Tire pressure question

I run mine on 35 / 36 psi without any adverse affects. In fact I think the mileage is a little better than running them at the lower pressure.

The pressures shown on the sticker are the recommended cold pressures. The max pressure stamped on the tire is the highest safe cold pressure as rated by the tire manufacturer and you shouldn't run the tire at or near that pressure for normal use.

Got it. Thanks!

Just got back from a defensive driving course, and was told that pressure depedns on who you talk to. Marketers who try to push nice soft rides will go for lower pressures, whereas tire engineers will reccommend higher pressures.

Saying that, the course suggested to inflate to 35-36psi for normal city driving, and if you're travelling long distances under load, then inflate to 40psi. Keep in mind, this is for modern cars / tires. I wouldn't try this with cars from the 80s.

It was also mentioned that it's far easier to get a blowout at lower pressures than at higher pressures, as tires will deform much easier and therefore create friction at lower pressures (don't ask the physics on it... I didn't pay much attention to the tire bit about the course - which was probably a mistake since on reflection, they're the most important part of the car).

HTH,

...Ric

OK... I see your point, but the "new" information was communicated to me via the Subaru Defensive Driving course:

This leads me to think that they market cars one way, but they suggest you set it up another way to get "more" out of it...

...Ric

I see no reason to consider any single pressure ideal for the majority of drivers due simply to the fact that vehicle use can be so dramatically different from day to day, week to week. Of course it is unreasonable to make adjustments to pressure every time the car is used

- so the practicality of the issue lies somewhere between considering the 'door/manual' pressure as a good minimum cold pressure - probably yielding a comfortable ride and decent, fairly even wear with good handling for average driving, and the sidewall pressure being considered the max cold pressure (though the engineers probably have a huge safety margin built in)useful for a heavily loaded vehicle driving at high speeds (this includes track day as well as moving day). There are a lot of folks with plenty of experience using slightly (5-10%) higher pressures than the recommended number from the manual and they experience no significant wear problems. Perhaps a silighly harsher ride sensation. That is my experience anyway. Even on my one track day, I was under the max pressure on the sidewall.

It's better to UNDERSTAND what tires and the stuff related to them are all about rather than blindly follow a static number for something as dynamic as automobile tires. Considering what they have to do and the environment/punishment they endure - they are amazing objects really.

(where does all that worn off tire rubber from millions of cars go? hah!)

Carl

Doing "number 11s" on the streets around where I live...

:-)

...Ric

On 2006-06-04, ric_man penned:

You can test it out for yourself by riding a mountain bike on gnarly trails with underinflated tires. The resulting phenomenon is called a "snake bite", as the matching punctures on both sides of the tube (where the rim made contact because there wasn't enough pressure in the tube to prevent it) resemble, well, a snake bite.

Granted, that's a fairly extreme example, and probably not exactly what your teachers had in mind. I suspect that car tires (which of course are tubeless) are designed to have maximum structural integrity with the shape they have when properly inflated, and when they're underinflated that shape is compromised.

tires are amazing indeed.

but they are much much more likely to fail due to underinflation than overinflation. the higher the pressure, the smaller the contact patch and also the less flexure the sidewall must endure per rotation. the latter is what causes failure due to underinflation, the sidewalls overheat and rupture, usually catastrophically.

the only downside I can imagine of running 10 or 20 percent higher than (or any increase so long as it does not exceed the tire's maximum marking) the vehicle mfgr. recommendation would be accelerated wear in the center of the tread, and obviously a slight decrease in absolute grip due to the decrease in contact patch area.

often the vehicle mfgr. will recommend a pressure that is lower than that which provides maximum grip, sacrificing this for improved ride. in hard cornering, the lower portion of the tire is bent inward dynamically as it rotates. this too causes heat buildup. if the pressure is below optimal (and assuming the suspension is doing a reasonable job of maintaining proper camber) the tire will bend so much that the "good" part of the tread is no longer in contact with the road. in the extreme the sidewall will begin to make contact. this is obviously not ideal or even safe operation.

I would not recommend running more than the maximum value molded into the tire itself. the tire companies know what they are doing. I use their maximum as a starting point and adjust downward.

YMMV

That about sums up things that were said during the defensive driving course...

...Ric