Hub-center
steering (HCS) is one of several different types of front end
suspension/steering mechanisms used in motorcycles. Hub-center steering is
characterized by a swingarm that extends from the bottom of the engine/frame to
the centre of the front wheel instead of two forks.
The advantages of using a hub-center steering system instead of a more
conventional motorcycle fork are that hub-center steering separates the
steering, braking, and suspension functions.
With a fork the braking forces are put through the suspension, a situation that
leads to the suspension being compressed, using up a large amount of suspension
travel which makes dealing with bumps and other road irregularities extremely
difficult. As the forks dive the steering geometry of the bike also changes
making the bike more nervous, and inversely on acceleration becomes more lazy.
Also, having the steering working through the forks causes problems with
stiction, decreasing the effectiveness of the suspension. The length of the
typical motorcycle fork means that they act as large levers about the headstock
requiring the forks, the headstock, and the frame to be very robust adding to
the bike's weight.
Hub-center steering systems use an arm, or arms, on bearings to allow upward
wheel deflection, meaning that there is no stiction, even under braking. Braking
forces can be redirected horizontally along these arms, or tie rods, away from
the vertical suspension forces, and can even be put to good use to counteract
weight shift. Finally, the arms typically form some form of parallelogram which
maintains steering geometry over the full range of wheel travel, allowing
agility and consistency of steering that forks currently cannot get close to
attaining. The hub center steering's Achilles heel, however, has been steering
feel. Complex linkages tend to be involved in the steering process, and this can
lead to slack, vague, or inconsistent handlebar movement across its range.
Hub-center steering (HCS) is one of several different types of front end
suspension/steering mechanisms used in motorcycles. Hub-center steering is
characterized by a swingarm that extends from the bottom of the engine/frame to
the centre of the front wheel instead of two forks.
The advantages of using a hub-center steering system instead of a more
conventional motorcycle fork are that hub-center steering separates the
steering, braking, and suspension functions.
With a fork the braking forces are put through the suspension, a situation that
leads to the suspension being compressed, using up a large amount of suspension
travel which makes dealing with bumps and other road irregularities extremely
difficult. As the forks dive the steering geometry of the bike also changes
making the bike more nervous, and inversely on acceleration becomes more lazy.
Also, having the steering working through the forks causes problems with
stiction, decreasing the effectiveness of the suspension. The length of the
typical motorcycle fork means that they act as large levers about the headstock
requiring the forks, the headstock, and the frame to be very robust adding to
the bike's weight.
Hub-center steering systems use an arm, or arms, on bearings to allow upward
wheel deflection, meaning that there is no stiction, even under braking. Braking
forces can be redirected horizontally along these arms, or tie rods, away from
the vertical suspension forces, and can even be put to good use to counteract
weight shift. Finally, the arms typically form some form of parallelogram which
maintains steering geometry over the full range of wheel travel, allowing
agility and consistency of steering that forks currently cannot get close to
attaining. The hub center steering's Achilles heel, however, has been steering
feel. Complex linkages tend to be involved in the steering process, and this can
lead to slack, vague, or inconsistent handlebar movement across its range.
Hub-centre steering on motorcycles is not a new concept – it has existed in
two-wheeled designs for nearly a century - the Ner-a-Car of the 1920s had a
simple hub-centre design. The best-known modern production example, and the
inspiration for the Vyrus, is the Bimota Tesi.
The Tesi 1D was introduced in 1991 under the technical direction of Pierluigi
Marconi and wowed riders the world over with its radical suspension setup and
high-tech components, not to mention its exorbitant price tag. Marconi had
developed the hub-centre design in 1982 as a design thesis (hence the name, Tesi
is Italian for Thesis). The design was based on the Difazio hub-centre assembly,
which uses a floating axle riding through a king pin inside an oversized front
hub.
Several prototypes were built and tested through the 1980s, including one that
used a Honda V4 and hydraulic steering assistance. The production version was
announced in 1987 but delayed until 1991 by financial difficulties (a common
occurrence at Bimota). The production model that was unveiled in 1991 ditched
the V4 and complex hydraulic arrangement for a simpler approach – tuned Ducati
Desmoquattro L-twin and mechanical linkages with no assist. The 1D saw several
updates over the years, finally finishing production in 1996 as Bimota geared up
to build the infamous V-Due.
The original Tesi was flawed but served as inspiration for Rodorigo who
developed an updated hub-centre design in 2002. This machine would become the
Tesi 2D “Millenium”, introduced in 2004-05 when the brand was resurrected (for a
second time). He adapted the principles of the 1D but simplified the overall
design. The engine was now an air-cooled Ducati Desmodue 1000DS twin. The
suspension was refined and tweaked to improve on some of the complaints leveled
at the original Tesi. The suspension was damped by a unique springless air
assist shock mounted on the side of the bike, pivoting off the distinctive Omega
frame spars that used the engine as a stressed member. Styling was decidedly
modern and in keeping with Bimota’s new aesthetic direction – gone were the soft
curves and enveloping fairings of the 80s and 90s, here were sharp angles and
naked engineering. The Tesi 2D was a spectacular industrial beauty that wore its
mechanical bits on the outside for all to see.