The drivers probably do it on purpose in order to get the bike's front
wheel pointed at the ground while in the air.
<begin physics teacher mode>
While in the air, there are no external torques acting on the
bike/rider system, so angular momentum is constant. The three main
sources of angular momentum will be the two wheels and the rotation of
the bike and rider. The rotation of the engine crankshaft and flywheel
would also contribute, but probably be smaller since they don't have as
much moment of inertia as the wheels or bike. By applying the brake to
one (or both) of the wheels, the angular momentum will be transferred
to the bike and rider, and the rotation rate of the bike is increased
and in the same direction as the wheels were spinning. Conversely, if
the throttle is opened and the rear wheel is accelerated, the bike will
start to rotate in the opposite direction.
<end physics teacher mode>
I don't really know why they preferentially stop the back wheel, but I
could imagine that it has something to do with what would happen if
they hit the ground with the brake still on. With the back brake on,
the back wheel will just skid along until they release the brake, but
with the front brake on, they'd probably flip over.
Having never actually done this (and never really wanting do), this is
all just speculation and I'd be interested in hearing what the drivers
would have to say about it.
Mark
On Feb 24, 2004, at 10:35 PM, Stephen Scharf wrote:
>> Nice Shot! What is curious though is the apparent motion of the front
>> wheel
>> and the lack of motion on the rear.
>>
>> Gord
>
> Yes, I wondered that too, because I have a no. of shots that show
> that effect, the only thing I can think of is that the rear is
> stopped from engine braking, being connected to the engine by the
> chain, whereas the front is free to spin or freewheel.
> -Stephen.
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