# Recommended: Tyre Sizes and Pressure values for E39 skid

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• posted on September 16, 2005, 2:29 am
Malt_Hound > wrote:

[...]
...at any given tyre pressure - it's the air that bears the load.
Lowering the pressure lets the tyre flatten more increasing the area of the contact patch, and vice versa.
My B10's 265/35 R18s at 3.0 bar actually have smaller contact patches than the narrower tyres fitted as standard to a 523 running at lower pressures.
The extra grip is down to the tyres generating bigger slip-angles due to the shape of the contact patch (plus stiffer sidewalls, sticky compound (with OE Michelin Pilot Sports or similar), progressive rate springs and twin-tube dampers tuned for the wheel/tyre combo).
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• posted on September 15, 2005, 9:24 pm

Because P = F/S. The tyre pressure being the same, the mass of the car doesn't vary and g is a constant -> F=mg = Cst P = mg/S ==> S=mg/P it is alco a constant.
You don't change the wheel dia. Since the tyre width varies from let say 225 mm to 245 mm, the lenght of the contact pad (the shortest dimension!) obvoiusly decreases.

I agree

Don't consider the groove design. It's obvious that on a wider tyre it's bigger. Let assume it doesn't change, just for theory.

First, on a wider tyre there will be some grooves more and due to the fact that the patch is of the same surface, whatever the tyre witdh is (see above) the length in contact with the road is smaller, leading in a shorter contact length.
|----------------| |---------------------------------------| ^ | 225 | | 465 ;-) | | length | | |---------------------------------------| v |----------------| width
Sorry, I like drawings !
Since the grooves are shorter there's a smaller volume of water in them, compare them to small pipes Losses are also reduced because the pipes are easier to empty.

nope: p=f/s

Grip, sorry

the laws of the physics !