Landracing Forum
Tech Information => Technical Discussion => Topic started by: Ron Gibson on May 03, 2017, 05:44:18 PM
-
First off, I don't build chassis, never have, but I have a question for those more knowledgable than me. This is not meant to be critical of anyones builds or practices, just curious.
One thing I have noticed on several car builds on this site and on a race car being built in KC a couple of weeks ago. I don't quite under stand why to locate an axle lateraly (front or rear) a mechanism is built that is on heim ends front and rear. The confusing part is at the chassis, generally it is built out of tubing, triangulated, cross braced, gusseted, plated or otherwise strenghtened and hell bent for stout and at the axle all that is there is one heim joint under shear at the threads (if that is the proper term), which is their weak spot. To me, like a robot in some long ago SiFi, "that does not compute". Any information or theories accepted.
I asked the guy building the car at KC about it and his only answer was "that's the way everybody does it".
Ron
-
Will Robinson :-D, It's desirable to have a stiff/non flexing chassis but the suspension needs to move within the desired geometry with pivot points & Heim joints are the most common pivots used in racing. Is this what you were asking?
Sid.
-
No, the member is hell bent for stout but the single heim is side loaded and not under linear tension or compression as designed. The socket on heim joints with that kind of load can and does seperate from the joint. That is why the rules specify large washers to keep them from coming completely apart. If the socket does move in the housing, it can't come apart because of the brackets, alignment can change, maybe only 1/4" or so but some tire clearances can be close.
As in the case of SD, I can see a reason for the sliding shaft but I think there should be a triangulated or gusseted bracket on both ends of the sliding tube not just one.
Ron
-
Ron, you are absolutely correct that putting bending loads into a Heim shank is very bad engineering practice, especially in a corrosive environment and in an important structural application. That Speed Demon and others have gotten away with it, so far, doesn’t change that fact. How many times do you want to roll the dice?
-
My guess: lateral location devices (track bar, Watts linkage, etc.) are lightly loaded under "normal" LSR vehicle operation, as opposed to "non-normal" (spins, yaw moments, etc.). Therefore they seldom see loads high enough to truly test them, and can "get away" with things like single-shear-loads on Heim joints- 99% of the time. Just my two cents...
-
Ron;
Usually race cars are designed for as high a strength/stiffness-to-weight ratio as possible in order to keep the car lightweight but still have adequate chassis stiffness and strength. Land speed racing is an exception; weight is a low priority and sometimes even more weight is added (ballast) to improve traction.
What would be not acceptable in other forms of racing might be OK in LSR. For example, a Watts linkage used to laterally locate a solid rear axle would normally be built as you suggested- bolts properly loaded in double shear by tabs welded to chassis members. If weight is not a problem, the same thing can be achieved by using a very thick tab and loading the bolt (even on the threaded section) in single shear. Granted that the bolt might need to be three sizes larger, but it would work.
One other point- LSR vehicles run in a mostly straight line so the loads on the linkages are very light. Even in a spin, the limited traction on salt keep the loads lower than a car running on asphalt on a road course or a circle track.
Anyway, that's my take on your question.
Regards, Neil Tucson, AZ
-
I understand what you guys are saying. I guess my point was, if a lot of strenght is not necessary, why build a Brooklyn Bridge structure on one end and one, side loaded bolt (Heim) on the other end.
Ron
-
"Even in a spin, the limited traction on salt keep the loads lower than a car running on asphalt on a road course or a circle track."
Neil --
You might be surprised how much load can be put on suspension parts going sideways on dirt at about 200 MPH. I was. The part that took the hit was not of Henry's fabled origin, but a steering part off a supposedly heavy duty Jeep.
Stan
-
Ron,
Keep in mind that a lot of the chassis designs have been developed over time. Race cars have wrecks, and wrecks breaks stuff. Some of the ideas that go into the design is to have things break that are the easiest to fix.
The main chassis of a car is much more work to fix than an axle, for example.
If every component is built to the same loading design it would be impossible to predict where or how it will break.
It consumes energy to break stuff, the more stuff that breaks up during a wreck means less energy absorbed by the driver.
Rouse
-
I understand the break first, weak link idea. Personally I don't want the part that breaks first to be what holds my axles in alignment, especially the steer axle.
Ron
-
Stan;
I wasn't minimizing the loads in a spin- salt or dirt. It is, though, lower than on asphalt.
Consider the loads on the rear suspension of a sports-prototype during cornering- way over 1G due to the spoiler/wing downforce... and this happens in every corner of every lap- for 24 hours at Le Mans. LSR is a different animal.
Regards, Neil Tucson, AZ
-
Double shear isn't that weak.
And some heims get really really strong. Look at the Johnny joints or ballistic joints (or the other 50 names they go by) on 4x4 trucks.
My f350 was 4 linked front and back and used Johnny joints. That was a lot of steel and diesel and torque and big heavy tires and those joints held it no problem.
From what I was told, the huge tubing the 4 link was made from would break/bend before the joint gave up.
I think also the chassis side is built like a brick "sh1t" house becaus wit houses your butt and all the fuel and crazy stuff that can bleed or go boom. A suspension piece in a yard sale wreck just isn't that important.
-
We're talking side loads on the Heim, not the normal compression-tension loads as in a four link.
From what I remember a Johnny joint is held together, to keep from separating, with a snap ring, a Heim is not.
The MacKichan-Schulz liner used large, laid on their side, Heim joints as ball joints in their front suspension. There was a problem with partial seraration of the joint body and the pivot ball socket. The socket was sliding sideways out of the body. Washers kept it from total seperation.
Ron
-
Ah roger that. You are talking about it essentially popping out of socket per se like a shoulder?
-
Talking about the pressed in and very lightly staked ball socket coming out of the threaded body.
Ron
-
I would never use a Heim joint unless the suspended member's intended (not accidental, as to compensate for rubber bushing compliance) motion path had some secondary alignment (3rd plane) to it. Think of a motorcycle swingarm with Heim joints, and shudder...
-
I agree, there should never be a significant load that would push the ball out. Spherical rod end bearings are meant for radial loads. Just FYI, here is some data from the Aurora catalog (lbs):
Model Number Use Hole Size Shank Thread Ultimate Static Radial Load Proof Axial Load
SPM-8 Low-cost commercial 0.500" 1/2-20 6430 10% max 3-pc
15% max 2-pc
MM-8T Gen Purp, Teflon liner " " 6660 "
ASM-8T Aerospace $$$ " " 19500 2040
Depending on the bearing you select, its strength can vary over a wide range but loading any type axially is a bad idea.
Regards, Neil Tucson, AZ
-
I'm not sure I completely understand your example. How about a pic?
Also, are you aware of many of these setups breaking or being damaged in normal operation? I'm not.
-
If you use a "uni-ball" bearing and install it into one of the weld cups that are available from people like The Chassis Shop you can use the uni-ball bearing as a locating bearing with the load being axial. The weld cups are typically 4130 and a very tight fit for the uni-ball and it is retained into the weld cup with a snap ring. This configuration is very common for lower A arm bearings on many race cars. The axial thrust load of the uni-ball is greatly increased by the weld cup as now for the ball to be displaced from it's outer race it also has to stretch the weld cup. I have never seen any testing of the increase in axial load capacity by using a weld cup but it could certainly be 2 to 3 times higher. My son has these on the lower A arm of his 510 Datsun and the next time he has a wheel off I will snap a pic.
Rex