Using Heim joints on fixed-width control arms.

Terry Oxandale

Skinny Man
I've got a question I've always had about spacing for inner Heims on fixed-spacing control arms.

Whenever I fabricate my suspension arms utilizing inner Heims, I always provide adjustability of the arm itself, for the centerline-to-centerline spacing so that there is never any preload on the inner race when assembled. Yet I see many examples where the fixed-width arm user is relegated to finding the correct washers/shims (and in some examples, not even shims) to get the Heim inner race preload to it's minimum. My observation has been that even being a couple of thousanths off of ideal spacing can lead to more stiction and wear on the joint itself.

What enlightening practice can you guys provide me, that would allow me to feel good about going forward with lighter and simpler fixed-width control arms (with or without bearing preloading)? Am I overly concerned about this preload issue, even though I know it is not an optimal practice?
 
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The main issue with arms with heim/rod ends/rose joints is that presumably the idea behind fitting heim/rod ends/rose joints is to be able to adjust the arms but as soon as you do anything other than moving both identical amounts the arm dimension changes and it as you say causes preload issues.

From what I've seen it appears that the way to do it is to have one side fixed (ie both radially and axially) the other only constrained radially but free to move slightly axially.

You can do that with heim/rose joints by run one side larger than the other and having one side "floating". Eg one side as a 5/8" and the other as a 1/2". Use 1/2" bolts and positively locate and fix the 1/2" heim/rose and a 1/2" to 5/8" spacer tube on the other side.

If you ever look at motorcycle swingarms this is similar to how they do it without having to piss about with lots of shims. A fixed deep roller bearing one side which is constrained by a circlip and a needle roller the other side. Axial location is just off the one side (which is sized accordingly).

The other way is to have the arms as a V rather than an A, their width can be modified slightly by a bit of bending of the arms but I'd think the preference would be instead to have two arms (ie one arm is a tie arm rather than a suspension arm).
 

Howard Jones

Supporter
Does this Q&A from Aurora rod ends web site help any?


What is the axial static load capacity of AuroraBearing Company’s rod ends?

For Aurora two-piece rodends, the maximum axial static load capacity is recommended to be 15% of theultimate radial static load capacity.For three-piece rod ends, the maximum axial static load capacity isrecommended to be 10% of the ultimate radial static load capacity. However, on three-piece units, factors suchas race material, body material, and dimensions may affect the axial staticload capacity. For further information,consult the Aurora Bearing engineering department
 

Terry Oxandale

Skinny Man
Interesting thought Howard. So the more rigid the control arm structure (e.g. transverse reinforcement between the rod-end receivers, or rigid alloy control arms), the more likely these figures from the bearing manufacture would be exceeded if extreme care is not taken in determining shim sizing...at static assembly.
 

Howard Jones

Supporter
I think that "pinching" the rod end axially is ok as long as they, two as in a upper A arm for example, EACH remain in a "free" loading condition in the axial direction. To adjust caster, as another example, both rod ends need to be moved for or aft in the same amount and remain unloaded in their new adjusted location. One important factor is to load the rod end with as minimum misalignment as possible. Another is to consider that even with a radial load rating of 20,000 pounds, that seams like a lot, dynamic forces multiply very quickly as speed goes up and shock loads increase. The math (don't ask me it's been far to long) quickly goes off the end of the slide rule when you drop your wheel into a 4 inch deep pot hole at 50 mph. Add misalinement or excessive axial loading to that and you can see how the bearing can get pushed out of the housing.

I looked at several rod end manufactures and tried to come up with a radial load rating. A 1/2 inch male for example varies all over the map from the chepos, 6lb K to the good ones (aerospace) at 40-50lb K. I would use the best ones I could buy for holding the a arms on the car or at least good chrome moly from a well known manufacture.

Here's a good example of a auto racing application from a well known high quality manufacture with a 1/2 alloy at 12.6K and a second at near twice that. So the axial max on the first one would be about 1-2K...…………..@ 90 degrees to the housing. So if you keep it square to the mounting system and "free" in it's through bolted condition the second good one should be OK for upper a arms I would think. I would not use less that 5/8 for the much higher loaded lower a arms.

https://www.fkrodends.com/products/rod-ends/precision-series/kmx/

https://www.fkrodends.com/products/rod-ends/precision-series/sjm-t-sjf-t-2/
 
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Go to this website. You can make the pickups really close and when they are fastened there is enough 'give' in one side of the pickup point that there is no issue. Though the spacing is still critical 'between' the fore and aft pickups. These chassis were setup and welded to pretty close tolerances and the arm tubing was CNC cut and TIG welded to pretty close tolerances also.

http://www.jblmotor.com/chassis.htm
 
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