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Tire Wear On Your 4x4 Truck - Willie's Workbench

Posted in How To: Tech Qa on July 1, 2009
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As I write this month, I'm just finishing up a dual winter tire test that we'll publish in the future. We tested the tires on numerous vehicles in order to get different driver opinions, to see how they reacted to a change of vehicle weights, and to accumulate enough mileage on snow- and ice-covered roads to be able to develop an honest opinion of them. This was a fairly long-term test to also establish wear factor (secret preview: these are recaps). But I'll stop here because that testing brought up the subject of how to get the best tire wear possible.

First off, don't spin the tires! While it sounds pretty simple, we don't always really follow that advice. You can actually burn off rubber on glare ice when you spin the tires. Yes indeed, as we've seen the black streaks left behind. Same thing goes with spinning them in the dirt or rocks. And we shouldn't even have to mention pavement. Treading Lightly actually works for your wallet as well as for the environment.

Second is proper air pressure. We're talking about pavement driving here, not low pressure off-highway. But what is proper? First, try the old trick of drawing a line of chalk across the tire's face, and then driving a half mile or so on pavement. If it's worn off more in the center = too much air pressure; worn off on the outside edges = too little air. Not a really true test, but a good starting point. The vehicle's center of gravity as well as cornering loads, such as driving on a very curvy road, will have an effect on tire wear. Plus, there is just the way the tire feels and how the vehicle handles to factor in.

While we are on the subject of proper tire pressure, there is another thing to consider, and that is tire capacity. I'm sure that that you have noticed that right on the sidewall of a tire (let's use a 33x12.50-15 for an example), it will have embossed something like "load range C, maximum capacity 2,245 lb @ 35 psi". Now, this is the maximum load/air pressure that the tire can carry, not the recommended air pressure. Your friendly tire dealer has a chart that will show you just what the load capacity is at a given pressure. Again, using this example, it would have the capacity of 1,755 pounds at 25 psi. So you can see that vehicle weight when fully loaded versus empty can play an important factor in determining proper air pressure.

Another factor is rim width. The Tire and Rim Association lists recommended rim widths for particular tire sizes. For instance, a 33x12.50 can fit on an 8.5- to an 11-inch rim. Lots of times, this tire will end up on a rim only 8 inches wide. A narrow rim causes the tire to "crown" across the face of the tread somewhat, which will lead to, naturally, more center wear. It will also cause the sidewall to tuck in and flex more, inducing some tire roll during hard cornering under minimum street pressures. A wider rim will place the tire's sidewall more vertically, resulting in a stiffer ride from less sidewall flex and a bit more stability at minimum street pressure, with possibly more wear on the outside edges.

Alignment is also very important. Toe-in, caster angle, camber angle, and wheel scrub radius all are important aspects of tire wear. Improper toe-in, and to some extent, camber angle can show up with the chalk test. More chalk worn off on the one edge of the tires-let's say on the street-side edge of each tire-indicates too much toe-in. This is especially true if the tread edges have a "feathered" look as you run your hand over the tread face. (It doesn't take much of a bend in a tie rod to change toe-in angle, either.) Smooth wear on either the inner or outer edges will usually indicate an improper camber angle. In other words, the tires are not making contact with the road at a true right angle. Most likely, the cause on a solid-axle vehicle is a bent housing, or on an IFS vehicle it would be worn ball joints or A-arm bushings.

Excessive caster: Caster is the angle formed from vertical of an imaginary line that runs between the upper and lower ball joints as viewed from the side. Caster is what causes the wheels to go straight down the road and return to center when turning a corner. Excessive caster will cause the wheels to "lay over" at an angle during a turn, causing more wear to the sides of the tire's face.

Cupping: Aggressive off-pavement treads have more of a chance to "cup"-that is, to make uneven high and low spots across the face of the tread-than, say, a performance street tread. Tread design plays a major role here, and the more aggressive the tire is, the more cupping may take place. The fault is usually the lack of sufficient tire control by the shock absorbers. The more aggressive the tread is, the more it wants to skip and hop across the road surface. Tire balance, and again worn suspension components-including loose wheel bearings-will contribute to this.

I know of other editors of 4x4 magazines who actually state that they never bother to balance tires. Well, tire balancing is important. OK, so you say the weights get knocked off on the trail. Well, maybe so, but if they are missing, they usually leave a mark where they were attached, so it's a simple matter of going to the tire shop and having them pound on a new weight of the same size in that location, making it not necessary to remove and rebalance the tire again. Once a tire starts to cup badly, it's almost impossible to even the tread out. The solution here is to rotate the tires to even out the wear pattern about every 5,000 miles. This will go a long way in extending the life of your tires, not only in cupping wear but in wear caused by slight differences in alignment.

Oh, and finally, invest in a tread depth gauge. They are only a couple of bucks. Measure across the tread face and record the readings in your notebook (you do keep a notebook on your vehicle listing repairs and such, right?). These measurements will give you a pretty accurate idea of any unusual wear and a good starting point as to what mechanical components need to be looked into.

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