Why You Don't Box Rear Upper Control Arms

[Clone TIE Pilot]

The stress loads are all longitudinal on the lower control arms. They do not support any side loads, so they don't need strength in those directions. There is no gain to be had from boxing the factory lower control arms. In fact, allowing them to twist will compensate for the poly bushings if you have them. Also, the strength of the tubular arms, combined with poly bushings on each end, will affect the roll stiffness of the car (turning the axle into an second swaybar) and transfer the twisting forces into the LCA mounting points on the chassis and axle. This is not good.

 

[pontiacgp]

the sides loads are heavy on the lower when taking a corner and boxing them does help stabilize the lower control arm. The side loads are heavy enough to snap axles in a 7.5" rear end in our circle track car. It happened to G bodies so often that they changed to rules to we had to run an aftermarket axle on the right side as a safety feature

 

[jiho]

No, I don't think G-bodies in general were designed with circle track racing in mind.

On the other hand, Regals did take NASCAR by storm, briefly. What kind of setup did those cars use?

 

[pontiacgp]

those cars in nascar used truck arms.....and the cup cars still use the truck arms

truck arms are bolted to the rear axle and there are no upper control arms....similar to this..

 

[Clone TIE Pilot]

the sides loads are heavy on the lower when taking a corner and boxing them does help stabilize the lower control arm. The side loads are heavy enough to snap axles in a 7.5" rear end in our circle track car. It happened to G bodies so often that they changed to rules to we had to run an aftermarket axle on the right side as a safety feature

But the lower control arms are designed to flex. Box them up and all you do is transfer the twisting and more stress to the LCA mounts so they twist and fail because they aren't designed for that. Something has to twist and it's better to be the part designed for that.

 

[marcar1993]

I'm planning on running spherical housing ends with rod ends on the frame side for the uppers. So would it be a bad idea to run boxed lowers with rubber or poly? Or should I just leave then stock with stock bushings until I can get a pair of roto joint lowers?

 

[jiho]

those cars in nascar used truck arms.....and the cup cars still use the truck arms

truck arms are bolted to the rear axle and there are no upper control arms....similar to this..

Those look like I-beams. Not much flex there.

 

[jociha]

http://youtu.be/HzoijYQeyY8
Hope this works it is from Hotchkis it shows less binding then I would expect.

 

[Clone TIE Pilot]

Trailing arm suspension requires deflection in order to work, and when poly bushings are used, the required deflection is still there -- in the bending of the arms, mounting points, and flex of the rear tires. This is why poly bushings appear to work fine for street applications. Also have to remember a guy pushing down on a parked frame is different from a speeding car going down a off ramp under it's own power.

 

[jiho]

However stiff polyurethane may be, it's a lot spongier than metal.