The Stovebolt Site Forums The Parking Lot Big Bolts 48 1.5 ton bad vibrations and bucking body over 30mph

Hello. I have a 48 1.5 ton flat bed, all original with single speed rear axle and 4 speed gearbox, that vibrates/shakes/bucks when driving up towards around 30mph. Along with that there seems to be a bad bearings type sound that I only hear when driving in 4th gear. Any advice on where to start would be great. Thank you.

Does it have a one or two piece driveshaft? If it's a 2 piece, check out the carrier bearing and universal joint where the two shafts join. Also try jacking up one wheel at a time and run 4th. gear at 15 MPH or so on the speedometer while someome listens for noise at the hubs. Don't exceed about 20 MPH on the speedo, as the spider gears will spin the hub at double the speedometer reading.
Jerry

I second the driveshaft investigation, mine has three shafts with two midship bearings. I would then suspect out of round or out of balance tire/wheel. The wife and I will be going right by you in Kalamazoo Friday morning, headed west to visit our son.

While checking the driveshaft, make sure all the U-joints are in the same phase (the cross between the yokes should point in the same direction as the rest of them on the driveline). If the shaft was ever off for some kind of maintenance it is possible the slip yoke was put back on while rotated a tooth or two over on the splines getting the joints out of phase. This can present as noise and vibration at higher speeds but seem to have little or none at slow speed.

It is a two piece drive shaft which i doubt has ever been removed (under 5k original miles). I am putting new rubber up front soon. Sounds like I should start with the carrier bearing for the noise and new rubber and see if that makes any difference. I would like to do a rear axle swap to get a better gear ratio and easier to replace wheels/tires which I assume will require a new rear driveshaft too. The truck was infrequently used and sat for many years. I have only redone the starter, ignition system and diff/trans/engine fluids. Starts easy and runs great but shakes up and down once you pick up momentum and the bearing noise only starts when you shift into 4th regardless of speed (it seems)

Old bias ply tires also develop flat spots from sitting unused for a long time. Until they warm up and work the kinks out, the truck will shake like it's got square wheels. Back in the dark ages we called it "Nylon Thump". I had a 1959 3800 dually with 18" bias ply tires that would rattle teeth for 10 miles before the tires warmed up and got round!
Jerry

Tire tread separation can also cause the situation you describe.

took the truck out today and the bearing noise seems to be coming either in front of me or right under me as I am driving in 4th gear

Driveline and gear noises will transmit up and down the length of the truck and appear to be coming from places they aren't. Put the rear end up on some heavy duty jack stands, chock the front wheels securely, and roll under with a stethoscope or a broomstick to check out possible noise sources. 4th. gear might eliminate noises from inside the transmission, as there's no gear reduction going on other than the rear end, or it might just be speed-related.
Jerry