Re: [MV] Overdrive units
Mon, 4 Jan 1999 01:08:20 EST

Regarding the old Warn overdrive, as well as the newer unit if it is
identical, please consider the following:
Although the unit was advertised as workable in a CJ-5 length chassis, I had
one which I attempted to use in two different jeeps of that length. The
overdrive was super to have, but it required the rear driveshaft to be
shortened .(or it came with a shorter shaft, I forget now) The shaft wound
up so short that it caused the U-joints to run noisy, and ultimately fail. I
never got it to work as advertised, and sold it in despair. Before spending a
thousand bucks on this unit, please check with the manufacturer to see if it
is warranted for use in the M38 and A1, which I believe are both short
Can anyone else comment on this problem?

It seems clear to me now that if you change the u-joint center to center
distance, the driveshaft angle also changes, and this forces the U-joint to
work outside its intended arc. In addition, I believe it causes an out of
phase condition...a driveshaft speeds up and slows down twice each revolution,
as the u-joints flex in an arc with a center of rotation that's different
than the center of shaft's very complex to grasp this, but it's
true. (it's the reason Cardan joints are used in modern 4 X4's.)
I now believe that the overdrive unit made the shaft angle so acute that the
shaft was wrapping up. There's a URL on the web which I don't have at my
fingertips that explains this with neat graphics. I'll post it later so that
this stop-start action is more understandable.
OTOH, if the overdrive in question was the one which just "plugged in" to the
transfer case with no shaft modifications, like the Fairey unit for the Land
Rovers, then, as Emily Litela used to say, "nevermind"!

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