Young men of the 1960s liked to refer to graying AMA track officials as “the belt-drive boys” that dismissed them as hopelessly out of date. Yet in fact early motorcycles were belt-driven for a very good reason: When people did the obvious thing and put strong steel roller chains in place of laced-up, slip-prone leather belts, the chains spit off their rollers, tightened up, and broke. Belts worked, chains didn’t. Pragmatism made belts the wise choice.
Early bikes had one-cylinder engines that propelled them by a series of sudden pressure events, the combustion of a compressed mixture of air and fuel vapor. Connect a roller chain to all that banging and impact and the result was as described. Belts slipped just enough to survive.
Once builders had worked this out, it wasn’t long before compensating sprockets and other forms of spring drive were placed between the mighty thuds of the engine and the rear wheel. Roller chain took its place as the favored positive drive for motorcycles, and it remains so today.
When Harley-Davidson brought its VR1000 Superbike to Daytona in 1994, I was treated to the sight of a garage filled with personnel, every one of whom was on one of the new-fangled cellphones. All had veins standing out on their foreheads. Seems they were breaking clutch hubs. Yoshimura wasn't using spring hubs in the clutches of its four-cylinder Suzukis, so why would Harley's big racing V-twin need them? Uh, maybe because its thuds were twice as big as those of the GSX-R.
That wasn’t all. Those engines had a single balancer whose eccentric weight swung between the flywheels when the big ends of the rods weren’t in the way. The balancer, driven by gears, received the engine’s big thuds without the protection of a spring drive. Oh dear, the balancer “dephased” right at start/finish; it’s said a weld broke. Could that weld have given up just as had the first attempts at chain drive so long ago? The average load is small, but the peak load could be 10 times as great, as in the cam drive of Keith Duckworth’s original Ford Cosworth DFV Formula 1 engine. The brief contest between the VR’s crank and balancer wasn’t good for the surrounding crankcase, and spectators heard a brittle crunch as though someone had stepped on a really big insect.
Another time, a team decided to radically reduce crankshaft mass in order to boost acceleration. Suddenly the single-row special racing primary chain drive that had been completely reliable began to tighten up—“just throw on another; there’s a couple in the truck”—and spit rollers. Soon the team had to go home and regroup. The lighter the crank, the greater the resulting crankshaft speed variation at each thump and the harsher the peak force transmitted to the drive. It all goes back to the early 1900s and belts versus chain.
On yet another occasion, Rob “Mr. Superbike” Muzzy said, “I don’t know why, but every time I’ve tried to run a light crank at Daytona it hasn’t worked out.” And before that, Harley-Davidson’s former racing manager, the late Dick O’Brien, said the same, adding that light-crank engines didn’t top end as expected.
Many engineers and Herr Professor Doktors tend to dismiss statements like these as folktales. But when Muzzy's Kawasakis in the hands of Eddie Lawson and Wayne Rainey won AMA Superbike titles despite Honda's substantial spending, and when Kawasaki then decided to take time out from racing, Honda hired Muzzy to run its Superbike program. His success rolled on. Not bad for folktales.
There’s more to it. In a fascinating paper entitled, “Development of Valve Train for Formula One Engine,” Honda engineers Shuichi Hayakawa, Kazushi Ogiyama, and Masanori Tate describe many steps taken in trying to stabilize an F1 valve train. They note that once this was done to the degree possible, the effects of torsional oscillations (shaft windup) and crank speed fluctuations from cylinder firing and spring resistance, they were able to make the existing cam profile operate 1,500 revs higher than before. Why? Because, before being stabilized, the cam lobes were not rotating smoothly but micro-accelerating and decelerating several times per revolution and thereby beginning to “toss the valves” (cause valve float) as much as 1,500 revs lower than they were designed to.
Fifteen hundred revs chopped right off the top of your powerband.
By golly, that could take the edge right off your carefully built Daytona engine. But if you replaced your lightweight “acceleration crank” with a heavy stocker, its greater inertia might have a stabilizing effect, toning down the worst valve-tossing tendencies. It’s kind of soothing to imagine your crank revolving more steadily like a great millwheel rather than advancing in a series of jumps and jerks that confuse and upset the valve train.
