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

[hutchenc]

Lazlo...I had delrin bushings in my '56 up front before I tore it all apart. I had the Global West stuff. The car was already pretty stiff up front to be honest and the ride wasn't good (with the poly bushings I had before the delrin). After I put the delrin bushings in, I honestly didn't notice any ride quality difference, but it wasn't exactly a Cadillac to start with. I don't know how it will be on a C4, but I will likely find out (on the front at least). I've had two bad experiences with poly bushings so I'm gonna avoid them from now on. Just my personal opinion. Once it's all done you can see what you think...deal?

Also, as far as wear and tear goes on the delrin, without sleeves yeah I agree. It would break down. The Global West ones have sleeves on the ID and the OD of the delrin piece. http://www.globalwest.net/1060.html

[55 Rescue Dog]

Poly bushings work great on some pivot points, like a-arms where they rotate on the same plane, or a single link. The issue I see is mostly on the tandem C4 dog bones where the bushing needs to rotate, but, with camber change it also has to twist in conflicting planes, to follow the arc of the upright through full suspension travel. If there is very no camber change it would be fine, but it does change?

[Rick_L]

I think you guys are missing Rescue Dog's point (more on whether it's a good one in a bit). The delrin bushings are a great setup for a joint in rotation only. The advantage of a spherical rod end is if the link had a twisting component also. An example of that would be a 4 bar suspension with a live axle. There the bars have to twist as well as rotate. But often even those use a rubber or polyurethane bushing. They gain a little bit of compliance by using a big bushing with lots of plastic or rubber.

With the IRS though, my guess is that any twisting component is minimal, but enough to preclude a delrin bushing. It would be easy enough to check out by removing the spring and observing the action of the links and seeing if there is a tendency to bind. Because the dogbones are so short they would be the most suspect.

[chevynut]

I think you guys are missing Rescue Dog's point (more on whether it's a good one in a bit).

No, I'm not missing his point. The IRS goes through arcs in both directions so there is some side minor load on the strut rods and the dogbones. However, with the shorter upper dogbone and the longer lower one, the side loads on the strut rods are minimized due to the geometry. And there is some slight misalignment at the dogbones, which I'm sure GM recognized but didn't consider it a problem. My point is the urethane is soft enough to absorb those slight misalignments just like the original rubber bushings do. The hardness of the stock rubber pieces isn't that different than the urethane, and the rubber is going through the same loads. And the rubber is actually a little thinner than the urethane. However, with the stock pieces the rubber has to twist in the shell and the sleeve and that inhibits movement. The stock rearend doesn't articulate that freely with the rubber bushings either. But they built millions of cars like that.

If you ever messed with the stock C4 front end you would see that it doesn't move very freely either. I had to loosen the a-arm shaft nuts on my setup suspension parts that we use in the frame jig to get some movement to set up shock mounts. Those rubber bushings also twist inside of a shell. So urethane bushings will help with the front end articulation as well.

BTW, I suspect Newman put that rearend together without lubing the urethane bushings where they contact the sides of the suspension components, or possibly not lubing them at all. The sleeve has to rotate in the bushing, which means the suspension bracket that doesn't contain the bushing has to slide on the bushing. It's not "binding" as much as the fact that there's friction in the system. And look at what range of travel he's moving the suspension through...it doesn't do that while driving as you can see in the other video. Remember also that the video is a sales pitch for their bearings. I'll be assembling mine before long and maybe I'll video what it does.

The delrin bushings are a great setup for a joint in rotation only.

I agree. I think they would work well in something like an a-arm, but again they're harder than urethane and should transmit more road noise to the frame. Some guys go to the extreme and use aluminum bushings in their a-arms, but you wouldn't do that in the IRS.

The advantage of a spherical rod end is if the link had a twisting component also. An example of that would be a 4 bar suspension with a live axle.

I agree it's obvious that a bearing will absorb those slight twists. But there's a downside, and that's harshness of the ride, wear, noise, and cost. With some urethane surrounding the bearing it would improve the vibration problem, but there's no room for something like that in the dogbone ends. Also, urethane works to accommodate some slight movement where a harder bushing like Delrin or aluminum would not. There may be something better than urethane out there, but I don't know what it is.

With the IRS though, my guess is that any twisting component is minimal, but enough to preclude a delrin bushing. It would be easy enough to check out by removing the spring and observing the action of the links and seeing if there is a tendency to bind. Because the dogbones are so short they would be the most suspect.

My point from the beginning has been that the "twisting" happens, but it's small. I think Delrin or anything hard like that would cause the dogbone itself to twist slightly which wouldn't necessarily be a good thing.

I don't see a lot of "twisting and binding" in the video RD linked, and the suspension travel isn't that much....I'd say less than 2" up and down. Once he comes up with a measurement or calculation of how much misalignment is induced in the dogbone bushings we can discuss the loads on the urethane.

[chevynut]

Here's some data you might be able to use for your calculations, RD

Halfshaft length: 18.125"
Upper dogbone length: 10.875"
Lower dogbone length: 12.75"
Vertical distance between dogbones at knuckle: 6"

[55 Rescue Dog]

No calculations needed, to simply visualize that whatever degree. the camber change is through travel, will twist the dog bones, (like a driveshaft), and the mounting points. The rubber bushing will take rotational, and torsional forces in any direction. A poly bushing is only meant for rotational movement. Sure it can move like rubber with enough force, but that is not an improvement on the original design by far, and will needlessly be adding more binding force on the dog bones, and mounts. The urethane will add even more stiffness under cornering, accel/decel loads. I see no benefit in making it worse than stock. It is a lot like try to rotate an engine on a stand. Rotates easy, until you put a load on it. But, I'm probably wrong again. The C4 rear suspension appears to be a 5 link design, where there are many different forces in opposing arcs, unlike many other designs. It's like a modified C3, trying to mimic, what turned into a total re-design on the C5+
What looks great on smooth paper, does not always work so great on rough pavement. Calculate that.

[chevynut]

You have a weird mindset imo, RD. You seem to think that because a double shear design is better than a single shear design, that there's something wrong with any single shear design. The fact is, a single shear design is fine as long as the stresses are below the strength of the members, as I showed you. But for some reason you still can't accept it.

Same with this question you brought up. Sure there is some misalignment in the dogbones as the suspension moves. But the FACT is that the urethane isn't solid, and it can absorb some misalignment. I suggested you calculate how much misalignment there really is, so you can see that what you're worried about is minor. Obviously you don't have no interest in doing any calculations, so you agonize over some made-up "problem" and try to imagine how things move. MILLIONS of guys use these poly bushings in Corvettes and other cars, and they're proven to work.

The rubber bushing will take rotational, and torsional forces in any direction. A poly bushing is only meant for rotational movement.

The rubber bushings don't take any force without being stressed. They're stretched with ANY movement of the suspension members. A poly bushing rotates freely, and it can take some side or "twisting" force as well. Poly is not a solid, it's more like a very viscous fluid. The rubber adds just as much or more "binding" force to the suspension members as polyurethane.

What's the durometer of the stock rubber? What's the durometer of the polyurethane? If you don't know, you can't make the statements you're making.

The urethane will add even more stiffness under cornering, accel/decel loads. I see no benefit in making it worse than stock.

What's the durometer of the stock rubber? What's the durometer of the polyurethane? If you don't know, you can't make the statements you're making. What's "worse than stock"? The urethane allows more rotational movement, thus more articulation of the suspension. The stock rubber stretches and binds with ANY movement.

What looks great on smooth paper, does not always work so great on rough pavement. Calculate that.

How did I know you'd eventually say that?

I don't understand why you even started this thread. If you think the bearings are so great, buy them.

[chevynut]

"In the case of a sleeved bushing, the rubber pressed into the inner and outer sleeves requires some give to absorb the flexing of the control arm or spring under load. While this rubber is where all your suspension flexibility is found, it is also where suspension component failure first occurs. The constant loading and unloading of a rubber bushing causes it to lose its ability to rebound quickly and could eventually lead to the rubber tearing within the inner and outer bushings. Because they are not designed to rotate as surrounding suspension components rotate, deflection occurs, which can damage the bushings prematurely, in addition to being harmful to the control arms and shafts that they are buffering. Other factors also contribute to deterioration, including road salt, rain, dirt, snow, temperature, ozone and UV light, but most failures occur from age or long periods of use. As the rubber ages, its durometer rating also decreases, causing suspension creaks, less responsive body roll in turns and inability of the car to track accurately down a straightaway because of changes in caster and/or camber. These factors brought about the use of harder substances in suspension bushings based on polyurethane."

"Polyurethane is less likely to be affected by oils from your car or off the pavement. It also withstands changes in temperature better and has a higher load-bearing capacity than rubber. Polyurethane also is less likely to tear, and can withstand abrasion better than conventional rubber bushings. Because polyurethane bushings are usually assembled by you, rather than pressed together by the manufacturer, the bushings function independently from their sleeves, allowing for more flexibility and less binding than a rubber bushing. These factors also add to the lifespan of polyurethane suspension components; they typically last over twice as long as conventional rubber bushings."

http://www.hemmings.com/magazine/mus...s/2293331.html

[chevynut]

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.

[chevynut]

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