Tire pressure in the cold

[RedDevil]

I run 175/65/R15 winter tires on my 2nd gen Insight, and they are a bit narrow and small for a car of that weight; other lighter cars often have way fatter tires, even as OEM.
So all the more reason to keep the pressure up way above factory recommendation (2.4 Bar, while I run them at 3.2 / 46 PSI.)
Or so I thought.

The last few weeks we had a cold spell here; Siberian wind 6 Bft dead east. down to -9 Celsius all day.
I found my car to be somewhat loose on the shoes. Wind sensitive, prone to not corner as tight as I'm used to. Also, it was not as economical as I expected it to be, even taking the cold into account.
As I was about to embark on a long trip I decided to check the tire pressure never mind the cold. And lo and behold, they were down to 2.6 Bar all round...
Upped them to 3.2 Bar again.

Now that was funny. I swap my tires myself. They've been stored all summer and held their pressure. The dealership lowered them at MOT, but I put them back up soon afterwards and expected them to stay there.

But that was on a lukewarm autumn day, and this was the coldest day in 6 years. I know the cold drops tire pressure, but this much?

Time to do the math.
I filled them to 3.2 Bar at 17 degrees Centigrade, that's about 290 degrees Kelvin.
I checked them at -9, that's 268 Kelvin.
3.2 bar tire pressure is actually 3.2 bar overpressure compared to the outside air, so it is 4.2 bar combined.
The tire pressure should be ((1 + 3.2) / 290 * 264) - 1 = about 2.8 Bar at -9 degrees Celsius.
Yup, I should have lost 0.4 Bar or almost 6 PSI due to the cold.
Another 0.2 Bar or 3 PSI went south for no reason whatsoever.
Yet most of the loss was due to the cold.

Anyway, after I raised the pressure again the economy gained considerably. And as the wind has changed and the frost is gone, the economy gets better still; almost summerlike.
Best of all, it grips the road really good; sharp steering and confident cornering. There's more road noise and vibration though.
Better not check the pressure now again or I'll see numbers I haven't seen before on car tires

We also had some snow. I was wondering whether the high pressure would impact grip in the snow negatively, but it did not. On the contrary, really.
Then, we also had snow just after its MOT whereat the monkeys had lowered the tire pressure; at that time it seemed to have very low grip...
I know you need to lower tire pressure in loose sand, but snow seems to require high pressure?

So, check your tire pressure when it gets really cold.
Also, wear gloves if you can while doing that. I did not. My hands hurt when I think of it.

[mpg_numbers_guy]

Good to know! I've been gradually inflating my tires from what they were when I bought my Civic (32 PSI) to sidewall (44 PSI). They WERE at 40 PSI....probably down now due to the cold weather...but the air compressor I use is broken now.

[Frank Lee]

Seems all my tires leak down at various rates.

[mpg_numbers_guy]

Originally Posted by Frank Lee

Seems all my tires leak down at various rates.

Same; I find a slight variance in how "fast" they deflate in cold weather. And it's a different tire each time that deflates the fastest.

[davelobi]

I was running the tires on my old crx at 50 psi they even went to 55 for a couple tanks. Sidewall (max) pressure posted on the side of the tires was the common 44psi. Being new tires, I felt that they could safely handle that much pressure and they never had any problems. Higher pressure rolls much easier. Don't believe it, try peddling a bicycle with a flat tire. There is a word of caution however with highly inflated tires. With a much smaller patch on the ground they will not hold near as good braking or cornering. Hyper milers don't brake much anyways so it never turned out to be an issue for me.

[Frank Lee]

Nobody wants to buy a bike with a flat tire unless it is at a nice discount.

[freebeard]

Siberian wind 6 Bft dead east.

Six out of twelve ain't bad.


http://kifissiameteo.gr/Lesson02_Bea.../Cartoon06.PNG

[davelobi]

Originally Posted by Frank Lee

Nobody wants to buy a bike with a flat tire unless it is at a nice discount.

buy?? Junk pick em till you have a pile of non-usable bikes that you have to get rid of when you move like I did.

[Hersbird]

Originally Posted by davelobi

I was running the tires on my old crx at 50 psi they even went to 55 for a couple tanks. Sidewall (max) pressure posted on the side of the tires was the common 44psi. Being new tires, I felt that they could safely handle that much pressure and they never had any problems. Higher pressure rolls much easier. Don't believe it, try peddling a bicycle with a flat tire. There is a word of caution however with highly inflated tires. With a much smaller patch on the ground they will not hold near as good braking or cornering. Hyper milers don't brake much anyways so it never turned out to be an issue for me.

I think they will hold braking or a corner very close, certainly nothing to worry about. what they may do is actually "feel" more responsive because of the stiffer sidewall. This may lead you to believe you actually have more grip and push it closer to it's limit without getting the feel that you are about to lose it. Still the traction really hasn't changed.

In a racing application you want as large a contact patch as possible but that is because of repeated cornering and braking at the limit. This really builds heat in the contact patch so the bigger it is the more heat is distributed over a bigger aera. During normal driving encountering an emergency situation the tire has plenty of ability to absorb the heat until after the event is over. So a smaller contact patch has the same grip because more weight is pushing down on smaller contact patch.

Now this goes well with snow or deep puddles as well. Smaller contact patch means more weight per square inch and it cuts through the snow or water better. On hard packed snow and ice the lower pressure can be better because it forms around little irregularities better finding something to grab. The problem then is the tire will run hotter so when you stop you actually melt the ice a bit an make it slipperier. The colder ice stays the stickier it is. Then you run into the problem of summer tires being very cold the rubber gets too hard. Winter tires are designed to stay flexible when frozen.

I say air them up, winter and summer. Even autocrossers air up their factory tires, the races are too short to build much heat (at least most classes, there are some cars that can melt tires in sort order).

[RedDevil]

Actually, cornering speed increases slightly over higher pressure. Low pressure allows more sideways movement in corners which deforms the contact patch.

Under braking there should be less difference, as there deformation is not much of an issue.