Landracing Forum

Tech Information => Safety => Topic started by: jbryant200 on June 20, 2014, 11:50:26 AM

Title: Weight and Balance
Post by: jbryant200 on June 20, 2014, 11:50:26 AM
Hi, my name is Jeff Bryant. I live in Northern California and am a second generation land speed racer. My father started coming to Bonneville in 1957 (Tom Bryant). I have been coming since the mid ‘60’s.

I have been a private pilot since 1991. I hold (have held) 200 mph records at Bonneville, El Mirage, and Muroc. On the way to the Bonneville 200 mph record I drove our D Competition Coupe. I crashed it at over 200 mph. Fortunately, I was only bruised in the ordeal. We made many changes over the years and learned about concepts called “Weight and Balance” and “Center of Pressure”. These are physical truths that apply to landspeed racing and flying (Private pilots are required to know their “numbers” related to centers of pressure and balance). If you don’t pay attention to them, you can load your plane in such a way that you can take off but not safely land. This information is a matter of life and death and not theory.

Over the years we have found that these physical “truths” apply to landspeed racing in the same “life and death” way they do with private pilots and their planes. This became painfully clear when we lost my youngest brother, Barry, in August of 2009 at Bonneville.

Since that day, my “therapy” has been to immerse myself into safety at Bonneville. I have been a tech inspector for several years now and am reaching out through this forum to tell other racers about the important calculations called “weight and balance” and “center of pressure”.

This information is not official and SCTA does not require that racing teams include this information in the design and modification of their vehicles. But I believe it is imperative that we know about these things. Here’s why:

Every landracing team should assume that someday their vehicle might leave the ground. Denial of this possibility puts your life at risk. A properly configured vehicle will keep its nose relatively straight when off the ground, much like a dart. That way the chute remains effective and helps stop the car. There are exceptions to this, but the odds of safely coming to a stop increase with a properly configured car. How your vehicle handles in crosswinds and the effectiveness of your steering are also affected by these calculations.

So, if you are building a new vehicle, have a vehicle that progressively gets harder to handle the faster you go, have a vehicle that seems to spin easily, and especially if you plan to exceed 200 mph, you should know about and apply this information. KNOW YOUR NUMBERS. We can discuss your question here or I can be reached at jbryant200@hotmail.com.

P.S. We knew our numbers on our competition coupe and still got in trouble because we did not RECALCULATE after making modifications to the car. Don’t you make the same mistake. Always recalculate whenever you change the weight distribution or shape of the vehicle. You always want the center of gravity to be four to six inches in front of your center of pressure.

I’ve heard many in landspeed racing say they have trouble with traction and handling. The common thought is to add weight to the rear of the vehicle when tires spin. This can be seriously dangerous in some cases. ALWAYS make changes to your vehicle with the proper configuration of "The Numbers" in mind.

Title: Re: Weight and Balance
Post by: Seldom Seen Slim on June 20, 2014, 12:10:05 PM
"My father started coming to Bonneville in 1957 (Tom Bryant)."

Jeff, I think that at least one or two of us on this Forum know you and your family. :roll:  Okay, maybe a bunch of us.  Hey - dang near everyone knows Tom Bryant and his family and the very successful racing career - that has turned into legend.  Thanks for taking time to broach this subject.

Oh, yeah -- and say hi to your dad for Nancy and me, too.  Thanks.
Title: Re: Weight and Balance
Post by: SPARKY on June 20, 2014, 01:56:15 PM
Jeff,  thank you SO much for taking the lead on this  Center of Pressure in relation to Center of Gravity is so critical if our cars step out on us  ---Scrub Radius with to much CASTER has a lot to do with them getting out of shape in the first place.  On my new car I am in the process of working out the CP/CG before I go to T&T in July   I would like to have more weight in the back but before I can put more in the back I need more in the front---I am looking at a SOILD BAR frt axel  ---I have few other easy  :? options
Title: Re: Weight and Balance
Post by: manta22 on June 20, 2014, 02:51:41 PM
Jeff;

I think your admonition about adding weight to the rear of a car changing its CG in relation to the CP speaks for using a wing to add rear downforce. True, the additional drag is a penalty but if you have excess HP creating wheelspin, it may be a good tradeoff.

Regards, Neil  Tucson, AZ
Title: Re: Weight and Balance
Post by: dw230 on June 20, 2014, 03:26:12 PM
Neil,

You still have to within the rules for your class.

DW
Title: Re: Weight and Balance
Post by: toclub on June 20, 2014, 03:31:26 PM
How do you calculate the center of pressure?
Title: Re: Weight and Balance
Post by: Sumner on June 20, 2014, 03:34:11 PM
Jeff;

I think your admonition about adding weight to the rear of a car changing its CG in relation to the CP speaks for using a wing to add rear downforce. True, the additional drag is a penalty but if you have excess HP creating wheelspin, it may be a good tradeoff.

Regards, Neil  Tucson, AZ

Jeff good to see you posting here and bringing forth a topic that many more people should become conscious of.

Niel you make great points and we are moving in that direction but have been very conservative in the shape of and the use of the wing so far as a wing at speed can generate some really large numbers in regards to downforce.  The really big advantage to using....

(http://purplesagetradingpost.com/sumner/Hooley%202013/wing--84.jpg)

..... a wing in my opinion is that now....

(http://purplesagetradingpost.com/sumner/Hooley%202013/wing--60.jpg)

...you are allowed to have vertical stabilizers that can play a huge impact on increasing the center of pressure on the car immediately which as Jeff pointed out can make the car a lot safer.  The increase in CP can let you add more weight forward of the rear axle for added traction and as you become better acquainted with the use of the wing you could remove some of that ballast for better acceleration since weight hurts you there.

Not all classes allow the ....

(http://purplesagetradingpost.com/sumner/Hooley%202013/wing--87.jpg)

...wing and verticals but if you are in a class that does you can add a lot of safety to the car.  On one of the license runs at about 180 the throttle stuck wide open and the car immediately started to go around with the tires spinning for over 3 seconds according to the data log.  I thought the car was too far sideways to pull the chute at first and didn't want it wrapping around the car so didn't.  Just then the vertical stabilizers caught the car and sent it the other way with the tires still spinning.  The verticals caught the car again and as it came back towards middle the chute went out and caught the car.  I'm a really big believer in them now.

There are two videos of the run here...

https://www.youtube.com/watch?v=xaGf2Dh7bPo (https://www.youtube.com/watch?v=xaGf2Dh7bPo)

https://www.youtube.com/watch?v=mKGZrCyLSEI (https://www.youtube.com/watch?v=mKGZrCyLSEI)

It would be nice to have on here and possibly on the SCTA site an explanation of how to figure where your car's center of pressure is with relation to the center of gravity with some examples of how to roughly figure this for those people who 'kind of understand' what we are talking about here.  You can get a fair idea of all of this with a good side view of the car and a couple jacks,

Sum

 
Title: Re: Weight and Balance
Post by: Sumner on June 20, 2014, 03:36:45 PM
How do you calculate the center of pressure?

I mentioned briefly one way in a post here...

http://www.landracing.com/forum/index.php/topic,9404.1710.html (http://www.landracing.com/forum/index.php/topic,9404.1710.html)

Sum
Title: Re: Weight and Balance
Post by: bubruins on June 20, 2014, 04:16:57 PM
Is Center of pressure calculated by side profile surface area the best indicator for how a car will react sideways in the air? I am just learning here, but I've got to think that the coefficient of drag over different parts of the car as you would look at it from the side has to influence it. For example - on the stabilizers of Hooley's car they are completely flat. Imagine trying to push a car with a flat front through the air forwards. I think that in the same way a big flat surface is difficult to push through the air sideways, it would be easier to push a round surface through the air. If the surface was round it would still count towards the center of pressure calculation, but may slip through the air with a much less drag. Does that sound crazy?
Title: Re: Weight and Balance
Post by: Sumner on June 20, 2014, 04:22:50 PM
Is Center of pressure calculated by side profile surface area the best indicator for how a car will react sideways in the air? I am just learning here, but I've got to think that the coefficient of drag over different parts of the car as you would look at it from the side has to influence it. For example - on the stabilizers of Hooley's car they are completely flat. Imagine trying to push a car with a flat front through the air forwards. I think that in the same way a big flat surface is difficult to push through the air sideways, it would be easier to push a round surface through the air. If the surface was round it would still count towards the center of pressure calculation, but may slip through the air with a much less drag. Does that sound crazy?


What you are saying is all true but show me an easy way to figure that  :-).  Using the surface area will probably get you close and probably show a worst case situation and then take in the surface contours like you said to see if you are erring on the safe side or not.  Talked about that in Sparky's build where the link above went to,

Sum

Title: Re: Weight and Balance
Post by: manta22 on June 20, 2014, 06:05:33 PM
Neil,

You still have to within the rules for your class.

DW

That goes without saying, Dan. Right you are.

Regards, Neil  Tucson, AZ
Title: Re: Weight and Balance
Post by: 631 on June 20, 2014, 09:12:25 PM
Note the article on CP CG in the tech & FAQ section.
Title: Re: Weight and Balance
Post by: WOODY@DDLLC on June 21, 2014, 10:44:38 AM
You CANNOT get the CP of a complicated 3D object with a 2D side view! FYI: Sparky's CP is right on top of the CG!  I will have more to say on this topic later - just really slammed right now! Meantime take a look at this: http://www.youtube.com/watch?v=WLm4wkTBbuc
Some of you will know this guy has real life experience with CP/CG location!  :-o
Title: Re: Weight and Balance
Post by: Sumner on June 21, 2014, 11:16:02 AM
You CANNOT get the CP of a complicated 3D object with a 2D side view! FYI: Sparky's CP is right on top of the CG! ...

While I agree that you aren't going to get a totally accurate CP with a 2D side view I think it is better than doing nothing.  With that in mind we need some method that we can use were we can at least see if we are close to having a problem.  The majority of us don't have the money for wind tunnel tests and/or CFD studies, yet we will still take our cars to the salt.  What do you suggest we do to fill that need?

(http://1fatgmc.com/car/misc-pics-1/Sparky-lakester-2.jpg)

On Sparky's car above what is going on that moves the CP up to the CG if the CG is where he said it was?  There is a lot more body behind the CG than in front of it and it is a lot flatter also for much of that length at the back of the car.

Thanks,

Sumner

Title: Re: Weight and Balance
Post by: jl222 on June 21, 2014, 11:29:45 AM
You CANNOT get the CP of a complicated 3D object with a 2D side view! FYI: Sparky's CP is right on top of the CG!  I will have more to say on this topic later - just really slammed right now! Meantime take a look at this: http://www.youtube.com/watch?v=WLm4wkTBbuc
Some of you will know this guy has real life experience with CP/CG location!  :-o

  That test shows good results for a road car and ignores why spoilers were invented.

  Any hi hp Bville car set up like that would spin from spinning the tires;

  I know the Lindsley's and Liggett Camaro had a big setback for the engine and a bunch of weight on rear tires, small spill plates and short spoiler and had an average time of 330+ mph. How fast was it going at the end of the mile? They didn't run the last mile.

  Set up as in the video they would have melted the tires or spun.

             JL222
Title: Re: Weight and Balance
Post by: Sumner on June 21, 2014, 12:08:31 PM
Note the article on CP CG in the tech & FAQ section.

Thanks, that is a good article ....

http://www.landracing.com/docs/CG-CP_Hakansson-Dube_4-2014.pdf (http://www.landracing.com/docs/CG-CP_Hakansson-Dube_4-2014.pdf)

and is a little more technically orientated finding the CG vs. placing two jacks under the car on each side to find where it balances and where the CG should be above that point.  If I had weight scales available I'd try that and compare it to the 'jack method'.

=======================================================

SSS if you can you might want to fix a small mistake though. There was a mistake in the part on figuring the CG.  It should be....

Rear Left     117.25 ...... 1130 ....... 132492.5  (not 112492.5)

Total ......................... 4674 ....... 292446.0 

292446.0 [inch-lbs.]/4674 [lbs.] = 62.56 [inches]  

I also turned the numerator and denominator around in the line above as that is the way I'm use to seeing them, but maybe I'm wrong,

Sum
Title: Re: Weight and Balance
Post by: jl222 on June 21, 2014, 02:29:56 PM
You CANNOT get the CP of a complicated 3D object with a 2D side view! FYI: Sparky's CP is right on top of the CG!  I will have more to say on this topic later - just really slammed right now! Meantime take a look at this: http://www.youtube.com/watch?v=WLm4wkTBbuc
Some of you will know this guy has real life experience with CP/CG location!  :-o

  That test shows good results for a road car and ignores why spoilers were invented.

  Any hi hp Bville car set up like that would spin from spinning the tires;

  I know the Lindsley's and Liggett Camaro had a big setback for the engine and a bunch of weight on rear tires, small spill plates and short spoiler and had an average time of 330+ mph. How fast was it going at the end of the mile? They didn't run the last mile.

  Set up as in the video they would have melted the tires or spun.

             JL222


 It seems that air flowing over a down sloped hood would add weight to front and change the cp. But it also adds down force flowing over rear spoilers.

  Lindsley Liggett had trouble with front end rubbing on tires.

        JL222 
Title: Re: Weight and Balance
Post by: SPARKY on June 21, 2014, 02:49:34 PM
As have others  George Fields comes to mind
Title: Re: Weight and Balance
Post by: tortoise on June 21, 2014, 03:13:39 PM
The increase in CP can let you add more weight forward of the rear axle for added traction

I don't think there's anything magical about the rear axle that makes adding weight behind it bad.

The same total weight and center of gravity location can be achieved with weight concentrated near the CG or far from it.  More weight at the ends gives a higher polar moment of inertia, having a good effect; being slow to get out of shape when wheels lose traction, and a bad one; a tendency to keep spinning once you've started.

Road racers like the lowest possible polar moment of inertia. Does anyone have a well-founded opinion as to whether ballast for a land speed racer should be placed to increase or decrease polar moment of inertia?
Title: Re: Weight and Balance
Post by: Sumner on June 21, 2014, 04:24:00 PM
....  More weight at the ends gives a higher polar moment of inertia, having a good effect; being slow to get out of shape when wheels lose traction, and a bad one; a tendency to keep spinning once you've started....

I don't have a scientific answer but I don't want that weight at extremes.  It might help initially to keep it from coming around but as you mentioned once it does it will increase the chances that it will go around and sooner or later any car with HP is going to spin the tires and start around.

I once made a pump trailer to pump deep wheels with and it had a lot of weight at the ends of the trailer and was a handful to tow as once it started fishtailing it didn't want to stop.  The next one I designed totally different to keep the weight central and it was a dream to tow.

Other than the run where the throttle stuck on the other runs when I experimented some with WOT and the tires spun and the car started around as soon as I lifted enough to stop the spinning the car immediately straightened out with the new vertical stabilizers.  We have a lot of weight just ahead of the rear axle on both sides and more forward of that and more in the nose.  The car was very easy to drive,

Sum
Title: Re: Weight and Balance
Post by: tortoise on June 21, 2014, 04:43:49 PM
I don't have a scientific answer but I don't want that weight at extremes.  It might help initially to keep it from coming around but as you mentioned once it does it will increase the chances that it will go around and sooner or later any car with HP is going to spin the tires and start around.

That makes sense. I'd suspect that many builders, deciding that they need x pounds more on the rear, and y pounds more on the front, add those amounts in two slugs, each around one axle. They might do better, if there's room, to put it all in one place.
Title: Re: Weight and Balance
Post by: Interested Observer on June 21, 2014, 06:21:23 PM
Quote
and the tires spun and the car started around as soon as I lifted enough to stop the spinning the car immediately straightened out with the new vertical stabilizers.

So---was it the re-established traction or the vertical stabilizers that regained directional control?  If the stabilizers were effective in providing aero stability it shouldn’t have gone very far around and should have corrected the yaw even with the rear tires still spinning.
Title: Re: Weight and Balance
Post by: John Burk on June 21, 2014, 06:46:46 PM
Sprint cars have the engine and driver close to the center for a low polar moment so it can be snapped sideways going into a corner so rear tire thrust adds to cornering Gs . The price of low polar moment is less time for the driver to react when things start going wrong . For land speed you'd think you'd want spins to start more slowly .
Title: Re: Weight and Balance
Post by: Sumner on June 21, 2014, 06:49:22 PM
Quote
and the tires spun and the car started around as soon as I lifted enough to stop the spinning the car immediately straightened out with the new vertical stabilizers.

So---was it the re-established traction or the vertical stabilizers that regained directional control?  If the stabilizers were effective in providing aero stability it shouldn’t have gone very far around and should have corrected the yaw even with the rear tires still spinning.

On the brief WOT tests I think it was a combination of both that settled things right down.  The car never jumped around to the same extend that it did when the the throttle stuck WOT with the tires continuously spinning.  That time I think the fact that it did not go all the way around indicates that the verticals did stop it. 

I feel there is a big difference in if the CP is just a little behind the CG or a lot behind and the results you could expect to see on the aero stability.  At this point we don't know where the CG is as it was a thrash to just get the car to the salt even 3 days late.  We didn't have corner weights or where the CG was after all the changes to the car and still don't have a good side view to work with to try and figure the CP.  We need to get that done and if someone has a side view from a distance I'd sure like to have it.

With the 8 lb. springs in the wastegates the car was probably gaining around 250-300 HP in less than a second as the car went from no boost to 8 lbs. with the throttle going from 40% to 100% instantly.  We now have 2 lb. springs in the wastegates and that should help since we can then use the E-boost controller to increase the boost over the 2 lb. in more controllable steps.  So a more normal application of the throttle would of probably not resulted in the car jumping out at all.  I was just playing at the time with 'what if I do this'.

I'll share one other item here.  Our two turbos have an AR of 1.32 which most people will shy away from thinking they will spool too slowly.  A second to spool at the drags might be slow but for what we are doing you don't need them to spool faster.   Granted the 572 is big but I'd stay with the same AR even with a much smaller motor.  We are trying to slow the power coming on in our case, if you can hook all the power you can make with a turbo up right away then go with a smaller AR, but I doubt that is the case most of the time,

Sum

Title: Re: Weight and Balance
Post by: Seldom Seen Slim on June 21, 2014, 06:49:56 PM
I understand your request -- but I'll let the authors of the essay make the changes.  I don't want to do it -- and get it wrong in my own way just from a typo or something.  Rex, if you're seeing this, how 'bout asking Eva or Bill in to check to be sure -- and make a change if necessary?  Thanks.
Title: Re: Weight and Balance
Post by: Sumner on June 21, 2014, 06:56:22 PM
Sprint cars have the engine and driver close to the center for a low polar moment so it can be snapped sideways going into a corner so rear tire thrust adds to cornering Gs . The price of low polar moment is less time for the driver to react when things start going wrong . For land speed you'd think you'd want spins to start more slowly .

John I can see what you are saying but in our case at least we have a lot of weight barely ahead of the rear axle and a lot more quite a bit further forward and a lot longer wheel base and weight to begin with than a sprint car. 

Everyone still has to do what "they believe in" but I don't want to see us with a lot of weight hung out back behind the rear axle.  We do have a large and a smaller battery in the trunk area, but that is all,

Sum
Title: Re: Weight and Balance
Post by: Sumner on June 21, 2014, 06:58:18 PM
I understand your request -- but I'll let the authors of the essay make the changes.  I don't want to do it -- and get it wrong in my own way just from a typo or something.  Rex, if you're seeing this, how 'bout asking Eva or Bill in to check to be sure -- and make a change if necessary?  Thanks.

Jon that is fine, but it is just a multiplication error on the one line that then carries forward to the rest of the calculations.  Probably just a slip on the keyboard at the time,

Sum
Title: Re: Weight and Balance
Post by: RidgeRunner on June 21, 2014, 09:07:22 PM
The increase in CP can let you add more weight forward of the rear axle for added traction

I don't think there's anything magical about the rear axle that makes adding weight behind it bad.




     Wouldn't the axle location on an unsprung car [and relatively close to it on a sprung car depending on the type of suspension] act like the pivot point/fulcrum on a playground teeter board?  Add weight behind and all would be on the rear end plus some transferred from the front being unloaded.  Add it ahead and most would be on the rear with some going to the front.

      A pound might always be a pound but the location can have a large effect on the end results.  My first drive in an early Tempest with the rear transaxle , hitting a bump while cornering on a slick road comes readily to mind almost 50 years later..........

             Ed
Title: Re: Weight and Balance
Post by: tortoise on June 21, 2014, 09:25:43 PM
You always want the center of gravity to be four to six inches in front of your center of pressure.

Can the center of pressure to be too far behind the center of gravity?
Title: Re: Weight and Balance
Post by: SPARKY on June 21, 2014, 09:58:08 PM
not for LSR  in my opinion---I spent a lot of time with Ms Liberty today  I think I have figured out a way to add   a bunch of weight to the nose---but will be a pain---would have to pull the frt body off and build a weight box in the nose--  or build a weight box that will fit the nose area---our it full of lead---slide it forward and bolt and weld it in---sure hope we do not have to do this.  I have one more opinion I am waiting on---I have an opinion, Sum, Doug & Woody have an opinion---they diverge---my job is to decide where and what---if we have a decent track and I do not need more rear weight for traction---ok NA

When we turbo we will need more frt weight and more rear surface area   
Title: Re: Weight and Balance
Post by: 631 on June 21, 2014, 10:28:55 PM
good points on the complexity of 3D shapes and flat side surfaces.  The on course airflow may not agree precisely with the paper area calculations for a particular vehicle and all of our cars are going to have some divergence from our simple calculated results.  The point of the exercise is to get a base line idea of the high speed stability of our cars.  Wind tunnel time will validate (or not) our stability assumptions but not all of us have access to these great devices.  Grease dots are a time honored land speed method of seeing where the air goes around a vehicle at speed, taped yarns and an on board camera are the information age replacement for grease dots,  suspension data acquisition is great info as well; stiff wire, zip ties and tape can provide simple data in place of sensors.  Weigh the car, figure out the area center of the side of the vehicle, do the cut out test then make some assumptions and next summer spend a few work up or test runs proving or disproving the stability assumptions. The 411 streamliner was designed by a young mechanical engineer using paper and pencil proving to me that good results are possible applying the basic principles of mass in motion traveling through the air.   
Title: Typo in calculation/equation. (Re: Weight and Balance)
Post by: killajoule on June 21, 2014, 11:51:37 PM
Eva Hakansson here.

Sumner - you are absolutely correct. There is a typo in the calculation that carried through and the equation is flipped (it is correct in the text and wrong in the equation). Sorry about that.
It's the result of late night typing. For some reason, making a living always gets in the way of racing and you end up working late nights...

The example calculation was a fictitious example we made up for the article. I have all the calculations for the KillaJoule, but we decided to not use that as an example because the three wheels and the asymmetric body makes it a bit more complicated.

I have sent a revised version to Rex Svoboda. I hope it the current version will be replaced soon.

// Eva


Note the article on CP CG in the tech & FAQ section.

Thanks, that is a good article ....

http://www.landracing.com/docs/CG-CP_Hakansson-Dube_4-2014.pdf (http://www.landracing.com/docs/CG-CP_Hakansson-Dube_4-2014.pdf)

and is a little more technically orientated finding the CG vs. placing two jacks under the car on each side to find where it balances and where the CG should be above that point.  If I had weight scales available I'd try that and compare it to the 'jack method'.

=======================================================

SSS if you can you might want to fix a small mistake though. There was a mistake in the part on figuring the CG.  It should be....

Rear Left     117.25 ...... 1130 ....... 132492.5  (not 112492.5)

Total ......................... 4674 ....... 292446.0 

292446.0 [inch-lbs.]/4674 [lbs.] = 62.56 [inches]  

I also turned the numerator and denominator around in the line above as that is the way I'm use to seeing them, but maybe I'm wrong,

Sum
Title: Re: Typo in calculation/equation. (Re: Weight and Balance)
Post by: Sumner on June 22, 2014, 12:00:13 AM
Eva Hakansson here. ...The example calculation was a fictitious example we made up for the article. I have all the calculations for the KillaJoule, but we decided to not use that as an example because the three wheels and the asymmetric body makes it a bit more complicated.....

Thanks for taking the time to do the article.  I think it explained everything very well and hope others take the time to read it,

Sum
Title: CP-CG - simple is better than nothing... (Re: Weight and Balance)
Post by: killajoule on June 22, 2014, 12:15:10 AM
Thanks, Sum.
The article is just a "rule of thumb" type guide, but we find the method very useful.
Most of us Bonneville racers will never get anywhere near a windtunnel. If this simple method tells you that your CG is behind your CP, then you will know why your vehicle was handling badly. There is no point spending money on windtunnel time until you have made some attempt to fix the CG-CP issue.

// Eva

Eva Hakansson here. ...The example calculation was a fictitious example we made up for the article. I have all the calculations for the KillaJoule, but we decided to not use that as an example because the three wheels and the asymmetric body makes it a bit more complicated.....

Thanks for taking the time to do the article.  I think it explained everything very well and hope others take the time to read it,

Sum
Title: Re: Weight and Balance
Post by: SPARKY on June 22, 2014, 08:29:56 AM
"There is no point spending money on windtunnel time until you have made some attempt to fix the CG-CP issue. "


Maybe that should depend on what speeds one is trying to run---a lot of the faster cars have had frt. end lift issues---some very successful ones ---have had body changes in route to that success

some have just wrecked without changes---if the ride just gets better and better---it may not be a smoother track  :-o
Title: Re: Weight and Balance
Post by: wobblywalrus on June 23, 2014, 12:38:43 AM
One problem with having a lot of weight back past the rear axle or near it is how the vehicle reacts to bumps or dips on the track.  Weight behind the axle can actually lift the front end of the vehicle if it hits a bump.  My experience with this is with pick-up trucks overloaded with fire wood.  It is not racing, although the principles are the same.   
Title: Re: Weight and Balance
Post by: RidgeRunner on June 23, 2014, 07:27:29 AM
     A buddy who had a general repair garage put together his own roll back truck back in the early '70s using a truck with a marginal WB.  Having borrowing it for a move once, I can testify that the results were the same on a larger scale, front end could get scary light real fast.  He didn't use it any more that he had to until he could get it on a truck with a longer WB.

              Ed
Title: Re: Weight and Balance
Post by: SPARKY on June 23, 2014, 09:10:20 AM
On my car I start out with most of my cooling water 20" behind the rear axle---during the run I actually move about  80# from behind the rear axle to 108" in front which really helps on trying to get CG more in front of the CP.
Title: Re: Weight and Balance
Post by: maj on June 23, 2014, 08:08:20 PM
My only experiance with this sort of thing is 10 yrs in dirttrack sedans, and how relevant it is to salt is anyones guess
But it left an permanant impression of weight in front of rear axle = good
and weight behind axle =bad
So much easier to steer in a slide if the back is not acting like a pendulum, leverage over contact point sort of feel

I still apply this to the bike rear fairing i had previously built and in the process of doing again, i feel it needs to be long for better aero shape minimising separation , so i put the tire right back out there too and this gives me more room to put weight in front of the tire, and transfers more of the total to the front tire   
Title: Re: Weight and Balance
Post by: Richard 2 on June 23, 2014, 09:41:06 PM
I found this 101 site on CP and CG of rocket stability and thought I would share it.
Shows a simple way to find CP and why it is necessary to find it in relationship to CG.
Title: Re: Weight and Balance
Post by: Graham in Aus on June 23, 2014, 11:38:28 PM
This is Rocket Science!  :cheers:

(http://www.zazzle.com.au/unstable_tee_shirts-235378000992244454)

http://www.zazzle.com.au/unstable_tee_shirts-235378000992244454

 :-D
Title: Re: Weight and Balance
Post by: wobblywalrus on June 23, 2014, 11:53:33 PM
Sparky, that is smart thinking.

Greg, I read somewhere that when one increases the wheelbase the front wheel trail needs to be increased, too.  The trail vs wheelbase on the Triumph was 8% before and after it was lengthened.  It was a lot of work to increase the trail and it was harder than stretching the wheelbase.  Was what I did really needed?   


 
Title: Re: Weight and Balance
Post by: jbryant200 on June 24, 2014, 11:54:46 AM
How do you calculate the center of pressure?

The center of pressure is complex. There are considerations that include strakes, whether or not you have open wheels (if you do, BOTH sides of the wheels are included in the calculation), wings, spoilers, and other items not included in the rough estimate using a profile.  The profile approach, as already discussed in this forum, is a surprisingly good estimate.  There are a number of us in the LSR community that are willing to provide an unofficial calculation for those interested. We are also willing to provide additional feedback depending on your handling experiences, speed goals, or plans for body changes.

I have found that the LSR community is truly a supportive network. We want to be a part of this, especially since we have seen such a large increase in worldwide interest. There are many new projects in the pipeline and we hope that forums like this serve to help these new teams learn the safest ways to approach design.  Help get the word out!
Title: Re: Weight and Balance
Post by: SPARKY on June 24, 2014, 12:13:41 PM
"I have found that the LSR community is truly a supportive network."

This is so true  you will find there are LOTS of folks that will keep you from making "numbing" rookie mistakes!!  LSR  is
" SO non COOKIE CUTTER"  as some one has as on their sig.

"If the objective is to skin a cat---SO LITTLE TIME SO MANY CATS"

We are all dealing with exactly the same thing:

  " TOTAL DRAG VS TRACTIVE EFFORT"  :cheers:
Title: Re: Weight and Balance
Post by: Richard 2 on June 24, 2014, 06:04:10 PM
I found this 101 site on CP and CG of rocket stability and thought I would share it.
Shows a simple way to find CP and why it is necessary to find it in relationship to CG.

http://www.rockets4schools.org/images/Basic.Rocket.Stability.pdf

Forgot the address, up to late I guess.

Richard 2
Title: Re: Weight and Balance
Post by: Speed Limit 1000 on June 24, 2014, 07:18:53 PM
 :cheers:
Title: Re: Weight and Balance
Post by: Jack Gifford on June 25, 2014, 04:53:08 PM
It seems to me that every traditional-layout drop-tank lakester has the CP infront of the CG. Yet I've never gotten the impression of them being notoriously evil-handling. Perhaps because the distance between CP and CG is typically quite small? [Please keep in mind that I'm just an LSR wanna-be]
Title: Re: Weight and Balance
Post by: Sumner on June 25, 2014, 06:18:13 PM
It seems to me that every traditional-layout drop-tank lakester has the CP infront of the CG. Yet I've never gotten the impression of them being notoriously evil-handling. Perhaps because the distance between CP and CG is typically quite small? [Please keep in mind that I'm just an LSR wanna-be]

If one never really spins the tires it isn't going to probably hurt.  It is only then that the car wants to swap ends or if it gets up in the air, say you blow a tire or something, which could happen to any of us on any run (if you have ever done a course sweep there is stuff out there and stuff coming off cars the whole meet).  It just takes a lone dzus or some other small object to possible cut a tire down,

Sum
Title: Re: Weight and Balance
Post by: tortoise on June 25, 2014, 06:36:07 PM
It seems to me that every traditional-layout drop-tank lakester has the CP infront of the CG.

It seems to me that you can't see the CG unless you're Superman.