From: lotto@laura.harvard.edu (Jerry Lotto)
Newsgroups: rec.motorcycles
Subject: Stopping distances (was Re: ZX-11 front brake)
Date: 19 Nov 92 14:43:46 GMT
	<1992Nov18.150911.24658@exu.ericsson.se>
	<1992Nov18.230735.23129@rd.hydro.on.ca>
Organization: Chemistry Dept., Harvard University
In-reply-to: jlevine@rd.hydro.on.ca's message of 18 Nov 92 23:07:35 GMT

>>>>> On 18 Nov 92 23:07:35 GMT, jlevine@rd.hydro.on.ca (Jody Levine) said:
Jody> In article <1992Nov18.150911.24658@exu.ericsson.se> lmcstst@noah.ericsson.se (Stamos Stamos) writes:
Stamos> I can lock the front wheel of my ZX no problem, in fact it's too easy 
Stamos> to lock up the front wheel. 

Depends on the situation.  At high speeds at maximum weight transfer
it should be pretty difficult to lock up the front tire on any
motorcycle.  At low speeds or by sudden application of the front
brake, most brakes should be capable of locking the front wheel.

Stamos> A ZX is much heaver than a CBR900, but then again someone in
Stamos> rec.autos posted some formula on braking distances and he claimed
Stamos> that the weight of the vehicle didn't affect the distance

Jody> Yes and no, minister. Naively, the heavier the vehicle, the more down
Jody> force you have, and since traction is related to down force (no, in

Right on the money, at least static traction (no skidding, shear,
etc...) is.

Jody> Sports cars with ABS stop in the shortest distance (Motorcyclist). The 
Jody> average biker stops better than the average cager (riding course).

Actually, the statement made in the RiderCourse is that a SKILLED
operator can stop a motorcycle in less distance than (non-ABS
equipped) cars.  The "non-ABS equipped" addition is a recent update to
the MRC:RSS.

Skilled motorcyclists can stop in shorter distance than car drivers
because they have independent control of both wheels.  Since a
motorcyclist can modulate brake pressure to account for individual
tire traction, they are more able to approach the theoretical minimum
for stopping distance.  A car driver that has a single control to
operate four brakes will not be able to accomplish that without
mechanical (ABS) assistance.

As long as the tires are not skidding, the weight of a vehicle affects
traction and momentum in a way that offset each other.  This is not
precise, but it is good enough for a generalization.  Therefore, the
additional weight of an automobile is not an advantage.  Neither is
the contact patch or number of tires (basic fizziks).  After you bring
shear forces, hydroplaning, heat dissipation, friction with the air
(profile), and a dozen other variables into the equation, the previous
paragraph is a still a decent approximation for non-skidding, dry,
straight-line braking.

Stamos> If two brake systems(for the same bike) both have the power to lock
Stamos> the front wheel but one is better, what is it that makes it
Stamos> better/stop in a shorter distance?  Isn't the front tyre (and
Stamos> suspension) the limiting factor??

Jody> That sets the limit, the other task is to find it. How easily you can
Jody> find the point of almost-lockup, where you have the most braking force?

The problem is more complicated since weight transfer is a
time-dependant phenomemon.  The "point" of impending skid is a moving
target that you need to hit from the onset of braking to complete stop
on both tires to accomplish the theoretical maximum stopping distance.
In real life situations where the surface is not ideal, impending skid
can still be approached but a greater reserve is required to take up
surface changes and imperfections that the suspension cannot
completely take up.  It is not easy at all.  On a given motorcycle
under a controlled set of conditions (like a track), you can get
pretty close.  Specifically, you can do better than ABS!  Once you
start adding variables like mottled pavement, unfamiliar equipment,
and non-ideal tire condition or inflation, it is MUCH more difficult.

Stamos> Is it the feel of the brake that allows you to use maximum pressure
Stamos> without locking the front? 

Jody> This is the bottom line.

Well, I would say that it is more the feel of the front tire that
clues you into impending skid.  As long as you are applying the brakes
properly and approaching (as opposed to rapidly exceeding) impending
skid, the feeling is unmistakable!

Stamos> Is it the lead time of the pads from when you first pull on
Stamos> leaver to when they really start to grip? 

Jody> If you have nothing nothing nothing then lots lots lots, then your
Jody> brakes are grabby and difficult to control. This is part of feel.

(lots of discussion of the physics of brake system fade and onset deleted)

This is the true bottom line.  Issues like brake system changes are
not really important under normal traffic conditions.  When you first
apply your brakes on a wet motorcycle, or if you are talking about
drum brakes, this stuff is important.  On most modern motorcycles,
after the brakes have been used once or twice and the tires are
marginally warm, rider skill is 98% of the battle.

The "feel" of the front brake is very important, but YOU have to
modify YOUR technique once you are familiar with it.  You should be
able to apply the front brake gently unless it is operated by an
electrical switch :-)  Ideal emergency braking requires that you
apply (and maintain) both tires at impending skid.  In the real world,
you have to leave some reserve but the technique is the same.  Use a
firm PROGRESSIVE squeeze on the front and lighten pressure on the
rear.   People who complain of grabby brakes or jumpy throttles are
comparing the behavior of a motorcycle to something THEY consider to
be a standard.  As long as the motorcycle is not in need of
maintainance, "grabby" brakes can be solved by a lighter touch.
--
Jerry Lotto <lotto@lhasa.harvard.edu>         MSFCI, HOGSSC, BCSO, AMA, DoD #18
Chemistry Dept., Harvard Univ.  "It's my Harley, and I'll ride if I want to..."