I have
read recently that today's GP motorcycles have moved from the
traditional mc steering that keeps a high degree of inherent stability,
to a steering geometry that has none of that, but requires conscious
effort by the rider to run straight.
.Bill/ other MC racers or any one interested
The main ingredients in motorcycle directional stability are ( the up side)
1. forward motion (inertia ) which tends to keep the bike moving in a straight
line & the higher the bikes speed and weight the less it will be effected by
outside forces trying to change its direction, inertia is a function of mass
as well as velocity
2.Trail is the distance the contact patch of the front tyre lags behind the
point where the steering axis intercepts the ground, the greater the distance(
up to the point of diminishing returns) between the steering axis intercept &
the contact patch the stronger the self centering force becomes and this
increases as speed rises
3. Gyroscopic forces ( primarily the two wheels) gyroscopic resistance to a
change in lean angle increases as the rate of rotation of the wheels increase
,gyroscopic forces increase as speed increases ,also gyroscopic precession
plays a roll in the bikes stability
4. long wheel base increases stability as it allows for high polar moments
which makes for slow steering but high stability
So the stabilising effects of inertia , trail and gyroscopic effects all
increase with speed
as an aside bikes steer using camber thrust where as cars use slip angle so
car and bike tyres are designed very differently and should not be swapped
whether this applies for straight away racing I know not
now the down side ( aerodynamics and directional stability)
as we all know a lower Cd and effective frontal area reduce drag but in our
attempts to reduce drag using ( partial streamlining ) our bike now suffers
much more from side winds than our non faired cycle, also even if we manage
neutral lift from this streamlining we will still have potentially dangerous
lift over the front wheel produced purely as a by-product of drag, drag force
acts through a single point at a finite distance above the ground , this
produces a torque trying to rotate the bike around its rear wheel thus lifting
the front wheel ( so we need to keep the height of the bike to a minimum )
Our directional stability can also be upset when the C of P ( centre of
pressure ) is in front of our C of G ( centre of gravity ) , as our speed
increases separation from laminar flow to turbulent happens sooner ,which
moves the C of P forward of our C of G causing instability, if we move our C
of G closer the front we alter our forward / rear mass bias then we get
problems with to little weight over the rear tyre and traction problems arise
Unfortunately the rules for partially streamlined bikes do not allow effective
measures to overcome these problems i.e. tails, fences , splitter plates etc.
and we have to move on to a full streamliner
anyone got any thing to add or correct if my reasoning is wrong , as I posted
earlier this speed game is all about compromise and finding the best ones
Gary
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