mythbusters tests tyre / tire pressure

[UFO]

Originally Posted by Fat Charlie

Using tires will cause tread separation and blowouts. The amount of air you put into them can affect this in some way. Your vehicle's weight and suspension settings can also affect this. Road conditions and driving style can affect this. Why on Earth would you pick out one of these factors of tire wear and announce that it does not affect the tires?

We are talking semantics now. "Cause" is a primary factor in failure, where "affect" is merely a contributor. Tires are designed to carry weight up to the load rating AT the sidewall pressure, so how can the claims be made (without failure analysis) that this is "over-inflation" or higher pressure "causes" a failure??

Of course tire pressure can affect handling and grip, and care must be taken to drive within the vehicle's limits. These may vary with inflation pressure among many other factors. It's interesting how tire pressure is the only variable receiving attention.

[CapriRacer]

UFO, I hope you'll recognize I am not singling you out, but the way you worded something is part of the confusion surrounding the inflation pressure issue and I think it is important that this be correctly stated.

Originally Posted by UFO

.....Tires are designed to carry weight up to the load rating AT the sidewall pressure......

Tires are designed to carry weight up to the load rating at SOME inflation pressure - and that pressure is not necessarily what is imprinted on the sidewall.

For example: For Standard Load Passenger Car tires, the load carrying capacity increases as the inflation pressure increases up 35 psi. Put another way, the maximum load carrying capacity occurs at 35 psi. It is permissible to use higher inflation pressure as specified by the sidewall maximum.

Here's a copy of a page out of the 2005 Tire and Rim Association Yearbook that shows a typical load vs inflation pressure relationship. This is commonly referred to a Load Table:

www.barrystiretech.com/2005traloadtable.jpg

Notice that for "Standard Load" the table ends at 35 psi and for "Extra Load" it ends at 41 psi.

[UFO]

Originally Posted by CapriRacer

Tires are designed to carry weight up to the load rating at SOME inflation pressure - and that pressure is not necessarily what is imprinted on the sidewall.

No, it specifically says on the tire, rated for xxx load @ xx psi maximum. Your link confirms that and makes my point: the load capacity of the tire increases with pressure all the way to the "sidewall maximum".

[Endless44]

im going to keep my tire pressure to 36 -38 psi to be on the safe side

[CapriRacer]

Originally Posted by UFO

No, it specifically says on the tire, rated for xxx load @ xx psi maximum. Your link confirms that and makes my point: the load capacity of the tire increases with pressure all the way to the "sidewall maximum".

No, there are 2 ways the sidewall information is expressed:

1) Max Load XXXX at YY pressure

2) Max Load XXXX, Max Pressure YY

For Standard Load Passenger car tires with speed ratings T and lower, the first configuration is sometimes used.

For H and higher speed ratings you will see the 2nd configuration. But sometimes you'll see the 2nd configuration for T and lower.

These 2 configurations are not equivalent statements.

And I should warn you that for some reason Kumho uses the first configuration in a misleading way. So for the purposes of this discussion, let's stay away from the way Kumho is doing it, as it just confuses the matter more.

[fidalgoman]

I run our Sienna Mini-Van in the low forties. It makes 2+ MPG over the 32 Costco set them at. As a side note I remember growing up as a kid all the Pickup trucks with 8 ply or greater tires had 45 PSI written on the sidewalls. Any guesses where most people ran them all the time. It was just too inconvenient to run by a gas station to fill them up when you need to haul a load of firewood, gravel, etc. Just information.

[UFO]

Originally Posted by CapriRacer

No, there are 2 ways the sidewall information is expressed:

1) Max Load XXXX at YY pressure

2) Max Load XXXX, Max Pressure YY

For Standard Load Passenger car tires with speed ratings T and lower, the first configuration is sometimes used.

For H and higher speed ratings you will see the 2nd configuration. But sometimes you'll see the 2nd configuration for T and lower.

These 2 configurations are not equivalent statements.

And I should warn you that for some reason Kumho uses the first configuration in a misleading way. So for the purposes of this discussion, let's stay away from the way Kumho is doing it, as it just confuses the matter more.

What is misleading about the first designation? It is a very common way of expressing exactly what I am asserting, load peaks with sidewall pressure maximum.

I just read the ratings off 6 different tire brands I have.

Yokohama (LT), Kumho (ZR), Big O (T) and Bridgestone (H) are printed as #1.
Michelin (V) and Dunlop (ZR) are printed as #2.

The second way of printing the rating does not state the load rating decreases with increased pressure. That is your position, correct?

[CapriRacer]

I just realized the missing piece of this puzzle:

For Standard Load (SL) Passenger Car tires, there are 3 possible pressures on the sidewall: 35 psi (where the max load occurrs), 44 psi, and 51 psi - the last 2 of which could be considered over-inflation.

So referring back to the page out of the Tire and Rim book (www.barrystiretech.com/2005traloadtable.jpg), if the sidewall of a P225/60R16 were to say "Max Load 1609# at 35 psi", that would be correct, but it could also say "Max Load 1609#, Max Pressure 44 psi", or "Max Load 1609#, Max Pressure 51 psi."

[UFO]

Originally Posted by CapriRacer

I just realized the missing piece of this puzzle:

For Standard Load (SL) Passenger Car tires, there are 3 possible pressures on the sidewall: 35 psi (where the max load occurrs), 44 psi, and 51 psi - the last 2 of which could be considered over-inflation.

So referring back to the page out of the Tire and Rim book (www.barrystiretech.com/2005traloadtable.jpg), if the sidewall of a P225/60R16 were to say "Max Load 1609# at 35 psi", that would be correct, but it could also say "Max Load 1609#, Max Pressure 44 psi", or "Max Load 1609#, Max Pressure 51 psi."

But what does it actually say? To me it looks like it's not rated for more than 35psi, so that should be the stated maximum. On that site, the author states:

Increasing the inflation pressure increases the maximum load carrying capacity up to a maximum value. This maximum could be called the "Rating Point", but generally this is referred to as separate entities - the "Rated Load" and the "Rated Inflation Pressure"

Regardless, any pressure lower than the stated maximum is not "over-inflated". However, any load greater than the rated load at intermediate pressures is certainly overloaded even if the load is less than the maximum sidewall load rating.

[euromodder]

Originally Posted by CapriRacer

For Standard Load (SL) Passenger Car tires, there are 3 possible pressures on the sidewall: 35 psi (where the max load occurrs), 44 psi, and 51 psi - the last 2 of which could be considered over-inflation.

I asked Michelin folks about the max. pressure for their Alpin A4 winter boots, as I couldn't find it on the tyres, and got back that it was 3.4 bar / 49.4 psi .

No strings attached, no "don't do it", no "you're going to kill yourself".