Air pressure in Tires

mfrogg

So, here's my logic, and the subsequent question:

In cold climate, air is more dense, yah?

If Air is more dense, then the tire that said air is in would shrink.

If the tire surface is heated up (through driving), the air would heat up, and reinflate the tire to the pressure it was at before.

If you fill a tire when it's cold, to the proper pressure, then drive, it would become over-pressurized and provide less footprint for the tire.


Should you fill up the tire when it's cold, or after you've driven on it and heated it up through friction?

Improvolone

I have a tire guy who has been in the business for probably 40 years. He says you the rated PSI is safety max, not target. So fill to about 5 under cold or hot and you should be fine, though I can't imagine the temperature difference is so great that it's going to expand or contract that much.

Killgrimage

I've always been told to keep my tires a little under recommended PSI during the winter, because they will get better traction on ice. However, I have been told that they do indeed have "less air" or rather "less PSI" in them during colder months. I don't think driving around with them will cause that much expansion because there isn't really that much friction causing heat (I mean, rolling doesn't cause friction/heat, sliding does right? Dammit Jim I'm a biologist not a physicist!)

RUNN1NGMAN

First, you should be filling your tires to the manufacturer's recommended PSI, which should be on a label on the inside of the driver's door frame or inside the gas cover. Don't go by the max PSI marked on the tire.

Second, you should always fill your tires when they are cold, regardless of the season. The recommended PSI is based on cold tire pressure. If it's cold out your tires warm up, but that's already accounted for.

The tire pressure will in fact go up a few PSI when the tires warm up, and tires do warm up quite a bit from the constant rolling and friction with the road.

mfrogg

First, you should be filling your tires to the manufacturer's recommended PSI, which should be on a label on the inside of the driver's door frame or inside the gas cover. Don't go by the max PSI marked on the tire.

The rest of your post makes sense to me, but if you get a new tire, the PSI may be different for the tire itself. How's the manufacturer supposed to know each tire you put on? What if you get new rims? o_O

This turned out to be more interesting than I originally thought. Fantastic!

Jimmy King

Killgrimage wrote: »

I've always been told to keep my tires a little under recommended PSI during the winter, because they will get better traction on ice. However, I have been told that they do indeed have "less air" or rather "less PSI" in them during colder months. I don't think driving around with them will cause that much expansion because there isn't really that much friction causing heat (I mean, rolling doesn't cause friction/heat, sliding does right? Dammit Jim I'm a biologist not a physicist!)

But there is friction and your tires do tend to feel warm after driving awhile on them, if I remember right (I actually checked once when I was first learning to drive, but that was a long time ago, I could be remembering wrong). It's not going to raise air pressure to the point of risking damage to the tire, but there is still friction. If there was no friction, you wouldn't need to replace your tires due to the tread getting worn down.

RUNN1NGMAN

mfrogg wrote: »

<Quote>First, you should be filling your tires to the manufacturer's recommended PSI, which should be on a label on the inside of the driver's door frame or inside the gas cover. Don't go by the max PSI marked on the tire. </Quote>

The rest of your post makes sense to me, but if you get a thin profile tire, the PSI would be different than a standard profile tire. How's the manufacturer supposed to know each tire you put on? What if you get new rims? o_O

This turned out to be more interesting than I originally thought. Fantastic!

If you're getting performance tires then you're not really going to care about the recommended PSI for a number of reasons. The recommended PSI is based on safety data, and sacrifices things like handling in high-speed turns for stability, comfort, and fuel efficiency under normal driving conditions. There's lot of data out there on what your tire pressure should be for specific situations if you have custom wheels and tires. Most people adjust based on whether they are on the track or on the road.

But for stock wheels and tires, go with the recommended PSI.

an_alt

If you really want to get the proper amount of air in your tyres, chalk them up on a dry day and go driving. If the chalk on the sidewall is worn off, you're underinflated. If the chalk on the edges of the tread is still there, you're overinflated.

Or if you want to be close enough (for the vast, vast majority of cases), just go with the recommended pressure and only change it if you see wear problems.

WildSpoon

this is actually a tricky question i have issues with as well. My car has simple tires, but not the ones that came with the car. when i fill them to the reccommended 34 PSI (according to the door sticker in the car) i swear that they look (and feel when driving) to be flat / under-inflated. i fill them to 38 psi and they FEEL better, but still sometimes look slighly bulged at the bottom (ever so slightly).

so ya... im a confused soul when it comes to this as well

RUNN1NGMAN

WildSpoon wrote: »

this is actually a tricky question i have issues with as well. My car has simple tires, but not the ones that came with the car. when i fill them to the reccommended 34 PSI i swear that they look (and feel when driving) to be flat / under-inflated. i fill them to 38 psi and they FEEL better, but still sometimes look slighly bulged at the bottom (ever so slightly).

so ya... im a confused soul when it comes to this as well

Tires are supposed to be bulged at the bottom. If they look totally symmetrical they are way overinflated. Overinflating tires a few PSI will make your car handle better, generally, but you are sacrificing road noise and how much you feel minor bumps and cracks in the road. More inflated tires will make your steering feel more responsive, though.

The recommended tire pressure is based on the stock tire size and speed rating for your car, so if you buy new tires with the same specs then you should be fine following the recommendation.

edit: Tirerack.com has a lot of great information about tire maintenance. This is how to perfectly set your tire's pressure, if you're interested.