Cobra Vibration Problem

[Cobra #3170]

Hi Everybody,

I encountered a serious problem today while track testing CSX3170 prior to the Monterey Reunion. I am hoping that with all the gear heads on this site someone will have an idea or a similar experience. The following describes the problem and what I plan on investigating, any other thoughts would be welcome.

Problem: Excessive Vibration in drive train under moderate to heavy acceleration:

The car has a vibration that is so violent that it blurred my vision in 2nd, 3rd and 4th gears.
The vibration is directly related to applied torque in that if I eased off on the throttle but maintained speed the vibration goes away. It was so violent that I was afraid to use anything over half throttle in my initial test session. The car is equipped with vintage Goodyear rubber that was dynamically balanced. After checking the car over and finding nothing out of the ordinary I tried raising the tire pressure to 30 PSI from 25, this
reduced the vibration by about 50%. I am not sure if the reduction is do to less tire distortion or just less torque through the drive line because of slippage at higher pressures.

What I plan on checking:

Flywheel spokes for cracking in the spokes, happened before but may have been a symptom not a cause.

Half shaft phasing I checked at the time I installed new u joints but maybe they are off

Drive shaft phasing I had it built at a local shop and they may have it miss indexed

Tire run out

Center section mount distortion, the front of my cradle mount (used with a Jag center section) shows signs of rubber damage and the cradle mounting bolts seem to loosen
every session. I replaced the original 3/8” bolts with 7/16 studs and welded reinforcements in to the cradle structure because of previous problems with this area.
The cradle is from Brian Angless and was built in the early 1990's.

Differential damage on the ring and pinion

I will check engine and transmission mounts too.

The U joints were replaced on both half shafts and the drive shaft last month with heavy duty Spicer units.


I don't think it is a tire balance or engine balance issue because it only shows up under extreme torque loading.

The kicker, I had this problem with Hoosier A-6's at Fernley race way but it was only in high gear. When I took the car apart I found cracks in 3 of the six spokes on my Mcleod Magnum Force flywheel the cracks emanated from a stress riser caused by rivet holes in the flywheel. I have a new flywheel but the vibration is back in spades