69_427_SBC

I'm going through the worn out rear suspension in my '87. I'm more focused on lateral performance than I am accel/decel improvement, so I had figured I would go with heim-jointed camber and toe rods, and just upgrade to poly bushings (with zerks) in the trailing arms.

This sounds great on paper, but I'm worried about the combination of those two causing binding in the suspension. Where the rubber bushings all around could accommodate the few degrees of camber change in the spindle as it articulates, now the Heims will force the camber change, and the poly trailing arm bushings will resist being twisted.

Has anyone ever run this setup? Am I making this a bigger deal than it would be in reality?

Thanks for the expertise!

grandspt

I originally installed poly in the rear of my 1996 GrandSport but I did get a bit of binding (felt like the rear was not moving or in sync with the front) when I made abrupt lane changes. I called Tom Urban at Banski and explained my issue he agreed that it sounded like binding and he recommended only changing the Camber Rods with the Banski Camber Rod kit (he could have recommended his whole Banski set up and I probably would have bought it but he seems to be an honest guy). He felt that the rest of the suspension was probably fine with the polyurethane.
So my car now has poly in the rear (dog bones poly, sway bar poly, toe rods are still OEM) except for the camber rods which are Banski heim rods. After an alignment the car responds very good without any binding that I can feel!

Hope this helps

BOOT77

I actually looked into this for drag(acceleration only). I came across some info that explained the poly rear are better suited for drag because they bind when cornering and heim joint rod end style have more free movement. The poly will be fine in and up/down movement you see in drag vs side loading of cornering. Still from a launch perspective I'd think the rod end would also be better and not compress any, especially with high power. But my build is more of just a budget and I don't plan on going too fast so I'll prob just poly my trialing arms to save $. Pretty much what you have now rod end camber, I'll get rod end toes and install polys in my dog bones.

ddahlgren

The problem is the silly design sold as new. Rubber was the band aid to cover it up. The dog bones need to be installed to follow the same arc as the half shaft to not bind with a rigid bearing and they are not. In my mind poly is one of the most over sold patches ever devised as they fail on a variety of conditions. yes they are better than worn out rubber but come with their own problems too.

Tom400CFI

Yes, there will be binding. Why? As the suspension moves through it's travel, the camber changes; the knuckle rotates when viewed from the rear. That movement is dictated by the axle and the "camber rods" AKA lower control arms. The Dog bones AKA trailing arms aren't designed to twist to accommodate that rotation of the knuckle. They accomplish that movement in the stock set up, with the rubber at each end.

The best poly/heim solution would be spherical bearings in the trailing arms and poly in the lower control arms, since the lateral deflection to the LCA bushing is minimal. The best solution over all is spherical bearings at all pivot points.

,

ddahlgren

+1

JrRifleCoach

Heres a nice mod for the dog bone mount.

69_427_SBC

Thanks all for the input. Sounds like there's general consensus that poly trailing arm/heim camber rod is not a good combo, as I figured.

Looks like by piecing together parts from Banski and VB&P, I can go full-heim in the rear for ~$850. I'll justify the cost to myself as "fixing a design flaw in the original suspension!"

Tom400CFI

I would piece it together and NOT replace the toe rods. They factory toe rods are already a spherical type joint....just not a "precision" one. The benefit of the stock toe rod ends is that there is already no "mush" in them (b/c they're a steel spherical joint, rather than a rubber bushing), they are well sealed, they are long lasting/robust, and they are free. The disadvantages are that they're not a "blingy", and they have higher friction -however the friction isn't a meaningful factor.

Ideally, I'd want to see spherical bearings on both ends of the LCA's and all 4 points of the dog bones. I'd save money and not care about the toe rod/ends.

.

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ddahlgren

Are there any dimensions or angles for setting rear spindle when changing so called dog bones / longitudinal links?

whalepirot

Street driven, been setup this way for >15 years, with a recent R&R of all 16 aft Heims. The car is very precise, a stock Vette feeling very loose, by comparison.

Zerks on the F a-arms and lubing the F sway bushings at each oil change, improves the 'feel', deemed less binding of those poly sway bushings.

MatthewMiller

FWIW, I have the Banski camber rods installed on my C4 and the Banski trailing arms not installed. The camber rods are by far the most important, because they allow a real range of camber and won't ever move due to cornering, whereas the stock eccentric can move (especially if you compete on sticky tires where cornering happens) and you lose your rear camber setting.

I have not really noticed a need for the trailing arms, so I didn't prioritize them too high on my to-do list. But I will try them when the competition season starts up in spring, I think. Banski was reducing stock on some items, so if you can't get the trailing arms don't fret too much. Get the camber arms by all means.

I emailed Tom Urban (Banski honcho, super helpful and knowledgeable) about this when I started contemplating installing the Banski trailing arms. The directions say to duplicate the measurements of your stock dogbones. But with all the hoopla surrounding C7 rear caster settings (those cars are hyper-sensitive to it and have a stock adjustment), I wondered if he had any thoughts about using the rod-end trailing arms to change caster. In short, he did not have any info on that. Either nobody has tried it or they haven't broadcast their findings.

So your best starting point is stock. You could experiment from there. The main thing you're going to be impact is rear bump steer. I go a little cross-eyed just thinking about all the permutations of those rear caster angles! You could also go shorter or longer overall on all four trailing arms to shorten or length the wheelbase a bit. But I don't know why one would.

ddahlgren

My point exactly with a factory non adjustable setup there is no need to publish a target spec.

ddahlgren

Thinking out loud I wonder if DRM has a handle on this?

Rob31

Wow , my post was deleated .Well see how long this one lasts . All this info about poly /rod ends ...
Not one mention of the rubber bushings in the batwing . All the effort spent in bushings and alignment wont mean squat in a lateral situation if the rear end is moving around in 25-30 year old rubber bushings .

MatthewMiller

Interesting point. In the year that I've been on this forum I don't think I've seen this point raised for C4s. I don't know how much it moves laterally, which is down to the two round rubber mounting bushings for the batwing, as you say. It is well constrained longitudinally and in rotation (around the axles) by the C-beam. Does anyone even make poly or Delrin batwing bushings?

69_427_SBC

I can see why that would make sense, but my toe rod ends are both pretty shot. So I am weighing a factory replacement against this "upgrade," which makes it more logical.

You must have a full Heim rear setup then? I'm not talking poly control arms (in the front), but poly trailing arms (in the rear)...

I agree with this, with the possible exception of...

I'm definitely going for poly batwing bushings, and would probably go solid aluminum or delrin if I could find any. The only argument for these being less important than camber rods is that movement in the camber rod actually effects the camber angle, whereas movement in the batwing wouldn't, as the half shaft and the camber rod would both move identically. Still, seems like the whole rear end sloshing around under the car could lead to some scary dynamic behavior, so they'll get swapped for sure.

MatthewMiller

The thing is you have to figure the magnitude and direction of forces acting on the bushings. Lateral cornering forces in the rear are reacted by the camber rods and driveshafts (acting as lower and upper control arms, respectively). The direction of that force vector is mildly upward from contact patch toward center on the outside tire and downward from center to contact patch on the inside tire (but at a lesser magnitude because it is lightly loaded). So in my mind's visualization of this, the cornering forces are acting pretty close to a direct line through the batwing bushings. That means they probably move a lot less than we think. It would be different if the forces were acting well below them, setting up a significant lever arm that would torque the whole batwing around. The other good news about this is that the batwing is probably only moving laterally, not up on one side and down on the other. This means the basically rear geometry doesn't really change when it moves - literally only the body/frame moves a bit on the batwing, which is not nearly as bad as other types of movement.

So that's all a long way of saying I doubt that the batwing bushings are all that critical. But all of this is just an educated guess.

69_427_SBC

Agreed on all points. I'm picturing this: if the batwing bushings have 1/4" compliance off center in both directions, then as I transition left to right doing a slalom, the whole rear suspension is "sloshing" 1/2" total back and forth under the chassis. Not as big a deal as uncontrolled camber change, but still unwanted.

Rob31

the stock batwing bushings move around more than you would think . I was hesitant on changing them mid season as a Championship was on the line . After counting threads on the spring bolts , changed the bushings , put it all back together I gained 3/8 in ride height . the bushings sagged that much . Tires rubbing the inner fender wells brought this to my attention .I run 315s /Rcomp tires and they rubbed in high
G loaded corners .Im still sorting this out , I have only run the new set up once and it was in the rain . ( I was second fastest time of the day btw )