78 R Mack bounce at 45mph

[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.I have also replaced front shocks Thanks for anyone's help!!!!

[blackdog2]

Show me a mack that dosent unless its air ride

[72U685ST]

My u-model hops like a bastard at 40mph unless its pulling a load. They all do it

[2stacksuperdog]

Yep its a mack thing.

[sdt]

I would understand better if it did it empty but the bounce gets worse as load weight is increased

[blackdog2]

Check drive shaft yokes are inline.

[sdt]

I was woundering about the driveshaft the frame was lengthened for the 20' bed any guides for checking the drive shaft was installed correctly ? I did not use balance beeds in tires should I. The drive shaft was built by a machine shop that builds drive lines so I think it was made correctly

[thomastractorsvc]

sounds like out of balance steer tires, thats where I would start look for mud or dirt packed in the front hubs/spokes

how bad can you feel it in the steering wheel? you probably have the bounce unloaded but dont notice it

[sdt]

I can barley feel anything empty I have tried a different set of front tires same bounce, I will look for mud in the spokes I would have thought that something out of balance the bounce would get better as I loaded the truck,I don't know what to think any more . I can haul 750 bushels of corn or 42,000lb. I cant really say I feel it in the stearing wheel as much as it feels like the cab of the truck is bouncing like front tires are way out of round,

[redrider]

My bet is the steer tires. I had the same issue with my Granite right out of the showroom, although my steers are 425's. At 50mph loaded it shook so bad it almost literally made me throw up...empty it was barely noticeable. No shake in the steering, no shake in the driveline, just a bouncing airride cab and hood. Dealer checked the driveline and found nothing, tire shop checked the tires and found nothing. I went through 3 sets of Goodyears and one set of Dunlops, same problem. Broke out the wallet and went with Michelins, problem solved. That was 400,000 miles ago and the truck has never had an alignment since.

Edited May 15, 2013 by redrider

[wastenafarmtrucking]

my 85 Rd did the same thing at the same speeds. I replaced the springs, spring hangers, king pins, tie rod ends, changed the fronts from daytons to hub pilot, re-bushed the camel back suspension, changed all of the rear torque rods, changed the front tires twice, spun balanced the steers, put on new drums, wheel bearings, bought those centering washers and put 3 different sets of front wheels on it.

know what I had when all of the above was done?

A rebuilt truck with a god damned bounce that would shake the fillings out of your teeth from 43-53 mph!

the only thing that I hadn't tackled, and I knew there was a problem, was the drive shaft. I knew it had 2 bad yokes on the shaft, I had replaced all of the bolt on yokes. I was going to pull the whole shaft out, replace the yokes and have the whole thing balanced.

it was stolen on Feb 6th. I hope that the S.O.B that took it to NYC pissed blood for a week!!!

[macattack]

i have much the same problem on my mr688s. around 40-45 mph. starting to think i either need an alignment or maybe replace my front steers with a higher quality tire. i have hercules 425 65r 22.5 fronts. maybe i should try a goodyear or michelin?

did alot of work to try and solve this. replaced springs, pads, bushings, torque and transverse rods, tie rods hubs etc....... all new tires fronts and rears. all with balance beads.

dont know if its common with the mr models as well. as stated before by other poster . no shake in the steering wheel at all. i can remove my hands and she will drive straight. just a wobble feel at 40-45 then goes away. does the same coasting.

did notice that i have only one spring wedge on the driver side front and none on the pass side front. maybe an alignment would be my best bet,

what do you guys think??