Backdriving a hypoid gear during towing

[jbender-zanoni@domain.elided]

As a mechanical engineer I am inclined to disagree with Simon and support
John Hertzman's position. 

Depending on the angle at which the gears act on each other, sliding
contact gears such as a worm gear or a hypoid gear can be more efficient
in one direction as opposed to another. Thus backdriving can be very
inefficient, generate a lot of heat and possibly cause mechanical damage.
Some gearsets with very low drive angles are even described as
"self-locking" (0% effeciency), they cannot be back driven (except under
some conditions like vibration). That is the essence of Simon's point as
I see it.

The problem with Simon's position is that virtually no power is going
through the gear mesh during towing. The drivetrain is only working
against friction. Even for an ineffecient  gear mesh  the power losses
are low. Consequently the heat generated and mechanical damage are not
significant.

The issue usually comes down to the heat generated. That is why when a
Spider differential was adapted to the Montreal a heat sink housing is
added to cool the oil. 

So a good test to settle this question is: Does the differential get hot
when you tow it?

Joe Bender-Zanoni
Ann Arbor, MI
164L