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THE DIFFERENTIAL
There are six types of differentials being used in racing; the open
dill, the cam and pawl or ZF type, the clutch locker, the Detroit
Locker, the Weismann and the spool. The only reason to run an open
diff is if the regulations require one. Locking the open diff is
very simple--you weld the spyders solid. You will get caught, eventually.
I have not personally run an open diff since the days when I didn't
know that there was anything else and I do not expect to ever run
one again. Therefore, I know nothing about the techniques used to
trick them so that they will partially lock. I do know, however,
that this has been done in SCCA production racing. I neither know
nor care how it is done. The reader who is interested should be
able to find out without too much trouble.
So much for the open differential.Street cars need
differentials between the driven wheels because the outside wheel
in any cornering situation must travel on an arc of greater radius
than the inside wheel, and so will have to revolve more times in
negotiating any given corner. If the two driving wheels are locked
together, the unladen inside wheel will be forced to rotate at the
same speed as the inside one and will therefore hop along like a
rabbit. This makes a funny squeaking noise and upsets the handling
of the vehicle. Street cars typically operate at low force levels
so the open differential does not normally present a problem--and
it is maintenance free. However, there are times--like trying to
get up a steep hill in winter when one wheel happens to be on glare
ice--when the limitations of the open diff become very apparent.
With the open diff, the torque from the engine takes the easy way
out and if, for whatever reason, one of the driving tires has exceeded
its thrust capacity, all of the torque will be delivered to that
wheel and it will spin--while the other tire does nothing and the
vehicle goes nowhere. This condition occurs on the race track all
of the time-- while we seldom end up with one tire on a good surface
and the other on a slippery one, lateral load transfer accomplishes
the same end by unloading the inside tire. Since no one tells the
open differential about this state of affairs, as we try to accelerate
out of a corner, the diff keeps transmitting drive torque to the
unloaded tire until the torque becomes more than the tire can bear
and it starts to spin. About then the diff sends all of the torque
to the spinning tire and none to the laden tire and we go nowhere.
The problem becomes more acute as the power to weight ratio raises,
but even Formula Fords get inside wheelspin out of slow corner.
Wings, by keeping the unladen tire partially loaded with aerodynamic
downforce, make the problem less acute.
However, all racing cars, in order to realize
their potential, require some sort of limited slip or locked differential--and
always have. What we need here is a differential that will be open--or
will differentiate--on a trailing throttle, so that the rear wheels
can rotate at the required radius speed during corner entry, but
will start to lock as the driver comes back on the power to stabilize
the car, thus providing a degree of built-in understeer by driving
the inside rear wheel, and which will gradually lock all of the
way as the power is increased so that there will be no inside wheelspin.
At the time of writing, no one has quite achieved this goal.There
are five types of differentials in use in racing cars today--the
locked dill, the cam and pawl or ZF type, the clutch pack or Salisbury
type, the Weismann locker and the Detroit Locker. With one exception,
each has advantages and disadvantages. We'll start with the exception.
To my knowledge, the Detroit Locker has no advantage
over any other type of differential except the open diff. It is
an abortion. Its functioning can be compared to that of the ratchet
on a chain fall. As load is transferred it is forever locking and
unlocking, causing great lurches and changes from understeer to
oversteer. The best thing to do with a Detroit Locker is to remove
the center cam and run it locked. Period. End of discussion. We
all know that the Indy cars, the dirt cars, Nascar and Porsche use
a totally locked diff and they go like stink.Most of the IMSA type
large sedans also use it. Why then, is the spool not used in sophisticated
road racing machinery except by Porsche? Not because people haven't
tried it! The problem has to do with corner radii, weight distribution
and how much we are willing to sacrifice. The high banks and the
two-and-one-half-mile ovals tolerate the spool because at the corner
radii we are talking about there is virtually no difference in rear
wheel rpm--and the tire stagger makes up for most of that--when
we only need to worry about one corner radius, we can, by making
the outside rear tire larger in diameter than the inside (stagger),
arrive at an equal tire rpm situation and therefore neutralize the
drag moments about the center of gravity on a trailing throttle
corner entry situation. So long as the driver picks up the throttle
smoothly and progressively, we can then tune out the full throttle
understeer caused by the drive on the inside rear wheel. It is also
very important that the driver not apply sudden power during a time
when he has understeer lock (toward the corner center) on the front
wheels or he will understeer immediately into the wall--thump. When,
as in road racing, the radii of the various corners vary considerably
and the amount of the braking and turning combination taking place
also varies with the nature of the corneL it is no longer possible
to achieve equal rear wheel rpm in most of the corners
extract from "tune to win" Printed in
about 1978?
written by
Carrol smith
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Welded center |
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Slipper Center |
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LSD Exploded Diagram |
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