Fishtailing: Can tire psi help?

[byez]

Today it was quite slick out on the roads, ice & snow everywhere. I would constantly feel the back end of my RX8 fishtail the tiniest bit, I just get that feeling in my gut that traction has been lost. Thankfully I made it him by letting off the throttle and doing small steering adjustments every time it would fishtail.

The RX8 is not a high torque car and this was happening in even 5ht & 6th gear.

Currently running a few season old Nokian RSI's in the OEM 225/45/18 diameter.

Would modifying tire pressure front & rear help with the fishtailing? I would like the vehicle to be less tail happy. If tires get better traction on ice with a lower psi (correct me if I am wrong) then I assume that a lower psi in the rear will reduce oversteer. I am considering a 35 psi front and a 30 psi rear. Any problems in doing so?

Thanks for any advice I can get out of this and I hope others can learn along the way as well.

 

[Throckmorton]

As I recall, Jeremy Clarkson on Top Gear had a similar problem with the RX8 in the rain.

I think the problem here is that your car has a low polar monent of inertia. Consider this, your car has a 50:50 weight distribution, but unlike a barbell where the weight would be concentrated at either end, in your car it is all at the center. This makes for agile handling on dry roads but instability and dartiness on slippery surfaces. S2000 drivers have observed the same thing.

Try weighing down the back end. That might help a bit. Snow tires would also help tremendously.

 

[Ramblin Fever]

It does sound like you have too much air in your tires, and/or could just be the effects of a low profile tire on slippery surface.

I drive a company truck with low profile tires, this truck has 100% less traction capability then my personal truck running normal tires. 90% of it's due to low tread depth, [censored] tire pattern as well. 10% due to company truck being a lot lighter.

 

[1sttruck]

Is the RX8 rear wheel drive ? Does it have a limited slip ? If so it's doing what it's been designed to do. To stop the fishtailing you need to keep within the limits of traction, either by driving slower and/or increasing traction on the drive wheels.

 

[IndyIan]

I was going to say go down to 25psi all round but 45 series sidewalls are pretty short... If you have steelies on then go as low as you dare looking at the sidewall bulge, 28 26 psi.
I imagine your other problem is rear toe in and camber, the toe in is making the tires fight each other all the time and the movement back there is one winning then the other making the back end move a bit. I had an escort GT with enough toe in to do this, kinda odd feeling but not really an issue with fwd. Softer tires are worse at speed in slush but better on hard packed, plus it will help negate your toe in.

I still say, get the smallest steel rims that fit over your brakes and get the skinniest highest profile snowtire that isn't to much bigger than stock tire diameter. P185/65R15 would be perfect for your car, the tall sidewalls should negate your toe in and they are $100 at CT. The narrowest 16" Nordic snow tire is 205 which would be better than what you've got.

If you are semi mechanically inclined and don't want to get new tires, try changing the rear toe in yourself, change both sides the same and count the number of turns on the adjuster so you can put it back in the spring or get a shop to do it. Your car with the right tires and alignment should be pretty good in the snow. Better than a 2wd pickup anyways.

Ian

 

[Bror Jace]

In most cars (typically front-wheel-drive) I put 2psi less in the rears than the fronts. Since I run my tires on the high side of normal, the fewer psi enhances rear traction just a tiny bit and lessens the likelyhood the rear end will go out around on me in abrupt wet-weather maneuvers.

Not sure if this would work for a RWD car. The RX8 would be one of my last choices on roads covered by snow and ice ... all RWD sports cars would be in this same category.