42 / CYCLE WORLD
Increasing friction causes the
steel discs to turn, slowly at first,
then faster. They turn the inner
hub, rotating the gearbox input
shaft and the rear wheel, whose
large torque—through a 10- to
15-to-one overall first-gear reduc-
tion between engine and rear
wheel—accelerates your bike. The
“slip phase” of starting is the pro-
cess by which clutch friction forces
the steel discs to gradually catch up
with the outer drum and its friction
discs. When slippage ceases, the
drive has effectively become solid.
Motorcycle clutches have worked
this way for a century, with vari-
ations. The lead photo shows the
chain-driven four-spring clutch of a
1954 pre-unit Triumph 500, move on
to a 1974 Norton Commando clutch,
which employs a single diaphragm
spring instead of the more common
multiple coil springs, then leaps to
that recent development, a 2019
Husqvarna FS 450 “slipper” clutch.
Slipper clutches were originally
created to deal with engine brak-
ing in road racing. When a rider
brakes and closes the throttle, the
rear wheel, through chain, gearbox,
clutch and primary drive, must spin
the engine against its internal fric-
tion. As the bike, still braking, leans
into a corner, this friction torque
might exceed the grip of the rear
tire, making it slide out or hop.
A slipper clutch uses the reverse
torque of engine braking, acting
through spiral ramps built intothe clutch inner drum, to reduce
clutch spring pressure. Essen-
tially, when the momentum of
the motorcycle pushes against
engine braking, the forces act to
“unscrew” the clutch, forcing the
pressure plate to lift. This allows
clutch slippage to take the place of
tire slippage or hop, making the
bike more stable.
Later it was realized that such spi-
ral ramps could work both ways—to
reduce clutch grip when the rear
wheel drives the engine, and also to
increase clutch grip when the engine
drives the rear wheel. This so-called
“assist” feature, by generating added
pressure on the clutch stack, makes
it possible to reduce lever pressure
by 40 to 50 percent—a welcome
result. Such slipper/assist clutches
are to be found on many recent
motorcycle models.
Today, all these secrets reside
under that round bulge on your
right-hand engine cover. QABOVE:The heart of a slipper clutch.
Balls-and-ramps reduce clutch grip in
proportion to engine-braking torque. Re-
verse torque from engine braking drives
the balls up their ramps to reduce clutch
spring pressure. In a slipper/assist clutch,
forward torque adds to spring pressure,
increasing clutch grip and allowing a
lighter initial clutch pull.