When’s a Circle an Ellipse?
Q I had a driveshaft made to fit a ’65 A-833 four-speed trans and Jag rear end in my ’32 DeSoto coupe. I noticed it had run out of 0.090 inch at the slip yoke about 6 inches aft of the U-joint attached to the Detroit flange replaced the old ball and trunnion joint. I asked the builder what he had as runout when he made the shaft and I was told it was 0.003 (TIR) inches. I measured runout at the tranny flange (it is a casting that has a machined interface surface), which is 0.010 inches TIR. I found an article on the web that said when a driveshaft spins through its U-joint centers, it spins in an ellipse not a circle. The driveline guy said he would recheck the shaft if I bring it back to him, but he is pretty sure it is correct. It will be at least a year before this project is running so to check for vibrations will be a while down the road. Should I be concerned? Thanks, once more.
Via the Internet
A First off the chassis looks extremely well done and very stout, however, as you suspect the driveshaft may be a problem. Dynotech recommends driveshaft runout be 0.020 inch or less over its entire length. We’re suspicious that the splined section of the driveshaft may not be true, causing the excessive runout. Have it checked out.
As for the driveshaft running in an ellipse, it’s actually the U-joints that do that when they run at an angle— they wobble back and forth and as a result they speed up and slow down two times each revolution—that action is canceled out by second U-joint and the result is the smooth transmission of power.