I don't know how to quantify the severity of my caution. I would call it "strong," I guess. The vast majority of drivers are probably never going to encounter the snap oversteer I'm talking about, because they never drive near the limits of their cars (many think they do, but they don't), and if they ever do one time they won't know the difference because they don't regularly drive at the limits of any car. If you autocross or track your C4, and test it back to back with and without hard poly bushings, you'll feel the difference. This isn't lawsuit material for a bunch of reasons I won't go into here.

The best way is to assemble the rear suspension without the spring, swaybar, or shocks installed and then actually measure the force it takes to move the suspension through its entire range from full droop (extension) to full compression. Do this with heim joints, stock rubber, SuperPro, and hard poly installed and compare the force curves as force over suspension displacement (force on the Y axis of a graph and displacement in inches or mm on the X axis). My prediction is that you'll get nearly no force required for heims (pretty much just the flat weight of the suspension components), which is ideal (they have no "rate" in that condition, so the wheel rate is provided only by springs, sway bars, and shocks). As you go up the order of bushing types you'll see higher force rates with more extreme upward curves (instead of a straight line angled upward - IOW a rising rate, which is bind). I have never seen anyone do this with a C4 chassis. I know an engineer in the automotive aftermarket who has done this for some solid-axle suspension setups, and he has measured considerable bind (rising rate) with hard poly bushings. It is a real thing.

 

I just wanted to throw out some real world experience that I’ve had with running threaded trailing arm and other links with heim joints. They are extremely durable. So for those of you worried about wear and durability, rest assured, they may outlive you. I had a circle track race car with a solid rear axle that had a 4 link set up. Two trailing arms, a lateral panhard rod and a top radius rod. All were threaded aluminum links with heim joints on both ends. We sprayed them with WD-40 after every race weekend and wiped them off. That’s the maintenance, period. Don’t put any grease on them that stays wet or sticky as it only collects debris and will cause wear. The heim joints were torture tested on asphalt and dirt tracks every weekend for years and we rarely found one with any play. The only thing that could bend or break them was a hard crash where the axle took a bad shot. Most of the time the aluminum links bent, not the heim joint. If you spend the money on links with heim joints it will likely be a one time thing.

 

This reminds me of my recent quest for reviews/feedback on tires. Often, writen descriptions didn't match numeric counterparts. I'd read noise/handling might be an issue...then see (numerically) tires rated about the same -- not night/day difference. If Poly bushings aren't going to cause a crash AND since "the vast majority of drivers are probably not going to encounter snap oversteer", I gotta loop back to my original post in this thread. Combined with 5yrs of driving, I'd conclude poly is a reasonable option -- depending on use/budget. Put another way, I'm not hearing reason enough to go rip them from my car.

As for wanting an example, I was talking about DRIVING example. Say....in a parking lot turn a HARD corner pushing through it with maximum acceleration -- without losing it sideways. You know...at 25mph, 30, 40, whatever. FWIW, I was sufficiently impressed by the Newman's video. And, if I hadn't (yet) gone to poly, I'd give more serious thought to heim trailings arms. Control arms too -- though I'd prefer press-in bushings [for those] that would do the same thing on the control arms.

Note: Since I'm getting tires/alignment next week, I wanted to "vigorously" counterpoint this issue. With any serious desire to redo the back, I'd hold off on any alignment.

 

If you already have poly bushings in the rear and don't autocross/track or drive your car hard on windy roads, it's probably not worth pulling them out and replacing them with SuperPro or rod ends. If you do any of those things with your car, you should. Those are driving examples: you will run faster times with fewer spins and other issues if you have proper bushings in there. That's really all I can tell you. A steady-state turn with power applied is not where snap oversteer is likely to happen. It will happen in transitions such as slaloms, emergency lane changes, and corner entry. It's really beyond the scope of this thread to go into more detail about driving dynamics, so you're just going to have to trust me on this.

If you have old rubber bushings that need to be replaced, there is no good reason to use hard poly bushings when you can use SuperPro that are not way more money and are actually easier to install. Rod-end links are probably overkill unless you're using the car in a competition environment.

 

Unless you had enough foresight to purchase a set the 1st time that Banski thought that it was the end and deeply discounted them, hee hee hee ....

 

Hell yeah! I wish I would have seen THAT! I'd buy a set -- on the cheap for sure!

Not sure why Poly is harder? Most of installation (for me) was pressing the old rubber out, cleaning the arms THOROUGHLY, then polishing the arms. I can't remember if the were 2-part (like superpro's vid suggests). If they are, it's just lube and place in rod end before reinstalling the rods. If they're 1-pc, I don't remember any additional trouble pressing them back in. All I can remember is using 2-3 ratchet sockets (depending on rod end/bushing size) as shown in the superpro video.

I would have thought snap oversteer required acceleration vs just braking/turning? You know...something to shift the weight back onto the rear. Otherwise hard braking/turning is down mostly with the front tires, right? Maybe you're saying poly can cause the rear to "spring" out and around during an emergency brake/turn? Even that seems counter-intuitive since the weight isn't shifting down (in the rear) then back up. I would have guessed it to happen coming OUT of a HARD corner then accelerating. (Similar to hitting the gas and going around a car in front of you)

All this said, I was 100% sold on heim link's value watching the Newman's video.

 

Banski still has all the documentation up, including how to build your own arms. If it was me (in fact I just downloaded them to this machine) I would download the documents.

 

Nope. Acceleration induces understeer, not oversteer...at least up until the point that you ask the rear tires to accelerate harder than can hold. Again, I think it's beyond the scope of this thread to get into a discussion of vehicle suspension dynamics. The bottom line is that more rear roll stiffness relative to front roll stiffness leads to oversteer, and hard poly bushings in the rear of a C4 induce a rising rate to the rear roll stiffness. There are plenty of good books that explain why the former is true, and I've done my best in this thread to explain why the latter is true.

 

I just ordered a complete SuperPro set June 7 from Dino"s Corvette Salvage for $348.23 total. When I spoke to him he said they had plenty of bushings but the problem they had was getting the metal sleeves and that he had no idea when I would get the kit. Well I got an email from him yesterday and they have shipped. I will have them Monday.

 

C&S/corvette parts center.com seems to have some available. Says limited quantities in the description but I did not try to add them to cart. So if you’re looking…$325 for early cars $349 for late model C4’s.

 

Hey Matt....Yesterday I read that understeer is increased by lowering rear tire pressure, oversteer increase by raising front tire pressure, etc...

In my tire thread, you suggested 34/30 (or 33/29) PSI might be a good combo for a street car -- if not mine specifically? I'm guessing it can smooth the ride but I don't think you were suggesting pressure as a means to alter steering? Maybe so? I'm curious if fatter rear tires (315s) -- combined with lower pressure -- would essentially offset all/most of the "snap oversteer potential" for owners running poly bushings?

Even without fat rear tires, maybe owners running poly should seriously consider lower rear tire pressure?

 

That's another way of saying that the lower a tire's pressure, the more grip it has. It's not that simple: if it were, then we all run 0psi in our tires! Every tire has an optimal pressure for best grip depending on its size, the wheel size, and how it's loaded. Any deviation either higher or lower from that will decrease grip. And also, your cold PSI is way lower than your PSI after you drive around, especially if you're really wringing the car out in lateral acceleration while doing so.

[/quote]In my tire thread, you suggested 34/30 (or 33/29) PSI might be a good combo for a street car -- if not mine specifically? I'm guessing it can smooth the ride but I don't think you were suggesting pressure as a means to alter steering? Maybe so? [/quote]
I probably suggested that for your particular setup, which I believe was involving 315 rear tires. You would run less pressure in a 315/35/17 vs a 275/40/17, all else being equal.

It's not going to fix the problem. For one thing, it may or may not improve rear grip (see above). For another, no matter what pressures you run, you won't solve the issue that handling balance will be non-linear with the added bind from hard poly bushings.

 

I understood there are "limits" in under/over-inflation. Futhermore, I'm sure there's "ideal" tire pressure(s). Maybe the ideal pressure for drag racing (straight-line traction) is different that cornering? I'd bet money on it.

If it sounded like I was implying the problem would be "gone", I misspoke. For other readers, I can't say I regret the choice of poly. If you're needing a lower cost bushing replacement -- especially for a street-driven car -- I wouldn't shy away from it. I've had mine for 5yrs and haven't been "surprised". With a new set of Michelins, response has gone way up -- as has my ability to run more pressure COMFORTABLY. In itself, I feel there's been a great difference in tire choice -- than my conversion to poly. I tried making some fairly aggressive swerves today -- just to "test". You must REALLY need to so something drastic for poly to be "dangerous" -- as implied. Because, I don't see the problem. With poly bushings and new Michelins, I'd just say mine feels damn "tight". And, I like it. (Who knows, tire pressure might also mean more than bushing choice?)

Given the choice for a redo and with an extra grand lying around, I'd be all over heim links. Superpro's are probably nice but total, free articulartion does sound the best. All that said, until poly is deemed as unsafe as the Pinto, I can't tell others NOT to try it. After all, there has to be a TON of C4's needing new bushings where they don't want to spend a buttload for a bushing upgrade.

 

GREGGPENN is that your car in the Avatar? Where did you get that hood??

 

Answered via PM