Rear Wheel Hop

[blueknight0517@yahoo.com]

A friend of mine has a 1981 Mack R600 twin screw tractor. I belive that it is a Mack camel back. He has replaced the bushings, New Dog bones, New tires and rims and replaced the rubbers at the spring ends. When he approaches 42 MPH the rear suspension develops a wheel hop. It feels like the wheels are jumping forward. Above 42 the problems go away. However when he hits 62 MPH, the rear wheels start to hop again but doesn't set in hard because the govenor holds the truck at 65MPH. I belive it is a harmonic problem but all the things; that spin have been addressed. The drive shafts have new universals and the shafts have been correctly timed. (In sinc) The new wheels and tires have not been balanced. Could this be the problem? He says the problem is reduced when he puts the low boy trailer on. Any suggestions are greatly appreciated.

Jim

[Lmackattack]

tires are unsprung weight. think of them like a basket ball. problem is we have 8 of them out back all bouncing. try this to see if it helps. First reduce the air press in all 8 down to about 20 psi then go bobtail around. if the bounce went away. it could be a tire balance issue. have them all ballanced. Most people dont balance drive tires but it can make a diffrence. I know it does on a car so it has to on a truck. other issue is that the springs really dont move much and they pivot at one point on the suspention. If you dont have shocks to help control tire bounce you will feel it. If the vibration is bad with a load on the truck it can be bad motor mounts, or drive train issues. I have even heard a bad power divider can cause bad vibrations ?

[speedy]

A badly out of round tire? Are the wheels unimount or daytons? Could be they are wobbling too.

If you suspect balance, maybe rotate the outside tires 1/3 - 1/2 of a turn (or thereabouts) one at a time and test drive in between.

Would axle alignment have anything to do with this?

[blueknight0517@yahoo.com]

Thanks for the information. You guys have given me some ideas to pursue. I'll let you know what sovled the problem. Thanks again. Jim

[blueknight0517@yahoo.com]

Lmackattack Thanks for the information. He put a bag of Equal balance sand in each tire. The truck now drives like a Caddy at 42MPH and is steady as a rock at 65MPH. thanks again.

[Speed]

I was gonna suggest maybe a driveline out of phase;glad you got it sorted out without much time or expense!

Speed

[Lmackattack]

glad its fixed!

[Rob]

I'm a believer in the tire balance powder and run it in all steer tires at a minimum. I've heard the "Centramatics" work well also, but I've no experience with them as most of my medium and heavy carriers had Dayton hubs. The two midliner Macks used the "Equal" and yielded excellent tire life.

Rob