R MACK BOUNCE

[sdt]

I have a 1978 R Mack with a 20' grain box, after I load it and driving 40mph to 45mph it has a very bad front end bounce until you hit about 50mph and it smoothes out. The truck unloaded drives very smooth.This is my first big truck . I have looked at all suspension and can not visually see anything badly worn, the rear tandems track in-line and had front checked at a tire shop. When I first got the truck it had 10.00x20s on back I thought that was the problem so I put 8 new 11r 22.5 radial retreads on the back(Dayton wheels) and still same bounce.The front has 22.5 radials .I can be driving down the road about 55mph and take it out of gear and let it coast down to 45mph and it will bounce . I have jacked up the truck and the only tire out of round is less than 1/8 ",it is the front,and very min side to side.Thanks for anyone's help!!!!

[rsb502]

I may not be reading that right but do you have a front tire out of round? Even 1/8" is enough to bounce and without balancing and good shocks it could be a pretty nasty bounce, I have Equal balancing powder in all my tires and as long as they are at the right pressure they run smooth but if big tires are under inflated they will bounce like a ball, make sure your tires are within 5 psi of the sidewall rating on the steer and 10 psi on the drive, if they are recaps I would run them 10 psi under the sidewall till its over 90 degrees then I would drop them to 15 psi under the sidewall marking. I hate to say that any out of round tire can cause a bounce especially on dayton wheels, make sure the wheels are straight, jack the wheel up just a little off the ground then set a hammer or block or whatever up next to the tire and spin it to see if it gets closer or farther from it, if so the lugs need to get backed off and re torqued in sequence to straighten the wheel. Make sure your shocks are good they wont control those small bounces if all the fluid has leaked out and they arent expensive so I normally swap em every couple years even if they arent leaking, its worth it for the ride. If your steer tires are not balanced got o a tire shop and get them done or have tire balancer like equal added to the tires it helps A LOT.