Hub-center steering

Hub-center steering (HCS) is one of several different types of front end suspension/steering mechanisms used in motorcycles and cargo bicycles. Hub-center steering is characterized by the steering pivot points being inside the hub of the wheel, rather than above the wheel in the headstock as in the traditional layout. Most hub-center arrangements employ a swingarm that extends from the bottom of the engine/frame to the centre of the front wheel.

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 systems have only appeared on a very few production motorcycles, and not with any great success. Evolution, rather than revolution, tends to drive advancements in new models, and dictate sales. After so many years of telescopic forks, people are used to riding a bike that handles in a specific way, and almost expect the limitations, and compensation is part of the experience. Also there is a depth of knowledge known about fork based chassis design that attempts each year to get around the limitations through technological advances on the current system. Thicker and thicker fork tubes are used to reduce flex, special coatings are used to aid stiction, and greater and greater steering angles are used to counteract dive.

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Wikipedia