Bimota Tesi And Supercharged Kawasaki Are A Rad Combination

Revisiting the much-tried concept of motorcycle hub-center steering.

Tesi H2
At the Tesi H2’s EICMA unveiling in Milan, Kawasaki planning division manager Hiroshi Ito told the backstory of the Japanese manufacturer’s investment in Bimota: “A small investment bank approached us inquiring if we were interested in an Italian motorcycle manufacturer,” he said. “When I checked the documents, I instantly noticed, ‘Oh, it’s Bimota. Yes, that Bimota.’ ”Bimota

Why did respected publishers Blanche and Alfred Knopf hotfoot it into Europe, with the smoke of World War II barely cleared, to be first to sign French existentialist writers Camus, Sartre, and de Beauvoir to book contracts? Because they were raddest of the rad, representing a revolutionary new hack on life.

Sportbikes are finished. Adventure, hailed as the “next big thing,” has failed to whisk us all to Patagonia. And lockable storage? Testosterone-proof small wheels? Don’t make me laugh.What happens next? When the conventional puts people to sleep, you wake them up with something so different it can’t be ignored. That at least gets their interest, and that’s what motorcycling needs at the moment: People, potential new riders among them, paying attention.

Bimota’s Tesi (Thesis) came forth in 1980, breathing new life into the very old concept of hub-center steering. The 1980s were a period of intense motorcycle chassis experimentation by many innovators, but experience, including a sustained effort in 500cc Grand Prix racing by ELF Honda, failed to reveal any of the many prototypes built and tested as a path to the future. All that remains of that decade of work is the single-sided swingarm.

Bimota Tesi H2
“Combining this engine and that chassis,” Ito told former Bimota engineer Pierluigi Marconi, “we can make a great bike.” Kawasaki established Italian Motorcycle Investment as a subsidiary of Kawasaki Motors Europe. Bimota S.A.’s trademarks will registered in Kawasaki’s name, after which IMI is to be renamed Bimota S.p.A., with B and Motion S.A. (formerly Bimota A S.A.) owning 50.1 percent and KME as 49.9 percent shareholders.Bimota

It turns out that doesn’t matter to most people. They like radical motorcycles even if they fail to dust off conventional designs in racing. People prefer extremes to middle-of-the-road blandness.

Kawasaki saw an opportunity to burn its brand into our minds. The Japanese manufacturer revived that variable star of radical motorcycling, Bimota, then combined its most radical design, the Tesi, with Kawasaki’s most radical powerplant, the mechanically supercharged H2.

Forget World Superbike. Forget racer-replicas. The Tesi H2 brings to two wheels what the 1,000-hp supercars from outfits like Koenigsegg or Bugatti have brought to four. They are about being “the most.”

Let’s now review what hub-center steering is and the motorcycle problems it was created to solve.

Nearly every automobile made has a form of hub-center steering. Each front wheel spins on a stub axle that is made in one piece with what’s called an “upright,” a vertical member with ball joints at top and bottom. Projecting sideways from the front structure of the car on both sides are a pair of A-frames, each able to swing up and down on frame-mounted pivots. The apex of each of these A-frames attaches to the ball joints on the upright.

Symmetrical aluminum beams
Tesi lives again: Symmetrical aluminum beams reach around the front tire, providing steering clearance. The outside diameter of the non-rotating hub carries a pair of large-diameter but slim-section bearings. The front wheel itself, having a hole through its center of suitable size, spins on these large-diameter bearings.Bimota

Now we have a design that combines suspension (the up-and-down swinging of the A-frames) and steering (the upright and its stub axle can rotate on its ball joints).

To provide steering control, a steering arm is attached to each upright, and the two steering arms are connected to the driver’s steering box by links with ball joints on either end. As the driver steers, the links are moved to right or left, causing the steering uprights to rotate around their top and bottom ball joints to steer the two front wheels.

The weight of the front of the car is supported by springs that typically bear against the lower A-arms, often with a tubular suspension dampers centered within the springs.

This brings us to the natural question: What’s the advantage in all this complexity?

Brake torque from the front discs is reacted to the chassis by mounting the calipers on the uprights. The “deep-dish” construction of auto wheels allows the upright and associated parts to be mostly located inside the dish.

One form of motorcycle hub-center steering explored by the ELF Honda group in the 1980s is a modification of this, in which the A-arms are quite long and bent around to pivot behind the wheel on the lower front of the motorcycle’s frame.

This brings us to the natural question: What’s the advantage in all this complexity? On a conventional motorcycle with pivoted telescopic-fork suspension, tire grip forces originate at pavement level and are then transmitted up through the wheel to the axle, into the fork, and from it into the steering-head bearings located nearly 3 feet above the pavement. These forces must then be transmitted back down into the motorcycle’s frame. This is a long and flexible stress path!

Back in the 1980s, when alternative front ends were being furiously investigated, the powerful incentive was tire chatter—a rapid hopping of (usually) the front tire in hard cornering, which only got worse the harder the rider tried to “push through it.”

This was the basis of the stiffness argument in favor of hub-center steering. Instead of stress from tire grip traveling the long way as in conventional bikes, a hub-steerer’s stress path would go directly through the two A-arms, straight back into the lower frame of the motorcycle. This is the efficient structure argument for hub-center steering, with the hope that without the springy conventional structure, tire chatter would be prevented.

The two other principal motivations for seeking front suspension by other than telescopic fork are the stiction argument and the pro-dive argument. The telescopic fork moves on sliding bearings, and especially when side-loaded by hard braking, lubricant is squeezed out of such bearings to leave them in a condition of mixed lubrication, part oil film and part surface-to-surface contact. It is characteristic of surface-to-surface friction to stick and slip alternately, as when a heavy crate must be slid across a floor. This stick-slip friction, when occurring in a telescopic fork, has come to be called “stiction.” Such irregular motion interferes with front-tire grip by causing large variations in tire-to-pavement pressure (tires hate that). Pivoted suspensions, of which hub center is one type, all but eliminate stiction, and as one common result display somewhat shorter braking distances.

The pro-dive behavior of telescopic forks arises from the angling of the fork legs, which can be seen to convert a part of braking force to fork compression, which would not be the case if the fork’s rake angle were zero. Pivoted front suspensions can be designed to have any desired behavior—pro-dive, anti-dive, or dive-neutral—through the action of suitable linkage.

One of the ELF Hondas, powered by a four-stroke engine, has both A-arms reaching into the front wheel from the left side to carry an upright at hub center. The front wheel is deeply dished to make this possible.

Ron Haslam
British star Ron Haslam leaves pit lane on the hub-steered ELF Honda NSR500 at Donington Park in 1987. He qualified sixth and placed seventh in that race, 32 seconds behind winner Eddie Lawson on a Yamaha. Haslam rode three versions of the innovative two-stroke over three seasons, 1986, ’87, and ’88. He podiumed at Jerez and Hockenheim in 1987.Gold & Goose

Another variant, used by Bimota in the Tesi, is stress-symmetrical in that the front wheel is supported from both sides. A conventional two-beam swingarm projects forward from the lower front of the bike. Its forward ends contain bearings in which an axle with a kingpin at its center can pivot. The short kingpin (perhaps 100mm long) is carried at a rake angle of close to 15 degrees and at its top and bottom it carries steering bearings.

Pivoting on these bearings is a non-rotating hub in the form of a short piece of large-diameter tubing. It is pierced at right angles to its axis to fit onto those steering bearings. Because the hole through this hub is large, it is possible to “steer” it by 20 or more degrees to either right or left.

The outside diameter of this non-rotating hub carries a pair of large-diameter but slim-section bearings. The front wheel itself, having a hole through its center of suitable size, spins on these large-diameter bearings.

The front axle also carries a lever, connected back to the front of the chassis by a rod with ball joints at either end. Its purpose is to keep the kingpin at a largely constant rake angle as the swingarm rises and falls over bumps. This rod and lever also react front-brake torque back to the chassis.

To allow the front wheel to be steered, there is a steering shaft on the bike, connected to the rider’s handlebars, and there is a sideways-projecting lever attached to the non-rotating inner front hub by which it can be steered. As patent applications so often declare, the rider’s bars and this hub-attached steering lever are connected by a “multiplicity of shafts, bearings, levers, and ball joints.” Also connected to the non-rotating inner hub are brackets to carry the calipers that grasp the two front brake discs.

The two beams of this front swingarm are “elbowed” outward, like the arms of a person embracing a beach ball. This is done to provide steering clearance for the front tire. Look at the photos and it will all make perfect sense.

Sadly, ELF Honda’s experience with an evolving variety of such ambitious designs was that they improved as they moved away from the complexity of rods and levers and moved back toward simplicity. In racing, riders found that front-end feel could get lost in all the ball joints, rods, and pivots. The bikes were perfectly rideable but none was able to show clear superiority over the conventional.

But what if that’s not the point anymore? Today is not the 1980s, chatter is no longer a huge roadblock issue, and sportbikes rest with the dodo in museum displays. Today we can have radical high tech for its own sake, without requiring it to solve any problem other than our thirst for super-rad knock-out novelty. High tech has become a style, an entertainment channel, a social movement. As in, let’s phone Boston Dynamics and see if its engineers will build us a bike with legs instead of wheels. Can’t wait!