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Rear end bounce Ford F150

I am having a bounce in the back of my old pickup. One shop rebuild the drive shaft u-joints. It did not help. Shaft was taken to machine shop, where balance was checked. It was fine. They said they did not know anything else to do.



I took to a second shop where they kept for a year working on in spare time. They put in used drive shaft, moved transmission around, checked motor mounts. Nothing has changed the symptom. Used rims were installed. Tires balanced twice. They didn’t seem to think axles were bent. This is a long wheel based F150 with hanger brg. It has 315000 miles on it, so don’t want to spend a fortune. It is ok until 50 to 75MPH. then vibration bounce etc. set in.



Any ideas?

Has anyone checked your rear suspension? Leafs springs can break but need close inspection to find them if in the bundle. How are your shocks? Does it bounce more loaded or unloaded?

I have not run it loaded with this problem. I told them to check out everything since drive shaft was not solving anything. I don’t know if they actually checked springs or not. You can’t see any obvious damage to springs, but I will look again.

Thanks

My first guess would be bad shock(s). How old are they?

Several years old. Shop thought they were ok.
Thanks

I’d try two things.

I’d put the wheels on a macine that does “road force balancing”. That’s a process whereby the wheels with tires are spun with a simulated road force applied via a spinning drum pressed against the tread. That can detect internal faults and anomolies in the tires that normal balancing cannot.

And I’d change the shocks. Normally I don’t advocate just changing psrts without being able to verify that they’re bad, but shocks are inexpensive and easy to change on this vehicle, so it’s a good thing to try.

I assume they checked all the bushings over well.

I am not sure they checked the bushings. I will do something about the shocks.

Thanks

Will check and see if I can find a shop that does road force balancing. I don’t think there are many in this area.