Did you ruin a set of performance tires?

[Chiketkd]

Based on this thread, I went out and changed my tire pressures to 40/36 today after completing my monthly oil change. Handling does feel better, and the higher tire pressures will actually improve my gas mileage slightly...

-Chike

[UberRoo]

There are general rules for tire pressure for a specific application. A good tire shop will know. The general rules however, are just that: General. The only way to find the ideal pressure is to experiment. This isn't hard, but it's something most people won't find the time to do.

There are different pressures for different applications. For drag racing, lower pressures are sometimes better, but this isn't always true, especially on low-profile tires. For top speed, higher pressures are better. For best cornering, and tread life, moderate pressures are best.

The tread life test can be done in two ways. The best way is to carefully monitor the tread over time to see which part wears out first. If it's the outside, the pressure is too low. If it's the inside, the pressure is too high.

The other test is a footprint test. This is usually done with a special machine, but you can use carbon paper or thin sheets of Styrofoam, or even both simultaneously. You simply drive over the test material. This test just shows how much pressure each part of the tread exerts on the road surface. If you use carbon paper, the dark spots exert the most pressure. If you use Styrofoam, the most deformed parts exert the most pressure. If the pressure is to the outside, inflate the tires - and vice versa. (The texture of the surface you set the carbon paper on will affect the results.)

The cornering traction test is done simply by measuring how well the car can corner. You can use an instrument to measure lateral G-forces, measure the fastest speed through a corner, measure the smallest radius you can achieve at a particular speed, or simply measure track times on an especially twisty course.

Usually the ideal pressure for cornering traction will be very similar to the ideal tread wear pressure, but slightly higher. This is because when you corner, the weight shifts to one side of the car. Since a very large percentage of the traction is on that one side alone, the excessive pressure on the unladen side is trivial to the inadequate pressure on the loaded side, which makes up the majority of the traction. To combat this, the overall pressure is increased so the laden [important] side is at it's optimum. The consequence is that when driving straight, all the tires are overinflated. This is fine, if not good, for racing purposes, but for daily driving, it leads to poor tire wear patterns.

Overinflation in general is preferable to underinflation. Economy will be better. The propensity for overheating and sidewall damage is reduced. Excessive wear in the center of the tread is less likely to cause hydroplaning than wear on the outside. Handling will be better. Tire balance will remain good, and wear should be relatively even about the circumference. Underinflation adversely affects all these things.

In all cases, the ideal pressure vary between sizes of tires, the vehicle weight and weight distribution, the manufacturer and model of tire, and even between each tire on the car.

For the rough numbers, call a few tire shops and ask.

Originally posted by Bwana
So what pressure would you guys recommend for 225/40/18's?

[immortal_suby]

Originally posted by Bwana
So what pressure would you guys recommend for 225/40/18's?

I'd be running at least 45psi on 40 series tires - to prevent damaging a sidewall or rim if for nothing else.

When I was looking for where to begin with my +1's I went to nasioc.com's tire forum and poked around - the autocross guys know their tire pressures well.

[SVX10]

Originally posted by seasvx
33/29
37/33
40/36
45/42:

...what's the reason (or the physics) behind running the rears @ 10% lower pressure than fronts? and does this logic apply equally for fwd vs. awd variants? (ok, you engineers; here's your chance)

I'll take a stab at this. It's got lots of reasons behind it, but the most simple has to do with the weight distribution of the car. The SVX is 59%front, 41% rear if I'm not mistaken. That means that 59% of the vehicle weight is on the front tires, but only 41% of the weight is on the rear tires. The area of the tire contact patch equals the weight on the tire divided by the tire pressure. The front tires have more weight on them, so their contact patch will be larger if they are inflated the same as the rear tires. The end result of this larger front contact patch is increased wear on the front tires, and a higher rolling resistance at the front tires. Basically, the tire wears out faster, and the engine has to work harder to overcome the friction between the tire and the road. By bumping up the pressure in the front tire, the contact patch is reduced, tire wear is reduced, and rolling resistance is reduced.

A very surface level explanation that isn't even 100% true just because of the very very complex dynamics (that I sure as hell don't understand) behind tire performance.

[UberRoo]

Originally posted by SVX10
...because of the very very complex dynamics behind tire performance.

The reason high pressure is useful for maximum traction while cornering is also useful for front / rear weight distribution. Since we are not traction-limited in acceleration (except for the FWD folks) braking ability is the factor to focus upon. Higher pressures up front are important for this because so much weight shifts forwards under braking.

Just because the weight distribution is distributed 59 / 41 doesn't necessarily mean tire pressure should be balanced the same. The argument above for [relatively] higher front pressure is countered by the fact that low pressure in the rear tires can lead to stability problems, so the rear also should have plenty of air. Again, back to the "overinflation is better than underinflation" viewpoint. All four corners are better off that way.

We could get into motorcycle tires if we wanted to get really complicated. I know their dynamics are far more complicated and varied between designs. My understanding is far less keen in that school - as is the understanding of their designers too sometimes.