When’s a Cir­cle an El­lipse?

Street Rodder - - Shop Manual -

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.

Tom Keyes

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.

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