58 FEBRUARY 2020 scca.com
With the wide variety of limited-slip differentials available for most
popular RallyCross vehicles, selecting the
diff that’s right for your application and
driving style can be difficult. Luckily, we’re
here to help – or at least clarify things. To
that end, this month we’re going to cover
the various types of differentials and how
they operate, and next month we’ll be back
to cover what vehicles come with factory
LSDs and what type of LSD may be the
best choice for your particular application.
A differential’s basic job is to allow one
drive wheel to rotate faster or slower than
the other while turning. When a vehicle turns,
the outside wheel is making a larger arc
than the other and thus traveling a longer
distance, so it must rotate faster. This is
what an “open” differential allows under all
circumstances. This is great for initial turn-in,
especially when no torque is being applied,
but not so much when you’re applying torque
to accelerate. In contrast, a locking or limited-slip differential limits the difference in speed
between the output shafts (your axles), and
by proxy, the differences in wheel speed.
There are three different types of LSDs
in terms of operation. They include 1-way,
2-way, and 1.5-way units. A 2-way differential
will limit wheel speed differences under
acceleration and deceleration. A 1-way
differential will only do so on acceleration
or deceleration. A 1.5-way differential limits
differences in wheel speed under both
conditions like a 2-way, but at different rates.
In addition to the types of LSDs,
there are numerous different styles of
LSD. Some of the common styles include
clutch or cone types, helical gear (like
the Torsen), viscous, electronic, and
fully locked or welded differentials.
A clutch or cone LSD uses a stack of clutch
discs to limit wheel slip. The more the clutch
stack is compressed, the more “locked” the
wheels become. By contrast, gear-driven or
mechanical LSDs utilize worm or spur gears.
As torque is applied to the gears, they are
pushed against the walls of the differential
housing, creating friction, which in turn
limits differences in wheel speeds. A viscous
LSD operates using friction generated by
discs suspended in a fluid. As the wheels
change speeds in relation to one another,
the discs will be pressed together with the
fluid between them. The friction generated
by this is what provides the locking action.
An electronic LSD, or “E-Locker,” typically
utilizes an internal construction similar to
an open differential combined with a clutch
pack. The clamping force of the clutch is
then electronically engaged or disengaged
based on surface conditions and/or a preset
value. Some systems allow the user to
control and adjust the differential’s limiting
torque, like in Subaru’s DCCD system.
Finally, a fully locked or “welded” differential
allows no difference in wheel speed between
the output shafts. If you ever hear or read a
reference to a “welded differential,” this means
the spider gears inside of the differential have
been welded together so they can no longer
spin, thereby locking them in place, which
forces both wheels to spin at the same speed.
And, with that basic grasp of the topic,
next month we’ll help you make the right
selection for your car and driving style
because, well, differentials aren’t cheap, so
you’ll want to do it right the first time.
WHAT’S THE DIFF?
(LEFT) A limited slip
differential helps turn
power into motion, which
is essential when the
surface is lacking grip.
(BELOW) There are
a number of different
types of limited slip
differentials, and many
of them are tunable.
WORDS Matt Wolfe | MAIN IMAGE Dave Green
Before we dive into RallyCross tuning tips, you’ll first need
an understanding of differentials – so here it is