Okay, so a couple weeks ago I stomped my gas pedal getting onto the highway. About 20 feet into it I hit a dip in the road and heard a loud "crunch" sound and from that moment the car vibrated at highway speed. Before it tingled a bit at 70, but now it's more like a low-level shaking. I drove home thinking I had destroyed one of my new whitewalls but when I got home everything looked fine. I jacked up the car and it turned out that my rear u-joint hit the floor pan. (I just lowered the rear 3/4" and didn't realize the front pinion was close enough to smack metal.) So I thought I'd ruined my rear u-joint. The drive shaft itself did not hit anything and there are no marks on it anywhere except at the very ends were the rear u-joint mounts. I can't imagine that it bent because it's a big, heavy duty member out of a '78 Impala station wagon. The hit wasn't even that hard - it just hit sheet metal and did a fine job of clearancing that area by wiping away the metal in the few places where the contact was made. I replaced the rear u-joint it a week or so ago and hammered the floor pan for a little more clearance and thought that would cure it. The vibration is still there. Any thoughts?
"...my rear u-joint hit the floor pan. (I just lowered the rear 3/4" and didn't realize the front pinion was close enough to smack metal.) So I thought I'd ruined my rear u-joint. The drive shaft itself did not hit anything..." Get the driveshaft checked for straightness and balance....
I only replaced the rear u-joint because it wasn't worn out, just (I thought) damaged, but I suppose I can pull the shaft, get it checked, and replace the front joint all at the same time.
Time to cut that floor up, mine only hit if I had all the gear loaded up, full cooler, chairs, pop up tent. but even then it was a floor brace kissing the driveshaft, about 4" ahead of the pinion. If you're lucky, it just knocked a weight off. Inland Truck will balance it pretty cheap.
Good chance it bent the ears on the driveline that hold the U joint. Need to have the driveline checked for straightness and balance.
_________________________________________________________ Right there, don't assume that it isn't bent just because it doesn't have a big ding or bow in it.
The wisdom of the HAMB has spoken! This winter I will pull that driveshaft and get it checked. I've been thinking of having one made anyway because this one is really too big and heavy for my car. (The wagon weighed something like 4,500 lbs and this driveshaft is the same diameter as the one on my '85 C-10 with the heavy tow package.) Thanks, guys!
Yeah, I guess a 3" instead of 4" OD (for argument sake) would be the difference in flange clearance between cars.
Check the rear end pinion yoke. There are small tabs on the ends of the area that hold the cups. A blow to the U-joint can shear one of these tabs off which will let the joint slide side to side under load causing vibration.
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