Kevin Cameron has been writing about motorcycles for nearly 50 years, first for Cycle magazine and, since 1992, for <em>Cycle World</em>. (Robert Martin/)
I’ve written before about how motorcycles used to deal with vibration. The British parallel twins that would by 1972 numb riders’ hands, feet, and backsides, began life as little 500cc twins whose pistons were a small fraction of total vehicle weight. As a result, the vibratory “excursions” of bars, pegs, and seat were small.
“More power!” came the cry, and 500s became 650s, then 750s, and finally Norton’s 830cc. Those heavier pistons, plus the rise of revs beyond the 6,500 rpm of the original Triumph Speed Twin, added up to greater excursions.
My friends and I were busily building British twins in the mid-1960s and foolishly admired the gleam of aluminum fenders. Was it Dixie Cycle Supply that answered our call? Whatever the source, our engines effortlessly broke those light alloy fenders and their “universal” mounting brackets. It seemed that aluminum was a metal that remembered every insult. It was like the film badges worn by people working with radioactive substances, darkening with every hour of exposure.
Norton, under the stewardship of whichever greedy holding company was calling the shots at the time, did the obvious thing and developed the “Isolastic” system for separating the rider from much of the vigor of engine motion. Because both pistons of those British twins moved together, the engines orbited in a single plane; they did not rock from side to side. That being so, the engine and swingarm could be built as one unit, and the front end and rider/passenger accommodations could be built as another. The only relative motion permitted between the two units was in the vertical plane through the wheels; the engine’s orbital motion as it vibrated was restrained and damped by bonded rubber elements. It was a beginning.
When I first struggled with Yamaha’s furiously vibrating TD1-B production roadracer, I could see the chassis cracking in a pattern repeated in every TD I worked with. As the chassis was already quite heavy, I was sure they had originally prototyped something lighter and had increased tubing and engine mount thicknesses, probably more than once, until it took a season’s use to fail. And it was heavy.
We started to talk about “old metal,” for we knew that even as the lower left rear engine mount had broken, its mate on the right had undergone change that would soon break it in turn. Years later, in the pages of Bela Sandor’s Fundamentals of Cyclic Stress and Strain, I would learn that every stress cycle causes damage to metals. Individual atoms under strain are “uncomfortable,” and when a good-sized “jostle” comes along, they may shift slightly to reduce that strain. Rather like myself, trying to sleep in the passenger seat of a van somewhere in Nebraska. Over time, the sum of all those tiny relaxations, plus the occasional bond breakage, adds up to serious fatigue damage. That’s why airframes and racing valve springs have a predicted life. Continuing to operate a high-hours airframe beyond its three score and ten takes you into the territory in which cracks which lived below the surface for 10,000 hours are now emerging, ready to move more and more rapidly. Just think of the stress on the material at the very tip of a crack!
Yamaha had been making parallel twins for 18 years when the extra weight of the 1976 RD400′s 64mm pistons finally became too much. Many of us know that Yamaha engineer Masao Furusawa turned the company’s MotoGP program around in 2004, but less well known is the fact that it was this same vibration specialist who created the rubber mounting system for the RD400. A single-plane system like Isolastic could not work for the RD, because its engine also rocked like a double-bladed kayak paddle in vigorous use.
For a time, the big four-stroke literbikes were protected from their own vibration by being so heavy. Vibratory excursion depends on the ratio of the engine’s total weight to its reciprocating weight: pistons, rings, wrist pin, and the small end of the con-rod. Their rpm was limited as well, by their roller crankshafts being pressed together. As Rob “Mr. Superbike” Muzzy said in 1982, “With this engine [a Z1-based Kawasaki] things start to go bad in a hurry at eleven.”
Why didn’t those big engines travel the time-honored road to the ever-bigger bores and shorter strokes that would reduce piston acceleration and permit higher revs? It was because those engines were air-cooled, which sadly means not very well cooled. In those days before piston-cooling oil jets, the bigger you made the piston, the hotter the center of its crown operated and the closer it was to failure. The very same problems affected the air-cooled Wright R-3350 radial engines that powered the B-29 bomber in World War II. Their pistons suffered all the ills of excessive operating temperature: scoring, detonation, sagged ring lands, broken or stuck rings, and seizure. No surprise that piston-cooling oil jets were quickly adopted once they were made to work on aircraft piston engines.
As soon as water-cooling was widely adopted in bike engines, and with piston temperature under control, big bore/short stroke became a reliable option. Revs were free to shoot up. Vibratory force increases with the square of rpm, so pretty soon designers were penciling in gear-driven balancers and arguing for them in the endless meetings that are the bane of engineering work. Having a good idea is just the beginning. Then come the politics!
In the “tween years” separating the big air-cooled literbikes and their twice-as-powerful water-cooled descendants, some strange excesses took place. Not only were engines rubber mounted, but seats, bars, and pegs were smothered in rubber as well. As I can see from my son’s CB900F, if you pile up the parts made necessary by all this rubberizing, it is a great many pounds. Add to that the extent to which the frame and other structural parts had to be made heavier to survive vibratory fatigue and you have many times the weight of any gear-driven balancer. Figure the extra power needed to get all those extra parts moving and you are losing far more power than is consumed by the tiny friction of a balancer shaft and its gear pair.
Go Faster on Skinny Tires?
When I first got involved with 125cc racing the Europeans insisted that you must fit the very smallest possible tires to your machine, as any extra rubber reduced top speed. Then we found things were different on our home track, Loudon, New Hampshire’s 10 turns in 1.6 miles. There, extra grip raised corner speed and dropped lap time. Maybe the skinny tires helped on the few long European straightaways, and with limited power. Not at Loudon. And so it turned out to be with 250s as their power increased. The ancient TD1-B of 1965 rolled on 2.75 x 18F, 3.00 rear, but by the time the 250cc two-strokes left the Grand Prix scene in 2009 they were wearing the same rim widths and tire sizes as TZ750s (and they were making roughly 10 hp more than those first TZ750As).
The same game was played at top level. The tires at Daytona for the 1972 200-miler were the same-old, same-old; narrow and hard. But Suzuki and Kawasaki had brought new bikes with revolutionary power, 90 to 110 hp. They made short work of the tires and the race was won, that year and the next, by light, easy-on-tires Yamaha 350 twins. Narrow tires boost top speed? Maybe in some other world than the one we know, but down here Dunlop’s Tony Mills got busy drawing much larger-section tires with a round profile. To keep “centrifugal force” from expanding them until they rubbed the insides of the fenders, he wrapped them with an under-tread belt. To reduce heating, he made the tread surface almost slick, molding in just a few little squiggles to prevent conservative AMA officials from suffering apoplexy at the sight. By late season ’73, the Goodyear men were handing out actual slicks to certain users—”for evaluation.” Bang, roadracing tires were 100 percent big slicks overnight, and rims and tires for streetbikes gained width with just enough tread pattern to say so.
Don’t Say “Front Fairing” Too Loud
Right now in MotoGP engineers are imaginatively chiseling away at rules dating to 1958. Commercial aircraft cannot extend their landing gear above ~ 250 knots because of the danger of parts carrying away in the wind blast. But at the recent Qatar test at least 218 mph was achieved, and not because the bikes are well streamlined. No, that speed is achieved by brute force, engine straining to shove shapes with the aero coefficient of bread trucks through the air. Small wonder they are trying anything that offers to reduce the staggering drag of that “nosewheel.” The clear goal of what Yamaha tested this past week is to guide air past the disturbances of wheel and fork legs, but the parts have to be called something other than “front fairing.” Front fairings are illegal! But a sector-shaped piece of carbon fiber can be fitted between the rim and the “fork leg guard.” What is a fork leg guard? It is a piece of streamlining that a half-decent lawyer can call a fork leg guard because it protects the hard-plated surface of the exposed fork tubes from the seal-damaging dings and dents that would otherwise result from the steady impacts of small stones from the track surface. That sector-shaped piece can be called a “brake cooling deflector” or some such—just as long as it is not actually joined to the “fork leg guard” to become what it is—streamlining for the front wheel. Fenders have shown a tendency to shape themselves as if to smooth airflow to the giant array of water and oil coolers behind the front wheel. Calipers have been gobbled up in this piecemeal attempt to smooth the disturbances created by the exposed front wheel.
Disc wheels, anyone? Oh no! That would cause dangerous aerodynamic steering effects! But if we separate that function into little separate carbon fiber pieces, and give each piece its own fairy-tale name, airflow and officials alike are fooled and the bike is stable.
Being a tech inspector in that situation is a bit like being traffic policemen who have been told, “OK guys, listen up! We know everybody’s doing 85–90 out there, but the state legislature would be pretty unhappy with the extra lanes they’d have to build if we actually enforced the speed limits during rush hours. So be reasonable.”
So it is that when there’s a persistent design problem, but the Big Solution hasn’t appeared yet, a tall pile of Band-Aids is applied. Fun to watch!
