Re: torsens

[Orin Eman <orin@wolfenet.com>]

> >
> >if you put one axle on a dolly, the locked center diff will redistribute
> >torque 100/0 to the axle that is not on the dolly.

> nope eliot is quite right, just one wheel off the ground will stop the
> drive to
> that [virtual] axle (generation 1 locked centre diff, generation 2 torsen),
> and
> to the whole car (generation 1 open centre diff, not so with the torsen).
> <
> Actually, with the locked Gen I center diff and 1 wheel not contacting the
> ground, the power(torque) split between the front/rear axle is still
> 50%/50%, it is just that the 50% going to the axle with the wheel off the
> ground is wasted turning that wheel. So only 50% (not 100%) of the power is
> transmitted to the axle with both wheels touching the ground.

With any diff, 100% of the torque input to the diff is being
transmitted... somewhere.

I started considering the physics of the open diff and quickly
concluded that for steady state, ie wheels turning at a constant
speed, the torque split is 50/50, EVEN if one is stopped and the
other spinning.

The Bosch Automotive Handbook puts it this way:

...this balance effect limits the effective drive torque to
a level defined as twice the tractive force available at the wheel
with the lower coefficient of friction.  This wheel responds
to the application of excessive torque by spinning (a great
oversimplification, the whole drivetrain and engine accelerates).

BTW, you cannot look at this by saying there is x torque
produce by the engine going into the diff, you have to
look at it as a complete system... ie, if the engine is
producing x torque, the diff can only transmit twice
that at the wheel with the lower coefficient of friction,
say y torque, then the difference x - y is going to
accelerate the engine, drivetrain and wheel that is
(about to) spin.

Now lets look at the locked diff.  Simply looking at the
force vectors indicates that the effective drive torque
is the sum of the tractive forces available at the wheels (outputs).
If the engine is producing more torque than this sum, then
the whole drivetrain and engine accelerates along with
both wheels (outputs).

In summary, open diff means 50/50 torque split at the output,
locked diff means the split varies between 100/0 and 0/100
at the output.

In both cases, if the engine torque is greater than the
diff output torque, the whole system will accelerate...

...until you let up on the gas pedal, hit the rev limiter,
the engine self destructs due to the rev limiter being
disabled, or if you were light on the gas to start with,
the RPMs reach the value at which internal engine losses
match the torque that cannot be transmitted.

As for torsen, it changes things slightly from the open
case... my belief is that if the output torque is split
say 30/70, then input can only be 100/30 * traction at the
wheel which is slipping...  100/30 or 3.33 is better than
2 which is the case of the open diff when trying to
get going on a slippery surface.  Personally, I don't have
a clue what it means on the track, except I did spin the
95 90Q at Bremerton a couple of years ago.

This could get as bad as the piston acceleration thread.

Orin.