Once upon a time, we had a perfectly good 2000 Yamaha R1. Wait, once upon a time, we had a perfectly good R1 with a bent frame, so we sent the bike off to Evan Steel Performance in Arizona for a little “freshen up.” Before we knew it, the thing had been stripped to its elemental components. The original idea was to build a gnarly old carbureted R1 that would run with modern literbikes, to the tune of 160 horses or so on pump gas (even if it might not meet the strict government regulations that new bikes must).
To do that would require more than a gentle port, polish and pipe. The heart of any engine is its crankshaft, and ESP farmed the R1’s out to APE (aperaceparts.com) for a thorough going-over.
APE’s Jay Eshbach points out that, unlike in most automobiles, a sportbike’s crankshaft also serves as its flywheel. Flywheels store rotational energy, and their inertia helps keep an engine from stalling at low speeds—and for that reason, most bike cranks are heavier than they need to be. Once the bike is rolling, that extra weight is just baggage that consumes power when accelerating and brake pads when it’s time to stop. Once the crank is lightened, the engine can spin up faster—and a faster-spinning crank equals a faster-moving motorcycle. Furthermore, with the special contours that APE machines into the counterweights (commonly called “knife-edging”), the crank, it’s claimed, spins with less drag through the oil mist draining back from the top of the engine.
Also, a transverse motorcycle crank acts like a gyroscope (as do the bike’s wheels and all its spinning gears), which makes the bike resistant to leaning. People pay big money for wheels that are a few pounds lighter; APE removed three pounds from our R1 crank for just $225, which should greatly increase its “flickability.” And dynamic balancing is included, using a computerized machine that measures imbalance thru an X plane running lengthwise through the crank. Should have the old beast running nice and smooth.
Naturally, I asked Kevin Cameron for some crank-lightening enlightenment (after the fact) and got this:
“Light cranks may help in drag racing, and they seem like a good idea, but a lot of good people have had opposite experiences. Muzzy had three weights of crank for the 1982 Z1-based Superbike, and he said it top-ended best with the heaviest of the three.
“The lighter the crank, the greater the speed change each time a cylinder fires. Back a few years when Buell was trying to get one of their 1340 air-cooled V-twins to qualify and race in the 600 class, one thing they did was to lighten the crank by 10 pounds. It then began to beat up and wreck primary chains that had previously been reliable.
“In dirt track and in roadracing, too light a crank allows the engine to tach out immediately once traction is lost. Kenny Roberts said the small crankcases on the 1981 0W54 Yamaha prevented them from increasing crank mass, so it was hard to keep that one hooked up. This is especially important in dirt track, and I’m sure you’ve seen the add-on accessory external flywheels for some MXers—same idea, to keep the tire hooked-up by making the engine a bit less ‘lively.’
“Finally, Jerry Branch tells the story of some big Twin guys who sawed two cylinders off a small-block Chevy and added the necessary covers, etc., to make a motorcycle engine out of it. I think it was 1500cc or so. They had done the same with the crank—just sawed it off, which left the engine with no flywheel. Jerry said it made about 35 horsepower.
“Another point: The greater the crankshaft’s ‘rpm flutter’ as a result of cylinder firing impulses, the easier it is to toss the valves when the crank is at its momentary peak revs.”
There you have it, two sides to every coin. Anyway, what’s done is done, and we’ll probably spend more time standing around talking about going fast on our streetfighter R1 than actually going fast on it. And “lightened, knife-edged crank” sure sounds cooler than “stock.” We’ll see how it works out soon enough. Knock on wood!