Hey guys -

Tom's tutorial:

For a multi-track human powered vehicle (HPV) to remain
upright, the projection to the ground of the combined
rider/HPV center of mass must remain within the area
defined by the tire/ground surface contact points.

For fairly static vehicles on tilted surfaces this is a
valid comment. For the more common tipover threat of
taking tight corners a bit too fast, I think it might
be better stated as:

For a multi-track human powered vehicle (HPV) to remain
upright during lateral acceleration (flat wide turns), the
coupling moment resulting from that lateral acceleration
with a moment arm effectively the same as the height of
its center of mass must not exceed the counteracting
moment of the vehicle's total weight (acting downward)
with a moment arm equaling the distance between the
center of mass and a line between the two outboard wheels.

While on a quad with equal front and rear track width,
this line is parallel to the vehicle's longitudinal
axis, for a trike it's an an angle to that axis, and the
farther the center of mass is from the two-wheeled 'axle'
the easier it will be to tip the trike.

For a trike and a quad with the same weight, height of
center of mass and track width, the trike will be more
likely to tip since the stabilizing moment arm will be
shorter for any practical design. For example, in a
tadpole with equal weight distribution, this distance
(and therefore the vehicle's maximum lateral g-force)
would be about two-thirds that for a quad. Under real
world conditions, an aggressive tadpole rider will
compensate for this condition by shifting his body
weight (fairly radically) into the turn.

Regards,
Wayne