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Brave New World
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Bimota Tesi 2D & Vyrus 985 C3 4V
Jeff Buchanan
11/01/2006
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Photography by Cordero Studios/corderostudios.com
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Motivated by rudimentary
engineering principals, Marconi and Ugolini focused on how to most
efficiently transfer energy absorption through the front wheel of a motorcycle
to the center of the vehicle’s mass. Their design also targeted reducing
steering mass. These goals resulted in a design utilizing an oscillating front
swingarm for maximum stability. This was the first phase of separating the
steering and suspension functions into two, wholly independent processes in
order to obtain maximum performance from each element. The ensuing experiments
and study became the aspiring engineers’ thesis paper, which in turn, lent its
Italian translation, tesi, as the project’s name. Tamburini was sufficiently
impressed with the design on paper to commit a good deal of Bimota’s precious
resources to developing the machine.
There are multiple reasons why the
implementation of a swingarm for the front suspension may be superior to a
telescopic fork. An oscillating front swingarm with a pivot point close to the
vehicle’s center of mass presents a more direct route for the transference of
kinetic energy absorbed through the front wheel. The weight of a motorcycle in
motion can generate an enormous amount of energy. Any time there is an attempt
to redirect that motion—deceleration or lean angle, for example—the resulting
inertia needs to be dissipated. Increasing rigidity in a motorcycle’s chassis
and forks help, but energy absorption will always naturally move toward the
center of a vehicle.
Typically, motorcycles are fitted with movable forks for
steering at a pivot point on the frame. Those forces of energy are absorbed
through a somewhat inefficient, circuitous route up through the steering stem
and down through the frame. This indirect energy transference results in
potential instability that dramatically unsettles a motorcycle, especially under
the tremendous loads exerted at high speeds.
 Bimota Tesi 2D. (Click image to enlarge)
Telescopic forks, regardless of
their diameter, are prone to flex and have a tendency to twist torsionally due
to their longitudinal, unsupported structure. Perhaps the single largest
challenge for telescopic forks is that they dive under the weight shift of
braking. By design, as forks compress they change length, altering chassis
dimensions and affecting a motorcycle’s handling. Also, as a suspension
component, with the telescopic tubes sliding inside one another, forks are prone
to lateral forces (such as braking or turning) that can compromise their travel,
resulting in uneven movement known as stiction.
Another negative is that
forks, with triple clamps and wheel assembly, represent a large, movable mass
which can render a heavy handling feel when weight is shifted forward during
deceleration, multiplying that mass. All in all, despite the superlative
performance of today’s telescopic forks, there are strong arguments that
contradict the accepted wisdom of traditional front fork assemblies.
 Vyrus 985 C3 4V. (Click image to enlarge)
In
contrast, the most impressive aspect of the Tesi design, due to the dispersing
of energy laterally into the machine’s center of mass—at a low center of
gravity—is the resultant lack of dive under braking. This means the attitude of
the motorcycle remains consistent and the full suspension travel is available
even when the forces of braking are applied, allowing the front wheel to
continue to absorb uneven pavement while decelerating—without question, the most
crucial moment that suspension is needed. In addition to the practical
implications of transferring energy more efficiently with the oscillating front
swingarm, there is the added advantage of structural integrity offered over the
relatively flimsy nature of forks.
The Tesi’s alternative design necessitated
a radical departure from the conventional motorcycle steering system. The front
swingarm negated the typical steering stem pivot for turning the front wheel.
Hub-steer was the logical solution. With hub-steer, a king-pin inside the front
hub allows the front wheel to be turned on the axle through an auto-motive-style
linkage. Technically, this makes the steering “indirect” and lends a slightly
surreal aspect, in both appearance and function, to front wheel response.
However, riding a hub-steer machine is not as alien as you might think. All
the customary principles of counter-steering, balance and rider input still
apply, but there is a slightly detached feel at extremely low speeds due to the
indirect nature of the steering linkage, which is a little slow (we are talking
milliseconds) in transmitting input to the front wheel. The system, just like
the automotive world it is borrowed from, has some floating tendencies resulting
from the natural play in the steering rods. However, adaptation by the rider is
rapid.
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