Why You Don't Box Rear Upper Control Arms

[jetsetw31]

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.

No... Actually they are designed to flex also. The original truck arms are close to 2 C channel rails bolted together to make a I channel with rubber between them to allow flexing. And Nascar uses them religiously today because, they are the most stable and least expensive at high speeds. This pic is a G body with truck arms. The only real problem is running exhaust pass them.

 

[Clone TIE Pilot]

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

That's the problem, poly forces the metal to have to twist more. The factory rubber bushings in the lower control arms accommodate this rotation/twist by design, poly bushings force the lower control arms to twist more. Tubular or boxed control arms - with poly bushings - force the LCA mounting brackets on the chassis and axle to bend. In other words the poly bushings bind, forcing the metal to twist instead of the rubber compressing.

Through deformation, rubber bushings allow a large range of angular motion along a primary axis of rotation. Some bushings pivot only along the primary axis, others like 4 link LCA bushings along two or more axis through compression. Nearly incompressible, polyurethane binds along any secondary axis making it a poor choice for a 4 link suspension. The engineers that design these cars employ rubber bushings because deflection is required in most locations. This is why you need either rubber bushings or some form of roto-joint.

Poly will cold-flow, meaning it will deform under pressure and not return to normal, as it lacks the elasticity of the rubber bushings. Over time, they will loosen and then rattle. Check out the shape of your swaybar's poly end-link bushings after only a few months. This cold-flow issue can also lead to alignment problems on the front control arms, as the bushings deform.

 

[pontiacgp]

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

That's the problem, poly forces the metal to have to twist more. The factory rubber bushings in the lower control arms accommodate this rotation/twist by design, poly bushings force the lower control arms to twist more. Tubular or boxed control arms - with poly bushings - force the LCA mounting brackets on the chassis and axle to bend. In other words the poly bushings bind, forcing the metal to twist instead of the rubber compressing.

Through deformation, rubber bushings allow a large range of angular motion along a primary axis of rotation. Some bushings pivot only along the primary axis, others like 4 link LCA bushings along two or more axis through compression. Nearly incompressible, polyurethane binds along any secondary axis making it a poor choice for a 4 link suspension. The engineers that design these cars employ rubber bushings because deflection is required in most locations. This is why you need either rubber bushings or some form of roto-joint.

Poly will cold-flow, meaning it will deform under pressure and not return to normal, as it lacks the elasticity of the rubber bushings. Over time, they will loosen and then rattle. Check out the shape of your swaybar's poly end-link bushings after only a few months. This cold-flow issue can also lead to alignment problems on the front control arms, as the bushings deform.

How many G bodies have you built for circle track racing.? Most tracks you have to use stock rear control arms. We left the upper open and boxed the lower and used polyurethane bushings. I could move the rear end up and down through it's full range with no binding at all. You can use all the theories you want but you cannot beat practical on hands knowledge.

 

[jiho]

I just pulled a polyurethane bushing out of the box. I was able to compress it with my fingers, and it immediately returned to shape. Granted, I haven't actually installed them, let alone run with them for a while, I'm just sayin'....

EDIT: Maybe I should say "bend it" rather than "compress it." It is pretty stiff, but like hard rubber.

 

[Clone TIE Pilot]

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

That's the problem, poly forces the metal to have to twist more. The factory rubber bushings in the lower control arms accommodate this rotation/twist by design, poly bushings force the lower control arms to twist more. Tubular or boxed control arms - with poly bushings - force the LCA mounting brackets on the chassis and axle to bend. In other words the poly bushings bind, forcing the metal to twist instead of the rubber compressing.

Through deformation, rubber bushings allow a large range of angular motion along a primary axis of rotation. Some bushings pivot only along the primary axis, others like 4 link LCA bushings along two or more axis through compression. Nearly incompressible, polyurethane binds along any secondary axis making it a poor choice for a 4 link suspension. The engineers that design these cars employ rubber bushings because deflection is required in most locations. This is why you need either rubber bushings or some form of roto-joint.

Poly will cold-flow, meaning it will deform under pressure and not return to normal, as it lacks the elasticity of the rubber bushings. Over time, they will loosen and then rattle. Check out the shape of your swaybar's poly end-link bushings after only a few months. This cold-flow issue can also lead to alignment problems on the front control arms, as the bushings deform.

How many G bodies have you built for circle track racing.? Most tracks you have to use stock rear control arms. We left the upper open and boxed the lower and used polyurethane bushings. I could move the rear end up and down through it's full range with no binding at all. You can use all the theories you want but you cannot beat practical on hands knowledge.

You can't beat facts and physics with so called "on hands knowledge". Poly bushings with boxed arms is a recipe for snap oversteer, very poor engineering and if the tracks had any brains they would ban poly bushings.

Let's remember one thing: in a bonded-rubber bushing it's the rubber that twists between the inner and outer shell. The rubber is the pivot. Nothing slides, nothing rotates. This is why rubber suspension bushings must be snugged when the suspension's at normal ride height - you want to ensure the bushing is in its neutral position most of the time, and that the rotational travel is roughly even in both directions from neutral. Poly bushings are not flexible enough to do this, so they rotate on their inner sleeve. This is bad. They will squeak, they will gall and egg-out and can cold-flow. Poly has proven drawbacks including stiction, binding, squeaking, harshness, and need for regular greasing. 1LE rubber bushings are a far better choice than non-roto joint poly bushings. Your own photos proved what I have stated is true.

Here is some good reading about the subject.

http://www.elephantracing.com/techtopic/binding.htm

http://elephantracing.com/techtopic/polyurethanefriction.htm

http://www.crystalridge.net/cars/bushings.htm

http://www.netbug.net/blogmichael/?p=103

http://www.pro-touring.com/threads/16050-Explanation-of-bushings-binding

 

[pontiacgp]

all those articles are generalizations except for one that talks about an f body. They do not give specifics. One of those articles did say this.."If you are going to modify your suspension, understand common causes of binding. Select properly designed performance parts and combine them in ways that do not bind. Check operation over the full range of motion" If you recall that is what we checked with our metric so you can dismiss the knowledge that I gained over building and crewing on 4 metric circle track cars, a couple of sportsman cars and now a pro late model but I do know what I am speaking of. UMI designs and manufactures control arms and offers different types of bushing including polyurethane. UMI tests their products on their own cars and there are other aftermarket companies who make similar products and using polyurethane bushings. You might want to rely on articles you found on the internet cause we all know everything on ht internet is correct but I rather rely on what we have tested and found to be true. Oh and you forgot to tell me how many metric circle track car you have been involved with cause I'd like to know what you know and not what you have read online.

 

[Clone TIE Pilot]

The number of cars I worked on is nothing more than a red herring, besides you should be focusing on quality instead of quantity . Your boasting on working on so many cars is mere hearsay and appeal to unqualified authority. It has no basis on the truth, the facts do. The fact is poly doesn't compress and deflect like rubber does, which is required for any suspension that requires muti-axis movement like a 4 link. This has been proven over and over again, even the Rustang guys know this. That is why you need either rubber bushings or roto joints, non-roto poly only has one axis of movement and binds on any secondary axis movement, period. You can claim whatever bunk you want but you can't argue against physics and science. As I said before, your own photos you posted in this thread proves what I have said is true. So you are arguing with yourself LOL. Do you even understand what binding and snap oversteer are? It really seems like you don't.

I am done arguing with you, I can't force someone to accept the truth. If you want to believe in the magic red bushings go ahead and corporate advertising and PR spin never lies. :wink:

 

[pontiacgp]

You don't understand what hearsay is but regardless you can call me a liar and chose to believe what you read online......

 

[pontiacgp]

The issue with the uppers is really not an issue since there are products to buy that cure any binding and give you the performance that you are after. I have a set of the UMI in the conversion style for my A body rear end. With the roto joint that lets the upper twist as must as it needs with no binding. There are other manufactures out there that produce similar products so there is no reason to modify what you have since any mods you will do won't be as good as a set of these..

 

[tc1959]

The number of cars I worked on is nothing more than a red herring. Your boasting on working on so many cars is mere hearsay and appeal to unqualified authority. It has no basis on the truth, the facts do. The fact is poly doesn't compress and deflect like rubber does, which is required for any suspension that requires muti-axis movement like a 4 link. This has been proven over and over again, even the Rustang guys know this. That is why you need either rubber bushings or roto joints, non-roto poly only has one axis of movement and binds on any secondary axis movement, period. You can claim whatever bunk you want but you can't argue against physics and science. As I said before, your own phots you posted in this thread proves what I have said is true. So you are arguing with yourself LOL. Do you even understand what binding and snap oversteer are? It really seems like you don't.

I am done arguing with you, I can't force someone to accept the truth. If you want to believe in the magic red bushings go ahead and corporate advertising and PR spin never lies. :wink:

I guess he told you.......