Tire Pressures

[Sylvan]

The other day I was autocrossing, doing ok, about the middle of my class. Getting better as I get more practice. Anyway, I had my Megan coilovers set on the stiffest setting, and there were some rough spots in the course. The back end felt pretty bouncy and loose on some turns, and especially powering out of the corner. Felt like the tires were just kinda bouncing off the parking lot surface. Everything's tight in the suspension, but when the car is sitting still and I shake it around I see a lot of sidewall flex especially in the back tires. I see the wheel moving around quite a bit either in/out, up/down, etc within the tire. So I'm thinking this could be any of 3 things:
-Bad tire choice
-Bad tire size
-Improper pressure

or some combination of the above. So I'm running Toyo proxes T1R. Size is 205/45 16 in the front, 225/45 16 in the back. Pressure is 32/36. Max psi for those tires is listed at 50 psi.

So, I didn't have a pump with me to play around with the pressures, but I might before I go next time. Should I bump up the pressures a bit? Should I go with 225/40 instead of 225/45? Maybe go with a 17" wheel so I can go wider, maybe 245/something? The fronts don't seem to be too much of a problem right now. Thanks for your inputs.

[Bak@EMSPowered]

What are your tire pressures at now? Have you checked your suspension components such as balljoints, swaybar endlinks, chassis braces, etc...?

[Sylvan]

Pressures are currently 32 front 36 rear. I recently replaced all the ball joints and put in new polyurethane suspension bushings. Swaybar endlinks were recently installed/inspected, as were the steering tie rods. Chassis braces looked good, but I never removed them. Everything appears and feels rock-solid. The only compliance I can see is in the tire sidewall. I wish i had a video camera so I could post up what it looks like.

[NONACK]

Up the pressures to the manufactururer's maximum and do s skidpad-type run in a parking lot. Put chalk on the sidewalls of the tires. If the chalk is gone/way effed up after that, the sidewalls suck ass, and it's just your tires. In my experience, all an MR2 really needs to do well in Autox is a 205 f 225 r set of r-compounds. Check out the Kumho v710 for a budget race tire.

[Sylvan]

Crap. I was doing some work under the car today and I noticed the rear chassis braces were missing. Every time I work on this car I notice more evidence that this car was raped and it just pisses me off. Anyway, I'll make some new ones and see if that makes a difference.

[Gairloch]

Mine were missing too and I didn't see any difference when I replaced them. I did see a significant reduction in chassis flex when I put in a turbo strut brace though.

What tires do you have? It could be that your pressures are too high too. I run my Hankook RS-2 215/40-17 235/40-17s at 29 front and 32 rear at most tracks. I have found that a very small change in pressures, as small as half a pound, can make quite a lot of difference in how the car handles. Generally more pressure in your tires is going to give you less compliance over uneven surface.

If the car is bouncing through turns over uneven pavement it's because the high speed damping isn't able to keep up with the changes in the surface. Reducing the pressure in your coilovers will likely go a long ways towards resolving this.

[Sylvan]

Thanks for the input. I will probably end up investing in the engine bay X-brace and see if that helps. My car came with just a straight bar, looks to be an aftermarket type.