Vibration through floorboard at 40-43 and 55 to 60

[DooSPX]

1999 C1500 Suburban, stock suspension. New front and rear brakes. New U-joints front and rear, new axle bearings and seals. Still have a vibration that you do not feel in the steering wheel, do not feel it if you accelerate quickly through those speeds, or slowing down through those speeds. Basically you only feel it when you accelerate slowly or hold those speeds constantly. Take your foot of the gas and you don't feel it. What could it be? It actually feels like going over rumble strips. Transmission problem? Rear Diff?

Please help!!

 

[Nad_Yvalhosert]

High speed vibration, 45mph and faster, is commonly a tire balance issue. Seeing as you just replaced the u-joints, I'd suspect you bent the yoke, or knocked off a driveshaft balance weight.
Try this. Lift the rear off the ground, remove the rear wheels. Put a couple lugnuts back on to keep the drums on, and start the engine. Run 'er up to those speeds and see if the vibration is still there.
If yes, driveshaft is the problem.
If no, wheel/tire is the problem.

 

[Nad_Yvalhosert]

Also if yes, take off the drums and retest. Just dont step in the brake pedal...

 

[DooSPX]

Thank you! I will try that!

 

[DooSPX]

Got under there to check the driveshaft weights are still on the driveshaft, and I noticed the trans mount is just about flat. Probably original at 212k. Would a trans mount being bad cause the harmonics/vibration at certain speeds?

 

[Nad_Yvalhosert]

If the mount is to blame, you would have had the vibration before you changed the u-joints and brakes.
Did you?

 

[DooSPX]

If the mount is to blame, you would have had the vibration before you changed the u-joints and brakes.
Did you?

Yes, it has had the Vibration before we changed the u-joints.

 

[SUBURBAN5]

Also check engine mounts

 

[JSlezak83]

I had a similar issue, and it turns out when I went to have the tires balanced, the wheel was bent. It would cause an oscillating vibration at sustained speeds of about 40 or higher. I think we added like 4oz of weight and it only at best masked the issue. I need to swap tires with my spare.