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

[55 Rescue Dog]

After trying to visualize all the different arcs the rear suspension arms have to move in, during bump travel, it seems like there would be a tremendous amount of binding/twisting forces applied to the anchor points, if you replaced the original compliant bushings, with poly bushings. It would be great if there was a available spherical bushing that could be pressed into one the pivoting ends of all 6 control arms. Newman had the right idea, but it could surely be done for less than $700, I would think.

[Rick_L]

Doesn't Cnut build some of the links custom with spherical rod ends? Not sure what pieces you get for $700 from Newman, seems like if you could do your own you could save. No idea what Cnut charges, but it's probably somewhat reasonable. Some other commercial solution may or may not be as expensive.

[chevynut]

How much "binding" is there over 2-3" of travel? Let's see some numbers.

[hutchenc]

I can only comment on this from what my father knows about polyurethane from a manufacturing perspective (but what he says is corroborated by some pro-touring communities, and one Mary Pozzi who is no dummy). Polyurethane will ovalize over time (that is, it doesn't have the same properties as rubber) so, personally, I think it's a poor choice for any suspension component that needs to rotate/articulate/house anything that needs to maintain alignment on an axis that's subject to shock (which is the case with all control arm and leaf spring bushings). It's great for body mounts and sway bar bushings though. I honestly think they'll all squeak so I wouldn't even consider using them without a greasable zirk fitting.

My pop has poly bushings on his 67 Vette...they squeak something fierce. He greases them every year or so and while that helps for about 3 months, the poly bushings push out the grease eventually and squeak again.

I think delrin is where it's at for control arm and leaf spring bushings unless you want to go with rubber. Just my opinion. Delrin is more expensive, but it's a far superior material if you're not concerned about noise and vibration transfer. Delrin also doesn't wear out like rubber or poly so it's a one-time deal.

[55 Rescue Dog]

Unless the rear C4 spindle upright stays completely vertical during its 5 inches of travel, something has to give. Any change in camber, caster, or toe, is going to try to twist, and turn every bushing. Since the OEM rubber bushings can only be replaced with poly, it will no longer work as designed. I would think a poly bushing would be fine on one end of the locating links, if there was a spherical bearing available that could be pressed into the other end. Or, replaceable OEM bushings. The poly bushings are just to hard to work properly with an IRS, IMO.

[chevynut]

Chad, I personally think Delrin is too hard for a suspension bushing for a street car. For racing it would be good. Polyurethane has been around for decades and millions of cars have used poly bushings. Your father's vette is probably squeaking in some place other than between the sleeves and poly, like where there's a washer over the end of the poly. That's why greasing them doesn't help much. As far as "ovalizing" most of these bushings are designed to be constrained between washers so if they're used correctly I don't see that happening. In fact, it's probably more likely to happen at the swaybar bushings that aren't as constrained. I use them on my pinion support. It's made of two bushings 1.25" OD and .75" ID so there's .25" of poly all around the sleeve. At the ends it's constrained by a bracket that compresses the poly. IMO it's not going anywhere. Most suspension bushings are designed similarly...or should be.

Poly bushings improve the articulation of the suspension by allowing the inner sleeves to rotate in the bushings. Rubber bushings are molded solid to the outer case and the inner sleeve so the rubber has to flex. Eventually it will tear. Poly is soft enough to allow some misalignment too, as is rubber.

There are no rubber replacement bushings for C4 suspensions, so poly is the common replacement that most people use. I think using steel bearings is ok for a race car, but they'd probably rattle your teeth on a street-driven car and they will eventually wear out causing alignment issues...not to mention the cost. And what's the point if polyurethane improves articulation? How much articulation do you need over a 2-3" up/down movement at the wheel? That's only a few degrees at the bushing. Small misalignment angles are easily accommodated with poly bushings. Put one in a dogbone end and move it side to side....it moves a bit. IMO polyurethane is a better choice than rubber or steel. I used polygraphite on my Nomad, greasng everything well, and time will tell how well it works.

RD, you have it backwards. Any deflection of rubber or poly bushings is going to change camber, caster, or toe. Rubber deflects more than poly.

[55 Rescue Dog]

It looks to me for example, if you had solid bushings in all the rear suspension links, and just camber gain alone would cause the dog bones to try and rotate lengthwise for one, and the suspension would be bound up. If the toe changes while turning that would make it bind some more.

[chevynut]

"Some" is the key word. Nothing "binds up" with poly bushings. Go with the spherical bearings if that makes you sleep at night. ;)

[55 Rescue Dog]

The bushings appear to bind in this video, and they are not even loaded by weight or g forces.https://www.youtube.com/watch?v=6qgAXtVCauY
Here is a good under car video of the C4 rear in action.
https://www.youtube.com/watch?v=MhK5eKXC8Og

[destroyertta]

When I did the bushings on my Camaro I used polyurethane my friend suggested using powdered graphite instead of grease worked great I never heard it squeak I plan on doing the same when I do the bushings on my c4 parts

[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/2009/11/Polyurethane-Suspension-Parts/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/showthread.php?2102368-Custom-RTAB-and-upper-inner-rear-bushings-project

https://infopart.org/images/9b/c/lemforder-3642601.jpg

http://img.photobucket.com/albums/v188/joshh/IMG_6429_zps9ae29f56.jpg

[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/showthread.php?2102368-Custom-RTAB-and-upper-inner-rear-bushings-project

https://infopart.org/images/9b/c/lemforder-3642601.jpg

http://img.photobucket.com/albums/v188/joshh/IMG_6429_zps9ae29f56.jpg

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.