I hate it when that happens. No really, I hate it.
I'm glad your friend is okay.

 

Actually, it doesn't. The pressure on TOP of the rim pushing down is the same as the pressure on the BOTTOM of the rim pushing up (and for that matter, all the way around the rim), so the forces cancel each other out. And it doesn't matter what the pressure is, the forces from the AIR on the rim are in equilibrium at all times.

What the air pressure inside the tire DOES do is push the tire away from the rim. Because the air pressure is so much higher inside the tires, there is a very large net force pushing the sidewalls and tread away from the rim. This stiffens them up and enables the sidewalls to carry the weight of the car from the rims down to the ground through the tread. The cornering and braking loads are carried the same way.

 

Hah, your fancy science does nothing to my simple answer!

However the part about "doesn't matter what the pressure is" does seem at odds with rest of it, since there seems to be a relationship (per your answer) between air pressure and surface area on the tire (and perhaps that is a better way of saying it).

I've always used volume as the explanation since tires with a greater amount volume inside the tire require less air pressure to support a given load (best example I can give was an old camaro of mine that was fitted with P295/50R15's vs the old F- blah-bitty-blah- 14 size tires of yor, the 295's only required about 16 or 17 psi to carry the load vs the 28 or so psi with the smaller tires) and tires with an otherwise identical amount of surface area but very different in shape required generally the same amount of air pressure to carry the same load.

 

ok...well since you brought up the topic.. i have drifted around several corner on accident, never hit a curb*yet*...but one time instead of the rear breaking loose..the front tires slid and i went strait into a field, then i drifted back out of it...but i did not go to that general area after i seen two women laughin their butts off at me waiting at the light...funny story with the truck too..snowy outside, slippery roads, 4wd fun, both diffs are custom, 99% locked, i goosed it around a corner slid around just like the movies, i had a grin, but then it grabbed a bit to much, i shot over a ditch and ended up in a field with no way out except driving down about 50 yards to where the tractor comes on and off....embarassing lol

 

I usually set mine to 34 psi this time of the year at the service station but I forgot about the Cold tires vs Hot tires difference. I think you gain about 3 psi after driving at least 4-5 miles, so maybe I should think about 35 psi. when they are warmed up. Good advice on checking them at least once a month. You really need a Good Gauge though, Those cheapy ones aren't reliable

 

Glad you didn't get hurt or cause too much damage.

 

I did that once driving moms '79 El Camino. I went around a turn (wet road) and the car did a 180. People were looking so ,I drive it up the street the other way, and did it again (on purpose this time), just to show them i meant to do that.....lol, deep inside, i was embarrassed as hell.

 

"doesn't matter what the pressure is" is referring to the forces on the rim. They are ALWAYS in equilibrium and therefore do not exert any net force on the car or tire. This just means it doesn't matter how much air pressure you have WITH REFERENCE TO FORCES ON THE RIM, because the air goes all the way around the tire, the air CAN'T hold up the rim.

As we have all seen, air pressure DOES play a critical roll in the shape of the tire and it's contact patch with the ground. This is because we DO have a pressure differential between the inside of the tire and the ouside, so changing the air pressure changes the forces acting on the tire.

Of course, I never should have replied to this thread in the first place. I went out to the garage yesterday morning to find a nice little flat on my passenger side rear tire. There was a nice, small nail in the shoulder of the tire, so it couldn't be repaired. Because they have 37k miles on them, I went ahead and replaced both rear tires. If it hadn't been this close to Christmas, I would have replaced all four with something a bit wider, but...