PDA

View Full Version : RUBBER, Versus POLY - The Saga Continues.



NickP
11-21-2016, 05:38 PM
Argue here

55 Rescue Dog
11-22-2016, 02:00 PM
I wouldn't think about using poly bump stops either. Squishy progressive bump stops work better, along with the rubber bushing adding some progressive spring rate over big bumps, or when it bottoms out, and the spring rate goes infinite, then something breaks. On smooth roads the rubber bushings offer very little rate to the spring. It might not help much, I don't know, but wouldn't the rubber bushing assist the spring in reverse motion, along with the shocks to keep the tire planted on bumpy roads? I've driven plenty of fast, great handling cars, on bumpy roads that felt well planted running on stock rubber suspension bushings. Not that the OE bushings can't fail, but I can't remember when I last had to replace one for a lot of years. I have had countless GM 4 link cars over the years, and they made some great ones. I think where people hinder the rear suspension action on those cars is when they switch all the rubber bushings over to poly, and use boxed control arms. They use an open channel on the stock links so they can twist too, when the rubber compresses so far in the bushing. Otherwise nothing can twist, binds up, spring rate goes sky high, and around she comes. Snap over steer is tough to drive out of. I still guessing of course, but with my imagination I could see that maybe Chevy went to the ugly, but effective composite small diameter dog bones, so they could twist and add compliance to the multi arc 5 link C4 suspension?
I don't know this either, but I would think the 96 C4's had the best ride? And hell yeah, I would love to hear all about the dynamics, and functions of the C4 IRS. It seems like there is a lot going on back there with all the different arcs, changing wheelbase, camber, caster, toe, roll center, and even track width. Who knows what happens for sure, when you start changing components of the original design? Heim joints seem to be the obvious choice for trailing arms, since the OE bushing are not available. All that is needed is a good well made OE replacement, instead of some sales pitched, poorly made plastic bushing that almost fits. A modified, narrowed "well engineered and tested" IRS that worked better would be an amazing feat. Especially with the smaller suspension arcs, and a pro-stock narrowed track. I'm planning on a full roll cage for mine with a much wider track, with a cheap paint job on the roof until it can hit 1G in a corner on at least 3 wheels. I don't know what the effects are for, but I have read that GM made the axels (upper link) on the C4 IRS as long as possible for some stupid reason.

Rick_L
11-22-2016, 07:23 PM
I am reluctant to reply because of the shit storm that will go on, but here goes.

Comparing the twisting action that has to go on in a 60s/70s GM coil spring/solid axle suspension to a C4 is comparing apples to oranges.

The "Chevelle" suspension has significant twisting motion throughout its travel. The suspension arms are a "hat section" and have little torsional rigidity. When you box them, the torsional rigidity increases probably ten-fold. Add in pu bushings that are stiffer than stock, and you have a recipe for binding - and probably eventually breaking something.

In constrast, while the C4 "dog bones" are a solid cross section and they are pretty stiff, the dog bones are not required to twist anywhere near as much as the bars in the Chevelle suspension. So even with stiff dog bones the bushings don't have to give much in twist, and they should work fine.

In making the decision on what to use, you need some good data on what the real difference in materials is. And that's not ever going to be clear.

As for your comment on rubber vs. pu bump stops - yes, rubber is not a linear material when you compress, tension, or shear it. But neither is pu. The biggest difference is that the pu material you can buy commercially starts out harder. But don't think it's not "progressive".

55 Rescue Dog
11-23-2016, 05:09 PM
I wonder how many of the great handling cars, supercars, or all out race cars use poly bushings? None would be my guess. It's cheap plastic garbage that sells well. If rubber bushings were cheaper to make, that is what the aftermarket would be trying to produce/sell, and that would be an improvement. No moving parts is where it's at.

55 Rescue Dog
11-25-2016, 03:07 PM
Interesting ad in the Danchuk catalog I got today, and what I've been trying to say about poly bushings in general.
6596

Rick_L
11-25-2016, 05:14 PM
Holy crap! $300 a set. From Danchuk, those performance guys, yeah right. I'll pass, they're a ripoff.

Like with any material you might want to choose for some application on a performance car, you might want to find out what you're getting. Whether the Delrin is polymer or copolymer, and whether the material is virgin, annealed, or double annealed, as well as whether there are any "alloying" materials in it. You might even ask if it's really Delrin (a Dupont brand) or some other acetyl.

NickP
11-26-2016, 05:03 AM
BTW and FIY, this product is actually a RideTech™ component. Having said that, it's still worthy of the same questions Rick.

55 Rescue Dog
11-26-2016, 02:32 PM
Looking at the at NP's drawing it looks like the dog bones create a pretty tight arc affecting the wheelbase lengths, which would also make poly bushings on the camber links a terrible idea too, since they have to move up/down, and for/aft all at the same time. The only things I see in the rear C4 IRS that wouldn't cause a bind issue using poly, is the batwing, and torque arm bushings. Driving the car easy cruising, the poly bushing probably feel great, but when the car is pushed to it's limits, it will bind up like a knot.

NickP
11-26-2016, 02:35 PM
Looking at the at NP's drawing it looks like the dog bones create a pretty tight arc affecting the wheelbase lengths, which would also make poly bushings on the camber links a terrible idea too, since they have to move up/down, and for/aft all at the time. The only things I see in the rear C4 IRS that wouldn't cause a bind issue using poly, is the batwing, and torque arm bushings.

Whaaaaaaaat?

Oldxr
12-12-2017, 08:23 PM
The suspension arms do not stay at a 90 degree angle to the mounting bolt through the suspension travel.Rubber bushings have enough give to not create a bind condition at the limits of the travel .There are companies that make heim jointed arms for c4 rear suspension .

chevynut
12-13-2017, 03:02 PM
I'll put my opinions in this thread to counter the BS posted about polyurethane bushings.....again.

When properly designed and used, poly bushings do not "bind" as claimed. in fact, ALL rubber bushings are ALWAYS in a bind when the suspension is moved. That's why you have to lower a car to ride height before you tighten the bushing fasteners. On rubber bushings the rubber is BONDED to the sleeve and shell. The sleeve CANNOT rotate relative to the outer shell without imparting shear loads (twisting) of the rubber. It's physically impossible.

Poly bushings are designed to rotate on the inner sleeve. That rotation facilitates articulation of the suspension, especially when a proper lubricant is used. There is no twisting of the poly like there is of the rubber. Properly designed poly bushings don't induce friction or constrain movement like rubber bushings do.

On my mockup C4 suspensions used for building shock mounts I have the stock rubber bushings. When the front a-arm shafts are tightened down, it's almost impossible to move the suspension by hand. I have to loosen the large nuts on the shafts to get them to move. Also, when the stock dogbones and strut rods are tightened down, the rear suspension barely moves by hand. When a rubber bushing tears, it squeaks like crazy.

I used poly bushings on both my front and rear C4 suspensions. However, I modified them so they fit the way they're supposed to fit. I measured the width of the dogbone sleeves and I made the bushings just slightly larger so they were lightly compressed (about 0.010") from the sides when the bolts were tightened down. I did the same on the front lower a-arms. After greasing and tightening, the rear suspension moves just like it had heims. The front suspension articulates so well I had to set it on boxes to hold it in position. The poly bushings are NOT rigid, so they absorb some deflection of the parts due to misalignment. There is no "binding" at all, and the displacements in the C4 suspension are small as I pointed out in another thread with this misleading BS posted on it.

The reason rubber is used for OEM applications is because it's cheap, and it's softer so it reduces NVH. Rubber doesn't hold the suspension components in alignment very well, that's primarily why they're replaced for performance applications. Poly is a compromise between harder materials like aluminum or delrin and rubber as far as NVH goes. It gives better control of the suspension components and still absorbs some vibration.

I personally think that using heims on the C4 dogbones for anything other than racing is a BAD idea as they absorb most of the shock from road irregularities and transmit it to the frame. IMO it's the WORST place to use them due to the shock loads and the wear that causes them to rattle over time. The strut rods don't have to absorb those shock loads, they just move up and down but they can still transmit some vibrations.

My experience is that manufacturers can't account for all the variables in some poly bushing applications, so they simply make them larger than required and people crank them down, compressing them too much. That causes a lot of friction and poor articulation of the suspension. The preload on a poly bushing is caused by making the sleeve larger than the hole, so it needs to be pressed in. There doesn't have to be preload from the sides, just a way to constrain the bushing.

If rubber was so good there would be a huge market for it and nobody would buy poly. But poly is known to control suspension deflection better than rubber for performance applications. It's been the material of choice for decades. You can hardly find rubber bushings for many applications, like the C4 and C5 Corvettes. All the aftermarket bushings for them are poly or delrin, or similar.