get away with a shorter wheelbase. But, before you run
out and lower your seat and slide the rear axle forward
to improve cornering, you must factor in the mass of the
vehicle and ask, “How will these changes affect the weight
bias?” Remember, moving the rear wheel forward in the
swingarm moves the engine backwards. Among the many
mass considerations involved are engine weight, engine
position, gas-tank volume, fuel position, seating position,
footpeg position, overall bike weight, wheel weight,
handlebar position, rider weight and the rated horsepower.
One change, say moving the rear wheel forward, will
beget many changes: (1) Moving the rear wheel forward
will lessen leverage on the shock and make it feel stiffer.
( 2) Moving the wheel forward will make the wheelbase
shorter but also change the weight bias from 50/50 to
something approaching 45/55 (by moving the gas tank,
engine and rider closer to the rear wheel and farther from
the front wheel). ( 3) Moving the rear wheel forward will
shorten the wheelbase and make the bike easier to fold
into tight turns.
CAN THE ENGINE AND WHEELS AFFECT A
BIKE’S HANDLING? Yes. Horsepower and torque
influence weight bias and suspension performance. The
rotating inertia inside the engine creates a gyroscopic
effect that impacts how much the bike resists turning
and how it behaves in the air. High-rpm engines slam
bumps harder and need to be mellowed out through frame
geometry. A designer must also take into consideration
the riding styles that will most likely be used on that
displacement bike. Smaller-displacement bikes are shorter,
quicker-turning and more agile to match the way they are
The weight of the tires and tubes affects cornering.
Factory riders can feel the difference between heavy and
light tubes. Heavier tubes increase the gyro effect. The
heavier-spinning weight keeps the bike straighter and
more resistant to leaning.
HOW CAN THE WIDTH OF HANDLEBARS
AFFECT HANDLING? The arc, that the ends of the
bar form when turned side to side, will be smaller with a
narrower bar width. Conversely, wider bars turn a larger
arc. That means the rider’s hands move a greater distance
to turn the wheel the same amount if his bars are wider.
On the track, narrow bars feel quick and nervous. They
also transmit more feedback from the track surface to
the rider. Conversely, wider bars slow down the steering
input (because the hand movement is longer and slower)
and mask some of the feedback that comes from the track
through the steering geometry.
Handlebars can also be used to tune the center of
gravity. Straighter, more upright bars or forward-set bar
clamps will move the rider forward on the bike and
shift weight bias to the front. Depending on the exact
suspension setup, this could make it easier to stick the
wheel into corners.
HOW DOES ENGINE SIZE AFFECT THE CHOICE
OF STEERING GEOMETRY? Steering geometry is not
the first thing on the designer’s mind. It won’t be until the
remainder of the bike has taken shape that the engineer
finally decides what the exact steering geometry will be.
With the input of test riders, it will be tuned for quicker or
slower-steering action or for more or less stability to best
match the characteristics of the bike and rider.