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T E C H N I C A L L Y   S P E A K I N G



How can Shaughnessy & Co. put such heavy loads on tires? The secret is a combination of the right tires, increased inflation pressure and very, very slow speeds. Tires can have vast, untapped reserves of load capacity, provided you inflate them properly and run them slowly enough.

 

 

How can a tire carry such a heavy load?

Remember, it's not the tire that carries the load, but the air inside. The tire is only the container. And, inflation pressure does not change with load. If the tire had 100 psi-jacked up in the air-it will still have 100 psi when you lower it to the ground.

Then why are there different load ratings for tires?

The tire has to have certain strength characteristics to be able to contain the amount of air necessary. If you have two tires-same size and same model, but with different load ratings-you can put more pressure in the one with the higher rating. And that allows a heavier load.

But at 100 psi, different tires can still have different ratings.

Absolutely right. It's not just the pressure. It's also the air volume. Tires aren't like balloons. They don't get bigger when more air is pumped into them. Their volume stays pretty much the same-the pressure just rises. You could say it's the number of air molecules inside that determines the maximum load. Higher pressure means more air molecules and higher capacity. And, a bigger tire can contain more air molecules-and therefore has more capacity-than a smaller tire at the same pressure. And there's one other factor.

What is that?

Speed. The speed you're going to run has a huge effect. In a previous discussion (in Real Answers, v. 3, issue 1, p. 32) we pointed out that one of the functions of correct air pressure is keeping the tire the correct shape. Basically, a tire has two shapes-its "unloaded" shape, and its "loaded" shape. Or, "un-squashed" and "squashed." As the tire rotates, some 500 times per mile, every part of the tire is constantly going back and forth between the squashed and un-squashed shape. This causes the sidewalls to flex, generating heat.

Why is that?

It's a bit like taking a piece of coathanger wire and bending it rapidly. Do it for a while, and the wire can get very hot. Do it long enough and fast enough, and the wire breaks.

Is that what happens when a tire fails?

Sometimes. With too little inflation pressure, the tire flexes between shapes that are too extreme. That generates too much heat. In severe cases, it can cause a failure. At the very least, it can severely shorten casing life.

How?

If you've ever tried our coathanger wire experiment, you knew the wire was going to break, several bends before it actually did. You could feel that the wire had lost its strength. All that flexing does permanent, irreversible damage.

When a tire is under load, its shape changes, but not its inflation pressure.
As a tire rolls, its cross section is constantly cycling between its loaded and unloaded shape, generating heat.

 

So inflation pressure controls flexing?

Yes, because it controls the difference between the two shapes. And speed, because it controls the rate of flexing, is another major factor in heat build-up. The faster you go, the worse it is. That's why many tire load ratings have changed with today's higher speed limits. And why, with some tire brands, you must increase inflation or decrease loads at higher speeds.

How does this relate to very heavy loads?

A heavier load means more "squash" at a given pressure. So, you need a higher pressure with a higher load. But, slower speeds mean slower flexing. That reduces heat. So, if you're going very slowly, you may be able to carry heavier loads than at "normal" speeds.

How slow?

The benefits start showing up at speeds under 50 mph for most tires. And at very low speeds, capacities can really increase. With a tire like an 11R22.5, load capacity can increase 185 percent when the vehicle is stationary-if you boost inflation by 40 psi.

Relationship of Speed and Pressure to Load
For Metric & Wide Base Radials [for example, 295/75R22.5]
Speed Range [MPH]

% Load Change

Infl. Pressure Change
71-75* -12% +5psi
66-70* -4% +5psi
51-65 None No Increase
41-50 +7% No Increase
31-40 +9% No Increase
21-30 +12% +10psi
11-20 +17% +15psi
6-10 +25% +20psi
1-5 +45% +20psi
Creep (2) +75% +30psi
Stationary +105% +30psi

For Conventional Radials [for example, 11R22.5]
Speed Range [MPH]

% Load Change

Infl. Pressure Change
71-75* -12% +5psi
66-70* -4% +5psi
51-65 None No Increase
41-50 +9% No Increase
31-40 +16% No Increase
21-30 +24% +10psi
11-20 +32% +15psi
6-10 (1) +60% +30psi
1-5 (1) +85% +30psi
Creep (1, 2) +75% +30psi
Stationary (1) +185% +40psi


* No change necessary for Bridgestone tires rated for use at 75 mph


1) Apply these increases to Dual Loads and Inflation Pressures.

2) Creep-Motion for not over 200 feet in 30-minute period.

Note 1:
The inflation pressures shown in the referenced table are minimum cold pressures for the various loads listed. Higher pressures should be used as follows:


A. When required by the above speed/load travel.

B.
When higher pressures are desirable to obtain improved operating performance. For speed above 20mph, the combined increases of A and B should not exceed 20psi above the inflation specified for the maximum load of the time. The maximum load and inflation capacity of the rim must not be exceeded.


The chart lists something called "creep." What is that?

"Creep" is defined as moving 200 feet or less in a 30-minute period. That works out to about 1/13th of a mile per hour.

What does it work out to in pounds?

It depends on the tire, but for a 12.00R24 J-rated tire, the maximum single tire load limit would be 9,230 lb. at 120 psi. Add the 40 psi for creep speed, and here's what you get:

Maximum Load for 12.00R24 Radial Tire at Creep Speed
Load = Maximum load at 55mph + 140% of Maximum
= 9,230 lb.+ (1.40) x (9,230 lb.)
= 9,230 lb.+ 12,922 lb.
= 22,152 lb.

So the load at "creep" more than doubles if you add an extra 40 psi to the tire. And, if you observe two other very important rules.

Which are?

First, the tire must be able to handle the extra pressure. Check the sidewall for the maximum permissible inflation pressure. Never exceed that limit. Second, the wheel or rim must be capable of handling the extra load and pressure. Don't exceed that limit either.

So the secret of moving heavy loads is slow speeds?

As Davy Crockett used to say, "Be sure you're right. Then go ahead." If you have the right tire, right size, right load rating, right inflation pressure, right wheel and right speed, you can handle incredibly heavy loads.

 

End

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