From: lotto@laura.harvard.edu (Jerry Lotto)
Newsgroups: rec.motorcycles
Subject: Re: Countersteering a shafty
Date: 28 Aug 92 02:19:56 GMT
	<1992Aug17.201252.21909@webo.dg.com>
Organization: Chemistry Dept., Harvard University
In-reply-to: x1sdup@esparc19.NoSubdomain.NoDomain's message of 17 Aug 92 20:12:52 GMT

>>>>> On 17 Aug 92 20:12:52 GMT, x1sdup@esparc19.NoSubdomain.NoDomain (Stephen Dupre) said:
Stephen> In article <3524@tegra.COM>, hull@tegra.COM (Andy Hull) writes:
Andy>  dana@locus.com (Dana H. Myers) says...
Dana> Who else has discovered that countersteering does not appear to work
Dana> correctly on shaft-drive motorcycles?

Dana>  It must have to do with the counter-rotation of the shaft cancelling
Dana> out the centripetal counter-steering force, but I'm not sure how the

Andy>   With chain drive, when you roll on the throttle the rear end squats
Andy> which will unload weight from the front end.  With shaft drive rolling
Andy> on the trottle (sic) will make the rear end stand up

Stephen> I agree more or less with the information above.  My experience is
Stephen> that MOST shaft drive bikes lift 
Stephen> ... the rider should not be 'rolling on the throttle' in a turn
Stephen> causes the rear suspension to extend (most shafts) and therefore
Stephen> decreases traction (less rubber on road).

This whole thread is getting pretty bizarre.

First off, shaft effect is not so great that it completely negates
weight transfer.  Are you saying that a shaft bike will do a reverse
wheelie on acceleration!?!  Sure, there is some rear extension, but
that doesn't reverse weight transfer to the rear wheel.

Second, countersteering works because of SIDE forces.  If shaft torque
forces were significant enough to affect countersteering, the
motorcycle would attempt to lean on straight-line acceleration.  This
problem is usually circumvented by using shafts which are not a
significant fraction of the motorcycle weight :-)

The main reasons for rolling on the throttle in a turn is to stabilize
the suspension and increase ground clearance.  Turning IS an
acceleration, and without a gentle throttle roll-on, you can't
maintain speed.  I don't believe that anyone suggested a drastic roll.

Finally, let's get this concept of traction dialed in.  If you ignore
heat disappation, sliding friction, surface inconsistancies, and
hydroplaning (yeah, I know, but let's get the simple part right first,
OK?) traction has nothing to do with the "amount of rubber on the
road".  Traction is a function of coefficient of friction and weight
(vertical force).  That's it.  Unless you are going uphill or
downhill, the overall traction is the same in a wheelie as it is
during normal travel or in a turn.  It is just distributed a little
differently :-)

On to countersteering.  The best way I have to explain the phenomenon
is imagine trying to turn a motorcycle left at speed by turning the
handlebars.  The front tire starts to track left and the bike "falls"
to the right.  As long as the tires have sufficient traction, this
falling is actually a lean to the right.  As soon as the motorcycle
starts to lean, steering trail takes over the angle of the front wheel
and camber thrust makes the motorcycle turn.  Precession (gyroscopic)
effects only come into play at higher speeds and always resist
steering input.  By the way, this is the reason that you want to roll
on the throttle in a turn.  It counters centripital force (which is
trying to move you in a decreasing radius spiral) and aids balance in
a lean.  Try rolling a single tire and watch what happens as it falls
towards the inside of a turn.

Now to be fair, I have presented a simplified picture.  Precise
analysis of motorcycle handling dynamics takes more like a book than a
posting to explain, and I doubt that I have a good enough handle on
most secondary effects to do them justice.  However, the descriptions
I gave above are a pretty decent approximation.  I hope that it (at
least) gets this "shaft" thread back on track.
--
Jerry Lotto <lotto@lhasa.harvard.edu>      "Have you hugged the curves today?"
Chemistry Dept., Harvard Univ.              MSFCI, HOGSSC, BCSO, AMA, DoD #18