38 / CYCLE WORLD
Unlike steam or electric power-
plants, piston internal combustion
engines cannot produce torque
from zero rpm. That means they
must first be started, and only then
gradually connected—“clutched”—
to the load. Although 98 percent
of new U.S. autos have automatic
transmissions, for motorcycles the
ratio is the other way around: most
bikes retain foot gearchange and a
clutch operated by the left handle-
bar lever.
Modern motorcycle engines are
of unit construction: powerplant,
clutch, and gearbox are in one unit,
with crankshaft and clutch geared
together. It wasn’t always so—Brit-
ish motorcycle engines and gear-
boxes were long separate, joined
by bolting to engine plates. The
clutch (mounted on the gearbox)
was driven by a primary chain from
a sprocket on the engine’s crank-
shaft. A chaincase enclosed and
protected the chain.
In a typical motorcycle clutch,
the engine directly drives the clutch
outer hub, which is a cylindrical
drum roughly six inches in diame-
ter. Its rim—perhaps 1-1/2 inches
tall—bears inward-facing ribs or
splines that engage matching tabs
on the outside diameters of several
annular discs faced with friction
material—the friction plates.
Centered in the outer hub is a
separate inner hub, which is direct-
ly splined to the input shaft of the
gearbox. It too is cylindrical, and
its outer surface bears axial splines.
Engaging those splines are another
set of clutch discs known colloquial-
ly as “steels,” which are interleaved
with the friction discs. Friction andsteel discs alternate in the “stack,”
the frictions keyed to the clutch
outer drum, the steels splined to
the inner drum.
This stack of clutch plates is
topped by a spring-backed pressure
plate that forces all the discs in the
stack into frictional contact with
one another. A mechanism uses the
rider’s pull on the clutch lever to lift
the pressure plate against its spring
pressure, separating the friction and
steel plates. When you have started
the engine and wish to depart, you
pull in the clutch lever, then click
the transmission into first gear with
your foot./ FUNDAMENTALS /
THE CLUTCH
Providing greater engagement with your motorcycle
By KEVIN CAMERON /Photography by JEFF ALLEN
U
OPPOSITE:Three motorcycle clutches, one
new (at left), and two scarred veterans of
many years’ use. At top right, a venera-
ble Triumph four-spring unit. Below it, a
Norton clutch with diaphragm spring (see
its 18 “fingers”). On the left, a modern
gear-driven Suter.