Extra Heft and "Stability"

["Tom Brown" <tbrown@xxxxxxxxxxx>]

Gentlemen:

I have a 400 LB sport bike, an Aprilia Mille R.   It's very stable at
highways speeds.  It's very stable at 140 mph too.  I don't see WSB riders
having much trouble holding their course in a straight line at 170 + mph
until they start pulling hard on the front brake and downshifting for
corners.

My RT is more prone to trying to kick itself off-line when a truck goes by
or in a crosswind than the Aprilia, which I can ride though all kinds of
wind and rain without constant correction of my line.   A lot of this is
aerodynamics and "sail", the amount of solid surface on the side of the
bike, especially up high, that can catch crosswinds.  Any bike with a solid
front wheel, like some Harleys, will have a severe cross-wind steering
problem. Many have complained about the solid aluminum wheel on the V-rod.
My RT has a large windscreen and usually a top trunk on it.  I'm used to
leaning into the wind and having to adjust my line when I go under a bridge.
It seems like part of riding for me.  When I do this on the Aprilia, very
little correction is needed.   This is due to the fact that the Aprilia is
all hunkered down low and has a low windscreen, no top trunk and excellent
aerodynamics.

Stability and weight are really pretty unrelated on a bike.   More mass and
longer wheelbase will give a more comfortable ride over bumps with the same
spring rates, given enough suspension travel, but that's not really
stability.  That's comfort.   You can get comfort by using a lighter spring
on a lighter bike.  The trade off is brake dive and acceleration squat.
This can be minimized with good rebound and compression damping rates.  Not
to hard, just enough.

Geometry of the front fork affects the ability to turn in and straight line
stability.   A longer angle (front wheel sticking out farther in front of
the bike, will give handling that doesn't like curves very much at all, but
allows you to ride all day without putting your hands on the bars.  (This is
one of the attractions of choppers).   A very pushed-in front wheel, gives
sport-bike turn-in and the ability to zig-zag quickly, but can make the bike
prone to tank-slappers at high speed.   This is why those steering dampers
were created for sport bikes and why the older Brit bikes had those friction
screws between the handlebars.

It's possible to change the effective angle of the front fork by raising or
lowering the rear suspension of the bike.  Raise the rear end and the bike
with turn in better.  Lower it, and it will feel more "stable" on the
highway.  You can do this to an extent with the spring preload adjustment,
but that will also affect other aspects of the suspension.  This is why some
bikes/shocks have adjustable ride height independent of the spring preload
setting.

With a telescoping front fork, it's common practice to raise the fork tubes
up in their clamps, lowering the front end of the bike, to get sharper
turn-in.  This is not so easy with Telelever.

- -TB

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