symptoms of wrong tire size

Whether or not changing tire width will affect handling depends a lot on the suspension geometry of the particular car. One immediate thought is that if the scrub radius is substantially non-zero, then changing the width of the tire also changes the effective scrub radius, whereas if the scrub radius is zero it does not.

Also, if you bought tires that were the same *profile* number (eg, the "60" in P255/60R15) but had a different width (making the "255" a "275" for example), then you also effectively had a TALLER tire too. The way tires are numbered and sold, the width and height are coupled through the profile number and are not truly independent. A P265/60R15 is both taller and wider than a P255/60R15.

I'm imagine that the car *could* be aligned to work properly with the wider tires if you wanted to do so. But if you're happy with the stock size, no need to change.

Michael Mossey wrote:

Wider tires mean more surface on the road at the contact point. This might sound like a good idea until you consider that there's now LESS weight per square inch of contact surface on the tires. (Same weight, but spread over more square inches of surface.) That can mean less traction.

No, it means a differently shaped contact patch.

This

Primarily a function of wheel loading and tire pressure.

Is that right? If pounds is the same (weight of the car) and pounds per square inch is the same (tire pressure), then square inches (contact surface) should also be the same.

Narrow tires are better on snow because they have to push less snow out of the way.

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Actually, narrow tires are better on snow because they concentrate more weight per square inch onto a narrower contact patch. That weight both pushes through snow and deflects it.

-ChrisCoaster

We seem to agree that the narrower tires are better on snow because of the narrower contact patch.

But why, given my proposed calculation, do you think that narrow tires have more weight per square inch? If both tires are inflated to 35 psi, wouldn't they both be 35 psi?

Yes.

Contact patch shape chnages from wide and thin (wider tire) to long and narrow ( narrow tire)

It depends on the stiffness of the sidewall and tread surface. With a very elastic sidewall/tread only the pressure has any bearing on the contact patch area. With an infinitely stiff sidewall/tread the pressure has no effect at all. A real tyre falls somewhere between the two.

Dave Baker - Puma Race Engines

No. They will both start collapsing when the contact patch pressure EXCEEDS 35 PSI, but as long as the sidewalls are straight, both will have a contact patch pressure less than 35 PSI. But the skinny tire will have a higher patch pressure than the fat tire.

To take it to the extreme case: Jack one tire up off the ground. It still has 35 PSI inside it, but its contact patch pressure against the ground is ZERO!

NOT tire pressure!

If a P205-75R14 has 8 square inches of contact patch (number picked out of the air), say a P215-75R14 would have 9.5. That's either 32 for four tires or 38. If the car weighs 2000 pounds, and it's sitting on 32 square inches, there's 62.5 pounds on every square inch of contact patch. Widen the tires, to 2000 pounds on 38 square inches, and you now have

Again, I picked the contact patch area numbers out of my hat.

And so is the weight on it so what does that prove?

Dave Baker - Puma Race Engines

Reducing contact pressure in this fashion generally increases grip, not reduces it.

Todd Wasson Racing Software

Good thinking. I think though that changing tire width will change scrub radius regardless of whether it was initially zero (if you're measuring from the contact patch center), however, which would really serve to validate your point even further, Steve.

Back to Mike: There are a million things that could be interacting to cause this, so it's tough to say for sure. Steve's idea has merit. Other possibilities include increased sensitivity to camber, which might cause you to need a toe adjustment (I'm not real sure on that one). Also, the wider tires effectively lowered the wheel rates, causing it to act as though the springs and shocks had been softened. You've also increased the effective track width of the car with the wider tires, which would tend to reduce stability. Those by themselves shouldn't really cause the car to wander though, but just feel less precise (except for the toe angle bit).

It's also possible that the front tires were too stiff laterally compared to the rears. This could cause a car to wallow around when you want to drive straight, much like increasing front tire pressure too much might. The front tires build force with slip angle too quickly below the peak compared to the rears, so the car acts oversteers a bit at small steering angles. This could cause you to chase the car a little bit.

Those are just a few possibilities though.

Still, if you didn't report those problems I never would have said this. I would have guessed the car would handle better with the wider tires :-)

Todd Wasson Racing Software

It proves that the only time the contact patch pressure equals the air pressure inside the tire is when the tire is overloaded and the sidewalls are collapsing.

Of course if the car were carefully re-aligned with settings tailored to the wider tires, it might ultimately handle better than it would with the optimum alignment settings on narrower tires. The fact that it handled more poorly with wider tires just slapped on there with no alignment changes really doesn't say ANYTHING about the relative capabilities of the tires themselves.

Maybe to you...

What do you think is a typical contact patch pressure, for say a 3000lb car, with equal weight distribution F/R, L/R and 35lbs/inch in the tires?

These will help.

But if a patch of tire has 32 psi pushing down (the air in the tire) and 62.5 psi pushing up (the road), it would move up. Since cars don't float, wouldn't the force pushing up and the force pushing down have to be equal?

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He already said that he pulled the numbers out of a hat, the whole argument is spurious from that point on. Why argue with it?

While I claim that the contact patch pressure is generally LOWER than the air pressure in the tire during normal operation and normal amounts of side-wall deflection, let me make it unequivocally clear that the contact patch pressure can NEVER significantly (I include that word to allow for sidewall stiffness) EXCEED the air pressure in the tire, as his argument (with numbers from hats) would imply.

Yep, I agree.

Todd Wasson Racing Software