Tires: winter tires, traction surface, inflation pressure

Question
QUESTION: Hello . I noticed my car manual mentions to inflate winter tires 4.3psi more than the recommended for regular tires (which is 32psi, on my car).  

Could you tell me the reasoning behind this?  I thought lowering your tires pressure would increase your traction in snow tires.

Thanks.

ANSWER: Hugh,

There are 2 different types of conditions in snow.

The first is where the snow is on top of pavement and if you could get down to the pavement, you'd get better traction.  Raising the inflation pressure makes the tire's footprint smaller and increases the likelihood that you will reach pavement.

The second is where the tire can not get through the snow to the pavement, and in that case lowering the inflation pressure increases the size of the footprint and puts more of the tread pattern into contact with the snow.

So here's the problem in a nutshell:

If you raise the pressure, you'll more likely get to a good traction surface, however, there will be some situations where you will not be able to get the good surface.

Compared to:

A situation where you are less likely to get to the good traction surface, but have better traction than you would if you couldn't anyway.  Plus if you operate the tire at higher speeds, the operating temperature of the tire goes up and that reduces tires durability.

So the best "One size fits all" situation is to use more pressure - and if you get stuck in the snow, reduce the pressure to get unstuck.

---------- FOLLOW-UP ----------

QUESTION: Thank you for your quick reply!!

4.3psi, seems like alot to be raising tire pressure.?? is it not?  how much will this affect my tire wear?

Going in more detail...  If it is just cold out side and there is bare pavement, Should I stick with the 32psi(normal) and only raise the pressure when there is snow on the ground?

thanks

Answer
Hugh,

Dimensionally, radial tires are relatively insensitive to pressure.  The steel belt really restricts changes in shape.  So, no, 4 psi is not too excessive.  In fact I recommend that 3 to 5 psi more pressure than the placard calls for in pretty much all situations.

Raising and lowering pressure depending the weather is a lot of work and because of the cold temperature might be problematic due to water freezing in the valve.

So I recommend you set it at about 40F - a little higher than what you'd normally use and forget about it.