sperkins

I recently swapped to VBP's T1 poly sway bar bushings front and rear thinking they would be of some benefit, but since then the car has much more oversteer.
I did some digging and came up with this theory posted in some fancy chassis tuning article:

I'm on T1 bars and try to run the rear as soft as possible.
Did I just effectively and noticeably increase the rear roll stiffness by swapping to fresh poly bushings in the rear?

Aardwolf

Theory aside, how did you install them? How tight is the bar in the bushing? If you clamped it real tight it will change the bar quite a bit. It's possible one bar is loose and the other tight, if you didn't check. I had to put a washer between the mount halves to get mine equal. This is on a C4, not sure how the mounts work on your year. Do the bars rotate freely?

The grease would come out of mine pretty quickly, I added zerks. It's possible one bar is greased better than the other?

sperkins

I should have mentioned that they are shimmed.

redtopz

Interesting question . I have a similar question that maybe I can piggyback on yours. Does anyone else have issues with the rear sway bar clunking loudly when transitioning from one direction to another. I guess it's just seizing up in my poly bushings and instead of sliding smoothly it's suddenly popping from one side to the other. Pretty annoying. I lube the bushings frequently and have washers spacing out my brackets so they aren't too tight on the bushings.

Aardwolf

Put shaft collars on the inside of each mount so the bar doesn't slide.

mgarfias

the bushings don't change bar stiffness at all. Once the car has rolled over onto the bar there won't be a change in rate. What does change is how far the car rolls before it it loads the bar. Stiff bushing = immediately on the bar, soft bushing = wallows over until the compliance is taken up.

redtopz

Good idea. Thanks.

Yes, as long as the bar is free to turn in the bushings that should be correct.

Aardwolf

http://www.mcmaster.com/#catalog/116/1154/=9ait8p

NP! Link above for them. Beware the places selling two for $75 in a "kit". Nice $60 mark up!

froggy47

Which material do you suggest?

Does front need them also?

travisnd

With T1s do you guys shim the front bar with washers too, or just the rear? If you take the endlinks loose, but leave the bar clamped in place, should you be able to easily rotate the bar?

On a stock car with stock bushings and bars the bushings clamp tight. However, with T1s everyone recommends shimming... why?

Aardwolf

With my C4 I have found the front bar slides side to side very little (3/8"), but I put bar collars on anyways. The rear bar really slides around, it goes about as far was it can and hits the rear wheel well (1"). I would think you guys with C5's would have different movement, though if you notice any at all I would use bar collars. I use aluminum collars from the link above. I have always been amazed there is no mention of this or an install kit, sold with sway bars.

Travisnd, stock bushings are rubber and must be clamped tight so that they twist. Poly won't twist, so the bar cannot be captured so tightly. I shim the bar so that it rotates freely with poly, and use bar collars. Depending on what grease you use you will feel the stickiness of it resisting movement. I don't think collars are needed with rubber bushings.

I tried that thick suspension grease and found that it didn't stay long. I've also found that it's difficult to clean away and replace. Dirt gets it so sticky it's like tape glue. I switched to using what I use for chassis lube, Redline synthetic grease. I also put zerks in. Here's a pic:

mashinter

The bushings affect stabilizer bar "effectiveness". Completely rigid bushings would be 100% effective. Rubber bushings can be 50%. What's happening? The rubber bushing is deflecting, which reduces the torsional deflection (stiffness) of the bar.

Solofast

The short answer is yes, changing from rubber to poly will effectively increase the stiffness of the bar. The roll stiffnes of the sway bar SYSTEM is based on the rate of the bar as well as the compliance in the end links and in the bar mounts. Stiffening or softening any compoent will change the effective stiffness of the system. How much change in going from rubber to poly is based on how much roll angle is present, the width of the spacing of the bushings on the bar, the length of the bar overall and the spring rate of the bushings. While a rule of thumb is a nice thing, as you go to heavier cars with big sway bars, the amount of deflection in the bushings can have a very large impact on overall roll stiffness.

The rear bar is softer, but it could be that the rear rubber bushings were softer to start with (either thicker or of a softer material) when compared to the front bushings, and the change in mount stiffness, as a percentage of overall stiffness turned out to greater in the rear than the front. That is what is throwing off the balance. The easy fix might be to put the rubber back into the rear sway bar and then tune it with the bar length adjustment to get the balance back.

The comment about rubber bushings having slop and essentially no stiffness until the slop is taken out is not correct. Rubber bushings are generally clamped (I know that's not the case with the C5, but bear with me here for a bit) and bushings that are clamped are just more complex in that at low loads (loads below the clamping load) they have infinite mount stiffness. That is, at loads below the clamping load there is no motion in the mounts at all. After that the bushing is a spring that is non-linear since rubber doesn't have a linear force deflection curve. The clamped bushing also has a torsional stiffness of the bushing that is added to the oveall sway bar stiffness. This can be considerable, and that's why in some cases changing to poly doesn't change the stiffness as much as one might think since a lubricated poly bushing doesn't have this component of the stiffness in the equation anymore. As we have talked about in these threads before the amount of stiffness in a rubber mounted bar can be significantly effected by the clamping load on the bar.

Unclamped, free turning rubber bushings, like those found on the C5 have a spring rate based on the stiffness of the rubber and that is non-linear, but it is real and measurable and in these cases the engineers tend to use a stiffer rubber to make sure that the loading is more linear than a softer rubber bushing that is clamped. Changing from poly to rubber in a C5 is a major change since the change in mount stiffness is the only aspect in play here and it is pretty big because we are talking about pretty large sway bars.

froggy47

My memory is not what it used to be, but having owned both a c4 and a c5 and having shimmed the clamps on both and changed bars on both, I have to ask why you are saying the fastening systems are different?

C4 you call clamped & c5 unclamped?

Can you post a pic or illustration?

As I recall on my 1996 LT4 and my 2004 C5Z the (stock) bars are fastened to the sub frames the same. With clamps like shown in the (Aardwolf) pic above.

Sizes of the bars and the rubber isloators varied with the suspension options over the years.

The isolators (GM term) have a cut or split at one spot & you open it up & slip it over the bar & it closes. They are different sizes for different diameter bars. The metal clamp goes over the isolator and tightens (clamps) down. If you want it less tight you add washers between the clamp & the surface it snugs down onto.

Or am I not understanding something? Maybe you meant C6's? I have not owned one of those so am not sure if they changed the sway bar fastenings.

drivinhard

don't discount all the weight you shifted to the rear since you have been jenny craig'ing the car and running those big 100 lb fuel loads

Solofast

As you noted, on a C4 the bar is clamped in the rubber "isolators" and the clamping load is sufficient that the bar does not rotate unless it twists the rubber. On my 2004 C5Z, the isolator is lubricated and, while there is some clamping of the rubber, the inside of the bushing has lubricant in it and has some plastic "teeth" that look like the hook side of velcro to retain the lubricant. As a result, the bar can rotate without putting torsional loads into the bushing. That's what I meant. There is a bit of compression in the bushing, but it isn't near what it is on a C4, and the bar rotates in the lubricated bushing on a C5.

sperkins

Yeah I've been meaning to talk to you about that. Last weekend was the first time I've run with a full fuel load so maybe that's what the difference is and I'll have to change my driving style. The car wasn't loose on corner exit, but mainly on entry which would confirm or support your theory. I may take a slightly different ballast approach than you and load my tunnel area more and the rear a little less than I had planned - maybe 50/50 between the two. I always felt like braking/entry was one of my few strong points. I need to set the car up to find the right mix between entry and exit. Either way, I have to get some more right side weight as my split is not good at the moment.

Falcon

Unless the hole made in the bar collar for the zerk go completely through the rubber bushing, the grease has no where to go and will have no effect, will it?

drivinhard

have to bring up harry every now and then

Aardwolf

Yes I drilled through the bushing mount, through the poly bushing, then made a very slight groove around the ID connected to the hole.