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Why is it so hard to make tires that can run both on and off the road?
Consider the differences between the environments:
"On-highway" means hard, flat asphalt or concrete, smooth surfaces, relatively few puncture hazards and relatively high speeds.
"Off-highway" can mean anything from muck to sharp, broken rock - big pieces or gravel - high abrasion, tons of things that can distort and puncture casings and enormous heat from flexing.
How do tires differ for those two applications?
On-highway tires usually wear pretty slowly. Treads can last a long time - even several hundred thousand miles - and the real problem often is irregular wear.
In fact, many on-highway tires are removed from service not because they're worn out, but because of severe irregular wear.
With wear so slow, a relatively shallow tread works fine and may even help reduce irregular wear.
You want a tread compound that wears slowly, but isn't so stiff that it generates a lot of heat or fails to grip. You want excellent traction and handling, plus a smooth ride.
To run cool at high speeds, you need a casing that's fairly stiff, so it doesn't change shape very much as it goes around.
And for an off-highway tire?
Take everything we just said and turn it around. Off-highway tires tend to wear very quickly. The environment may be incredibly hostile, with chunks of rocks or those razor-sharp broken shells called "caliche."
Tires tend to wear so fast that if there is irregular wear, you may not notice it. And when they come off, it's because they're worn out - or have been severely damaged.
So, tire makers can use much thicker treads to add life. And a variety of things to help the tire stand up to abuse.
What helps a tire resist off-road abuse?
Sometimes, it pays to be tough, and sometimes it pays to be soft. There's our first example of balance.
On treads, you want high resistance to cutting, chipping and chunking by using an extremely tough tread compound.
In the casing, however, being tough is like being the proverbial oak tree in the windstorm. Rough, uneven surfaces off-road put enormous stresses on casings.
Make them too stiff, and those hazards, being stronger than rubber, poke their way through. So, make the casing flexible and resilient, and it can actually "wrap" or "envelop" an obstacle, springing back to its correct shape once it has moved on.
What about speed?
Often, vehicles traveling off-road aren't going very fast, if only because they can't. But all that wrenching and twisting of tire treads and casings still generates enormous heat.
So it's probably just as well that off-road speeds tend to be lower than highway speeds.
And if you have to go both places?
That's the trick. The characteristics of an on-highway tire tend to be bad for an off-highway tire, and vice versa.
So, what tire design engineers do is balance characteristics to get toughness and long mileage.
So on/off-highway tires can be used equally in both areas?
Not exactly. It depends on the type of off-road use, but most engineers would recommend that for even as little as 5 to 10 percent off-road use, you should consider on/off-highway radials, like the M850.
And at that, the M850 is intended primarily for vehicles that spend most of their time on the highway, and only some of it off the highway.
What kind of vehicles?
There are lots of trucks that service work sites, bringing supplies and equipment - dump trucks, flat bed haulers and the like. These vehicles spend most of their time going back and forth on pretty good roads.
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