I’ve been chasing a high speed vibration in the Race Wagon ever since I finished the build 2 1/2 years ago. It kicks in at about 100 to 110 mph and makes the rear view mirror pretty much useless. At first I thought it was a bad trans OD shaft, then an aero buffet, then the driveshaft, then the wheels, then the axles, ad nauseum. So after rebalancing everything to no avail, I just got used to it. But it was time to get serious about this issue as all the other items on my punch list were just about done. After some research and speaking to fellow racers and fabricators, I revisited the driveshaft and the u-joint angles. With the aid of a digital level, I took the measurements of the trans output shaft angle, the differential flange angle, and the driveshaft angle, and found the u-joints were working way too hard. Optimal angles have the diff and trans at the same angle but on different planes, with about 2-3 degrees of working angle between the driveshaft (I was way off). As the diff is actually higher than the trans in the Wagon, I needed to shim the nose of the diff up, and also shim the tail of the trans up. I also figured out that with a 7500 rpm motor, the driveshaft is turning about 8500 rpm in overdrive 5th, which is enough to possibly start an oscillation in a one piece, 2.5″ mild steel driveshaft. This issue is known as “driveshaft critical speed”, you can find a lot of info on the topic with a quick google search. So along with improving my driveshaft geometry, I had South Bay Driveline build me a fresh 3″ driveshaft to help prevent any driveshaft whip, they do good work at a fair price. I’ve got a race next weekend at Buttonwillow Raceway, hopefully things will be better after this fix.
(Editors note: The vibration is now greatly reduced. Further research and discussion reveals that with such a short driveshaft, it was probably the angles affecting the vibration more than the driveshaft diameter, but every bit helps.)