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RV Tire Cupping and Shocks

An RV Life and Travel reader asked....
(Problems with Cupped Tires and Bad Shock Absorbers)

My front tires are cupping; they are mud and snow. What are the best shocks for my 1982 Toyota 1/2 ton camper? One person says I need new shocks. Mine are gas and only 20,000 miles old. Another person, a tire man, says his heavy lugged tires just cup -- it is the way it is. Any more info?

Bob, the RVing editor's husband replies:

This is going to be hard to diagnose without seeing the actual tires and vehicle. Even actually seeing the cupped tires, these kinds of things are not always cut and dried, like some people think. Tires, loading, alignment, weight distribution, shocks, and other factors affect abnormal tire wear.

I'm guessing the tires on your Toyota are 14 inch. If that is true, you probably have very few choices for tires that are rated for the load you are carrying. To get a heavy enough tire, an aggressive lug design was maybe the only style available.

The tire man was somewhat right in saying that these tires will cup when used on the steering axle of a vehicle. An experienced, knowledgeable alignment person may be able to align your steering axle to minimize some of this cupping. Be warned, this person has to know what he is doing! If he isn’t fully knowledgeable in the mechanics of alignment, he can mess up your vehicle if he tries to experiment, and also make it downright dangerous to drive.

Cupping may also occur if your vehicle is loaded improperly, too heavy toward the front, or your front springs are week. Overloading is easy to do with an RV, and you should have your RV weighed by axle, or better yet, by individual wheel. This way you can be sure you aren't over the GVWR and that the
load is balanced.

Bad shocks may affect cupping, especially if only one is bad. Shocks are specified to handle a certain load range. An RV would be considerably heavier than the same vehicle with a pickup box. If shocks for a stock pickup were put on an RV, they wouldn’t properly do their job even though they were a perfect fit. Although shocks are usually warrantied by mileage, they might be worn out after only 20,000 miles.

If the only tire you can get to properly fit your application is a lugged tire, and you get a set of four or six at the same time, rotation or X-ing the tires will distribute a little cupping to each tire as they wear. I believe the more cupping a tire has, the faster more cupping will occur.

Many years ago I needed 14" tires with a high load rating and finally found a Michelin, commercial rated tire. It was expensive, but it was a very good tire, and we put a whole lot of trouble free miles on them.

Don't forget proper tire inflation. The tire makers have a chart on every tire they make. It shows how much pressure to put in the tires at what load. To do this you need to weigh your vehicle, loaded as you would have it running down the road, and a copy of the chart. I’ve been told these charts exist. They are hard to find sometimes, and tire dealers will tell you they don’t have them, just put in what it says on the sidewall. I believe they do exist, and had one for some tires on my class C.

Like always, these are my thoughts, opinions, and experiences. I hope they help you.

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