So I've had a set of cooper STs? on my truck (91 ext cab) for about 6-9k miles. They are 255/85/16's. I balanced them like I have the last 3 sets of tires, with balancing beads/bb's. Everything was fine until the last time I rotated them. I did the standard radial rotation going from rear to front and then crossing the fronts when I put them on the rear. When I did it this time I developed a vibration at about 65mph. It feels like it is coming from the rear of the truck, it's not really in the steering wheel. It can be a pretty significant vibration but it doesn't show up much below or above 65mph. It also doesn't always show up. I think on long curves at 65 I can feel it come on and get worse and then slowly fade out and get better, I don't know if this is 2 slightly out of round tires lining up in phase or what. I did not have this problem before the rotation. I have since replaced the lower BJ (the uppers were replaced less than a year ago when I did BJ spacers), and all the tie rod ends. These parts probably needed replaced (the inner tie rods had a little play, the outers and lower BJs still felt pretty tight). This appeared to have the effect of softening the vibration but it is still there.
So has anyone ever had tires go out of round that weren't like that initially?

 

Could be the tires or could be a bent rear axle shaft or two. You can put the rear axle up on jack stands, block the front tires and let the rear wheels spin while holding a pointer up to them to check for runout. If you see the surface of the tires moving up and down as they turn, pull them off and see if the same happens at the brake drums (or not). If the drums turn smooth and the tires do not, it is an out of round tire, otherwise it may be the axle shafts.

And yes, either out of round tires or bent rear axle shafts can cause the varying wobble on long curves as the two rear wheel wobbles go in and out of phase. I've had this exact problem with both out of round tires (bias ply Swampers) and bent rear axle shafts.

 

Thanks Crawler,
I talked with a tire guy today and I should mention that these are blemished tires I've got. Anyhow this guy said they used to mount BFG T/A blemishes on some 2wd trucks and after about 8-10k they started getting a wobble in them, but before that they had run nice. This guy also mentioned that the tread part may be out of true causing a side to side wobble...
I'm going to jack it up and check. I haven't done anything that should bend an axle that I know of, how hard is that to do?

 

Doesn't take a lot in my experience. I have 6 rear axle shafts in the back yard, all bent to some degree or another.

 

Also check tire wear for feathering/chopping/cupping/etc. Usually they will start to chop the edges first, and many times you'll never notice a vibration going straight, but on longer curves at speed, when you're loading up the outside edge of that tire and hitting the part of the tread that is chopped, it can cause a vibration.

If you're not sure what this is, do a google search and you'll see plenty of pictures and explainations.