Beg, borrow or steal a BMW boxer twin, the bigger the displacement, the better. Start it up, then – with the bike in neutral, your feet on the floor and the tank wedged solidly between your knees – whip the throttle open. Really give it a solid blip. To avoid a catastrophe, here's what to expect: The motorcycle will rotate along its front-back centerline, at the height of the crankshaft, hard to the right.
In simple terms, the crankshaft on the bike spins one way, the bike – in an equal and opposite reaction – tries to spin the other way. If you have never experienced it, and you're not expecting it, it will freak you out. The phenomenon is somewhat less noticeable once under way, but if you're downshifting, pull in the clutch and blip the throttle hard, you definitely will feel the motorcycle engage in the same motion beneath you.
There are a lot of advantages to a boxer engine configuration. Low center of gravity, cylinder heads out in the breeze. But virtually all motorcycle manufacturers have abandoned the boxer, and it's hard to imagine that the counter-rotational effect had nothing to do with it.
After the wheel/tire/brake assemblies, the crankshaft is the single heaviest rotating item on the motorcycle. Experiments with counter-rotating front brake rotors on motorcycles – discs that spin in the opposite direction of the wheel – have demonstrated that such devices do, indeed make steering the bike easier, but the additional cost, complexity and relatively limited benefit have combined to dampen major research into putting them into production.
And for International-level racebikes, counter-rotating disc systems run into the same problem that non-telescopic forks run into. A racer who has been on “traditional” brake discs since they were four years old has developed a superhuman fluency in the language of the feedback that such brakes provide and the skills required to steer such a system. It's simply too big a challenge to learn a new language in just a couple of races before the factory starts asking questions about what happened to their formerly-winning bike.
Hypothetically, swingarm front ends provide significant benefits over telescopic forks. But Rossi, Marquez, et. al., have ridden telescopic forks their entire careers, and would have to spend time to learn to “speak” swingarm front end (or counter-rotating brake discs). Ask those professional racers how many races they're willing to throw away trying to unlearn the skills that have made them successful and famous, and to spend those races at the back of the grid learning new skill sets that may or may not be better.
Anyway …
The crankshaft is always there. Where “there” is varies according to design preference, but it is carefully thought-out and examined. Its motion always affects the behavior of the motorcycle. One key reason 1000s handle differently than 600s that weigh only slightly less in race trim is the difference in the weight of the crankshaft and the impact it has on the behavior of the bike. Dirt bike riders know that they can alter the attitude of the machine mid-air with the throttle and rear brake. Even when the rear wheel is nowhere near the ground, grabbing a handful of throttle or stabbing the rear brake lifts or lowers the front wheel. Altering the spinning motion of a big, heavy thing has an impact on the behavior of the bike.
Racebike engine manufacturers mostly have chosen to spin the crankshaft in the reverse direction of the wheels. One of the key reasons is wheelie reduction. A forward-spinning crank tries to lift the front wheel. This is usually not such a big deal on a streetbike – longer wheelbases, more weight and less horsepower mean the engine's contribution to wheelies is less significant. Shorten the wheelbase, lose nearly 100 pounds and add nearly 100 horsepower, and the crank effect on wheelies suddenly takes on a new urgency.
So – one answer is to spin the crankshaft in the other direction. But that means an additional rotation-reversing shaft inside the engine. And that eats a bunch of power. Bad on a racebike. But until recently, it was the best compromise, and designing a racebike is an exercise in compromises.
Enter Honda. An engine company at its core, the horsepower lost by that extra rotation-reversing shaft was a carrot hanging there for its engineers. The company opted to build a forward-rotating engine for MotoGP and count on its software engineers to craft an anti-wheelie system. If it could do this, not only would the bike accelerate harder, it would have more power on the top and would eat a little less gas to boot. A more efficient engine pays dividends in a myriad of ways.
The company had it right, really, really right, in the first half of the decade. The bike was a missile, and it really only struggled on courses with rapid changes of direction. Then dumbed-down electronics were introduced for MotoGP, and Honda joined the field, building a reverse-rotating engine.
Until the day he retired, ex-HRC Vice President of Racing Shuhei Nakamoto railed every chance he got about the spec software and electronics hardware for MotoGP machines. Intellectually, I understood. Those electronics, and the spectacular work the racing engineers had done on them, had allowed his engine design department to build a better engine. And learning to build better motorcycles is one of the reasons Honda goes racing.
I recently got to spend a few days aboard a Boxer twin for the first time in my riding career. The crankshaft effect described in the first few sentences was unlike anything I'd ever experienced. But more than anything else, I was able to understand on a visceral level exactly why Nakamoto was so angry.
I could feel the sheer force of the crankshaft effect, and I started to feel the power of the force that his engineers had tamed. It was an accomplishment, one of the many engineering miracles that racing engineers perform. And it was all thrown away in the pursuit of more entertaining racing, a better television show.
Not only do I understand, but I can feel and respect, the fury of Nakamoto.
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