Re: Backdriving a hypoid gear during towing

[Simon Favre <simon@domain.elided>]

John,

I was also glad to see Joe step in with an ME's point of view. I think he
has a very valid question. Has anybody ever flat-towed an Alfa 100+ miles
and immediately jumped under the car to feel whether or not the diff got
hot? I may indeed be guilty of spreading old wive's tales, but the only
way to debunk them is to get them out in a forum like this. Even tho the
diff isn't transmitting an engine's power under towing, you are still
spinning the driveshaft faster than the wheels are turning and in the
less efficient direction for the hypoid gears. How much less efficient
and how much heat is generated by friction (and spinning the driveshaft)
are unanswered questions. I'll see what I can dig up from old owners
and shop manuals.

I am still concerned with the tranny issue. The bronze bushings inside
the gearwheels might not take kindly to having the shaft spinning inside
them with no oil. Needle bearings are a lot more durable. If it's a car
that isn't running that's going to be torn down anyway, it may not matter.
For a car that's been sitting in a barn for 20 years, there is a very good
chance that putting it in motion again will shed particles into the oil,
whether you start it or tow it. A car that's been running and well cared
for will have less of a problem.

JHertzman@domain.elided wrote:
> 
> It was comforting to see Joe Bender-Zanoni agreeing with me about the risks of
> towing- I don't mind being wrong (particularly if it leads to an illuminating
> discussion) but it is sometimes nice to not be alone.
>

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End of alfa-digest V7 #545
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