degrees beyond that so the actual weld occurs between similar thickness materials.
This latter method would avoid the issue of concern about the strength of attachment of the thin doubler plates, and give more inches of perimeter to the base plate and a more ductile edge which might give under extreme impact rather than trying to cut a disk of sheet metal out of the body work.
Larry
Excellent idea, Larry. The beveled edge (maybe as much as 60 degrees) would tend to yield and therefore spread the force over a larger area. Anything to promote yield or ductility rather than shearing seems prudent. The welding of similar thickness should encourage.
I’m having trouble getting small pictures be accepted. I’ll try the pics in another post.
-
Edit . . . The issues I have seen are not the roll structure penetrating the foot, it is the foot tearing out the flooring under it. It is my understanding that the lesser material will deform and stay attached to the sheet steel without tearing. I will try to model up a few plates and upload them later. -John
(Edit) I am having trouble getting the pictures posted. It could be late night server problems.
The cars in the pictures have the ¼” plates welded to 0.035” sheet metal floor pans. Both cars are tech approved for >200mph. Welding ¼” thick plate to thin sheet metal begs a few questions. Neither car is mine but I have the same model and have measured the pan thickness in those areas.
SCTA does not share publicly any technical analysis. In our litigious society I can certainly understand, respect, and support that position. However, sometimes the lack of technical information raises questions. I am not saying the rule is unfounded or without merit. I am just concerned, but I lack the technical or engineering data to discuss this with the SCTA principals. If good data showed my concerns were wrong I would be at peace.
John you can see what is going on here. In addition to the straight plate could you model Larry’s idea with a 30 or 60 degree beveled edge? FEA or anything else that brings a better understanding is greatly appreciated.
-
Here they are: Please no comments on the weld quality or anything else. The car owners graciously let me take these pictures to help me. Pictures are only to demonstrate the rule book requirement of 1/4" base plates. The floor they are welded to is .035" sheet metal. Is this the best solution?
-
My 2 cents (as a newcomer to this sport), but as a metallurgical engineer with 20 years of professional experience. My thinking is that the photos shown above are not the ideal situation from either a welding standpoint (welding thick to thin), or a structural standpoint (tearing of the weld seams).
My thinking is as follows, and it may not be 100% right, but I'll put it out there anyway.... Since the roll cage tubing thickness is specified as 0.120" nominal wall thickness, I would personally prefer to see the base plate thickness match more closely with the tubing thickness specification.
If the base plate requirement for unibody cars was for 1/8" thick material, but with a larger base plate size (more perimeter area), I think the attachments would be more structurally sound.
My opinion here is not intended to stir any political issues (if there are any), only to provide my own technical viewpoint on this issue.
Steve M.
-
Did they lose you at FEA?
If you have a single post and put a 50 pound weight on top you know what the load on the bottom of the post is going to be.
If you have something like a roll cage and try to figure out the possible paths of stress it isn't something you can do on a spreadsheet, much less in your head.
Finite Element Analysis (FEA)) is a numerical technique for finding approximate solutions of partial differential equations (PDE) as well as integral equations.
What? That didn't help either? Don't ya hate the guys that use big words????
OK, a picture is worth a thousand words.
(http://upload.wikimedia.org/wikipedia/commons/thumb/7/7b/FEM_example_of_2D_solution.png/220px-FEM_example_of_2D_solution.png)
FEA is a computer modeling program that allows you to simulate stress loads and calculate the stress through the part.
My input on the 1/4" plate is that the 1/8 plate was viewed as insufficient to carry the load. What it doesn't address is how the load is transferred to thin sheet metal. This doesn't require FEA to figure out. Weld the 1/4" plate to the thin sheet metal. Weld a 10 foot tube on the 1/4" plate and then wiggle the end of the tube. What part of the system is flexing? The sheet metal. You have to spread the load and stiffen the underlying structure to avoid tear out or punch through.
(http://www.millerwelds.com/education/articles/images/fig5.jpg)
Ultimately you end up at .035, you have to spread the load to reduce the stress to manageable levels. I would use a plate as large as shown (WRC rally car) with a similar plate on the other side.
-
Dean has got the bull by the horns here, and he even stopped and had a "sandwhich". The rule book allows you to bolt together 2 plates top and bottom (or weld) to the floor. This is deemed the best solution using a unit-body at this time.
If during this conversation a better process is proven to emerge two things can happen. 1) you can always add or have more safety features, no rule against that. 2) submit said information to the SCTA befor the due date for next years rules change.
-
I think we are thinking about the same thing Saltfever, regarding the bevel.
I just did not specify which reference the bevel angle was from.
I was thinking 30 degrees from the horizontal.
Note that in World Rally Cup they skip weld many of the sheet metal panels to stiffen the car, it is lighter, uses less weld wire and time, and is just as strong as a continuous weld and more resistant to unzipping a weld as it has to start a new fracture each time it breaks a weld.
The thinner edge at the weld point will also facilitate much better fitup on the weld seam as a bit of persuasion with a Mark I big hammer will get a close fit between the sheet metal base and the support plate perimeter.
Larry
-
Thanks for the clarification, Larry. I would probably not leave a sharp edge on the perimeter. It might be best to leave an edge thickness equal to 1x or 2x the floor pan thickness. You know, stuff like the HAZ or burn back on the edge would be less problematic and the filet would be more closely matched to the substrate.
Welding engineers your opinions are welcome.
-
Did they lose you at FEA?
Finite Element Analysis (FEA)) is a numerical technique for finding approximate solutions of partial differential equations (PDE) as well as integral equations.
What? That didn't help either? Don't ya hate the guys that use big words????
Dean, if you are going to cut and paste a definition from Wikipedia it would be nice to see them get the credit! But at the very least, if you are going to directly copy text from another source, adding quotes shows respect and not plagiarism.
No, they did not lose me at FEA. I am the one that started this thread looking for FEA help. Even though I am not a structural engineer I started using FEA with NASTRANS. In the late 80’s I moved to Algor and then moved to Inventor just when they incorporated FEA into that package. Unfortunately, I have been away from those wonderful tools for almost 10 years. You know the old saying “use it or lose it”! Well, that is why I am seeking help from anyone that can make a contribution to a better understanding of the base-plate design. I’m missing the tools and my grey matter is rusty.
I am quite used to the tone of most of your postings so I’ll try and describe the purpose of this thread in my reply to Mike below. Your interest and help is appreciated.
-
Hi Mike and thank you for taking an interest in this thread. I’ll fill you in on why I am posting here. After the plate thickness was changed a couple of years ago I had a very brief talk with Lee Kennedy. Of course, Lee had all the information leading up to the decision and I had zero. Last summer I again talked to both Steve and Lee individually trying to get a better idea of the technical thinking for the increase in thickness. Both of them are supportive but as you know neither of them could share any SCTA analysis or the causative factor leading to the change. I am not being negative; I respect the process and their careful regard for disseminating information. I have no vendetta and no mission. Poor Lee and Steve are constantly bombarded with opinions. “Without data I’m just another guy with an opinion”! :-D I simply want a better understanding about the design so I can discuss it more intelligently with Lee. I have fallen behind the technology but there are others here that are smarter and current. I am only in research-mode, hoping that others will help me develop an accurate picture of the thick plate welded to very thin sheet metal. The thick plate welded to thicker material is not a concern.
I agree that load paths and cage geometry are very complicated. I am simply focusing on one aspect of the design. Whether it is welded or sandwiched, under load the perimeter is introducing shearing force into the substrate. I would like to see data, and not opinion, to eliminate my ignorance.
-
Sorry, wasn't directed at you. There are lots of readers that don't know what FEA is. Trying to give some extra help. Wasn't pointed at any one person.
Didn't quote wikipedia because it wasn't pertinent. Just quoting the jargon.
I am quite used to the tone of most of your postings
Ooooh, didn't know I had a tone! I hope the tone is melodic, not jarring.
Peace, love, out. :mrgreen:
-
Thanks for the clarification, Larry. I would probably not leave a sharp edge on the perimeter.
Agree I was thinking of leaving a thickness of about 2x the sheet metal thickness you are welding to also.
As you mention a sharp edge tends to pull back as it gets up to welding heat so you would definitely want to leave a bit of material on the bottom edge of the bevel edge so you have stock to weld too.
I think we are very much in agreement on the general layout.
One interesting thing to include in the finite element analysis would be the ideal thickness of that weld edge to allow flex and energy absorption at the plate edge to minimize the possibility of shear failure of the floor pan sheet metal by allowing the base plate to work with the sheet metal under plastic deformation without pulling welds out of the sheet metal.
Sometimes stronger is actually achieved by allowing some planned flex in the design structure, so no single point overloads and fails completely. You also want it to fail gracefully which is what skip welds and plug welds would give you. A severe impact might break one or two skip welds or plug welds but the other welds would have to undergo the full fracture development process from scratch, where a continuous perimeter weld once is starts to fail in shear or tearing at a corner or some other stress concentration point can just unzip as the fracture migrates around the perimeter of the plate.
Larry
-
I did some work last night on it and ran into a huge issue. This is the problem with computer generated models.
The issue I ran into was how to acurately anchor the floor plate to show the deformation in real world terms, meaning that the attached structure has to deform also. You would not ever think about just placing a flat foot on the floor without some sort of doubler/structure under it.
I will run a couple with different anchors and you can make up your mind as to the results.
I will stick with my earlier comment, cage design and integration/attachment is far more important than anything else.
John
-
Edit . . . Sometimes stronger is actually achieved by allowing some planned flex in the design structure, so no single point overloads and fails completely. You also want it to fail gracefully which is what skip welds and plug welds would give you.
Agree, yield or elongation is far better than a failure. That is the essence of my concern and how to model it in a manner that promotes helpful discussions with tech. Stitch welding is preferred when welding outside gussets on the cage (pg.25). I have seen the base plate either continuously welded or stitch welded on cars tech-approved for >200mph. However, that would be interesting to iterate in FEA.
-
The issue I ran into was how to accurately anchor the floor plate to show the deformation in real world terms, meaning that the attached structure has to deform also. You would not ever think about just placing a flat foot on the floor without some sort of doubler/structure under it.
John, sorry you are running into trouble. Are you saying your FEA can’t transfer the load path through a weld to the sheet metal? Do we need to give you a solid model from something like Solidworks or Inventor?
On the few cars I have looked at none . . . repeat none . . . use a doubler between the ¼” base-plate and the thin sheet metal! You can see my concerns and why I need clever people to look at this.
-
The issue that John Neilson is referring to in “how to accurately anchor the floor plate” is one that affects any FEA model. The model requires that the “floor plate” be restrained in some manner to keep it from flying off in space when a load is applied to it. Should he just extend the floor plate an inch or so beyond the cage baseplate, and fix the edge there, or should it be 3” or 10” or what? No matter what you do, you will get results for that particular model and those results would not necessarily be indicative of the result obtained for a different model. You eventually come around to the conclusion that to really know the answer one needs to model a substantial part of the belly pan, at least to the extent that it reaches some a more structural portion of the frame.
And when he talks about having a doubler “under it”, he clearly means under the floor pan, not the cage baseplate.
-
Last summer I again talked to both Steve and Lee individually trying to get a better idea of the technical thinking for the increase in thickness. Both of them are supportive but as you know neither of them could share any SCTA analysis or the causative factor leading to the change.
I would venture to say that SCTA doesn’t divulge their “technical thinking” because there essentially is none. To summarily go from 1/8” to 1/4” baseplates without the sort of evaluation that this thread is attempting to accomplish shows that there has been no real engineering done by them on the problem. As will be seen, it is not a trivial problem, and the 1/4” solution may well have simply moved the perceived failure point and/or changed the failure mode. If they knew it was an improvement, why would they not disclose the rationale by which the decision was made?
What probably happened is somebody looked at the result of an “incident” and noticed that an 1/8” baseplate was distorted, said “Holy crap, we better make those thicker so they don’t bend.” End of engineering analysis.
-
OK, here we go. both of these show a 1 5/8"x.120 wall tube on a 1/4" plate 6"x6".
Both are 1020 cold rolled stl, and the load is a force of 5000# applied to the end of the tube.
What you will notice is the plate retained on 3 side edges shows quite a bit of stress on the edges fixed in space, this is because the plate is deforming but not allowed to physically move and follow the deformation.
On the other sheet structure the edges of the sheet are fixed and the plate attached is allowed to deofrm and follow the structure defomation.
The last criteria not addressed is the acceleration/time for the force application, here it is constant.
Also, it is possible to run many different angles of force applied, not tonight.
John
-
I am not used to looking at this sort of display, but if I am interpreting it correctly, the upper model with the plate restrained on 3 sides is a closer analog to a base plate set in a corner with one side along a door sill and the other along a fender kick up as that would be strongly restrained against movement on two sides and free to deform on the open sides of the plate.
The other would be for a base plate in an open floor area with some minor support under the floor.
The peak shear stress is at the base of the tube where it wants to cut a biscuit out of the base plate and or floor pan if it is stiff enough to resist buckling under the applied load.
Load spreading gussets at the foot of the tube to spread the load across the base plate would seem the solution to that issue.
Larry
-
To follow on Larry's point, I would think gussets oriented toward the corners of the plates but only extending one half to two thirds the distance to the corners. That would give extra support to the punching effect of the tube while not keeping the base plate rigid enough that it becomes the punch. My seat of the pants engineering on such a design would probably use 3/16" plate for the base.
I like the way this discussion is going I'm seeing a whole bunch of well thought out ideas. :cheers: :cheers: :cheers:
Pete
-
On the other sheet structure the edges of the sheet are fixed and the plate attached is allowed to deofrm and follow the structure defomation.
Very nice, John. What I don't understand is there is no highly stressed sheet metal around the perimiter of the plate applying the force to the sheet metal. If the load was high enough to deform the 1/4" plate the thinner sheet metal should have yielded long before the plate deformed?
To keep this simple the plate is pressing on a piece of .035 sheet metal bounded on all sides. It would be interesting to see the sheet metal stress around the perimeter of the plate just before the plate yields or deforms. Then what force is necessary to start the sheet metal tearing. Once we see that picture the goal would be to use a thinner base plate (eg. 0.187") to see how it behaves and its influence on the sheet metal integrity.
OK, that is a lot on your plate but your help is greatly appreciated. I'll buy you breakfast at the Red Flame :cheers:
-
To keep this simple the plate is pressing on a piece of .035 sheet metal bounded on all sides. It would be interesting to see the sheet metal stress around the perimeter of the plate just before the plate yields or deforms.
If I understand the issue with FEA is how to set up the model so it is representative of the real situation.
Perhaps you need to think about a 0.035 sheet of metal about 2 ft square bounded rigidly at its edges, with the 1/4 inch base plate positioned in the middle or toward one side, and its edge fastened to the sheet steel. Under load that sheet steel should look like a trampoline with a heavy plate sitting in the middle of it. Maximum stress would likely be at the corners of the base plate where it focused the load just before the 1/4 inch base plate started to bend.
It would probably start to cookie cutter the sheet metal at those corners and then punch through the sheet metal around the permeter of the base plate rather than the tube punching through the base plate center.
Which if true would imply that the safest base plate shape is not a square but a shape with rounded or truncated corners like a hex or a disk that would be less likely to concentrate stress at the corners.
I also much appreciate the modeling effort to facilitate the discussion.
Larry
-
The basic idea is to model a real world scenaro. You would never simply mount a plate flat on a unsupported piece of sheet.
The sheet model I show is .030, and it is 2 pcs bent 90° with verticals directly under the plate/tube location. While not a true model, it represents something possible. What is not shown is that the tube displacement on the sheet supported test moved about double the distance of the 3 side supported plate. This does represent the structure moving or absorbing some of the force. The pictures show very exaggerated deformation than actual, to make it easier to see small details.
You are correct about the sharp corners on the plate possibly being the stress riser and tearing the sheet.
There are many, many factors that play into the construction. I have a friend who was almost killed when the cage broke away from the chassis during a crash. The failure was not the footing plates tearing away from the sheet, instead it was the cage tubing breaking the welds from the plates. It seems that the welds were marginal and rusted. I don't remember the foot thickness, but the car was from a time when 1/4" was thought to be standard. I just confrimed the SCCA min spec is .080 thk ('09 GCR '07 spec).
John
-
Unfortunately it's somewhat hard to detect if a weld has penetrated properly but in most cases where a weld is not washed out into the main metal at the edge of the weld there is going to be a lack of penetration problem. I see this rather often in pictures and it makes me shudder when I know the vehicle has made it through tech with some organization. One of the problems with the ubiquitous mig welder is that it can make a really nice looking weld that instead of penetrating the plate tends to be cast up against the plate. Three things to help get effective penetration are:
1. turn up the heat
2. be sure to clean all the mill scale off the base plate. Mill scale is a pretty effective heat barrier and encourages the casting effect.
3. use some form of manipulation, usually a slight weave which helps to stir up the weld puddle and encourages penetration.
4. again, don't be afraid to turn up the heat. If you can't handle that without blowing holes find someone who can. Cold welds can kill! :oops: :oops: :oops:
Pete
-
To keep this simple the plate is pressing on a piece of .035 sheet metal bounded on all sides. It would be interesting to see the sheet metal stress around the perimeter of the plate just before the plate yields or deforms.
If I understand the issue with FEA is how to set up the model so it is representative of the real situation.
Perhaps you need to think about a 0.035 sheet of metal about 2 ft square bounded rigidly at its edges, with the 1/4 inch base plate positioned in the middle or toward one side, and its edge fastened to the sheet steel. Under load that sheet steel should look like a trampoline with a heavy plate sitting in the middle of it. Maximum stress would likely be at the corners of the base plate where it focused the load just before the 1/4 inch base plate started to bend.
It would probably start to cookie cutter the sheet metal at those corners and then punch through the sheet metal around the permeter of the base plate rather than the tube punching through the base plate center.
Which if true would imply that the safest base plate shape is not a square but a shape with rounded or truncated corners like a hex or a disk that would be less likely to concentrate stress at the corners.
I also much appreciate the modeling effort to facilitate the discussion.
Larry
Great discussion - here's my quick look based on comments. Lots of assumptions in the setup!
This is not a simple problem when you are dealing with structures. Which is why FEA is so useful! FEA lets you see how the various ideas respond then an optimization study can be run on the better ones!
Ultimately the cage and structure would need to be modeled and then drop tests performed at various angles to see the effects of the peak loads from the potential g-forces. But you would still start with simple models like these to get your head around it all!
Bridge builders were early hot-rodders of a sort. If the bridge did not collapse you were labeled a successful bridge builder. Then you hit the cost/benefits ratios, public liabilities and you have to start engineering for success. And even after we have "figured" it out we still have bridges collapsing!
Any engineering study would have to reviewed and certified by a professional engineer before you would go into the public marketplace. After a disaster the normal response is to over-engineer it! CYA!!! :cheers:
-
Two more!
-
The square/rectangular shape of the plate is only useful if the axes are normal to the force received (viz., presenting a long side with the edge to the vector).
A quartering impact (left front fender hits tree) converts the harmless left front corner of the plate into a shearing wedge.
Trying to estimate the effect of a formed 3-dimensional floor is pretty tough. The stiffness under the plate is asymmetrical left to right, with the outboard side almost always stiffer than the inboard side due to the proximity of a rocker, tube or other shape, where the inboard side has almost flat floor (subject to both bending and racking) unless there's a driveshaft tunnel.
-
I certainly appreciate the FEA work done by John and Woody, and I am sure that everyone that has posted on this subject can look at these analysis and see that there are very inherent and dangerous consequences from this design. I also think that all of the people that have posted recognize, without any FEA analysis, that the picture posted by Saltfever of the down tube welded to a 1/4 inch plate in the middle of an expanse of thing sheet metal is wrong and a disaster looking to happen but what is frightening is that this cage was built to SCTA rules and was in a car that had passed SCTA inspection at Bonneville. Whether SCTA will or can change the rule criteria for this type of cage connection is a question I don't think we can answer but I do think at all of us who have or will build cages for uni-body cars would and will not make this mistake simply because we can recognize the danger of this type of connection.
As John said "cage design and integration/attachment is far more important than anything else." These are word that we all need to keep in mind the next time we fire up the welder and tube bender. You have to get the cage to distribute the load throughout the car structure..
Rex
-
There is a big difference in vehicle types. Coupes and sedans and open cockpit type. Coupes and sedans have the advantage of the body absorbing much of the impact acting as a crush zone where a open cockpit type vehicle the roll cage takes the impact. I have never seen a tube to plate torn away from the body with the welded and in some cases bolted as well. In open cars I have seen roll bars bent and in one case the roll bar torn off. This was not because of a plate type attachment but thin wall tubing. Maybe SCTA will address this further or clairfy
it as the reason for the thickness change.
-
I think it is also important for folks to remember that many of these cars weigh 4000# - 5500# which is far heavier than most other motor sports environments. The only racing organization that I am aware of that has comparable vehicle weights is the desert truck racing series, and their roll cage regulations specify cage tube size and wall thickness based on GVW of the vehicle in race condition.
Larry
-
I don’t know what to say but a big THANK YOU to all of you that have recently joined in on this thread. All of your postings, on just about any subject, add value and are carefully thought out. There are always certain particular names I look for on this forum and you guys are coming on-board this thread. Please hang around. My apologies for the length of this post . . . I promise shorter ones to come.
John, re your post #25 I agree a real world scenario would be best but the complexity could easily overwhelm people’s generosity and sharing at this point. FEA still takes CPU horsepower and I don’t want to get so complicated that enthusiasm is killed. I don’t know the computer ET now days but I can remember starting calcs that ran for 8 hours. If CPU power if not an issue I’m still after simplicity if only to get a basic picture or understanding, to help filter through the complexity.
There is essentially no typical real world scenario. As Glenn pointed out, in the more than 500 entries last year, every one of those were different! The number of attach points on any one cage can be 6,8,10,12, or 14. All would be approved by SCTA. The geometry of cage designs is about as individual as fingerprints! Everyone is different and none will be the same. Even a design from someone like Chris Alston who replicates roll bar kits from a NC mandrel bender will be “personalized” by the installer.
Here is the only common scenario. The rule book forces us to weld (or bolt) a ¼” plate to a piece of sheet metal approximately 0.035” thick. That would be a Vega, Monza, Camaro, etc. I have not measured Ford products. That base plate used to be only 1/8” thick but for some unknown reason was changed to ¼” thick. The purpose of this thread is to try and see how the increased thickness changed the failure mode and if some other thickness is more beneficial. We do need to see loading that produces failure. Again, for simplicity, let’s keep the load orthogonal.
To further complicate the issue there is no data on a Bonneville crash. If you watched the Danny Thompson crash video, a good engineer may be able to infer approximate G forces by measuring time and distance but that would be only one data point. Regarding the landing attitude of the car . . . who knows? In other words; on a 10 point cage did one corner hit first, 4 corners, or more than that at the same time, etc? I have seen anything you can imagine and Glenn has seen a ton more. There is no typical accident at Bonneville. That being said, I “think” G-loading could be 10, 20, or 30Gs . . . on single or multiple members! So our model loads, most likely should be greater than 5,000lbs.
A real world model would be at least 500 plus individual scenarios and 500 solid model cage designs. We simply can’t go there. Let us focus on a generic sheet metal patch, bounded on all sides, a perpendicular force, and vary the base-plate thickness. Let’s get that picture first. Complexity can be added according to thread interest.
-
The simplistic model.
1.) 1 square foot of 0.035” sheet metal bounded on all 4 sides
2.) The rule book base plate welded in the center.
3.) A 1-5/8 x .120 wall mild steel tube welded perpendicular to the base plate
4.) Load applied perpendicular to base plate
5.) Stress & Strain at failure.
Change No. 1.
Base plate changed to 0.187: thickness
Change No. 2.
Base plate changed to 0.125” thickness
Change No. 3
1/2”radius corners on the 0.250” base plate.
Change No. 4
1/2”radius corners on the 0.187” base plate.
Change No. 5
1/2”radius corners on the 0.125” base plate.
Change No. 6
60 degree edge bevel on half the thickness with radius corners on 0.250” plate
Change No. 7
60 degree edge bevel on half the thickness with radius corners on 0.187” plate
Change No. 8
60 degree edge bevel on half the thickness with radius corners on 0.125” plate.
-
Ref. Reply #28, figure 1
Woody,
I am not convinced that the model is as representative as it should be--there seem to be some obvious peculiarities in the results. Granted, it was a rush job.
1) Why are the stresses at the ends of the 1/4” baseplate higher than those in the thin sheet metal that they are immediately attached to?
2) Why are the stresses at the ends of the 1/4” baseplate higher than those in the thin wall tubing that is welded to the middle of the plate (being thinner than the plate and much smaller than the length of the plate--i.e. having a much smaller cross-sectional moment of inertia) ?
3) Why are the stresses at the ends of the 1/4” baseplate higher than those in the plate itself at the area where the tube is welded to the plate?
4) Why is the stress distribution in the thin sheet asymmetrical?
5) Why does the stress distribution in the sheet have that odd banded pattern?
6) Seems very peculiar that the maximum stress is at an unrestrained external upper corner of the baseplate.
7) How about a plot of the FEA elements?
8) What FEA are you using?
-
IO this was just a quick & dirty in SolidWorks Simulation Express to emulate "pushing" on the pipe and wiggling the sheet metal!
As I stated there are a lot of assumptions in even a simple model like this. The .035 sheet is 24" square because it is restrained around the outside and we don't want to see that applied stiffness in our wiggle test. I applied a .035 "weld" bead around the outside of the .25" base plate so the only constraint was at the outer edges and the .035 sheet can distort under the .25 plate. If it were bolted then the contact conditions and reactions would change. Or if the plates were brazed together.
As you have deduced most of the visual anomalies aka errors are related to the coarseness of the mesh. Normally you start simple and then refine the mesh until the answer does not change much (converges) and that's your result.
It looks like Saltfever wants a load test that is straight into the sheet metal under more controlled conditions. This sounds more like what is the punch press force to poke a hole in the sheet metal.
I am stacked right now but I will try to model this in between jobs. I will try to post something simple later for feedback and then refine it.
-
Salt,
What you are asking for is not applicable to any real application. It is like asking which screw driver point will penetrate a paint can lid easiest.
Just to test this I modeled 2 scenarios, 1/4" plate and .080 plate.
Take a look, both fail, like the Mustang poking the cage through the floor of the car.
Best regards, John
-
Well John I’m not sure what to do. If you count just sedans there are probably 300-400 unique designs. The only thing they share in common is the attachment interface required by the rule book and a very thin sheet metal floor pan. Even at that there could be 8-14 attachment points.
When SCTA summarily increased the thickness of the base plate 100% I wanted to see the effect it had on only one point to keep the FEA effort manageable. The force and load path possibilities are almost infinite. However, with the knowledge you and others bring to bear an individual might use it to evaluate their unique case. What do you suggest?
What load caused the failures in your examples?
-
More than one way to model a cage :)
These were models I built out of 1/4 inch polyethylene tubing and hot melt glue.
I got a lot of lessons about how a roll cage structure shares loads with these tests.
Bought the tube in rolls, cut it in 36 inch lengths threaded them on welding rods and heat soaked them in the oven on warm then let them cool still on the welding rods to straighten the tubing. Then cut and notched with an exacto knife and assembled my prototype cage structure with hot melt glue (sets fast, is flexible and tough).
Allowed me to apply force to a real object and see what tubes gave under the load. Sometimes the load sharing was surprising. The rigidity of the side hoops is strongly effected by the stiffness of the fire wall bulk head mounts. If that is not stiff a downward force on the top of the side hoop pushes the bend at the front of the door near the firewall forward. Stiffen up that front fire wall area (why I inserted the cardboard bulk head) and the structure becomes much stiffer.
side hoops were heated then formed on a cardboard template and allowed to cool to freeze their shape, same for the main hoops.
Larry
-
More tests with stiffened bulkhead at firewall.
-
Buckle behavior without restraint at the firewall to prevent the side hoop from kicking forward at the firewall bend.
-
Awesome idea HotRod. Now I have some ideas that my 10 year old son and I can work on to model the cage in my Rampage.
It looks like you have some good scale modeling skills. Is the polyethylene cage made to a particular scale (1/10th or so)?
Steve.
-
Salt,
Like I said earlier, this is not a real world application.
The force applied to the previous models was #5000.
I did another at #50 load, it lived.
This shows a #100 force and failed.
John
-
Is the polyethylene cage made to a particular scale (1/10th or so)?
It is scaled so that the 1/4 inch tube is a scale 1 5/8 or 1 3/4 or what ever size tube I was planning on (simply can't remember now)
If you look closely at the cardboard floor plan on a couple of those pictures you will see scale dimensions.
I "think" I was using 1 mm = 1 inch, which would scale the tubes to 1.5875 diameter. It is not exact but a reasonable approximation of a scale size.
Metric scale in mm at 1 mm=1 inch works nicely because for all practical purposes it is a 1/25 scale (1 inch = 25.4 mm)
To hold some of the tubes in place long enough for the hot melt glue to set, dress maker pins worked nicely to "pin" the end of the tube in place long enough
for the hot melt to cool.
It was fun to build up the cage incrementally, and play with it then add a tube to try to limit a certain weakness and repeat. It is surprising how stiff it becomes with the addition of a couple critical tubes, and very interesting to see where the inherent weak points of a common cage. The most critical weak points in that design were always a front diagonal impact on the drivers side top corner of the windshield, and the middle of the top bar across the front windshield.
I was never really satisfied with how much less force it took to bring that front windshield corner down compared to the main hoop behind the drivers head.
That bar that runs across the top of the door and down the A post of the windshield needs to be a double tube (stacked vertically) to really stiffen it up to that front quartering top inpact.
Larry
-
Is the polyethylene cage made to a particular scale (1/10th or so)?
It is scaled so that the 1/4 inch tube is a scale 1 5/8 or 1 3/4 or what ever size tube I was planning on (simply can't remember now)
If you look closely at the cardboard floor plan on a couple of those pictures you will see scale dimensions.
I "think" I was using 1 mm = 1 inch, which would scale the tubes to 1.5875 diameter. It is not exact but a reasonable approximation of a scale size.
Metric scale in mm at 1 mm=1 inch works nicely because for all practical purposes it is a 1/25 scale (1 inch = 25.4 mm)
To hold some of the tubes in place long enough for the hot melt glue to set, dress maker pins worked nicely to "pin" the end of the tube in place long enough
for the hot melt to cool.
It was fun to build up the cage incrementally, and play with it then add a tube to try to limit a certain weakness and repeat. It is surprising how stiff it becomes with the addition of a couple critical tubes, and very interesting to see where the inherent weak points of a common cage. The most critical weak points in that design were always a front diagonal impact on the drivers side top corner of the windshield, and the middle of the top bar across the front windshield.
I was never really satisfied with how much less force it took to bring that front windshield corner down compared to the main hoop behind the drivers head.
That bar that runs across the top of the door and down the A post of the windshield needs to be a double tube (stacked vertically) to really stiffen it up to that front quartering top inpact.
Larry
Now that is a great idea! as well as the building of a scale model - :cheers:
-
Of course, you could look at a failure...............
Trees move at a pretty good clip when the car is on its side.
John
-
Do you guys think it would be a good idea to tie the A-pillar bars of the cage to the actual A-pillar of the car's structure (maybe by using several short sections of 1/8" thick sheet) to create a truss-like structure?
Steve.
-
Trees move at a pretty good clip
How did Lynnard Skynard phrase it -- "Oak tree, get outa my way"?
-
Steve, if you were going to do that one thought might be to use weak materials to try and create a crumple zone to help attenuate the G-force. Although you would not want to do anything that would weaken the integrity of the cage. AFAIK NASCAR doesn't do that and surely they have spent $Ms on testing.
-
edit . . . Like I said earlier, this is not a real world application.. . . This shows a #100 force and failed.
Well John I am not a structures guy so please evaluate my untested hypothesis. While failure is failure it can be either benign or catastrophic. AFAIK a tensile deformation-to-failure is less catastrophic than a shear failure. A design that fails in shear will fail with less force than a design with tensile deformation. When the base plate size changed to ¼” thickness (on very thin sheet metal) it changed the failure mode from tensile deformation to catastrophic shear of the sheet metal. I was hoping FEA would test the truth of that supposition. If true that data (and pictures) could assist in cage design and talks with SCTA tech.
Are those loads correct? #100 caused a failure of the thin sheet metal floor pan!
Thank you for your patience and continued support.
-
Salt,
The application of the 1/4" foot plate is to distribute the tube applied forces to the cars structure without just punching through.
If I am reading the rule correctly, it pertains to bolted application with top and bottom sandwich construction. If this is true, the 1/4" plate thickness is to retain the mounting bolts more than anything else.
Yes, at #100's this is why the test is less than desirable.
In answer to your comment about stitching the cage to the chassis, especially the A pillars, it is done all the time. Look at rally cars and road race cars.
John
-
Tony Foale has some useful hints on his web site http://www.tonyfoale.com/Articles/index.htm, samples from his excellent motorcycle chassis book.
-
If the failure mode of the 1/4" base plate is a shear failure the most direct solution would be to increase the perimeter area of the base plate so it has more material to shear. Perhaps the most straight forward solution would be to specify a larger base plate perimeter on unit body vehicle cage base plates or a pair of 16 gauge doubler plates 2x the size of the 1/4" base plate sandwiched between the 1/4" base plate and the body sheet metal and repeat on the bottom backing plate.
Stitch weld those 16 gauge doubler plates to the floor pan and you substantially increase the shear strength of the floor pan in the area of the cage base plates. You would have to do much more work to punch the base plate through that 3 layer sandwich of 16 gauge doubler plates and the floor pan.
Larry
-
John, yes your interpetation is the written part of the rule. However, the “interpretative” part of the rule allows welding a top plate only. Look at my pictures. Those cars have only a welded top plate! No doubler, repeat . . . no doubler on either the top or bottom. But here is the written rule.
On a unitized construction and monococque cars, the roll cage structure and braces shall have ¼” thick support pads on the top and bottom of the floor (or sill) in a sandwich construction and shall be of sufficient area to support an impact load equal to the weight of the car. For cars weighing . . . over 2500 pounds shall have at least 22 in perimeter.
So model it that way. Bolt two ¼” plates together, squeezing 0.035” sheet metal in the middle, and then load it to the cars weight (4,000 lbs) and show the results. Assume there is about 6” of fixed sheet metal all around. OK, it is not real world but please tell me if it is in shear or tensile deformation and if it fails, what was the load at failure.
Yes, the rally cages are beautifully engineered structures. I have taped a few programs and am looking for a frame grabber so I can print it out and study it further.
-
Saltfever and Johnneilson, you guys need to get together on what you consider “failure” to be.
It appears that John’s 100 lb “failure” was so-deemed because most of the thin sheet around the baseplate edges was stressed to the yield point of the material. Yet, I think we all would agree that if we built an actual version of John’s model and loaded it with 100 pounds, it would probably flex the sheet some, but would not produce a catastrophic failure.
It would be interesting for John to go ahead and do an elastic-plastic model with a realistic stress-strain curve for the material and see how much more load and deflection could be carried before exceeding the tensile strength.
With the direct downward loading, it is probable that the thicker baseplate detracts slightly from the load carrying capability of the sheet. However, that may or may not be the case for a bending load like Woody tried earlier. A number of geometric variables would enter into how that condition would perform for various baseplate thicknesses. If John has nothing better to do.....
In any case, these investigations show what should be fairly apparent--depending on a thin sheet to provide any substantial structural capability is hopeless. Roll cage tubulars should be routed to the strongest features of the frame, with spreader plates, gussets, and doublers as needed. If that location is not as convenient as the floor pan, just remember the 100 pound analysis results.
-
I think the real test is more the one shown on reply #20.
I have 2 pcs of .030 material bent 90° and the vertical pcs directly under the tube loading.
Take a look at the deflection and the loading, it clearly shows that even this simple sheetmetal can hold up to #5k.
Yes, 5000 as compared to the flat sheet at #100.
Is it ideal, no, it shows that the cage must be attached to structure, horizontal as well as vetical.
It sounds like someone doesn't like the 1/4" rule for what ever reason. I think it is irrelevant, if the construction isn't supported it doesn't matter what the foot plate is.
I now need to get some work done.
John
-
To my earlier point this is not such a simple problem! When I was working for the man we used to call this project scope creep! The more you discuss it the more everyone wants! :-D
If you restraint the floor material in the FEA model too close to the base plate then you turn the floor and base plate into a die set and you only shear the floor material. If you move it farther away then you can see the elongation before the floor gets "tight" and then fails. Think trampoline and the circus fat lady if we are going straight down! More stress in tension than in shear! If you look at my earlier shots that's a 500 lbf load on the lever and the material is buckling before it fails in shear.
Saint-Venant's principle says that several characteristic dimensions away from an effect the effect is essentially dissipated. In this case that means that once the base plate is 3~5 times as thick as the floor that's probably as good as it gets! So if the base plate is a little thinner (.12"~.19") then it will deform as well as the floor and help to prohibit the tube from penetrating. Actually a series of thin plates laminated together would probably work even better! When they tested for meteorite penetration on space vehicles they found that a series of thin copper plates worked better than thick armor plate. Each plate stretched then failed until the energy was completely dissipated and the last plate held.
Impact loads are what we really want to know. The peak impact load may be high but as the material stretches it absorbs the energy and the loads go down. Remember it's not the fall that kills you it's the sudden stop! :-(
Complex crash tests are done in the computer now but with lot's more HP than we have on tap!
Keep talking we may finger this out yet! :cheers:
-
Keep in mind that by definition any unmoving anchor point will give false results, if the model is too restricting.
In reality, the floor pan forms a sagging sheet with a point load in the center (roll cage base plate in this case).
If you work the geometry the tensile loads on the floor go to infinity as the floor approaches being completely flat.
As soon as the floor sags even slightly those tensile loads drop very rapidly.
It is basically a geometry of the angle problem. If you simplify the situation and represent the sheet metal by a single infinitely rigid cable.
If you specify the "dip angle" as "D" the angle below the horizontal that a cable forms under load (measured at the anchor end), then the tension in the cable is (F/2)/(sin D).
So for a rope or cable stretched between two anchors and loaded in the middle with a force F applied in the center, the tension in the cable will be:
D=5.0 degrees, tension = 11.474 x F
D=4.0 degrees, tension = 14.336 x F
D=3.0 degrees, tension = 19.107 x F
D=2.0 degrees, tension = 28.654 x F
D=1.5 degrees, tension = 38.202 x F
D=1.2 degrees, tension = 45.750 x F
D=1.0 degrees, tension = 57.299 x F
D=0.8 degrees, tension = 71.622 x F
D=0.5 degrees, tension = 114.593 x F
D=0.3 degrees, tension = 190.987 x F
As you can see, as the floor approaches being completely flat the stress in the sheet to support a weight skyrockets until the floor stretches slightly under the load to achieve a dip angle of a few degrees. Even small loads will generate huge stresses until the floor dishes slightly to carry the load.
This is one of the hardest things to model as stresses might be substantially increased or decreased by the stiffness of structure elements far away from the point of load application. One of the reasons this simplified FEA is only a very rough approximation. There are two stresses in the floor sheet metal, a shearing stress at the perimeter of the roll bar support pad that depends only on the perimeter length of the pad, the thickness of the sheet metal and the applied load. The second is a tension stress in the sheet as it sags to carry the weight. It can fail due to either stress.
These are only theoretical values because no real material is infinitely rigid (has no stretch), so even very light loads placed in the center of a sheet of material will stretch the surface so it is never completely flat. It is physically impossible to support the load without developing some dip angle in the surface.
As a rough back of the envelope analysis:
If you apply 5000 # to a plate with a 21 inch perimeter resting on .035 thick steel the shearing stress at the edge of the base plate will be 5000/(21x.035)=6802 psi shear.
Shear strength is commonly about 75% of tensile strength, so an assumption of shear strength of about 25,000 psi - 40,000 psi would be reasonable.
That means in pure shear that 21 inch perimeter base plate would shear the sheet metal at a load of about 18,375 - 29,400 pounds applied load.
On a 5000 pound car slamming down on the roof that would be an impact of 4-5 G's would be enough to blow the base plate through the floor if the full impact was focused on a single base plate due to circumstances of the impact.
Needless to say any side loading that tended to bend the base plate over to the side turning a corner into a can opener and that number drops significantly.
Larry
-
the best results I have had were with adding a 2x2 square tube welded in a channel cut in the floor to tie the front and rear sub frames together and then weld the ¼ reinforcing base plate for the cage from the 2x2, to the floor and then to the floor sills. do this for the front and rear mounting points.
-
Not having witnessed the failures around LSR, I can only comment on SCCA cars.
The trend has been to go from 1/4" plates down to .080 min thickness (last GCR I had '09).
The issues I have seen are not the roll structure penetrating the foot, it is the foot tearing out the flooring under it. It is my understanding that the lesser material will deform and stay attached to the sheet steel without tearing.
IMHO, the materials used are secondary to the design and attachment of cage. The last car I caged, Miata open top had 20 points of attachment to the car. Of course, it was to help stiffen the chassis so it picked up the front suspension points and the rear sub frame mounts. Also, because it is a small car the cage mounted not on the floor, but on top of the rocker panel structures. All the attachment points (footings) were .120 plate and about 36 sq/in.
somewhere I have a video of a Mustang going on its lid and you can see the cage feet punch through the floor.
I will try to model up a few plates and upload them later.
John
I think the is the crash that you are referring to of the Mustang where the cage punched through the floor. Here is a link to Jalopnik that shows a photo sequence of the crash.
http://jalopnik.com/5390934/mustang-cover-boy-tries-to-corner-flips-over-tire-wall
Here is a forum link that has a pic of where one of the tubes punched through the floor.
http://www.svtperformance.com/forums/pics-videos-buffet-149/646980-carfx-eats-hallett.html
-
This is the picture you are referring to. Notice how the uprights punched through the floor. This is a classic case of shear. Many good comments and suggestions have been made to this thread. Especially, how my simple model does not represent a real world scenario. I certainly agree that my model is unnecessarily simple and not representative of any SCTA cage. But the problem is there is no predictable or repeatable model for a Bonneville accident. The load path for any structure is completely random and unpredictable. See next post.
My concern is addressed in Larry’s post No.9. Obviously, there will be many attach points in any SCTA approved design. What I’m concerned about is that by adding extremely thick base plates, with thin sheet metal in between, the plates are acting just like blades on a shear. You have a thick, stiff, piece of steel transferring load to a very thin piece of sheet metal. (By definition the rule is for monocoque or uni-body cars, all which have sheet metal assemblies). As Larry points out (ref no.9); attachments in shear will have a greater tendency to unzip than a design in yield. For any given dimension a shear load path will always be weaker than a path in tension or ductile deformation.
-
Sorry about the small pic and thanks to Ray Therat for the photo. This was in 2010.Do you know how this will land? Will it be on single point, flat on the roof, front first, back first? Exactly! . . . There is no predictable landing and thus, as John has suggested, you would certainly consider other structural geometry in your base plate design. However, the rule book requires ¼” thick plates. I am concerned that these become “shear-plates” and I am asking for help with an FEA model that shows one way or the other if this is true. Would a thinner base plate accept more force before a failure? Will it deform and yield before failure? All of you are creative and can design an acceptable cage. But does the rule book put your design at risk from the start with a less than acceptable base plate? I don’t want to added complexity; I only want to see the essence of the rule. Load-path angles, supports, gussets, all can be added at your pleasure. That is not the issue. Would all those parts, with a thinner base plate make for a better attachment?
-
Saltfever,
You seem to be convinced that because the rollbar baseplate “punched through” the floor pan that it was a shear failure. However, to have a failure in shear there must be two opposing forces in close proximity acting across the material section to produce shearing stress. How many scissors have you seen with only one blade?
The floor pan failure is substantially a tension failure, but the failure is very localized, namely around the perimeter of the baseplate where the floor pan is being loaded by the edge of the baseplate. Load from the baseplate causes the pan to sag and develop tensile stress in the pan material, since the outer edge of the pan is presumably restrained from moving. As the load increases, the sag increases, and the tensile stress increases until the tensile strength is exceeded. This occurs at the edge of the baseplate because that hard edge creates something of a stress concentration there due to the bending of the pan around the corner, and also because away from the baseplate the pan has more material to carry the load, and the tensile stress is therefore less in those areas.
So, given this failure mode, it is apparent that the bigger the baseplate the more the load that can be carried--which is the reason for having a baseplate in the first place. Eliminating the stress concentration caused by the hard edge of the baseplate, by tapering or rounding the edge or using a thinner baseplate may make a small improvement in the load capacity, but probably not much*. And those actions may reduce the ultimate capacity if they, in effect, shrink the effective perimeter of the plate and cause even less of the pan material to be carrying the load. Those actions to reduce the stress concentration may, however, result in marginally greater energy absorption before failure since more material is being stressed.
*Because, once the material with the highest stress in the cross-sectional thickness reaches its yield point it begins to stretch, passing any increased load onto the region next to it until the whole cross-section has yielded and the whole section then stretches until it reaches its ultimate strength.
-
I can appreciate What your looking for here Saltfever, information is a good thing.
I would just point out the car in your photo sequence did not have a cage failure and the driver recovered to drive again. The crash happened at ~ 260 mph IIRC. Donny can add info if needed.
-
Saltfever,
You seem to be convinced that because the rollbar baseplate “punched through” the floor pan that it was a shear failure. However, to have a failure in shear there must be two opposing forces in close proximity acting across the material section to produce shearing stress. How many scissors have you seen with only one blade?
Without a careful examination of the fracture surfaces and surrounding area of the Mustang punch-through, we are just guessing about the fracture mode. However, in the case of a car which has flipped upside down and landed on its roof, there are definitely opposing forces in close proximity at the junction of a 1/4" baseplate and a 20 gauge sheet metal floor pan. The force of the baseplate in that case is in the "upward" direction, away from the salt, and the force of the sheet metal floorpan is in the opposite direction. My own suspicion is that "it depends" on the location of other support points for the 20 ga. sheet metal.
-
SteveM,
Just because there may be opposing forces doesn’t mean they are in sufficiently “close proximity” to produce a shear failure. Of course, it does depend on the “location of other support points for the 20 ga. sheet metal” but if that support is not within a few thicknesses of the sheet to the edge of the baseplate how does shear develop?
What is the origin of the closely applied opposing force you postulate? Inertia of the 20 gauge sheet? Bending stiffness of 20 gauge sheet? Both are negligible.
It is folly to expect a thin diaphragm to provide significant structural support to a concentrated load.
-
The origin of the opposing force would be inertia from the rest of the car that has just slammed into the ground, which is attached to the 20 ga. sheet. As far as I know, we don't have photos of the Mustang's roll cage installation before the crash to see how it was supported. A close examination of the pictured Mustang, both before and after the wreck would be most helpful for the purposes of failure analysis.
The FEA analysis is a great tool, but as someone said, in order to accurately model the loading and stresses would require a whole lotta time to accurately depict the actual conditions for each car, and would require a lot of computing horsepower. The stresses would certainly include tension, compression, bending, and shear at the various locations near the support point for the roll cage.
Steve.
-
Due to the fact that the roll bar feet both punched through, it is a pretty good assumption that it was a bolt in bar and probably 4 points of contact. It was probably made from decent materials, 1 3/4" dia? It probably had 1/4" plates? The tubing proved that it was adequate.
The failure was the fact that the roll bar didn't have any structure to concentrate 4 points of contact. If, the earlier model showed severe sheet deformation at #100 loading per foot it is not reasonable to assume that the design was sound. Car wt of #3000/4?? In this case, it wouldn't have mattered if the feet were .08 thick or 1" thick. BTW, from the picture the feet look to be very small.
Thinking back on this, years ago, when all I could do was get bolt-in bars and cages for my projects. I modified all the foot plates be radiusing the contact side to reduce the stress concentration against the existing car material. I also shimmed the feet with 16 ga and made them larger than the feet. It was something that I did and I don't remember where the technique came from.
I do not know this for fact, however it is a reasonable assumption that the SCTA uses SFI or NHRA as a guideline for specs in the rule making process.
John
-
Here's a look at the aftermath from the flip above, at 260 mph.
So, during all this talk, how is the cage "proven" in a monoque (sp?) chassis?
I guess the simple solution would be ban unit bodied cars..... :evil: :-D
-
Here's a look at the aftermath from the flip above, at 260 mph.
So, during all this talk, how is the cage "proven" in a monoque (sp?) chassis?
I guess the simple solution would be ban unit bodied cars..... :evil: :-D
Michael,
I think the solution is to read all of the rule specification and then build accordingly.
John
-
A photo of the Mustang foot well after being loaded on a wrecker with, presumably, the protruding part of the cage cut off. It is not clear if the whole floor panel portion was ripped out in the incident or if it was removed at the same time the tubing was cut off for transport.
It would seem to be a much better solution to have landed the roll bar on the much more substantial cross frame immediately behind the bar, or the rocker channel just to the outboard. But that is not as simple for the fabricator (seller) or installer, regardless of the probable benefit to the driver.
-
Just my opinion, the cage? was not designed well. It missed very substantial structure less than 4" away. I will reserve other comment.
J
-
From turboford.com on the Mustang:
"It's an AutoPower 4-point bar. (Was 'cage', changed it to 'bar'.)
The car was built by these clowns for a magazine article.
http://tulsacarfx.com/carfx1/index.php?option=com_content&task=view&id=14&Itemid=61 (http://tulsacarfx.com/carfx1/index.php?option=com_content&task=view&id=14&Itemid=61)
An article.
http://50mustangsuperfords.automotive.com/118359/m5lp-0912-2010-mustang-gt-car-fx/index.html (http://50mustangsuperfords.automotive.com/118359/m5lp-0912-2010-mustang-gt-car-fx/index.html)
$60K invested, a $450 roll hoop, and crappy-ass Corbeau seats, Corbeau lost their FIA approvals and certifications years ago because they went after the ricer seat market. Somebody's priorities are a bit lopsided, me thinks.
Complete specifications:
http://www.mustang50magazine.com/featuredvehicles/m5lp_0912_2010_mustang_gt_car_fx_web_exclusive/2008_car_fx_number_12.html
This car is nothing more than a bolt-on pile of cash, assembled by a "builder" whose skills are limited to using a ratchet and something with which to open cardboard boxes from UPS."
My first impression was the the tire barriers were much worse that useless.
Mike
-
Shows to go ya that the easy way out very often comes back to bite you.
Not directed at anyone cept that mustang builder.
just sayin
-
I talked to Joe Timney about this very thing while I was building the cage in a 98 Camaro.
His opinion was to not "sandwich" plates, but to attach route tubing through the plate,
then weld directly into a "frame" piece, such as 2x2 square tubing that is mounted under
the attachment and welded into the unit body. The tube could then be welded to the plate
as well, adding much for angular rigidity in the event of a crash, also making the OEM sheet
metal discussion negligible. If the car has an existing good cage, said "brace" could be welded
onto the existing plate/unit body structure, tying everything together. We used 2x2 .120 wall
under .250 plate, in lengths of 3 foot (or more where needed) to distribute load effectively.
-
I talked to Joe Timney about this very thing while I was building the cage in a 98 Camaro.
His opinion was to not "sandwich" plates, but to attach route tubing through the plate,
then weld directly into a "frame" piece, such as 2x2 square tubing that is mounted under
the attachment and welded into the unit body. The tube could then be welded to the plate
as well, adding much for angular rigidity in the event of a crash, also making the OEM sheet
metal discussion negligible. If the car has an existing good cage, said "brace" could be welded
onto the existing plate/unit body structure, tying everything together. We used 2x2 .120 wall
under .250 plate, in lengths of 3 foot (or more where needed) to distribute load effectively.
Too confusing....Do you have some pictures please?
-
Basically, create a thru-body frame by welding 2x2 or 2x3 box tube
to the unit body subframe. Then weld the cage to this created "frame".
More structure, less relying on .035" sheet metal for strength.
-
Actually, I was working with a guy on something like this today. The car is a Vintage SCCA type car that may be run at BVille eventually. Being an anemic Nash Healey, It won`t go fast, but the cage layout is interesting, and is all tied to a 2x2x120 underframe. It is all on the computer in a Solidworks type application, and the layout was able to be rotated and parts added/deleted. Pretty trick, and the tie ins and engineering looked good. Mostly 2'' by 120 wall tubing, which passes SCCA Tech. I would say that the car would be a perfect 150 Club example, so we`ll see what happens....
-
Just ramblin.
Tearing or cracking starts at little spots that don't show up in normal FEA models. A sharp edge, a minor gap in a weld, a small notch, etc. The narrower and deeper it is, the worst. Cracks start failures.
Square plates without large corner radii should fail at the pointy corners. Oval plates should be harder to tear with the same surface area. Modern epoxy should perhaps be stronger than welding when having to attach to thin wall material. Small areas of welding errors create stress risers. But doing a good job of gluing isn't easy. For those who don't know, modern aircraft are often glued together, and IIRC, so are newer Ferrari's.
When we did our our cage we had to to the body for 2 bars. So we made the bars redundant. We Y-tubed the main hoops so it attaches in 2 planes, and then put in kick bars to attach the bottoms, then extra 45° bars to do the job of the two bars attached to the body. The back bars (attached to body), are not adding any strength at that point. The NHRA allows you to remove those bars IF YOU do extra bracing as of 2008(?).
However, our truck is "heavy" compared to a car. So the extra tubing made sense, even if it hurts our drag race ET's, it will be much stronger than the minimums, perhaps twice as strong.
-
Wolfe Racecraft makes nice thru-body subframe connectors for our application,
we used it as a template for SFC's, seat mounts and cage mounts.
-
Here they are: Please no comments on the weld quality or anything else. The car owners graciously let me take these pictures to help me. Pictures are only to demonstrate the rule book requirement of 1/4" base plates. The floor they are welded to is .035" sheet metal. Is this the best solution?
Don't want to get into the details about the car or the owner or whatever but I would like to know what kind of car this is (make and model?).