32GO wrote:

Jeff and I got a bit off the topic in the TdPS. His last
post there said:

I have no problem piloting a trike in the mid to high
20's but when I go down a hill of any decent size,
I'm very quickly in the 30's and the trike does not
feel stable.

I'm often intrigued at
the conversations of 'bent bikers comparing their
impressions of long wheel-base, short wheel-base and
lowracers, and who feels safest at what speed on which
machine. Trikes are probably a bit different in that
they require almost nothing in the way of operator skill
or training at speeds up to 'casual cruising', and some
riders seem to be lulled into forgetting that as they
push the limits, trikes begin to demand some of the same
skills and heightened attention that it takes to ride a
bike fast.

I'm going to offer my observations based on lots of riding on upright
bikes and trikes. I have ridden 'bent bikes of different kinds, but I
must admit that they all felt dangerously unstable to me at any speed,
like they would dump me in a heartbeat if I even /thought/ about
letting go of the bars. I have almost no direct experience with 'bent
trikes, but I used to work with Rick Horwitz and I very briefly tried
one of his Thunderbolts.

Trikes as a rule have a natural "critical speed" that has more to do
with vehicle dynamics than it does with the rider's fear and
perceptions. Trikes can't lean, so they have a weight shift in turns.
They also have a natural steering response that is relatively
independent of speed. But as speed increases, the weight shift becomes
more sudden and eventually causes a violent response to even mild
steering inputs, up to and including high-siding the trike. Since the
effect of a harsh weight shift is to jar the rider, and this can result
in unplanned steering inputs, there comes a speed at which every trike
can be considered unstable. The lowest critical speed I have seen so
far was in a Trailmate E-Z Roll Regal adult delta trike, which tried to
buck the rider off starting at about 10mph. This was a result, I
believe, of several factors including extravagant frame flex, high C of
G, narrow track, and the misguided use of a normal bicycle fork offset
on a trike with a slack head angle. A trike will naturally become more
stable as it becomes lower, wider, stiffer, or more resistant to
steering input. That's why tadpole 'bents can be considered "sporty"
while upright deltas never are. However, while a sporty 'bent trike
might have a much higher critical speed than a senior citizen's
neighborhood trike, it still has a point beyond which it can be
considered unstable.

Bikes are different. Their front end steering characteristics cause a
natural self-stabilizing force that increases with speed. This force
causes frame, fork, and wheel flex, however, and thus can result in
shimmy problems as speeds and dynamic forces on the frame rise. But in
a bike and rider combination that does not exhibit shimmy, the ride
becomes steadier as speeds increase-- quite the opposite of what
happens with a trike. I used to have a roughly 1/2 mile downhill
averaging about 8% on my way to work, and with a mild crouch over flat
handlebars I routinely hit a measured 55mph on my upright bike--
without any unsteadiness or handling quirks whatsoever.

'Bent bikes tend to be longer, less triangulated, and smaller-wheeled
than uprights, and most of them suffer from compromised steering
geometry that does not not allow the sort of no-hands riding stability
common to uprights. Thus they will flex more (promoting shimmy) and
exhibit less self-stabilizing than a typical upright bike, and this may
contribute to a lower practical maximum operating speed. So because
'bent bikes are (generally speaking) worse than upright bikes with
regard to stability at speed, 'bent trikes are /more stable/ than
upright trikes, and this makes two- and three-wheeled 'bents comparable
to each other in their sure-footedness at high speed. In contrast,
there is really no comparison between DF bikes and upright trikes in
their handling at speed.

Chalo