Car Tire Inflation


#41

Let’s say the tire is at 30 psi (fixed) psi at 60, and 24 when it cools off to 0F. You want to use 24 psi as the recommended cold weather pressure, that’s incorrect. The tire should be at 30 psi.


#42

Ok I assume that you meant 30 psi at 60F so it would be 24 psi at 0F. Is that correct? 30 psi at 60F would be too low to begin with. If the tire placard called for 32 PSI, I would inflate to 35. It would then be 29 psi at 0F and that would be OK with me. Even if it was 32@60, 26@0 would be acceptable.


#43

Yes, psi, fixed it. So 32 is the recommended psi? Then overinflating when it’s warmer out will make up for some of the pressure drop when it cools off. Look, 3 psi isn’t going to make a big difference, but it’s not following the recommended pressures, and it’s not the ‘right’ way to do it. If someone inflates to the recommended pressure in cold weather, that’s the best way to go.


#44

How do you know that it isn’t. That’s my point. Everyone says thats the right way but has that ever been tested? I don’t think it has.


#45

Why would running tires at less than their recommended pressures need to be tested?


#46

The tire manufacturer should run the same traction/wear tests at different temperatures and different pressures to establish pressure/temp curves. The see if the same pressure works best at all temperatures or if different pressures work better for different temperatures. Maybe run a Taguchi array on these factors.


#47

I have experienced the same thing when taking my car to get the tires aligned. I finally just ended up doing it myself by eyeballing the tires and did a better job than the shops with the fancy equipment…


#48

I do my own too, but I use more than just my eyeballs. But I don’t mess with the alignment without a good reason. I carefully monitor the tires for unusual wear patterns. If the car runs true and the tires wear evenly, I don’t mess with it.


#49

I perform my own wheel alignments on the alignment rack every 10 years.

I don’t recall having an alignment come-back, how can a suspension and steering technician be incapable of performing a wheel alignment? I suppose skill and quality varies in the repair industry.


#50

I know this may seem weird but here’s what I do. About every 2 weeks or so I go to a local service station that’s less than 1 mile from my house and on my way to work. As a bonus it is the only place within 8 miles that has free air. Check the pressure and add or release as necessary to end up at 33psi (door jamb sticker says 29), at whatever’s the ambient temp may be. Seems to work okay. I got almost 86k out of my last set of Michelin Defenders and regularly get 33-36 mpg highway. BTW the car is a 2002 Camry 2.4liter automatic.


#51

A better goal would be to design the tire so that its traction and treadwear is relatively insensitive to inflation pressure and ambient temperature - and guess what? The way to do it is to discover what controls those things - then control them. Once discovered, that can be applied to every tire line that manufacturer makes.

So here’s what I know about that: A uniform pressure distribution in the footprint which is caused by the contour of the belt when loaded will result in even wear across the face of the tread regardless of the load/inflation used. That settles the inflation pressure issue.

The effect temperature has on traction and wear would be controlled in the tread rubber compound and discovering that would be a lab exercise followed by verification at the proving grounds. I am not a rubber chemist, so I can’t even speculate as to what is involved in controlling this, I only know that this has been a topic in the industry. Where they stand on it is unknown to me.

But once the manufacturer controls those things he needs to: Voila! Satisfied customers!

I am going to guess that since the issue of wear and traction vs inflation pressure and ambient temperature aren’t active complaints from the market (and it used to be my job to catalog those for one tire manufacturer), that by and large, the tire industry has a good grasp of this.


#52

The pressure should be at the recommended pressure when you start the car. Otherwise, you will have handling problems if the temperature is much lower than it was when you last checked the pressure.

No air escapes from the tire unless something is wrong. But the air changes its ability to make pressure with temperature. If you put 35 psi in the tire at 70 F, it will be 31 psi at 30 F.

The tire temperature and pressure increase when you drive, but this is designed into the tire. The important thing is the cold pressure. Too low and you get instability. Too high and the tire wears out faster.

The same thing happens to tires as happens to footballs.


#53

I’ve never met an engineer that wasn’t driven to prove his designs “hands on”. Engineers LOVE to test their ideas. It’s a part of our DNA.

There’s an old axiom that you can tell you’re a born engineer if you see a way to improve everything you see, no matter how perfect it already is. I very, very often modify things I buy. Every engineer I know does.


#54

Michelin says,
“Check your tires “cold” – before you’ve driven or at least three hours after you’ve driven.”

They also have a short blurb on nitrogen for tires.
"Because nitrogen replaces oxygen, less air can escape your tires, and your inflation pressure stays higher longer. "
https://www.michelinman.com/US/en/safe-driving/tire-safety/tire-pressure.html

So how much is the average pressure change cold vs say an hour of driving on the interstate, not enough to bother the tpms in my experience, are they programmed for fluctuations due to driving?


#55

Wow this is almost as good as the oil controversy.

I’ve been on the road for 50 years and I can think of about only once that I have been ripped off, and that was my own fault for pinching pennies.

My garage is about 45 degrees. Outside it is 13 below now. If we take the car someplace and it sits outside for a while, I’ll be sure to check the TPMS to see how low the tire pressure is. If too low, I’ll add a little air when I get back home. I really don’t worry much about it as long as I’m around 30# running around outside. On a long trip when its around zero out, I really haven’t seen much heat generated on the tires except to add maybe a pound or two.

I guess the main thing for me is that I’m not losing air in one of the tires due to a nail or something.


#56

I’m still waiting for Michelin to get back to me about the question I asked them.


#57

My tires are inflated to 33 PSI cold. After driving for a while, I’d say I almost always see a pressure between 36 PSI and 39 PSI, depending on how much warmer that part of the day is from the morning. This is according to my direct TPMS system that I believe is very accurate.


#58

I’ve never checked, but I suspect that how much the tire’s pressure changes varies depending on the weather and on the surface. I’d bet that a tire will heat up much more on hot pavement in Texas than on the roads in Bing’s hometown right now… or mine, since it’s been hovering continually around zero for days now.

In Bing’s case, if the car was garaged before driving the tires might even be colder after a drive to the mall than they were when he started out. :grimacing: And the pressure might thus be lower.

Engines have explosions to warm them up. And when I lived in North Dakota, sometimes even that wasn’t enough. Tires have only the innate squirming of the tire as it rolls down the road, friction with the road, and whatever heat can transfer from the surface it rides on. In Bing’s hometown, if the car was garaged I’d bet the heat transfer is from the tires to the roadway rather than the other way around… and at a greater rate than the heat from the roadway friction and the carcass squirming can replace.

I tip my hat to Bing for his excellent description of the phenomenon. I’m only glad that I don’t live in that part of the country anymore. It’s too cold up there for me. :grimacing:


#59

Well there are advantages. It was 13 below today and I went to the hardware store. Five guys standing around waiting to help me. No other customers fool enough to be out in the weather so you get good service. They were like the Maytag repairmen waiting to swoop down on a customer that came through the door. Of course it was almost a holiday too so maybe not just the weather. I did see three trucks pulling their ice houses on trailers heading out to the lake though. Until the last few days it hasn’t been cold enough to get much more than an inch or two of ice and you can’t drive on that.

Speaking of that, when I was a kid, they used to drive an old car on the middle of the lake and then everyone would bet when the car would sink in the lake in the spring. The one closest would win the pot. Those pollution control spoil sports put an end to that though. Should be some pretty nifty old cars sitting at the bottom of the lake though if anyone is interested.


#60

Sounds like a different planet. I have been wearing a long sleeve shirt for the last two days and it has been too warm, I switched on the A/C while riding in my sons car.