Quote:
Originally Posted by ardidi
 That's why it's impossible to find winter tires that fit the rears on the stock 4.8is setup. It's simple physics: pressure=force/area. With wide tires, you're splitting up the same weight of the car on more surface area, which is not what you want in the snow. For greater traction, you want the same weight on less surface area, so you want skinny tires. However, I'm assuming BMW recommends the 255/55s because it's a good balance between being decently skinny, compared to the stock tires, yet wide enough so that you don't lose that much performance during dry conditions, especially because winter tires have softer rubber than all-season or summer compounds. |
There are a couple of physics problems with this. The first is that pressure on the inside and outside of the tire are the same (putting aside sidewall stiffness issues). So, if your tires have 32 psi, then that is your ground pressure. It doesn't matter how wide the tire is.
Larger tires simply don't have more rubber on the ground, that is a common misperception. A wide tire and a narrow tire have exactly the same footprint in area. They just have a different shaped footprint, either short and wide or long and narrow.
A wider tire doesn't float because it has more area. It can hydroplane easier because it is harder to squeeze the water out the side. A wider tire does have to 'climb up' the leading edge in snow, and it has a wider leading edge. A narrower tire cuts through better, and the rest of the footprint follows in the track created by the narrower leading edge.
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