Re: Motorcycle Steering

[jmclean@chem.ucsd.edu (James Mclean)]

(Quoted bits from David Thiessen, rearranged a bit.)

> QUESTION: Should I even bother to respond to the article since the "common
> sense" (which isn't common) of the vast majority of the readers will agree
> that outward centrifugal force balances gravity and they will never have
> heard of angular momentum?
Although the *forces* of gravity and centrifugal force don't balance, their
*torques* do (i.e., about the pivot point where tire meets road).  Although
that idea would be easy to explain (even to someone who hasn't heard of
angular momentum), in this case I think most people would regard the
distinction between force and torque as a detail of little importance.  So
I would suggest that it would be more important to focus on the other
factors in a letter.

> For those unfamiliar with counter-steering, the process is known as: Push
> right, lean right, go right; Push left, lean left, go left.  "Push right"
> means pushing forward on the right handgrip in order to lean right into a
> curve going off to the right.  The amount of lean needed is a function of
> bike speed and the radius of the curve (Tan (theta) = v^2/Rg ).  Racers
> also shift body weight signficantly but touring bike riders do not.
>
> A SECOND INTERESTING ASIDE:  If you do not ride a motorcyle, you can
> demonstrate counter-steering to yourself while riding a bicycle.
I don't know the first thing about motorcycles, but I know for sure that
*during* a turn on my bicycle, the handle bars are car-steered.  I assume
that the same is true of motorcycles, and that counter-steering applies to
*starting* a turn?

> While
> riding at a safe, reasonable speed, gently push forward on one of the
> handgrips and notice what happens to the bicycle.
> ....
> Now for your own proof:  Remember when you were a kid riding your bike on
> the sidewalk because you weren't old enough to ride in the street?
> Remember when you got too close to the edge of the sidewalk and you tried
> to suddenly car-steer the bike back to the center of the sidewalk?  Do you
> remember what happened?  For some explained reason, the bike leaned the
> wrong way and you rode off into the grass, perhaps even falling over.

Re-performing the experiment confirmed my previously formed opinion based
just the sort of thing you mentioned above.  My explanation is somewhat
different than yours.

Pushing on the right handle bar causes the bicycle to turn left just as a
car does; there is no counter-steering effect directly attributable to the
push on the handle-bars.  However, the bicycle is now turning while
vertical: in the bicycle's reference frame, there is a torque from
centrifugal force tending to tip the bicycle over to the right, and no
torque due to gravity.  If this condition persisted, the bicycle would
indeed fall over to the right - ouch!

But then your bicycle-instincts kick in, and after the bike has tipped a
bit to the right, you push on the left handle bar, and you find yourself
turning nicely to the right with the torques due to gravity and centrifugal
force balanced.

To summarize, the initial counter-steer is just a way to tip the bike to
the right, so that a balanced turn can then follow.

To summarize the differences from David's understanding, the torque on the
front wheel (about the steering column) from the initial counter-steer is
not the main cause of the bike tilting - instead the tilt is due to
centrifugal force.  Also, coning is not the main cause of the ensuing
turn - instead, the turn is due to the driver following the initial
counter-steer with a car-steer.  Note that if coning were the main reason
for the turn, then there would be no need to re-adjust the handle bars from
counter-steering to car-steering.

I leave it to others to experiment with how a motorcycle differs from a
bicycle.

-- 
--James McLean
jmclean@chem.ucsd.edu
post doc
UCSD