Re: shifting one's weight

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

Leigh Palmer says:
> In the case of the bicycle one must neglect the mass of the bicycle
> itself.
> ...
> Pursuant to making your example more nearly analogous to the bicycle
> you should shorten the seesaw to a length equal to the pedal
> separation on the bike,...

My message carefully avoided trying to calculate anything for a bicycle,
because of uncertainties in the application.  Even if you consider a
massless bicycle, it is not clear that that is analogous to a massless
(see-saw + mechanism) in my model.  What if you were to remain motionless
excpet to stick your arm out?  Obviously, your hand and arm are too light
to have much effect, but that is an example where most of your body should
be considered as part of the (see-saw + mechanism).  Actual motions while
on a bicycle are too subtle for me to guess how to interpret them.

> > One feature of this model which connects to my experience, and therefore
> > convinces me that it's basically correct, is what happens to the board.
> > Note that, considering the (board + mechanism) alone, it experiences a
> > torque out of the page, so that it initially tips down on the *left*.
> > However, angular momentum of the (board + mechanism + mass) must remain
> > zero, so that once the extension has occured everything will tip down on
> > the right as described.
>
> That's a motorcycle see-saw. with only two feet on it you can't exert
> a torque on an inertialess see-saw.

Excuse me? I don't understand this sentence.

> Replacing the pivot with the road is unrealistic for a bicycle. A
> bicycle has an adjustable pivot. One can move the front of that pivot
> from side to side by turning the handlebars. That is how one balances
> a stationary bike.

Quite true.  But the existance of one mechanism does not prove the absence
of another.  And using that method is impractical while moving, as the
motion necessary to move the pivot under your CoM would produce a turn
which is destablizing rather than stabilizing.

> > Instead, I suggest an experiment.  Ride a bicycle standing on the pedals
> > (no seat contact) and only lightly touching the handle bars.  Now move all
> > your weight onto one foot.  This morning I found that this had a very
> > significant effect on the bicycle, leaning it over and tending to make it
> > turn.
>
> You will find that the results of this experiment depend sensitively on
> the initial condition, the lean (or displacement of COM from line of
> support) of the rider just before the weight is transferred. Try it
> *both* with the bike in a very gentle turn to the left and to the right,
> both times transferring weight to the left foot. I think you may be
> surprised at the different results.

I did not notice any such effects, but I will re-perform the experiment to
look for them.

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

> > PS.  While riding fast in a tight cicle (to check whether the front wheel
> > is actually turned a bit), I also found out that it is possible to break a
> > rear axle by skidding severely!  No damage to myself, happily.
>
> On the flat?! You must be even heavier than I*. I used to break rear
> spokes while laden with camping gear (always chainside, of course -
> Murphy rides a bike), but I have never broken an axle.

Well, it's a very old bike, and probably not the best quality.  It might
well have had a hollow axle, I had it repaired at a shop, so I don't know.
But I was surprised when they told be that too.  Maybe when they said
'break' they meant 'damage severely enough that we put another one in.'