Tire pressure -- who to believe?

Maintenance/Repairs
21

Actually, it was the type of tire construction over 50 years ago, that required such relatively low cold(not yet driven) tire pressures in cars - ‘bias-bly’(criss-cross) construction. Running the cold pressures specified for modern radial-ply(perpendiculary to the rim) tires in those bias plies could result in a very rough ride with severely reduced contact patch.

Conversely, inflating modern radials to the pressures specified back in 1957, or 1964, could result in the TPMS warning light coming on in your instrument panel. And the shoulders of the radial ply tread could wear out in a matter of weeks, let alone months or years.

Radial tires, as a rule of thumbs, usually require an average of 5psi more cold pressure than their bias-bly ancestors. So a 1964 Buick specifying 26psi on bias ply might require 31psi cold in replacement radial tires.

Sorry for resurrecting an older thread, but I was browsing threads regarding tire pressure, and came across this conversation.

22

OP makes good point, that wording is sort of confusing. However, 45 psi would make for a really uncomfortable & harsh ride, above posters are correct, best to use the psi on the door pillar.

I’ll have to check my car tires. Do they say “inflate to 45 psi max”? Or do they say “45 psi max”?

23

Ideally, it should say " do not inflate to over 45 psi".

24

I’ll bet it says " Max Load XXXX, Max pressure 45 psi". That’s the way it is usually done and the OP just misinterpreted it. In other words: Read what he wanted to read, not what was there.

5 Likes
25

Next thing you know, the wording will look like this:

Exceeding the max pressure may cause serious side effects including explosive air loss. Stop using these tires and get emergency mechanical help right away if you get any of the following symptoms or reactions:

  • Fainting, dizziness or feeling light headed

  • Swelling of the outer cords or belts

  • Trouble cornering or tightness in steering

  • Road rash or other adverse reactions

These tires at higher pressure may lower your ability to maintain control and increase your risk of premature wear. Before inflating to maximum pressures, consult your mechanic provider and seek immediate care right away if you develop any allergic reaction to using higher pressure.

:laughing:

4 Likes
26

Yep.

And that text can vary, per tire manufacturer:

(Max Load xxxkg/xxxlbs. Max. pressure 51psi) (to use my car’s example)

or

(Max Load xxxkg/xxxlbs @ Max pressure 51psi)

Some throw the word ‘cold’ between Max & pressure.

But it all pretty much means the same thing.

In my current and my last car, just for experiment, I drove to and from work, for one day(!) with cold pressures set 3 to 4 psi below that max cold value:

41psi on the Elantra tires(44psi max)

and 47psi on my current Accord(51psi max)

The experience in both cars was similar, although I still felt the weight of the bigger Accord despite the nervous ride and super-light steering!

In both cases felt like I was driving on ice, even in warmer months, and no steering weight, very little feedback from the road. Plus I actually burned a little rubber in both cars, jetting late from a green light…

…

I never again set my cold pressures anywhere near those maximum sidewall values, lol! And nothing anybody who does set their pressures that high could convince me otherwise. Even that famous cop article where the officer goes on to explain that all the cruisers tires were maintained at whatever the max pressure on their V-rated tires was. Barry I noticed that article is now nowhere to be found on the internet.

Next morning, I promptly set the Hyundai back down to 33psi door pillar value, and a couple years later, the Accord, to 32psi.

27

At track days I set my tires to 27 psi. Recommended is 30, for the stock size tires. Mine are bigger.

I typically set best lap times on hot tires at about 35 psi… the Mustang is heavy… and coming off track I read 37 to 39 psi all around. And these are street tires, not race tires.

Street use set to 30 cold, I measure 33 psi after highway driving in warm weather.

1 Like
28

Tire pressure is based on the tire’s optimal footprint and the weight of the vehicle. It’s really simple physics.

You can actually determine the weight of a vehicle by the footprint and the tire pressure.

Tired Weight: Math & Mechanics Science Activity | Exploratorium Teacher Institute Project

29

"Contents of this tire have been shown to cause cancer in the state of California. "

3 Likes
30

It isn’t that simple. A tire is not a balloon. @CapriRacer can weigh in on this but there isn’t a direct relationship between contact patch and tire pressure. The sidewalls themselves carry a significant amount of weight.

The best example I can give is run-flat tires. At zero pressure, they still support the car’s weight but do not have an infinite contact area.

2 Likes
31

Ah … Mmmmm… Not exactly

Barry’s Tire Tech (barrystiretech.com)

1 Like
32

You’ll always find examples that aren’t in the norm. Sure run flats have issues. But the formula works. I’ve done it in the lab of my physics class years ago. The tire tread will have a bearing on the calculation, but that’s solvable. Remove run-flats from the equation and the sidewalls really don’t matter too much. Using this method isn’t as accurate as weighing a scale, so there’s going to be a plus and minus factor. But it DOES WORK.

A Method for Weighing a Car or Truck (cuny.edu)

33

How do you know it works? Or how accurate it is? Neither post actually scaled the vehicles to confirm the relative accuracy of the method. What was your experience with the accuracy?

The only posts I can find Googling the method are from educational institutions and none verified the results with a scale. They all use the weight form the tags on the vehicles. I’ve never seen this suggested on any motorsports sites… where the corner weights are important for car set-up and scales cost $900.

34

Read my previous post. I’ve actually done the experiment in my college physics class. You can do a google search. That experiment is being performed by many colleges all over. My daughter did the experiment at her undergrad physics class at MIT. Some states still use this method to determine the load of trailers.

Yes, there is a margin of error. Not as accurate as a scale because you’re dealing with imprecise instruments and measurements.

35

I’m sorry, but perhaps you missed the webpage I posted that gave 4 examples where the math doesn’t work - particularly the quote “It is well-known, to tire engineers, that the average footprint pressure produced by a tire can be above or below the inflation pressure, depending on tire load.”.

1 Like
36

I saw it. And I agree is doesn’t work in THOSE cases. I also posted examples of where it works fine (with a margin or error). This is known physics. It can be applied to cars and trucks. If the science isn’t valid then there are probably thousands of tickets given out to truckers because some state still use that method to determine a Semi’s load. You can’t dismiss the math and physics because you found edge case scenarios. This formula is extremely useful for bicyclists because the weight of the rider could differ greatly with the same bike.

Vehicle weight determine by tire pressure

Tired Weight: Math & Mechanics Science Activity | Exploratorium Teacher Institute Project

AP Physics 2: Fluid Mechanics 4: Tire Pressure and Tire Footprint Problem - YouTube

37

You keep posting links where there isn’t any data - only a statement that it works. I’ve given you 4 examples with data where it does not and I expect from you a link to 4 different examples - with data - where it does. Not made up data, links to some people who have actually confirmed it works.

2 Likes
38

An older friend was very badly injured by an exploding tire during WWII. He was assigned to work on an aircraft carrier, repairing & re-inflating the airplane tires, which would often be damaged during hard landings. One of the airplane tires exploded during the inflation process.

39

weigh your car - YouTube

This isn’t something people do every day. But let me inform all the Physicists, college professors and Mechanical engineers that they’ve been wrong for decades. I’ll refer them to you, since you’re the expert on everything tires.

40

Please do inform them. Just because academics do this as an exercise, does not mean it is correct. Especially if no one ever measures to confirm the accuracy. That is why I asked what your personal experience was.

You’ve posted another video not verified by actual test data. It just shows the method is verifed by general load specs from the car. I’ll repeat my post… I have Goggled it and found no actual data confirming the accuracy or inaccuracy of the method with actual data.