_real_ benefits of "mass centralization"? (like buells)

i recently read an article where buell touts the virtues of their
low-mounted exhaust to help in "mass-centralization" in enhancing a
motorcycle's directional agility, beyond a normal set of rear-mounted
silencers can support.

this got me thinking about the axes about which a motorcycle rotates
when turning, and how the location of the exaust system (i.e., the
mass) affects the rotation.

the vertical axis about which a motorcycle rotates _directionally_ is
actually located at the rear tire's contact-patch. in this case, the
rear mounted canisters would be a benefit, as their mass is closer to
that axis. however, i think _directional_ changes about this axis are
relatively slow, meaning that the +/- of a particular canister
placement strategy vs. another are of minimal importance in the end.

that brings us to the longitudinal axis about which a motorcycle
rotates when _leaning_ into turns, which is basically a straight line
between the front and rear contact-patches. in this case, low-mounted
canisters would be of substantial benefit, as buell's canister
placement, for example, are as close to this axis as practical.
compare this w/ normal rear-mounted canisters which are fairly high if
mounted to the side(s) and very high if mounted above the tire in the
tail. directional changes along this axis, such as that seen in quick
left- and right-handers is VERY rapid, so the +/- of a particular
strategy in this department are definitely worth considering.

in the latter case, one can argue that the difference between
tail-mounted canisters and side-mounted canisters was significant
enough to drive their location on the side of the gsxr1000 despite the
market demand for a tail-mounted setup.

seems to me that "mass centralization" as a name is catchy, but
somewhat off-target. a more aptly named strategy would be more like
"reduced CG height", but it doesnt roll off the tongue nearly as well.

 

Mass centralization reduces moments of inertia around the various axes.
Jim Hall, the engineer/driver of the Chapparal Can Am racers said so in
a series of articles in Sports Car Graphic back in the 1960's. He
wanted to explain why mid-engined cars would corner better than
front-engined cars or rear-engined cars.

Mid-engined cars do corner best, up to the limits of traction, and the
engine location
that doesn't do terrible things at the tire contact patches doesn't
work to warn the driver that he's approaching the limits.

Motorbikes just add other axes of rotation to the problem. They have to
roll so they can yaw. Motorbike engineers are willing to compromise and
accept problems in the pitch axis, if they can just get the motorbike
to roll down to a cornering attiude quickly.

So the mass is centralized at a point so high it horrifies sports car
chassis designers. The center of mass is somewhere above the height of
the two axles.

Most sophisticated riders would immediately realize there were three
axes to rotate about, but one guy told me he recognized *seven*
rotational axes. Oh, well. The ultimate question is, "Does the friggin'
motorbike TURN IN QUICKLY or not?" A motorbike does not start turning
immediately, it has to be rolled down to a cornering attitude and that
takes fractions of seconds.

Mass centralization minimizes those fractions of seconds and requires
less physical strength on the rider's part. That's not to say that the
riders are limp-wristed sissies, even if they do wear ear rings in both
ears and dye their hair. Some of them may sound like girls with their
high pitched adolescent voices, but look at their necks and shoulders.
Those guys work out to develop the upper body strength to keep shoving
the handlebars this way and that, as they countersteer their machine
down and countersteer it back up through a long race.

Honda's engineer Irimajiri who designed such cutting edge motorbikes as
the 6-cylinder CBX and the oval piston 8 valve per cylinder NR-750
thought that lowering the center of gravity by placing the exhaust
system under a fuel tank shaped cover on top of the engine and placing
the fuel supply in a v-shaped tank underneath the engine would optimize
mass centralization and center of gravity location.

He was embarassed that his upside down NR-500 wouldn't turn as quickly
as a high center-of-mass 2-stroke GP bike with conventionally located
fuel and exhaust systems. Honda had wanted so badly to run their
trademark four-stroke machines in GP racing and dominate it as they had
done in the 1960's.

In order to reduce speeds and save tires on the motorcycle course at
Daytona, the AMA mandated a hay bale chicane at the end of the back
straight. Riders had to rapidly flick their racebike back and forth on
faith that they would make the turns. Long wheelbase motorbikes with
low centers of mass wouldn't do it. Short wheelbase bikes with high
centers of mass would flick back and forth quickly, and wouldn't
require as much lean angle to turn a corner of any given radius, but
they tended to wheelie on shorter courses. Riders found that longer
wheelbase machines were quicker on short courses because they could get
on the power quickly after the apex of a corner.