I am at my wits end. I have had the tires on my truck changed 3 times now and each time I get a vibration or bounce at 40-50 mph. I originally thought my first set of tires 315/75/16 were too big so I changed those over to 285/75/16's both sets were the big o brand XT. I took my truck to tranny shop, ailignment shop, and dealarship guess what all of them said "its the tires".So I had them change the tires to a set of big o AT's 285/75/16's less agressive tread. Much smoother but still bouncing around at 45.

Took the truck to a different tire shop ( with a really nice computer balancer) they said the tires would not balance and to take them back to where I bought them. So I had the original shop change them to BFG's 285/75/16's (the shop has been good about swaping them out...only spent about 100 dollars to upgrade to the BFG's and I am not the original owner)guess what poor ride and bounce at 45mph. IT AINT THE TIRES!!

Sorry for the long post but Im going kinda mental here

bubba

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98 F250 LD 5.4 dual exhaust, K&N, Big tires

 

The symptoms you describe definitely sound like some sort of balance issue since the problem only occurs at a certain range. Have any of the shops tested the rims? As in spin them on the balancer w/o the tires on to look for wobble. Often times warped wheels can make a tire seem out of round. Just a suggestion since nothing else seems to have worked.

 

I'm sorry to be on the fringe of usefulness, but I chased the exact same problem the same way. No one noticed that I had used that 'Fix-a-flat' stuff (two cans on the same tire in the dark) in an emergency. Got home and forgot about it. After a while in the heat, it turns water-like, and runs fine except at a certain speed, where it bounces the wheel badly enough so you think the suspension's coming apart. I doubt very much if you've used that approach twice with 2 sets of tires, but that's all I can come up with. Good Luck.

 

Have you tried adjusting the tire pressure? Just a suggestion!!!!

 

Have your tire shop rotate the tire 180 degrees on the wheel and then re-balance. I had the same problem once and this proved to be the needed fix.

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1999 F150 4x4, wedgewood blue, 5.4l auto, Manick brush guard, Duraliner, KC Hilites, surf rod holders, tinted windows, K&N air filter without airbox, Velocity air intake tube, Midas Performance muffler with dual pipes, toolbox, Uniden CB with whip, and a Superchip.

 

Just thinking out loud here. Don't know much about wheels and tires, but have a couple of questions. Did anybody check to see if the rims are round and/or not warped? Are the rims mounted properly on the truck? I never changed the tires myself and not sure if the '98 is the same as my old '97 was, but the '97 had a rubber O-ring on the hub that centered the rim on the hub. If the O-ring is missing or damaged, I hear it could cause vibration problems.

Is this a vibration or a bounce? Does it happen continuously (as in a vibration that lasts so long as you are driving between 40 and 50 mph) or just briefly? Some trucks had a problem with the rear bouncing when going over joints and bumps in the highways at certain speeds. Ford can install a damper under the bed that eliminates this bounce. I don't really have this problem unless I have a certain weight load (not much at all) positioned close to the rear tailgate. If I move it forward a bit, it disappears. When it happens, it feels like a bouncing motion that dissipates over a second or two until I hit the next bump/joint. Over certain roads, depending on the intervals between joints, it could feel like a vibration.

 

bubba,
Have your tires checked with a Hunter GSP 9700. It checks the roundness of the tire & wheel & shows the tech how to match the high spot of the wheel with the low spot of the tire, or if either is so out of round that it needs to be replaced. Ask the local Hunter service rep who the best is at using it. You can find the local rep @ www.hunter.com