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Is what's stamped on the sidewall of the tire the recommended inflation pressure?
Some people think that, but it's not true. What is stamped on the sidewall is actually the maximum load and the minimum inflation pressure for that load.
So what do those numbers really mean?
The load number is the maximum load for the tire in normal operation. When you select a tire, you should always make sure that this number is equal to or greater than the largest load you'll ever put on the tire.
Why not just go for the highest tire load rating available?
You can do that, but it really doesn't help, nor is it very economical. Tires with higher load ratings often cost more, but if you can't use that extra load capacity, there's no advantage.
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Why are there different numbers for dual and single?
Dual load limits are lower than those for singles to add a bit of a safety factor for inflation pressure mismatch between the tires, and because road crown causes each tire in a dual pair to be loaded somewhat differently.
But the recommended inflation pressure is the same.
Yes. And again, it's the inflation pressure for that maximum load. So, if your tire loads aren't that high, your tires are probably overinflated.
That's why the practice some people have, of looking at tire sidewalls to determine the inflation pressure for their tires, is not a very good one.
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Overinflated tires tend to exibit fast wear int the crown area, together with hard ride and increased tendancy to impact breaks. |
What harm does it cause?
First, overinflating tires makes them hard and stiff. That means a rougher ride.
And, in some situations, overinflated tires, because of their stiffness, can actually damage dirt roads, like logging roads. That's why some logging companies run their tires at different inflation pressures, depending on whether they're "in the bush" or out on the highway.
And, overinflation also means tires are at greater risk for an impact break.
What's an impact break?
When an overinflated tire strikes a sharp edge, like the edge of a pothole, at fairly high speed, it can literally "break" the tread and casing, often opening a huge gash across the width of the tread.
The damage is usually so bad the tire cannot be repaired or retreaded.
Are there any other disadvantages to overinflation?
Overinflated tires can suffer irregular wear, especially in the crown area, because the crown shape is distorted by excessive air pressure.
And, for this same reason, it's possible for overinflated tires to suffer a loss of dry traction, because the excessive pressure reduces the amount of rubber in proper contact with the road.
It was also this loss of traction, in the days before ABS became universal, that often allowed trailer tires to lock up and become flat-spotted. When a tire was inflated to its maximum, and the trailer was empty, brake lock and flatspotting were likely.
Can this inflation pressure ever be exceeded?
If you're running at speeds less than 50 miles per hour and need to increase tire loads, you can often do it by adding additional air pressure.
| So, if you're running a metric size tire, like a 295/75R22.5 with a "G" load rating, the maximum load at speeds of 51 mph and above would be 5675 lb for each tire in a dual pair. |
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