Tire bump

Just put on a new set of Goodyear tires on my 94 ford trk. At
30 to 40 MPH I get a small bump in tires. Took back to dealer
they re-balanced tires and drove it. Said I might have a drive
line problem. Called a Transmission shop, ask about driveshaft
and they say if it were out of balance it would vibrate all time.
I can take my hands and shake both back tires and I feel some
slack. Don't have this in front. Was wondering if I could have
something possibly loose or maybe bad bearings. No noise, and
no broken springs. Transmission is overdrive, and it bumps in
or out of overdrive and also in newtrel. Any suggestions would
be appreciated..

You shouldn't be able to move either rear wheels. Maybe axel bearings. I'd pull the wheels and see whats moveing and go from there

If it wasn't doing it before the new tires I would look at the tires a little closer. Could be something other than balance. If you can find another set of tires/wheels to replace them & the problem goes away................that would tell you for sure.

Quote:

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Originally Posted by rdobbs

I can take my hands and shake both back tires and I feel some
slack. Don't have this in front. Was wondering if I could have
something possibly loose or maybe bad bearings.

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What do you mean by "...take my hands and shake both back tires..."? Is the "slack" in & out, pushing & pulling away from the differential or do you mean pulling left, pushing right and reversing that motion? Pro70z28's idea of a borrowing a set of tires from another truck is a good one, and cannot be argued by the tire store if the "bump" comes back with the new tires. Another thought is to put a dial indicator on them, check the runout of the tires, the edge of the rims, and then the axles/rotors. How about your u-joints? They sometimes shake at a harmonic until they get really bad.

Is this bump happening when you drive over a pot hole or bump in the road or do you think it is in relationship to tire rotation. How old are your shocks. Check the shock mounts. What is the tire wall rating (ply) vs what you had on the truck. Do you have a spare? Use it a tire at a time if you cant find a loaner set.

Don't know what size Ford you have, but many later F250/350 4x4 models have a rear axle bump/vibration/etc at 40 to 50 mph. I had a minor one with the original Continentals at 48-50mph on my F350. With the new Kumho's it's even worse and they have been balanced twice on different machines. The vibration is at 49-52 now - whoopee, it moved up a mph. Supposedly a Hunter Road Force balance machine will fix it, but not always plus it's $25-40 a tire if you can even find a place to do them. Go to Ford Truck Enthusiasts Forums - Ford truck and SUV owners and enthusiasts Community And Information Source. and plug it in and you should have many search hits. Welcome to the world of fanny massaging via FoMoCo:whacked:

Trk is f150 2wd. Old tires were also out of balance. New tires are 15"
P235 Goodyear Wranglers 2 ply. The motion is back and forth from drive
shaft. Sounds like axle problems. Dont know how old shocks are. Trk has
103,000 miles..Thanks for all the good suggestions.

you should take it to a shop that has a balancer that measures "road Force" of the tires. Knowing what that measurement is will tell you wether it's the tires or not. Any thing over about 30lbs of road force will cause a vibration and the tire will need to be replaced