18 / CYCLE WORLD
Major elements of the motorcy-
cle—chassis, engine, fork, wheels
and tires—have become steadi-
ly lighter. This is not from substitu-
tion of expensive exotic materials
such as titanium and carbon fi ber,
but because improved manufactur-
ing technologies now let us use less
of the usual materials—steel, alumi-
num, and some magnesium.
Production motorcycles have
gone through cycles of weight gain,
then simplifi cation and weight loss.
From time to time I have felt dis-
couraged by so many tubby motor-
cycles. I know from the experience
of racing that lighter machines ma-
neuver more quickly and accelerate
and stop faster than heavier ones.
They are easier on tires and brakes.
That’s why, when Honda gave up
its complex oval piston NR
32-valve four-stroke GP racer, and
turned to two-strokes in 1982, they
built small, light and handy—not
big, power-laden, and tire-eating.
Pick up the cylinder head from
one of the original liter-bikes—a Ka-
wasaki Z1 of 1973, or a Suzuki GS
of 1976. Those air-cooled heads are
heavy. Why? Because there has to
be enough metal in them to store
heat for later conduction to near-
by cooling fi ns. That was the key to
knock-free operation at high pow-
er. Lots of metal keeps the valve
seats round and properly located.
When Harley-Davidson’s classic alu-
minum XR-750 V-twin racer was on
the drawing-board, racing manag-
er Dick O’Brien wanted an inch of
metal over its combustion cham-
bers. His instincts weren’t wrong.
Today’s liquid-cooled engines per-
form better in every way because 4
pounds of fl owing water carry heat
much faster than 20 pounds of solid
aluminum. Their heads are light, and
their valve seats stay where they be-
long because liquid cooling keeps the
metal around them cool and strong.The crankshafts of those early su-
perbikes were also heavy—there had
to be enough metal in them to pro-
vide the grip in the press-fi ts that
held them together. Yes, they were
assembled out of many pieces and
pressed together, because that was
the best way to allow use of the big-
ball main bearings and one-piece
roller rods that engineers of that time
believed essential to low friction.
Cranks today run on pressure-lu-
bricated plain journal bearings and
have split-and-bolted connecting
rods. As a result, they are one-piece
steel forgings—no pressed joints to
slip. And they are light.
Engine crankcases and entire
chassis and swingarms are now cast
by methods that came into use less
than 20 years ago. Previously, en-
trained aluminum-oxide fi lms in
metal being cast became crack nu-
cleation sites in fi nished parts—ex-
posure to air oxidized the surface of
molten aluminum as it was fl owing
through the mold and that oxidized
area was then trapped in the cast-
ing. To provide adequate strength,
parts had to be made quite thick./ TDC/
LOST
WEIGHT,
FOUND
PERFORMANCEPERFORMANCEPERFORMANCE
M
What have we done with motorcycle
weight savings?
ByKEVIN CAMERON