Wheels and tyres and pressure. Oh my!!

[Piwoslaw]

I'm looking for winter tires and would like narrower wheel hubs for them. So I find this page called "The Wheel and Tyre Bible", which has lots of cool info. Here's the location:
Car Bibles : The Wheel and Tyre Bible Page 1 of 2
On page 2 there is a section on tire inflation, starting from "Checking your tyres" about 2/3 down the page. Among many things, the author comes to the conclusion that door sticker tire pressures are on the low side and a better rule is max pressure -10%. This gives just under 40psi if the max is 44psi. The whole page is really nice to read.

Our car has a door sticker of about 36psi, but the max on the tire is 51psi. Going with the max-10% rule, I should pump up to about 45-46psi. BUT the tire also has a big warning, which states (in slightly different wording):
Terrible things will happen if:
1) You underinflate your tires,
2) You inflate them over 40psi.
I keep them at 40, but am wondering if I can go up to 45psi?

[Johnny Mullet]

All you can do is experiment. if the max pressure stamped on the sidewall is 51PSI, then it should be safe at that pressure. Are you sure the "inflate over 40psi" is not for bead seating? I know almost every tire machine has this warning on it. "Do not exceed 40psi to seat beads"

[theunchosen]

If the warning appears urgent enough test it carefully. The place its most likely to have issue will be in cornering or hard acceleration at which point the shear/slippage respectively will be greatest. I feel slippage is very unlikely to happen; however, shear is pretty common.

Tires that are under heavy stress and are overworn(roughly equivalent to drastically over inflating) typically blow out while turning or changing lanes. This(drastically over inflated) would be at that 51 PSI in your case or lower depending on tire condition and age. The trick is to avoid that happening while getting as close as possible.

As Johnny Mullet said "experiment."

If your tire is fine under normal turns and driving conditions slowly try it on harder turns or accelerations and if it doesn't blow its good. If not. . .well. . .don't blame me(mine hover around 37-38 to avoid overinflation in a high temperature change(its happened to me)).

[cfg83]

Piwoslaw -

Originally Posted by Piwoslaw

I'm looking for winter tires and would like narrower wheel hubs for them. So I find this page called "The Wheel and Tyre Bible", which has lots of cool info. Here's the location:
Car Bibles : The Wheel and Tyre Bible Page 1 of 2
On page 2 there is a section on tire inflation, starting from "Checking your tyres" about 2/3 down the page. Among many things, the author comes to the conclusion that door sticker tire pressures are on the low side and a better rule is max pressure -10%. This gives just under 40psi if the max is 44psi. The whole page is really nice to read.

Our car has a door sticker of about 36psi, but the max on the tire is 51psi. Going with the max-10% rule, I should pump up to about 45-46psi. BUT the tire also has a big warning, which states (in slightly different wording):
Terrible things will happen if:
1) You underinflate your tires,
2) You inflate them over 40psi.
I keep them at 40, but am wondering if I can go up to 45psi?

That's one of my favorite sites for tyre stuff. I have the URL in my garage.

From my POV, the "10% rule" is a great starting point for increasing tire pressure. I know I can recommend that to someone with tires in good condition. The person is increasing pressure for road handling *performance*, not MPG. The site also states that the rule a starting point. You have to find the sweet spot for your car/tire combination. Since our goals on Ecomodder are different from the URL, you could start with the 10% rule and bump up as needed.

CarloSW2

[Unforgiven]

Following the door plate suggestion is one of the things that got Ford in trouble with the Explorers and the rollovers that happened several years ago. My past experience in tire sales and installation lead me to follow a mental sidewall pressure template. Too low a pressure is never good on the sidewalls of the tire.

Tire sidewall max rating............................................ ....Preferred psi setting
35 32
38 35
41 38
44 41
50 45
65 55
80 70
(80 psi tires commonly found on 6wheelers or duallys here in the US often have the steers set down around 65 and rears at max of 80 and many of the drivers of such kept to that, but to keep the balance in the treadwear, I would suggest the steers to come up a bit)
110 100

Having said all that, I know some folks like to run even higher pressure in the hopes of easing the frictional component by trying to reduce the surface contact patch of the tire. Better off getting an actual slimmer tire or finding the actual LRR type to replace it with.

[BlackDeuceCoupe]

You know what?

It's probably a cosmic thing, but...

I run my tires at 50 PSI, and I don't care what happens, and...

Nothing ever happens! They wear like iron, and I get great FE!

When I used to worry about stuff like that, I had all sorts of tire problems - premature wear, out-of-round tires, thread separation, flats...

My suggestion is to run 50 PSI regardless, and quit worrying!

[Piwoslaw]

I had an even closer look at my tires today (my neighbor remarked: "I understand you've got a new car, but kissing its wheels??"). Johnny Mullet, you were right about the 40psi limit for bead seating. Here's the whole essay that the manufacturer wrote on the tires:

MAX PRESSURE 51 PSI (350 KPA)
SAFETY WARNING Serious injury may result from: tire failure due to underinflation/overloading -- follow vehicle owner's manual or the tire placard in the vehicle. Serious injury may result from: explosion of tire/rim assembly due to improper mounting -- never exceed 40 psi (275 KPA) to seat beads. After beads are properly seated inflate to recommended pressure as specified in vehicle owner's manual and the placard in vehicle.

The door sticker says 2.5bar (about 36psi) for all wheels with a normal load and 3.2bar (46-47psi) rear wheels with max load. So I pumped them all to about 44psi. Tomorrow I'll see how Svietlana handles and coasts.

I'm wondering how much the air temperature and pressure effects tire pressure. My elevation is 95 meters, so just over sea level, the outside temperature is around freezing (2-4 above during the day and 1-2 below at night). Say my tires are pumped to 44psi now and I drive to the mountains and park inside a heated garage. What will the tire pressure be at an elevation of 2000 meters and temperature of +25 degrees C? Will it be noticably closer to the limit?

Also, how much does driving effect tire pressure? I've read that pressure should be checked and adjusted before driving, when the tires are still cool. Is there enough friction heat in a working tire to noticably (or even dangerously) raise the pressure?

[MazdaMatt]

Yes, a driven tire will increase in PSI. However, the ratings are all in "cold PSI" so they take expansion into account.... as far as i know.

At least in racing, i can set my tire PSI to 36 and after a couple laps i'm at 40-41.