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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
psijacked up in the airit 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 tiressame size and same model, but with different
load ratingsyou 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 samethe 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 moleculesand therefore has more capacitythan 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 shapesits "unloaded" shape, and its "loaded"
shape. Or, "unsquashed" 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 unsquashed
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 buildup. 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 stationaryif 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 
7175* 
12% 
+5psi 
6670* 
4% 
+5psi 
5165 
None 
No Increase 
4150 
+7% 
No Increase 
3140 
+9% 
No Increase 
2130 
+12% 
+10psi 
1120 
+17% 
+15psi 
610 
+25% 
+20psi 
15 
+45% 
+20psi 
Creep
(2) 
+75% 
+30psi 
Stationary 
+105% 
+30psi 

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

Speed
Range [MPH] 
% Load Change

Infl.
Pressure Change 
7175* 
12% 
+5psi 
6670* 
4% 
+5psi 
5165 
None 
No Increase 
4150 
+9% 
No Increase 
3140 
+16% 
No Increase 
2130 
+24% 
+10psi 
1120 
+32% 
+15psi 
610
(1) 
+60% 
+30psi 
15
(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) CreepMotion
for not over 200 feet in 30minute 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 30minute
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 Jrated 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.




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