Truck Bounce

Make sure all spring hangers are tight on the frame, i went to do a test drive for a company, checked the truck out, hit the right front driver and the whole axle moved, the whole suspension hanger was loose on the frame letting it walk back and forth, just an idea and my 2 cents.

 

have you tried taking it out of gear and coasting to see what it does

 

check the u joints from the fire wall to the steering box just had that problem fixed on a t2000 have also had though symptoms with an out of aligned tandem axle

 

... have you tried to do front and rear alignment , checked pinion angles

 

Had the exact problem with mine , spend 3 months replacing parts, going to different shops , then one day bang it blow.
Replaced the diff. , the bounce went away.

 

Let us know what they find....

 

From my experience whenever I had to diagnose a bounce in a truck at a certain speed, (45 mph), on the throttle or coasting it was tires, tires, tires, everytime! If you can feel a shimmy in the steering wheel, it is with the steers. If you can feel it in your seat, it is in the drives. I know you mentioned they have been balanced and the truck has been up on jacks and run in the shop. Tire balance is really one of those things where you have to have a good tech with patience and good attention to detail. If you set up your equipment wrong or sloppy the tire could be out by a mile and the machine will show good! Also running a truck in the shop at highway speeds while on jack stands may not cause the bounce to show up because the tires are off the gound. Most bounces are causes due to a miss mount between the rim and tire making the tire out-of-round relative to the hub. This difference may be hard to see by the naked eye. Running the wheels slower will help. It doesn't take much, only 1/8" run-out causes major bounce applified from out-of-balance and centrifical force. To completely rule out tires, try to swap out the entire set of drives with another truck and go for a road test. Hope this helps.

 

Take the truck to a tire shop that trues tires. They will cut some rubber off, but the tires and wheels will now be matched round to the hubs.

I have done this on the last three trucks I have owned to keep bouncing away. You having a spring suspension, which will transmit bounce from flat spotted tires directly to the cab.

If the brakes have ever been locked up, you can guarantee you have flat spots. They are hard to see until checked with the wheel off the ground, and a guide bar against the tread as the wheel is slowly turned.