Vibration

[Sharp99]

I have run out of ideas and need help. Replaced the stock gears with 3.73 (Ford Motosports) in my 1999 Cobra (46K miles) and immediately noticed a bad vibration above 70 mph (through the seat). Re-indexed the driveshaft and this reduced vibration, but still noticable. Replaced the driveshaft with aluminum unit and no difference. Switched to a new set of summer wheels and tires (Enkei/Falken 615) and vibration was reduced fair amount. Even starnger, the vibration moved from 70 to 80 mph. Had shop put car on lift, ran speedo up to 80 mph and inspected driveline- differential felt smooth, tires and driveshaft looked to be true, but could feel the vibration on car body. Bearings felt solid with no play in any of the hubs. Another clue- vibration more noticable when I take foot off of accelerator. Before gear change I had car to +120 mph on race track with no vibration. Any ideas?

 

[Silver66FsBk]

I have run out of ideas and need help. Replaced the stock gears with 3.73 (Ford Motosports) in my 1999 Cobra (46K miles) and immediately noticed a bad vibration above 70 mph (through the seat). Re-indexed the driveshaft and this reduced vibration, but still noticable. Replaced the driveshaft with aluminum unit and no difference. Switched to a new set of summer wheels and tires (Enkei/Falken 615) and vibration was reduced fair amount. Even starnger, the vibration moved from 70 to 80 mph. Had shop put car on lift, ran speedo up to 80 mph and inspected driveline- differential felt smooth, tires and driveshaft looked to be true, but could feel the vibration on car body. Bearings felt solid with no play in any of the hubs. Another clue- vibration more noticable when I take foot off of accelerator. Before gear change I had car to +120 mph on race track with no vibration. Any ideas?

did you try replacing the U-joints? or maybe the gear wasn't set properly and have to much play in the pinion. or did you check your pinion angel?

 

[Sharp99]

New U-joints came mounted on the aluminum drive shaft. The pinion angle should be easy to measure and I will check that. My hope is that the gears were set correctly as this would be a real pain to have to redo. Thanks for the ideas.

 

[midlifecrisis99cobra]

If there were a big enough problem with the gear set-up to cause a vibration you'd be hearing something as well. Truthfully, this type of problem is the primary reason I haven't gone to 4.10s in mine. It looks like you've covered everthing I've seen as potential problem areas. Can you get a "road force" balance job on the tires in your area? Since changing the wheels made a difference in the speed range of the vibration, I wonder if that would help. Any chance of a bent axle stub or brake rotor issue?

The only other thing I can think of is adding the pumpkin weight they started putting on the later cars.