On Fri, May 16, 2003 at 11:44:07AM -0400, John F. Kelly Jr. wrote:
> My point is the contact patch on any given wheel and tire combo is only so
> big.
> How much power can you send to that wheel when the magic LSD does its
> limiting thing?
None. However, my point is that without a LSD, you can't send *any*
power to the other wheel either. That's a big problem. Without an
LSD, the amount of torque you can send to the outside wheel is limited
by the amount or torque that the inside wheel can take before it
spins. In addition, with an LSD, the inside wheel isn't spinning, so
that it is making some contribution to the cornering force and some
contribution to the tractive force, while a spinning wheel makes
almost none.
It seems that you are concerned that the outer wheel might not have
enough grip to handle all the torque that's being sent to it. On my
car, that doesn't appear to be an issue. It just doesn't have enough
power to spin the loaded outside rear wheel. On a CP Mustang it might
become an issue.
> I sincerely doubt it amounts to double of what was there before.
A wheel with more weight on it has more grip -- not quite 2X the grip
for 2X the weight. You lose some total grip when weight gets shifted
from the inside wheel to the outside wheel, but if the inside wheel is
not spinning, whether due to an LSD or due to a suspension setup which
keeps it planted, you do not lose all the grip of the inside tire
without gaining any grip on the outside tire. So no, not double, but
reasonably close. There is a continuous range of spring/swaybar
settings that trade off between more front grip/less rear grip and
more rear grip/less front grip. The point in that range at which the
inside wheel will start spinning if the car has an open differential
only has consequences if the car does have an open differential. If
the car has a limited slip differential, then there is not a sudden
transition in how much grip the drive axle has at that point; it is
in the middle of a continuous range of adjustments with smoothly
varying results. With an open differential car, it is the endpoint of
the useful range of settings.
> Bear in mind an LSD that locks up causes a distinct amount of understeer.
Torque sensing differentials don't lock up. Locking differentials
do. Lockers are used for off-roading and drag racing.
> The sharper the turn the more understeer you're liable to get.
Torque sensing differentials send more torque to the outside wheel, up
to a factor of 3-5. That seems to me like it would help the car
turn. In addition to that, carrying more of the tractive force will
increase the slip angle of the outside tire which again would promote
oversteer.
> But the key point is putting power to the ground. That's why fixing
> the spinning wheel problem must be solved first before spending any money
> on an LSD.
The spinning wheel problem may be solvable with an LSD. The
catastrophic point at which the inside wheel spins on a car with an open
differential, is just another usable point in a continuous range of
front/rear handling balance that is accessible to a car with an LSD.
The driver of a car with LSD may choose to tune for more grip on the
drive wheels, if the power and handling of the car is such that the
car oversteers too much under power -- but that may not be the case.
For example, I'll pick on Jake's MR2 Spyder. Stock, the car
understeers under power. It also exhibits wheelspin on sweepers.
This is a terrible situation, because stiffening the front suspension
in order to do away with the wheelspin makes the car understeer
*more*. Well, that's the tradeoff Jake is stuck with in the stock
class. If the Spyder had a torque sensing differential, wheelspin
wouldn't be an issue and Jake could choose whether he wanted the stiff
front bar and understeer, or a soft front bar and more body roll.
--
john@idsfa.net John Stimson
http://www.idsfa.net/~john/ HMC Physics '94
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