Experiment: coast down distances (rolling resistance) @ various tire pressures

[bertb]

Its enough "proof" that I need to check my air pressure more often. Even 100ft further is a lot of savings. Thanks for doing the test and showing the results.

[MetroMPG]

Originally Posted by HappyCamper

randomizing the order of the tests is a common practice.

Agreed - would have made for a better test.

As you guessed, it was just a case of not having the tools on hand to do it (a good, fast pump).

What I'm more interested in is at what psi does your tread wear evenly?

It depends (TM). Depends on the car, depends on the tire, depends on how far past spec you go. Caveat inflator!

With the Insight's OEM Bridgestone tires, the outside treads wear away before the center, even with the tires inflated WELL above the max. sidewall figure.

[CapriRacer]

Originally Posted by hackish

I think it's very important to point out to readers that tires are tested to a maximum recommended pressure by the tire manufacturer. They have a significant amount of wiggle room in this. A little birdy told me that premium brands will typically over-do this by 50% this and "drive" the tire at max weight and 25% over max speed to achieve a statistically small chance of a blowout.

Over-inflation can lead to less traction, especially during a panic stop. It can also lead to blow-outs at high speed. Ask firestone about that. I wouldn't recommend going over the max on the tire. Suppose you're in a crash due to a blowout or loss of control. If someone is injured they will check the tire pressures among other things. Just make sure you understand that if you're going to over-inflate, there are risks.

-Michael

Michael,

You have bits and pieces of this right, but there a lot wrong there as well - too much to comment on at one time. Keep your ears open for contradictory information.

CapriRacer

Visit my website:

www.BarrysTireTech.com

[justme1969]

I like the test but want to point out it isnt true if the transition from hill to flat is noted. a better test would have been by a preset mph coast down.
the 20 psi tire is going to nearly bottom out the rim on any more than a 10 deg. angle and thus loose much of its potential at bottom of ramp.
but we all do know that a harder tire should roll easier.
just dont over air your cheap tires as they break cords and get sidewall blisters.

[CapriRacer]

Originally Posted by hackish

I think it's very important to point out to readers that tires are tested to a maximum recommended pressure by the tire manufacturer. They have a significant amount of wiggle room in this. A little birdy told me that premium brands will typically over-do this by 50% this and "drive" the tire at max weight and 25% over max speed to achieve a statistically small chance of a blowout.

Over-inflation can lead to less traction, especially during a panic stop. It can also lead to blow-outs at high speed. Ask firestone about that. I wouldn't recommend going over the max on the tire. Suppose you're in a crash due to a blowout or loss of control. If someone is injured they will check the tire pressures among other things. Just make sure you understand that if you're going to over-inflate, there are risks.

-Michael

It occurred to me that my previous post shouldn't have just stated there was a problem. It should have pointed out what was wrong and explained why. So here goes.

What follows is about Standard Load Passenger Car tires. Extra Load PC tires and other types of tires are different in the details, but the general idea is the same.

There are load tables that delineate the relationship between inflation pressure and load. These max out at some inflation pressure. That point is called the rating point - although the terms "Rated Load" and "Rated Inflation Pressure" are the way it is usually stated.

The rated load will be imprinted on the sidewall of the tire. However, the rated inflation pressure may or may not be - usually not. If the term "Max Pressure" is used, it usually means this is NOT the rated pressure.

For Standard Load Passenger Car tires built to the US tire standard, the rated inflation pressure is 35 psi. For Standard Load Passenger Car tires built to metric standards, the rated inflation pressure 2.5 bar (36 psi) with appropriate difference in max load.

There are lots of tire tests, but the one of most interest here would be the industry standard "Step Load" test. The test is performed by loading a tire against a wheel. I hope you'll forgive me if I get the details wrong. I'm doing this off the top of my head, but the jist is correct.

The starting point is 80% of the rated load with the tire at the rated pressure with the surface speed of the wheel at 50 mph. The test is run for 2 hours (100 miles), then the load is increased 5% and run for another 2 hours, and so forth until the tire completes the 100% step. At that point, the tire passes the government minimum.

It is common for this test to be continued by increasing the load in the same stepwise manner. Tire manufacturers have a minimum value they use for QA purposes - say 125% of the rated load. They may stop the test at that point.

But sometimes the test is run to failure. Again, the load is increased in a stepwise manner - until failure.

When the tire is removed (whether at the QA limit or at the point of failure), it is examined. If there was a failure, was it heat related failure (not the goal of the test)? Was it a stress type failure (the intented result)?

My point? The standard industry (and government) test is NOT run at the maximum pressure (with a few exceptions).

There are a ton of other types of tests: Ride, handling, rolling resistance, force and moment, etc. Very few are run at the pressure listed on the sidewall. They are run at the pressure prescribed by the test.

For example, ride and handling tests are run at the pressure listed on the vehicle tire placard.

I hope this clarifies things. Oh, and you need to stop listening to that bird.

[hackish]

Originally Posted by CapriRacer

Michael,

You have bits and pieces of this right, but there a lot wrong there as well - too much to comment on at one time. Keep your ears open for contradictory information.

You're right, I rounded the actual numbers to the nearest 10 to protect the source but I do have the data directly from one of the largest tire companies in the world's test cell. They do have a tire specified maximum design criteria and they indeed tested it to +50% above their criteria. The point here is they leave some wiggle room but over-inflating your tires to 2x the published max is a bad idea.

The other info comes from a friend who is an accident investigator. I asked him directly about the over-inflation and his response was "we check the tire pressure 100% of the time when there has been any injury. It's on the checklist and if someone overinflated significantly it's not a detail you want a lawyer to have!"

So with your vague response it's hard to know what you take exception to but I consider it reasonably well researched given the price paid. The problem I have here is blind advice to try significant over-inflations leaves the reader quite uninformed with the risks involved, both in safety and liability.

-Michael

[rustfarmer]

Not sure why you want this information. I thought most folks knew raising tire pressure will reduce rolling resistance, but this would also change many other characteristics such as handling, uneven tire wear and perhaps most importantly braking. Varying tire pressure a few pounds can have rather large affect on ride comfort as well.

[PaleMelanesian]

Originally Posted by CapriRacer

There are load tables that delineate the relationship between inflation pressure and load. These max out at some inflation pressure. That point is called the rating point - although the terms "Rated Load" and "Rated Inflation Pressure" are the way it is usually stated.

The rated load will be imprinted on the sidewall of the tire. However, the rated inflation pressure may or may not be - usually not. If the term "Max Pressure" is used, it usually means this is NOT the rated pressure.

For Standard Load Passenger Car tires built to the US tire standard, the rated inflation pressure is 35 psi. For Standard Load Passenger Car tires built to metric standards, the rated inflation pressure 2.5 bar (36 psi) with appropriate difference in max load.

A certain major tire maker contradicts this.
Defender | Michelin Tires

Max Load, Single (lb @ PSI)
1433@44

Max load is rated AT the tire's max 44 psi.

[MARTINSR]

I was blown away at the difference in tires. I put a new set of tires on my Taurus SHO and lost about 150ft in how far it would coast. I would coast from a certain spot all the way around a few corners to the a stop sign, every single day I did this. I put a new set of tires on the car and never got close.

Now with my 59 Rambler that I drive every day. It has very narrow little bias ply white walls (we are talking 4") with drum brakes set up with zero drag and I have lost an entire block on that same test.

Brian

[CapriRacer]

Originally Posted by PaleMelanesian

A certain major tire maker contradicts this.
Defender | Michelin Tires

Max load is rated AT the tire's max 44 psi.

Yes, I am aware of that, and I disagree with the way they state it.

Proof? I could show you the source documents, if that would help.

But an easy way would be to ask Michelin what the load carrying capacity of a particular tire is at 35 psi if the tire is a P metric, or at 36 psi, if the tire is a Euro metric. If I am right, then the answer will be the same load as published, showing that what they published isn't quite accurate.

- OR - you could ask for a load table for the particular tire. Either way, I am confident of what you will get back.