Severe rear end bounce at 30 MPH?

[Paul Rattenbury]

Hello, I have a 1977 R685ST tandem dump. I just bought it, got all the paperwork done, drove 2.5 hours to pick it up, left where I bought it from, got up to about 30 MPH, and I was pretty sure something was about to come out from under the rear of the truck it bounced so hard. I stopped, looked around, again didn't see anything loose or bent or broken. So I got back in the truck, started off again, less than 30 MPH it might have shook a little but it was tolerable. Got back to 30 and the truck shook like the end of the world. I decided to see if I could go faster and make it go away, and it did. So luckily my whole ride home was mostly highway and at 50MPH things weren't too bad. I do know the truck had done a lot of sitting before I bought it. Like a couple years' worth. Could this be a flat spotted tire or 8? They are 10.00R20. Could the wheels be that badly out of balance? They are Dayton wheels. Is there a way to balance these? Anyways, thanks for reading. Any advice would be appreciated. 

[tjc transport]

it very well could have flat spots. 

 

[Steve L]

Are they radial or nylon tyres. Nylon tyres always do that after standing just overnight let alone a few months or years. It should even out after the tyres warm up. If they are old tyres and standing quite awhile flat spots may become permanent.

[Paul Rattenbury]

They are radial tires. 10.00R20.

Thanks for replying!

[tjc transport]

it took close to 150 miles for the flat spots on the radials on my pickup to go away after the truck sat for 4 years. 

[steve s.]

It would help to know what type of suspension your truck has, and what kind of road surface you were driving on. An unloaded Hendrickson walking beam on a concrete road will give the ride you describe. It's hard to imagine that tires with flat spots would perform any better at a higher speed. Good luck.

[MACKS]

59 minutes ago, steve s. said:

It would help to know what type of suspension your truck has, and what kind of road surface you were driving on. An unloaded Hendrickson walking beam on a concrete road will give the ride you describe. It's hard to imagine that tires with flat spots would perform any better at a higher speed. Good luck.

Or 58 rears,believe me I know..lol

[doubleclutchinweasel]

And, of course, improperly mounted Daytons can have BAD wobble to them.  That wobble CAN have better or worse bounce at certain speeds.  They can also go in and out of phase with each other as they get rotated to different positions relative to each other.

The '67 I used to drive would smooth out, start bumping a little, get progressively worse, then shake like the world was coming apart...then smooth out again.  It would repeat this cycle over and over.  Was particularly bad on concrete roads with expansion joints.  I took the time to true-up all the rims on the spiders, and it acted like a whole different truck.  Very smooth.  Well, as smooth as the 55K rears would let it be!

 

Somebody here will undoubtedly be able to steer you in a good direction.