Despite the meteoric advancements in motorcycle design and engineering, there is one concept in the annals of motorcycling that continues to intrigue. The concept possesses enough validity, in theory and practicality, to prevent it from being permanently swept under the proverbial rug of invention, periodically resurfacing in various incarnations in the decades since its initial invention. The fact that it has yet to succumb to any substantive lambasting or lasting, damning ridicule by the experts, suggests there just may be something to it. The enigma that refuses to go away is hub-center steering.
Hub-center steering is a creative alternative to an inherent engineering challenge in the traditional, accepted design of modern motorcycles that employ a telescopic front fork. In this common, almost standardized configuration, the front fork is required to perform two dramatically opposed functions: provide suspension for the front wheel and facilitate steering. Ultimately, in basic engineering principles, relying on one design element to provide two contradicting functions results in a compromise of optimum performance for both.
Although several manufacturers have dabbled with hub-center steering, scarce few have actually entered mass production. Certainly, most notable is Italian marque Bimota, virtually branding itself the faithfulpur-veyor of hub-steering design since astonishing the press at the 1983 Milan Motorcycle Show with their innovative Tesi. Yamaha entered the fray in the early ’90s with its impressive single-sided front swingarm, hub-steered GTS1000 sport/tourer. Despite mostly favorable performance reviews, sales tanked and the project was abandoned. Elf Racing campaigned a prototype hub-steer road racer for several seasons with a modicum of success, but that was purely developmental and never reached the public. (Click image to enlarge)
Today, there are two companies selling hub-center steering motorcycles—the aforementioned Bimota, and another Italian company, Vyrus Consultancy. Both of them sell hub-steer machines that mimic the original Tesi design platform. The Vyrus project was born as an independent endeavor, championing the Tesi design when Bimota fell on hard times and ceased production of its own machine. Vyrus continued to develop and refine the unique chassis, keeping the flame of hub-steer ingenuity from being extinguished. The work of Vyrus impressed Bimota. When the Bimota re-entered the market, the company contracted Vyrus to build its Tesi chassis.
Bimota’s Tesi 2D utilizes a Ducati L-twin, air-cooled, two-valve Desmodromic 992cc powerplant, whereas Vyrus tenders two powerplant variations—the 985 C3 4V, equipped with Ducati’s liquid-cooled L-twin, 999cc four-valve engine, plus the 984 C3 2V fitted with the same powerplant as the Bimota. Both machines are brimming with awe-inspiring, avant-garde presence—regardless of the beholder’s personal stance on their function or looks. (Click image to enlarge)
Over the years, there have been a number of experiments with regard to alternative motorcycle front ends, from leading and trailing links to girders and parallelogram designs. There was the Neracar of the 1920s (the first production example of hub-steer motorcycle technology) and, most recently, BMW’s current Telelever front suspension system. Engineers continue to explore paths to advance this aspect of motorcycle design. However, the majority of these concepts—with the exception of the Neracar and Tesi—are based on traditional steering fork designs and were primarily concerned with eliminating the effects of front-end dive, the resulting action of compressing a telescopic fork under braking.
Perhaps what makes the hub-steer project so interesting is that it is such a drastic departure from the traditional steering fork concept. As odd as the design may appear, it actually represents a sound, practical approach to basic physics. The Tesi project came out of the open minds of two students at the Plolitecnico of Bologna in the early 1980s. Engineering hopefuls Pier Luigi Marconi and Roberto Ugolini met with legendary designer Massimo Tamburini—who was then Bimota’s technical director—seeking insights into furthering future design elements. It was Tamburini who pointed the students toward the front axle of traditional motorcycle construction as representing the greatest opportunity for change and development.
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.
Once up to speed the most noticeable and welcome trait of the hub-steer/front oscillating swingarm design is that it is almost completely devoid of front-end dive under braking. The sensation, especially at racetrack speeds, takes a little getting used to. Sitting up at the end of the straightaway at 120 mph and aggressively getting on the brakes results in the bike merely decelerating rapidly.
Yamaha’s GTS put hub-steering to work on a sport/touring bike in the 1990s. Photograph by Yamaha photography.
Entering corners, the hub-steer motorcycle is extremely stable due to the fact that the front suspension retains its full range of movement. Whereas a telescopic fork collapses under the weight shift forward during braking and uses up the damping effect of the front suspension, the hub-steer/oscillating swingarm continues to absorb the forces being collected by the front wheel. The resulting stability is the machine’s magic, giving a rider a new awareness of how much speed can be carried on corner entry.
Because there is no compression of the suspension under braking, initiating turn-in has a deceptive ease to it. By design, the bike does not succumb to the changing rake encountered with compression of a traditional fork. This translates into highly desireable consistent handling characteristics. With the absence of dramatic attitude changes due to the unavoidable weight shifts incurred during braking and accelerating, the transitions are is smooth and controlled.
Photograph by Jeremy Henrie. (Click image to enlarge)
The advantages of the oscillating swingarm become apparent in the bike’s confident display of mid-corner stability. Once the motorcycle has been put onto its line it feels as though it is on rails. We have all read this interpretation before, but, for the first time, the description truly has merit. Another attribute of the Tesi 2D is the consistent, light feel of the front end. This is due to the forces of inertia on the steering mechanism being greatly reduced through the “indirect” design.
But, even with all these positives, the hub-steer/oscillating swingarm design has fought an uphill battle for acceptance. In the public’s eyes, it remains an unorthodox approach with odd looks. However, consider where the concept may have progressed to if the industry had placed the same level of focus and development that has been spent on the telescopic fork.
Problems for a racing application stem from the compli-cated design that inhibits fast wheel changes, a serious deficit given the frequency of tire changes required in competition. Another troublesome glitch is the danger of having the steering linkage exposed to crash damage. Continued development will be necessary to eliminate these design aspects.
Hub-steer motorcycles look odd because they defy our accepted perceptions of what is normal. But, remember, there was a time in the mid-to-late-1800s when all bicycles were built around the Penny Farthing design; the extremely large front wheel with a tiny trailing wheel. It is easy to look back now and think how odd the design platform was. By the same token, in 100 years, people may look back at our beloved motorcycles with telescopic front forks and ask, “Did they really ride those things?”