Does using polyurethane bushings on a C4 rear suspension cause a major bind?

[NickP]

Apparently BMW uses some sort of ball bushing in their rear suspensions....I have no idea what size they are but they could perhaps be used with some custom dogbones if one was so inclined.

http://www.bimmerforums.com/forum/sh...shings-project

That's essentially a "Johnny Joint". There are various iterations out there. That's a nice one too. Some out there are rebuildable.

[55 Rescue Dog]

RD, since you apparently don't want to make the effort to understand the magnitude of the misalignment in the C4 dogbones and expect someone else to do it for you, I'll do some calculations (again). But I know you think calculations don't mean anything and you have to actually drive the car on a rough road to know anything about anything. I'll make some simplifications to make it easier to follow.

The C4 rear suspension geometry is controlled by the halfshafts, lower strut rods, dogbones, and toe rods. Viewed from the rear, the strut rods and halfshafts control camber. Viewed from the top, the toe rods and halfshafts control toe. Viewed from the side, the dogbones control caster. Obviously these links all control roll center, anti-squat, etc., as well but that's not a subject of this discussion.

The lower strut rods are 18 3/8" long which is slightly longer than the 18.125" halfshafts and they are not parallel to the halfshafts at ride height. The inboard joints on the late C4 are separated vertically by about "5.5 and by about .75" horizontally. The outboard joints are separated about 6.5" vertically. I laid out the geometry and get 0.97 degrees of camber gain at 2" of upward travel with these numbers which are rough measurements off of a late C4 Dana 36 rearend.

Let's assume we start with the dogbones pointing straight forward with no left or right displacement at ride height. The halfshaft is fixed-length and swings in an arc so the center of the knuckle moves inboard as the suspension moves upward. At 2" of travel, the knuckle moves inboard 0.110". The vertical distance from the halfshaft to the upper and lower dogbones is about 3", and the knuckle rotates around the halfshaft. At 0.97 degrees camber the rear of the upper dogbone moves inboard about 0.051" and the rear of the lower dogbone moves outboard about 0.051". So I calculate the total displacement of the upper dogbone as 0.161" inboard, and the lower dogbone displacement as 0.059" inboard based on these numbers.

That's 0.85 degrees at the upper dogbone and 0.27 degrees at the lower dogbone due to horizontal offset. As far as "twisting", the .97 degrees of camber is absorbed by TWO bushings so they see 0.48 degrees each. Surely the urethane bushings can absorb that small misalignment without any issues. It's been demonstrated on "millions and millions" (as Trump would say) of C4 corvettes for decades of use.

Now are you satisfied that the numbers are small? You keep saying nobody here will help you, but has anyone ever tried to explain this or other technical subjects to you elsewhere to this extent? I calculated stresses in bolts to prove to you that single shear designs can be safely implemented, but you basically dismiss it. Nothing in this world is as perfect as you seem to think it should be. If you would just read what people say and think about it before dismissing it due to your prejudices, maybe you can actually learn something here. You ask a question, then won't listen to anything anyone has to say in response.

Like I said, if you think the misalignment should be zero for the IRS to work correctly, I suggest you pay the big bucks for the steel spherical bearings since you seem to think they solve a problem. And perhaps you should call all the polyurethane manufacturers and many Vette parts suppliers and tell them their poly dogbone bushings are junk.

It's sad when somebody can't ask question, have ideas, or opinions, and get ridiculed, and be insulted on every post, that doesn't have engineering proof. I wouldn't have had a career as a technician, if engineering got everything right every time.
But, thanks for the calculations, although I would suspect the camber changes quite a bit with full suspension travel, versus just 2 inches of travel in the sweet spot of the curve. I'm probably wrong, but the spec I saw on the C4 rear wheel travel was, 3.6 inches bump, 2.8 inches rebound. That is over 6 inches total travel. But. if the total positive to negative camber change is only .85 degrees with 2 inches of travel, it must be the same with 6 inches. So, if that true, it would not be a binding issue at all, I guess. Sorry, I've been proven wrong again. The toe change probably isn't a factor either I assume. And, the anti-squat geometry adds no force to the equation either, I'm sure. I have no idea, but mounting coil overs at an angle pressing outwards on the bottom of the upright helps the situation too, it seems, since it has now been totally re-engineered. I forgot, that changing the height of the toe links doesn't change anything either. Yes, poly dog bone bushings are junk. I'm just throwing it out there, hoping for somebody to invent a solution, I would buy.

[55 Rescue Dog]

After a lot of reading about it, from C4 comments over the years, I have come to the conclusion, that there is no way in heII, that poly bushings on the dog bones are a good idea. Sure, they kinda work. Way too many conflicts in motion for that hard of a bushing, and very unpredictable. Poly bushings cannot possibly move through the 3-4 different arcs of the dog bones, through 6 inches of wheel travel, without very inconsistent binding, and they will wear fast. The result will definitely end up with the car swapping ends, from unpredictable over steer, if it doesn't break a single shear mounted bolt first. Plus, it will not ride right either. OEM bushings would be priceless, if anyone could reproduce a replacement. The dog bone bushings have to absorb the forces from accel, decel, toe, camber, anti-squat, anti-lift changes, side loads, plus probably a couple more.