Trying A, Trying B

During my time in this sport I have met a few motorcycle visionaries who as young men were determined to cleave the dead layers of tradition and reveal the truly rational machine that the motorcycle should be. It’s very exciting (and, frankly, tiring) to be around these people, who are tightly wound and generally unmoved by any ideas not their own. That’s how it has to be. The successful surgeon cannot act without complete confidence in him/herself.

We know that around 1876 Nikolaus Otto, he of the Otto four-stroke cycle, was badly frightened when his high-compression prototype engine first fired with lab-shaking violence. His wife took him home, made him something good to eat, and expressed her confidence that he would get it right in the morning.

The safety bicycle, with both wheels the same size and roller-chain drive, became a craze in 1895, just as Otto’s engine hit the market. What could be more natural than combining them? In Paris, the Werner brothers gave it a whirl. Where does the engine belong? They tried everything, and later workers repeated their experiments. Up in front of the handlebar? It worked on 8 million post-World War II Velo Solex mopeds, but for larger engines it made the bicycle top-heavy. How about beside the rear wheel? In 1914 the Smith Motor Wheel appeared, and 30-odd years later Soichiro Honda would revive the idea. Mmm, not quite right. Cheap to make, but with the powerplant’s weight hanging off one side the vehicle felt funny. Honda then thought again and produced the practical, well-balanced 50cc step-through, of which more than 100 million have since been made.

Related: This Is Why Motorcycles Blend Art And Engineering

Or how about in front of the downtube (giving clearance to the bicycle’s pedals) and up high? Top-heavy again, with the added problem of getting the drive to the rear wheel without belts or chains snagging or tearing the rider’s clothing.

Easiest of all to adapt to any bicycle was to put the engine on a little trundle-buggy behind the rear wheel. This, however, fought the rider’s efforts to steer, especially at low speeds. Few takers.

Similar experiments sprouted all over the world, and consensus emerged: The engine belongs between the wheels.

Now the finer points. In 1950 engineers wanted the engine back against the rear tire, “for better traction.” But as engines grew more powerful, this configuration produced wheelies. Honda’s fearsome RC181 500 GP bike, with its engine mounted midway between its front and rear wheels, unloaded its front tire so much during acceleration that it weaved, greatly increasing rider workload. Giacomo Agostini, on MV’s new triple, with both rider and engine well forward, was competitive despite having less power. Ago was 500 champion in 1966 and ‘67.

Later, as the two-stroke era dawned in 1975, engines moved forward so radically that in some cases their coolant radiators were slotted to bring the engines even closer to the front wheel. Ducati did the same to improve the balance of its “ottovalvole” V-twin Superbikes.

Engine high? Engine low? Bikes intended for touring with large, heavy powerplants carry their engines low, even at the cost of seriously limiting their turning ability. Touring is accordingly conducted at very moderate maximum lean angles, for it’s essential to the heavy bikes’ handling to carry the weight low.

In racing and sport it was quickly learned that the lower the engine was located, the less weight transferred to the rear wheel during acceleration. The higher the engine—up to a point—the more weight transferred to the rear wheel, and the more throttle could be applied for early acceleration out of turns.

A few years ago Ducati’s former racing engineer Filippo Preziosi summed up the problem of motorcycle design by saying that physics already dictates the positions of the major masses. We know from experiment the best range of steering rake and trail for the front end, so we clamp a front wheel to the build bench and attach a fork at the desired angle. We know, also from experiment, the desirable wheelbase for a sporting or racing motorcycle. That positions the rear wheel on the table. Shorter makes steering quicker, but also invites the bike to wheelie at a lower rate of acceleration. The more dragster-like our product is (Hayabusa), the longer we make its wheelbase (so riders can save on broken taillight lenses). For lower-powered bikes whose strong suit is rapid maneuver, a shorter wheelbase—maybe even as short as Erik Buell’s fave, 54 inches, or Ago’s, at 53.5.

Experiments conducted since about 1990 have demonstrated the dynamics of rear-suspension squat, which dictate where the swingarm’s pivot and the engine’s drive-sprocket centerline should be. Set the pivot too low and opening the throttle squats the rear, taking weight off the front, making it “push” or lose steering grip as we accelerate out of turns. Make it too high and the rear “jacks,” as I saw Yamaha TZ750s do in the early ‘80s. But in the subjective world of the “new custom” the pivot can be anywhere because the driving force of anti-squat—chain tension—is usually moderate or zero.

Swingarm? Why yes. Twin swingarms have been tried, with attendant claims that the celestial spheres will pivot around distant axes with beneficial result. Twin swingarms that cross each other produce a straight-line vertical motion rather than an arc at the rear wheel.

This brings us to that fad of the ‘50s—constant wheelbase! Another reason for inventors to scorn the telescopic fork, whose angled legs shorten the bike’s wheelbase as the front suspension compresses. Away with it! In its place, the leading-link suspension, with either the long links favored then by BMW (after its thorough development of teles in the 1930s) or the short links used by Guzzi and certain British specials builders..

Related: How Engine Qualities Define How We Ride

And many others. In the 1980s, Honda and the French fuels giant Elf built a series of radical bikes to discover whether hub steering or MacPherson strut or anything but the telescopic could be used to some advantage. Big money was spent and many races contested, yet little survives but the still-fashionable single-sided swingarm. Die-hard defenders are sure that if riders or tire makers hadn’t been so accustomed to teles, a revolution might have emerged from the alternatives.

The parts we’ve attached to our build table now define a space within which the engine must fit. Make it too long and steering slows as wheelbase grows. Make it too wide and the cases or cylinders grind on the pavement, tending to lift our creation off its wheels and dispatching it into the weeds. So to be safe we add to our build bench a pair of defining planes in the form of two angled plywood sheets, simulating the pavement at full lean, with suspension compressed by cornering force. Everything must lie within that vee; footpegs, frame members, exhaust pipes. What angle shall we choose? In good conscience we can’t opt for less than what the tires are capable of giving. Do we pick the 25 degrees or less of today’s “gritty city cruisers”? The 45 degrees of 1965 roadracers? This is a moving target because the grip of the tires’ rubber compounds is constantly increasing—we’ve seen 63 degrees so often in MotoGP that those “defining planes” seem to be ever-closing, like the covers of a book. At its limit, must the motorcycle of the future become two-dimensional?

When motocross in 1973-75 sent its progressive wave of longer suspension travel in all directions, it solved old problems and created new ones. Before, bike suspension and damping had to be stiff to prevent bottoming, but that fatigued riders quickly and limited tire grip. Longer travel and softer springing increased tire grip by allowing wheels to rise and fall over irregular ground more easily. But now we encountered sudden forward pitch produced by braking. Motorbikes are a tight ball of compromise.

Returning to Preziosi’s remarks: Once all the major masses are positioned and the rider’s seat and footpegs are where they have to be, what’s left is tying all those parts together with structure. Up to about 1980 that structure was always steel tubing, but then aluminum arrived, and lately at trackside, carbon fiber swingarms and upper fork tubes have appeared.

Fortunately for those of us who keep our distance from the outer boundaries of performance, choice is less closely defined, leaving room for creativity, art, and even whimsy. I’ve always wanted to see a laminated wood chassis.

Like everything about the motorcycle, this is a compromise. The more we seek specific kinds of performance from this vehicle, the more tightly physics defines what we build. But if the goal is freedom of expression (let’s put the two wheels side by side!) we mustn’t be surprised if there is a cost in lost performance.