Mark,
Thanks for your summary, it was really helpful. I did a little research
and found a pretty good while paper on the Torsen differential. For
anyone interested it is at:
http://www.sonic.net/garyg/zonc/TechnicalInformation/TorsenDifferential.
html
I didn't fully understand the function of the worm gear in load transfer
until I read your explanation.
Regards,
Phil Roettjer
-----Original Message-----
From: Mark Palmer [mailto:mgvrmark@hotmail.com]
Sent: Sunday, November 17, 2002 10:24 AM
To: proettjer@charter.net; vintage-race@autox.team.net
Subject: RE: diff lube question
Phil,
Strictly speaking, the Quaife (or Gleason or Torsen) is not truly a
"limited
slip" differential (although we all casually refer to them as "limited
slip").
A true limited slip diff, senses the speed differential between the two
wheels and limits the difference. Clutch-pack type differentials are
truly
"limited slip" -- when one wheel "slips" (starts to rotate much faster
than
the other), the clutches tighten up and reduce the "slip" (speed
difference)
of that wheel.
Quaife, Torsen, or Gleason units are more properly termed "torque
biasing"
differentials. They do not sense or limit the "slip". Torque biasing
diff's work by directing more torque to the wheel that has more grip --
more
resistance. However, the amount of bias is limited (excuse me, bad
choie of
words -- the amount of bias is finite). I believe the bias is something
on
the order of 4:1
Therefore, if you completely unload one wheel (i.e. pick up the inside
wheel
in a corner), the Quaife unit will act nearly like an open diff. In
other
owrds, if the torque required to drive one wheel (the airborne one) is
nearly zero, then 5 times zero is still zero -- and you will transmit
essentially zero torque to the "loaded" wheel. After a while the
unloaded
wheel returns to ground and you can get going again.
The gear-type torque-biasing diff's do, in fact, depend on some friction
to
work -- as you surmise. It is friction between the gear teeth. In worm
gears, or certain other gears, it is easy to drive the gear train on one
direction but very difficult to "back-drive" the gear train. That is
the
basis for the torque-biasing units. If there were zero friction between
the
gears, it would suddenly become POSSIBLE to back-drive the gear train --
and
the torque biasing would be lost. So you do need some friction for
these
units to work.
How much friction? I don't know. Would a lubricant with friction
modifier
reduce the friction too much? I dunno. The guys with specific
experience
can answer. The answer may vary from brand to brand (of lubrication, or
of
diff -- what works in a Gleason might not work in a Quaife).
BTW, friction modifiers are basically intended to prevent "stick-slip"
conditions from occurring. You probably know there are two different
coefficients of friction -- static and dynamic. Friction modifiers
don't
reduce dynamic friction -- they just make the static COF closer to the
dynamic COF. The net result is that a driver will report "smoother"
action
or "less jerkiness". Friction modifiers are important in clutch-type
devices, to avoid having the clutches engage & disengage (stick & slip)
repetitively. In my experience I haven't seen gear-type mechanisms
where
friction modifiers were required or important.
Best reference I have found is in Carroll Smith's "Drive to Win",
appendix
on the racing differential. It is misplaced in this book, should have
been
in Engineer to Win, but I guess he thought about it later & put it in
the
later book. Drive to Win is worth having anyway.
Regards,
Mark Palmer
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