Description
This article is from the How to Become a
Motorcycle Roadracer FAQ, by Duke Robillard duke@io.com with
numerous contributions by others.
7.1 Why do Road Racers Stick Out Their Inside Knee?
(Thanks to Duncan Hardy (duncan@ducati.uk.sun.com) for the bulk of this
answer, and to Jobst Brandt (jbrandt@hpl.hp.com) for the effect lean has on
the tire performance.
1. It lets the bike lean less for a given speed. By sliding off the
inside of the seat, the rider's body weight is moved towards the
inside of the corner. This means the bike needs less lean for a given
speed and turn radius. As ground clearance is often the limiting
factor in cornering (particularly at higher speed) this allows the
rider to corner at higher speeds.
Here's some nerd info that describes turning geometry:
t= arctan [ v^2/(g*R) ]
v is your velocity, R is the radius of your turn, g is the
gravitational constant. t is the "lean angle." It's the angle between
1. the horizontal, and
2. a line from the contact patch of your tires through the center of
gravity of the bike-rider system.
Hanging off and sticking out your knee moves the CG of the system to
the inside, while leaving the bike more upright, so you don't run out
of ground clearance.
Aside from running out of ground clearance, a second limiting effect
is caused by excessive lean. At lean angles below 45 degrees from the
horizontal, tires no longer purely roll and are rotating more about a
vertical axis rather than a horizontal one. This makes them act more
like a rotary brush of a street sweeper than a rolling wheel. With
steep lean, the contact patch twists in place scrubbing away traction
and power so that leaning farther reduces cornering ability and
ultimately causes a washout in the turn. Incidentally, since cars do
not lean, they do not exhibit this behaviour, and can corner with
greater G-forces than motos.
2. Some riders like to 'feel' the road so they know their lean angle.
(You can't afford to look down). It doesn't hurt - they wear pucks on
their knees to avoid damage. Only problem sometimes is the rumble
strips on car circuits.
3. Top riders can use their knee to change the balance of the bike, for
example taking some of the weight off the rear wheel when they want to
induce a bit more slip.
4. Some will claim it also aids braking (because of air drag) going into
a corner. Every little bit helps....
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