Here's what happens at low pressures (stock) (archive)

You are correct with the physics of pressure vs pressure but when the car goes into a hard turn, a weight transfer from the rear to the front occurs and a low profile tire cannot absorb the lateral forces like a high profile tire can due to the stiffness and limited area of the sidewall. All the force is then transferred to the contact patch and sidewall edge which slides on the pavement as you notice this as accentuated wear on the outside edges. By increasing the interior tire pressure causes the edge on the contact patch and sidewall to be stiffer than with a lower pressure thereby increasing the edge life by forcing more of the contact patch toward the center of the tire into play.

For this reason staggered fitments in low aspect ratio application shouldn't be more than 1/2" difference. I have 10" Rear and 9" front. I have to order new outer lips and increase my front tires 20mm to help distribute the load better. But the tire pressure will have to remain high to get maximum service life out of them. Something to consider at 300/tire, not much albiet to some but with my budget it helps. You don't have to increase your tire pressure, it's up to the pilot. But you'll have to swap tires more often. But each operator must experiment on his own to achieve what works for him. Just because I say 50psi works for me won't necessarily work for you.

Guy

If the pressure recommendations on the sticker is for the 235/60-16 stock tires, it won't necessarily be the same with 18" tires, as the volume of the tires may be different. Remember that it's the amount of air that counts, not the pressure, so it will differ among tires with different volumes.

*** I do not agree. Pressure is pressure, and it is the pressure that counts, and that has nothing to do with volume. Of course the volume will be different, because the tires are of different sizes. But 33 psi puts the same force on the sidewalls of either tire.

BMW is recommending 33 psi, and while it is okay to go up a little, 5 additional pounds is a lot.

Also, low-profile tires don't react the same way as "conventional" tires when pressure is increased. Conventional tires tend to bulge out on the middle if the pressure is too high, but low-profile tires tend to expand sideways, in effect keeping the tire flat.

Big-T

*** Why would a 235/60-16 tend to bulge out any more in the center than a 235/50-18?

Have you ever run drag radials? Do you know what happens to the tread pattern on the ground, and your traction if you over-inflate? The tread becomes rounded, and the tread pattern does not make full contact on the outer edges. My position is that his tires will do the same thing at 5 pounds over.

Seems like closer to the factory recommended tire pressures as per the sticker in the door jam would be better.

38 psi for the front is too much IMHO!

Won't the tires wear more in the centers, as the tires will not be flat across the tread area?

DHG

ANYONE HAVE AN OPINION ON TIRE PRESSURE?
DRIVER'S DOOR SAYS 33 FRNT AND 41 REAR.
I just went with 38 and 41, it feels a little stiff but I like it and seems to handle better.
I have 18 in. Michelin Pilot All Weather XGT Z4
99 740il

*** Here he is saying what BMW is recommending for the 18" wheels (stock). I still believe that
38 cold for the front is to high, but it is not my car. Time will tell if he spots prenature center tire wear. And because of the different rim widths and tire sizing, he cannot rotate.

DHG

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Re: Here's what happens at low pressures (stock) Ascot Green 750 00:32 6/20/2001 (0)

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