Inadvertently exceeded driveshaft "Critical Speed" at Willow Springs.

While exercising my '68 convertible Barracuda during the Spring Fling Track Day a few weeks ago, I experienced a deep, high-resonnant frequency vibration when the car was in excess of about 105 mph. The vibration became more intense as I increased my speed increased past 130 mph, and went away completely under 105'ish mph. I immediately suspected a driveline issue, but know I had gone to great lengths to get everything lined up after installing a 200R4 mated to a stroked 5.9 Magnum. My current driveshaft is a Strange engineering built 3" chromemoly unit with forged ends, 1350 U-joints and a Mark William's forged transmission yoke. The transmission has a fresh tailshaft bushing, and there is 1" of clearance once the drive shaft is bolted in.

I measured and verified my driveline angle as "0" degrees at the transmission and "2" degrees down at the pinion. I am running slapper bars to control spring wrap. Dr Diff helped me get that dialed in. I'm learning now that i should get the pinion angle closer to just under 1 degree for very high speed use.

While researching this issue on-line the other day, I began to learn about the "critical speed" of driveshafts, and how that is dependent upon a variety of factors including length, construction materials and other items. Depending on where you look, it seems my current driveshaft has a critical speed of around 5500 to 6500 rpm. That correlates to the incidence of vibration I experienced at the top end of 3rd gears 1 to 1 ratio and beyond into 4th gear's .67 to 1 overdrive ratio. Theoretically, my current set-up would have the driveshaft approaching 10000 rpm wound out to the rev limiter in 4th gear.

I had no idea a high quality chromemoly driveshaft with 1350 U-joints and forged ends was essencially a fuse waiting to blow at high speed. This is obviously a very hazardous situation given that sudden catostrophic failure of a steel driveshaft can occur when critical speed is exceeded. That could have ruined my whole day or life. I thought I should pass this information on, given the amount of people adding performance upgrades to our old Mopars these days.

It seems the only solution is to buy a higher quality drive shaft with a much higher critical speed rating than a steel or aluminum shaft. That leaves an expensive carbon fiber unit from QA1 or another manufacturer to solve the problem.

The whole situation reminds me of the old saying, "Speed cost money, son. How fast do you want to go?"