Another consideration of the so called "jump rope" thing is whirling.

There is static run out of the driveline based on the initial straightness of the driveline.

Also, there is an additional runout resulting from driveline speed, called whirling, if the drivline is not perfectly straight.

The greater the rpm , the greater the whirling or out of plane displacement.

If the driveline was perfectly straight no whirling would result from rotation.
Believe some drive lines are straighter than others.

Agree with equal driveline straightness, the stiffer tube would result in less whirling or out of plane displacement from rotation.


Displacement from whirling results in stress in the material from tension to compression for each rotation. The higher the rpm the greater the displacement from centripedral force and the greater the stress level.

Stress reversals cause fatigue of the metal.

In general 6061 T6 alumumin is more sensitive to fatigue cracking than 1026 steel.

Bending stiffness in the structural world is measured by EI
where,
E is the material modulus of eleasticity
I is the area moment of interia

Comparing a 4" OD x .125" wall aluminum tube to a 3" OD x .095" wall steel tube,
the aluminum tube is only 7.7% stiffer than the steel tube.

The aluminum tube has a greater I than the steel tube, but ~1/3 of the E.

However ,the yield strength of the 6016 T6 aluminum tube is 15% > yield strength of hot formed 1026 DOM steel tube.

Combining the strength advantage and stiffness advantage of alumumin to steel results is nearly , 1.077 X 1.15 = 25% ,overall advantage of aluminm to steel.

Since the alumumin driveline is ~8% stiffer than the steel driveline, but 25% greater in diameter,
the resulting stress level from out of plane displacement from whirling would ~25%-8%= 17% greater than the steel drive line with equal straighness.

Believe the aluminum driveline must be fabricated to closer initial run out tolerance than the steel drive line to reduce the stress reversal level from whirling.

The only advange of aluminum is ~ 8 lb weight reduction, which only shows up in city MPG from frequent acceleration from stops.

The 15 inch long horizontal intermediate driveshaft and the u-joint behind the carrier bearing on the steel two piece drive shaft is basically a CV joint. The aluminum driveline has no CV joint and is a one piece drive line.