Does anybody know what causes tire cupping? I have a '96 Jeep Cherokee with Goodyear Wrangler Radials that have cupped and I am hoping to stop the tires on my new F150 from doing the same.

 

If only the search was working, I had an extensive reply to the same question previously. Np, I'll give you the abridged version...
Cupping is usually caused by two things. This is either a lack of rotation (the leading cause) or bad shocks. If tires aren't rotated ever 5-7500 miles, all sorts of stuff starts to go wrong, including the cupping. What happens is, as a tire rolls forward, the vechile (in pushing the car forward) is trying to "pull" the tire upward as it rotates around. Kinda hard to explain but it has to do with the tread blocks. As a single block rotates around, the leading edge is essentially "pulling" the trailing edge of the block off of the ground as it's contact with the pavement ends. This causes a kind of scraping that imposes additional wear to that specific portion of the tread block. Now look at your tires. As you look at the top of the tire, on the drivers side of the vehicle, the right side of each block should be lower than the front edge of the next block. True? If so, Cross-rotate your tires and it'll fix itself after a couple thousand miles (assuming it's not too bad). If this is not the case and the wear is more irregular, it's possibly the shocks...
Unfortunately, I'm just a tire buster, and not a mechanic. With the shocks, it's usually older vechicles (whose shocks are worn) that have the problems. it causes the same kind of cupping but usually the pattern is over larger areas. With your jeep, this is possible (depending on it's age and your driving habits) but with the truck, it's highly unlikely. If you have the Goodyear RTS or At or MT, these tires are just really prone to cupping, try to rotate ever 3-5k miles. The RTS is especially bad, and many people on this forum have stated such.
Well this turned out to be long again, once I get talking about tires, it's like selling a house, time to close escrow!
Hope this helps

 

I have kept the Jeep rotated every 5000 miles so that must not be it. I didn't really think the shocks were bad until after driving my new truck. The F150 feels really tight compared to the Jeep. My next question is, is it safe to cross rotate tires now? I have had one Firestone shop tell me that they won't rotate tires to the opposite side of the vehicle.

 

There is no problem cross rotating tires, it's even in the manual. With older radial tires, you were unable to cross them, but it's no problem.

 

I would bet on the shocks being bad. I sold tires for 5 years at Western Auto and that was usually the culprit.
Take one of the shocks off and see if it has any life left in it.

 

Worn suspension components, such as ball joints, springs, shocks. I had to replace my lower ball joints recently because of cupping.