Why different types of motorcycle handle differently
Because motorcycles are designed for a variety of applications—touring, cruising, sport, and
off-road—motorcycle handling must provide the performance that riders in each discipline
require
One approach to look at motorcycle handling is to compare stability and control response.
The sporting rider, seeking a highly responsive machine, quickly realises that response works
both ways. A bike intended to respond quickly to the rider’s control inputs will react
similarly quickly to other disturbances, such as road bumps or crossing railway rails at an
angle.
The touring rider does not want to be continually correcting. That motorcycle’s wheelbase
will be longer, allowing for more space between the rider, passenger, and luggage.
Wheelbase is also the lever used to guide the front wheel and swing the rear to a new
direction. The longer the wheelbase, the slower a steering input at the bars (or the effect of
any unintended disturbance) results in a change of direction for the entire vehicle.
The sports rider, who is more amused by curves and changes of direction, prefers faster
steering and direction shifting because manoeuvres take up less time and space, and there
is a better link between cause and effect. As a result, the wheelbase becomes shorter.
Why is there a trend towards longer? As engines improved from 40 horsepower in the mid-
1930s to twice that in the late 1960s, more and more time could be wasted on wheelies. It is
preferable to extend the wheelbase slightly in order to make better use of that acceleration
capability.
Comparing MX and other off-road designs to those from 50 years ago is shocking. When
suspension travel averaged 3-4 inches, MX bikes could afford a shorter wheelbase in
exchange for faster steering. However, because bikes with 8-12 inches of suspension travel
are so tall and move so quickly across undulations, the wheelbase has had to expand in
order to keep the front wheel on the ground at least periodically.
Dragsters have the longest wheelbases of any vehicle, again, to convert horsepower into
acceleration rather than wasting it on wheelies.
Where does the engine fit in the frame? Forward? Back against the rear tyre, “for traction”
as 1950s designers claimed? Higher? Lower? In the early 1960s, the Japan Society of
Mechanical Engineers (JSME) published a study that linked engine location to stability. Their
conclusion is based on testing on a motor-driven artificial belt roadway. Move the engine
forward for stability.
Take a glance at most motorcycles nowadays and you’ll notice that practice and theory are
in agreement. Honda’s Gold Wing and Harley-Davidson’s grand tourers both have engines
mounted well forward (OK, part of the rationale is to get them out from underneath!).
The same is true for sport and racing pavement bikes; what good is additional engine power
if the end result is wheelies that continuously interfere with control. Moving the engine
forward reduces the frequency and severity of wheelies.
Touring and cruising bikes have engines set so low that they only allow for 30-32 degrees of
lean in corners (any more and bright sparks fly from hard parts hitting the pavement). Riders
must set the weight of those engines as low as possible so that they can control it
confidently when stopped at a light or manoeuvring at a slow speed. The higher the engine,
the greater the sensation of “I’ve gotta watch it or this thing will pin me to the pavement.”
Similarly, the extremely low seat height—as little as 24 inches—allows the rider to plant his
or her feet firmly on the ground to manage the weight.
Sport and racing motorcycles, whose performance is predicated on rapid roll response, must
centralise their principal masses (engine, rider, and fuel). Mass centralization is at the heart
of speedy reaction.
One hundred twenty-five years of tyre and rubber development has resulted in fabulously
grippy tyres, but in order to fully utilise that grip, we require cornering clearance (the ability
to lean bikes to tremendous angles) as high as 63 degrees from vertical, in the case of racing
slicks on good pavement. That, too, necessitates engines mounted high in the chassis and as
small as possible.
Another issue is steering geometry, which involves two numbers:
The rake angle refers to the angle at which the steering axis is set vertically. The trail is the
distance between the centre of the front tire’s footprint and the steering axis projection on
the pavement.
In the 1970s, rake angles on pavement sport and racing motorcycles ranged from 27 to 31
degrees. A good explanation for these figures (unfashionably large today) was the higher
overall flexibility of the steel tube chassis at the time. The more flexible the chassis, the
more prone to instability, necessitating more stable steering geometry to control it.
As sturdier chassis and forks became available in the 1980s, smaller rake angles and slightly
shorter trails became practical. Today’s sportbikes have rakes of about 23.5 to 24.5 degrees
with trails ranging from 3.75 to 4.0 inches.
Bottom line? The more bendy material between your hands and the front tyre, the less
responsive your riding will be.