INFO: Winter Tire Pressure

[aboulfad]

As winter is almost here and many of us changing to winter boots, few questions are surfacing about proper winter tire pressure. Prior to this following analysis, I was running 34psi equally on my Nokian R2 245/45R18, and coupled with many other questions, posts and research, I have come to the following:

DISCLAIMER: not a tire expert, not responsible for any damage to your tires or vehicle handling
OBSERVATION: It is recommended that rear winter tire pressure to be higher than the front.
EXPERIMENTATION: i haven't done any as my winters are new, but it will be interesting if some can chime in.

My Tire Pressure: I have finally concluded and set my square winter 18" tires (at 20C in my garage) to 32psi (Front)/35psi (Rear) based on my installer recommendation and coupled with the fact that the car manual is off from the driver door pillar. I will increase this by 1psi once the temps dip to -10-20C because of the difference of temp between my heated garage and outside.

Here are some reasons that why it is recommended winter tire pressure to be lower in front, higher for rear:

1. Decrease oversteer
Ref 1: Tire Rack Air Pressure for Competition Tires

Ref 2: BMW Car Manual: see attached pic.
Few things to note from my car manual, section Wheels & Tires, page 173:
- manual lists 31 psi for the summer's but on the driver door pillar its 32 which i think is more relevant.
- Air pressure is higher for rear tires independent of tire size for M+S tires (not specifically winter tires, but one could infer that this is the OEM winter recommendation)
- Air pressure for all summer size is equal, exception to this is the tire pressure for speeds above 100mph for larger tires
- Air pressure is higher overall for larger size tires M+S (19")

2. Even wear
In a square winter setup (maybe staggered tire setup too ?), If tire pressure is higher on rear, there is less flex to the body and hence less heat generated so less wear and coupled that is a RWD car, you may end with even tire wear. Ref: VAG motorsport installer

Does it all matter ? i am really not sure either way... further any of this discussion is moot if your tire pressure gauge is not accurate and there's no way to know unless you have a master gauge ! But i hope this info is useful for some and that we may learn some more things along the way.

[AnujS2K]

Thank you for starting this thread. This question has been on my mind as well.

[aboulfad]

You are wc I think there is no right or wrong answer, as long that tire pressure is checked often and fits some normal range.

[coolmody]

I'd recommend doing the opposite of what you recommend if you are trying to decrease oversteer. Dropping the pressures on your winter tires gives MORE traction (to a point). I've tested this extensively in some pretty extreme conditions on a frozen lake.

I've dropped my pressures all the way down to 20psi, and the the grip levels are better as it allows the sipes in the tire to hold the ice/snow better. Ever see how trucks can run their tires flat when they are offroad? This applies if most of your traction comes from the rubber compound in your tire (studless). If you have studs and are relying on the mechanical grip from the studs themselves, things get much more complex.

My recommendation if you want more traction is to run the tire as low as possible while maintaining the integrity of the sidewalls, and not risking the tire coming off the rim If this is a mainly street driven car on dry roads, I'd recommend the pressures to be around the factory recommended range (32psi). As conditions get worse, and you see more snow/ice on the roads, start dropping that to the high 20's.

[aboulfad]

Did you post a video sometime ago driving on a frozen lake? That was cool. Although I can't disagree with your personal experience, dropping winter tire pressure for a tire like mine Nokian R2 to high 20's, would yield horrible dry road performance as it is a soft tire. FYI, the OEM recommended winter 18" tire pressure is 31/33. And I or most people will not change their tire pressure depending on the weather during winter, if it's snowy, dry, icy or slushy...

I've always had inflated the rears higher than the front, in order to help decrease oversteer, but honestly I have never done experiments to prove that, I have never inflated winter tires or any other tire below 30 on my BMWs. Most literature points in the other direction for winter application but all data is interesting to take in. Thanks!

[coolmody]

I have a couple videos floating around. It might be me

I agree with your point about running the tires low in dry situations. You would want to bump the pressures up during these conditions. BMW's 31/33psi recommendation is probably based on data of the type of driving behavior coming from normal driving. My experiences aren't based on normal driving, but in most situations lower pressures on a winter tire will give you more traction. This even applies to summer tires, where you want to run the pressures as low as possible before the tire rolls over the sidewalls. These are huge generalizations without getting into the details of tire dynamics, but is a good baseline to use if playing around with pressures.