Pensioner,

Well said and true! Howsomeever, there is yet another case. While your
treatment holds for the mechanical equivalent of the steady state of
continuum process, there remains the problem of instantaneous load.

Taking your poignant example of the 70 mph cruise and the urgent need to pass
the Explorer, the driver of which has just passed on a prurient digital
comment. When the accelerator is pressed forward the requested acceleration
ramps the engine load rapidly. Because of the rapid change in airflow (only
talking throttled gasoline motors here) the cylinder pressures quickly go to
the maximum for that rpm. Consequently, full torque is applied (or close)
until the brick has accelerated enough for the driver to release the demand.
Because of the high mass of the vehicle the ability to instantaneously drive
the vehicle to a full available torque application is exaggerated (made more
likely) with numerically lower final drive ratios. Taken to the extreme, this
is a subset of the classic lugging phenomena - high sustained cylinder
pressures at modest rpm. I think the probability of high instantaneous
pressures is higher with a normally aspirated motor than a turbocharged one
(necessary turbo lag), and higher still for diesel motors as compared to
gasoline (SI) engines. Hence, I have felt that the lore that TDi engines
break Vanagon transmissions is plausible.

So to simply agree with your well put analysis, it is the shaft torque (or
the shaft torque time spectrum) that breaks transmissions.

To proffer some anecdotal evidence, I have broken two 3/4 gears in my
Westfalia's diesel configuration and one in my 8V 1.8L Digifant
configuration. With each repair, I was assured I'd never have another
problem, because VW had improved the gear. And as I drove away from "The
Transmission Supershop" I heard a deep belch as the owner digested my check
and a cheery "Come back again, soon" from his soon to be college age daughter.

Frank Grunthaner