chrumck

Is replacing the bushings to polyurethane ones in 06 GT beneficial in terms of car performance on the road course? I guess yes, but the actual question is which ones should replaced, which ones not and what brand would you advise?

One note. I'm interested in replacing to poly bushings only. No spherical, no aftermarket suspension parts. The reason is that I try to come up with the setup for the next year which would keep me in NASA TTC class and I've got very limited amount of points to use up.

Norm Peterson

Quote:

Originally Posted by chrumck

Is replacing the bushings to polyurethane ones in 06 GT beneficial in terms of car performance on the road course?

It's something of a mixed bag.

Used up front in the lateral arms, you should benefit by reducing the compliance camber change that tucks the tire under further than the simple geometric constructions suggest. That's a good thing. If anything, front roll stiffness should drop slightly (OE rubber bushings actually do create a small "wheel rate", while poly does not if modified to eliminate its friction against the chassis bracket sides). This loss of front roll stiffness would be small, but would be a change in the "vehicle oversteer" direction.

With the strut suspension, they wouldn't be carrying much weight load, so cold flow shouldn't be a big issue. However, I'd still consider them to be consumables/wear parts.

Out back, you do get better axle end location and resistance to axle wind-up (which is good), and along with that you get some induced roll stiffness (which tends to be not so good). There will be some change in compliance axle steer, but I don't know offhand whether it would be an improvement. I think it would make axle roll steer become a relatively larger effect and maybe loosen things a little on throttle on corner exit.

Since you aren't doing much or even any launching from a complete stop, the anti-hop benefits of poly bushings are mostly wasted.

You can tune around the added roll stiffness as long as you have the freedom to swap springs and sta-bars, or adjust any adjustable bars. IMO, that's do-able but not preferable.

Quote:

but the actual question is which ones should replaced, which ones not and what brand would you advise?

First off, do the poly bushings by themselves count for any points?

Secondly, can you modify them without adding points? This is mostly a rear suspension consideration and it's the voided bushings thing, where you're trying to soften the bushings significantly in some directions/against some end rotations without much softening the directions you'd like kept stiff. You don't have to be an OE to do this sort of thing, but in your specific situation you need read the rules carefully, especially to the way the particular allowances and/or requirements are defined. If, say, the rule reads something like "the amount of metal relative to the amount of non-metal", you might have to find something really soft to pack the voids with.

There used to be some 3-piece poly bushings available from Maximum Motorsports that were probably the best OTS poly bushings for road course and autocross. I don't know if they are available for the S197 or not.


Norm

908ssp

J&M makes an interesting bushing. A hard poly ball in the center with two soft rubber bushings on either side. The ball gives good location and the soft rubber sides allow a natural twist to take place which full poly bushings don't allow. Full poly bushings in the back can actually hurt road course compliance cause shocks and increased roll resistance.

Norm Peterson

That's still a more or less spherical joint with internal pieces that move relative to one another, so there is probably a NASA points penalty involved. I know they aren't SCCA Solo legal in the 'Street Prepared' or 'Street Touring' categories, where limited suspension mods are allowed.

Sometimes it's tougher to build up a legal race car than it is to build up a legal street car.


Norm

908ssp

All very true and I wasn't even considering that sorry.

It seems everyday I miss the day when racing meant building something that was faster not building to a set of rules.

chrumck

Replacing the bushings gives no additional points and the exact wording is this:

NO-POINTS MODIFICATIONS:
29)Non-metallic replacement suspension bushings

and, as a side note, also no points:

10) Simple camber, caster, and toe adjustment by any method that does not alter suspension mounting points (unless the modification used is otherwise assessed points above). Bolt on camber/caster plates are not assessed points.

chrumck

If you look at the rule, using poly ball held by rubber inserts still constitutes a 'non metallic bushing' and is still within the rule

So to summarize:

Front LCA bushings are worth replacing to increase the precision of front aligment under load. Any specific brand and type you would recommend?

Rear LCA bushings require special solution to allow the twist of the axle in order not to double the effect of the rear swaybar. 908ssp, I couldn't track down that J&M bushing you mentioned, just complete control arms. Where have you seen them? Norm, I couldn't find anything like that at MM page but I'll look once again.
It looks for now rear LCA bushings are not worth touching unless we find some neat solution.

How about the diff control arm and panhard bar?

908ssp

Call J&M they told a friend of mine they would sell him just the bushings to replace the solid poly bushings in his rear LCA from another manufacturer.

Norm Peterson

http://www.maximummotorsports.com/st...roducts_id=996

The flat piece is the stiffer material and is sandwiched between the two pieces that have flanges (the softer stuff). Like I said, they may not be available for the S197 (yet). But you might be able to modify the ones for the earlier cars to fit, or make up a set completely on your own that follows the same line of thought. The DIY mods that I've done are aimed at doing generally the same thing.


It is interesting to hear that NASA assigns no points to the same kind of bushings that SCCA bans in limited-modification Solo categories. I stand corrected.


908 - I certainly hear you. That's probably a big part of why I don't even care that my street-driven cars have always ended up in either 'Prepared' or 'Modified' auto-X classes.


Norm

chrumck

Thanks for the input guys.

How about the panhard bar and the diff control arm?

My guess it's definitely worth replacing the panhard bar bushings to increase lateral stifness.

As for the diff arm, I would think the same rules apply like for rear LCA's.