"Morgans" wrote:

"Mark Hickey" wrote

(draw
a line through the steering axis, and if the wheel's contact point
with the ground is behind the axis, it's stable).

A bike is not in agreement with your statement. Yet it is stable. Any idea
as to why?

Actually, it is... the steering axis is canted (usually by 16-18
degrees). Remember what matters is the relationship between the
steering axis (a line drawn through the center of the head tube, or
fork's "steer tube"), and the point the front wheel touches the ground
(not where the steering axis crosses the wheel, which is ahead of the
steering axis line). Don't get misled by the centerline of the fork's
blades, which can be canted to the front, putting the axle ahead of
the steering axis - it's the contact patch that counts.

It's a lot easier to visualize if you're looking at a bike - the
steering axis line passes clearly ahead of the front tire's contact
patch (by 2.5 - 3" on most bikes). In effect, this produces the same
kind of results that a shopping cart's rear wheels does - the front
wheel wants to "trail" behind the steering axis. It can also get you
in trouble when you hit a very steep bump or get the rear wheel up in
the air a bit (since the contact patch can end up ahead of the
steering axis, and can take the bars out of the rider's hands if
he/she isn't paying attention).

Mark "and I always hate it when that happens" Hickey