I would like to estimate the safe rpm limit of my dual diameter steel drive shaft behind the B-W T89 OD in my 56 Ford Fairlane. Single diameter calculators are out there to estimate critical harmonic speeds but methods to deal with dual diameter configuration are not. At least I did not find any. Weld to weld, this particular drive shaft starts out 2"ODx10"L then transitions seamlessly over the next 4" to the next size which is 3.219"ODx29"long. The criticals of each section are well above the 7000 rpm safe ceiling I need, but I have no idea if that is a valid way of looking at this. Comments/suggestions?

Not a terribly good answer, but the likelyhood of exceeding 7000 rpm at the drive shaft is very unlikely. That is unless your name is Charlie and you have a yellow truck at Bonneville.

Frank/Rebop

Bristol, In ( by Elkhart)