LagunaPearlAP2

It sounds like tires are the problem to me. If you can identify a particular direction it is swerving to, you can always swap tires left to right on the back and see if the problem is mirrored to the other side.

When I got my Hankook V12s they swerved to one direction when I was on the gas, and then back to center when I got off the gas. All tire pressures were fine. Swapped them left to right and the problem was exactly mirrored. America's tire got me two new rear tires and problem solved.

Chuck S

Cheap ($60 each) all season tires are inappropriate tires for the S2000 which was designed around Extreme Performance Summer tires with extremely firm sidewalls to keep from squirming. In the 20 years since the car was first produced new tires are achieving the specs of those circa 1999 tires and modern Max and some Ultra Performance Summer tires are acceptable. Nankangs NS-25 tires are sold in North America by places like WalMart a bottom tier (albeit giant) retailer specializing in cheap products. For a beater-daily drive car. Maybe.

The S2000 does not handle mis-matched tires well. Same tire (and age) fore and aft with matching tire pressures (32psi).

-- Chuck

Car Analogy

What they said.

I will add regarding the left right swaying, tire diameter.

Tire diameter

Tire diameter

Tire diameter

This car is super sensitive to rear tire diameter. If the two tires have slightly different manufacturing tolerances, which wouldn't be surprising with low end quality all season tires, that can easily cause the sensation you are describing. Something that wouldn't cause the slightest issue on other cars can cause a disturbing lack of stability in this car.

Equally, if rear tire pressures are slightly off left vs right, this will also cause a difference in effective tire diameter which results in the same thing.

Ever see that experimental fighter jet with wings swept forward? Its inherently unstable. It requires a computer that can constantly monitor stability and constantly make changes to trim angles just to keep it from crashing. But its magnificently maneuverable! It can change direction like nothing else.

Imagine how changing something inherent in maintaining its stability could have a detrimental effect on its ability to fly. Something that on a regular plane wouldn't be a big deal.

The S is kinda like that forward swept wing plane. Hopefully this analogy helps in understanding why something so trivial can have such a dramatic impact on this car.

One problem is trying to convince someplace like Walmart tire center that an otherwise acceptable manufacturing tolerance is making your car dangerous to drive. They are gonna blame everything but the tires, and you're gonna sound nuts trying to explain it to them.

Which is why buying tires from someplace like Tire Rack is such a good idea. They understand why tires can make such a difference on a car like this, and will and have sent replacement tires in situations like this.

Hopefully its just rear tire pressure differences. Otherwise you're screwed.

s2000Junky

^^ All good points. Its most likely the flimsy pliable sidewalls though. and in that case dropping to 32psi from 40 will just make it handle even worse. I'm sure there is an excessive amount of tread squirm on these all seasons too which is adding to it further. You need a stiff 2 ply sidewall tire on this car above anything else. A good wide block summer tread design is helpful in handling sharpness as well, but secondarily to a proper constructed tire first.