The Rhythm of Power

V-Four, inline-Four, V-Twin—what do engine layout and crank design have to do with your ride?

The Rhythm of Power

With a few three-cylinder and V-Four exceptions, the sport motorcycle has crystallized into a choice between high-revving inline-Fours and only slightly less-revvy V-Twins. Yet if we look at World Superbike results, we see that they all have their innings and there is no clear winner, no dominant type.

Last year, Ben Spies ate up WSB on the revised Yamaha YZF-R1, a 1000cc inline-Four with a new-tech 90-degree crank. But dogging his every move was Noriyuki Haga on a 1200cc Ducati V-Twin. Showing amazing strength for a first-year bike was the Aprilia RSV4 Factory, ridden by Max Biaggi. Yet another first-year bike, the inline-Four BMW S1000RR, was far behind—often at the level of seventh to 10th, with a traditional 180-degree “flat” crank and amazing power.

Where is the sense? If we look for it in basic engine architecture, we will find almost nothing. If we look instead for trackside experience, we find a lot more. No one has more up-to-date racing experience than Ducati, and Aprilia is not far behind. Spies and crew chief Tom Houseworth had dominated Superbike racing in the U.S. with Suzuki, and on the world stage, Yamaha gave them what they needed. BMW is playing catch-up in experience, and it shows. Had the team simply bolted on Marelli throttle-by-wire, their race results might be better, but the company’s insistence upon developing its own electronics has value, too, as we shall see. And Fours with 180-degree cranks are no handicap, as the first 2010 WSB winners were Suzuki and Honda flat-crankers.

Even today, people associate Twins with torque, Fours with revs. Is it this simple? To engineers, the torque that matters is the one turning the rear wheel. The 1000cc Fours, revving to 14,000 rpm, are simply geared down a bit more than the 11,500-rpm Ducati Twins, and the results at the rear wheel are as closely equal as the race finishes.

But maybe non-engineers mean something different when they say “torque.” What they mean has more to do with where an engine’s peak torque is located and how easy it is to use. The smaller an engine’s cylinders are, the easier it is to give them adequate valve area. This makes it easier to achieve horsepower, which essentially is torque multiplied times rpm. And that peak torque tends to be high up the rpm scale.

The V-Twins fight back in two ways: One, the political way, is to make up power by having more displacement (1200cc vs. 1000cc). The second way is driveability. Twins, with less valve area per displacement, tend to have their rpm of peak torque lower in their rev band, potentially making them easier to keep in the sweet spot.

The reason I qualify this with weasel-words like “tend” and “potentially” is that so much depends on the details of design. Ducati has always given its Twins maximal valve area by going to the practical limit in big bore and short stroke. The four-cylinder makers have been less aggressive in this. Also, as bore is made bigger and stroke shorter, combustion tends to slow down because a tight chamber acts as a damper on flame speed. But, historically, Ducati’s engineers have been more clever at finding ways around this than have the four-cylinder brigade. To a great extent, development trumps design.

Now, how about the R1’s 90-degree crank and its possible advantages? In MotoGP, it seemed to make a big difference when first Yamaha and, later, Kawasaki adopted it. But when I asked Spies and Houseworth about this last year, they shrugged and said it didn’t change much. Ah, but wasn’t it in their interest to say this?

At Daytona in 1974, Kel Carruthers had the new Goodyear slicks on his bikes. When asked about these breakthrough tires, he answered, “I dunno. I guess it’s some new idea Goodyear has. You know, they’re tires.” But by the time of the next race, the rest of us figured out they were worth more than a second per lap. Tires would never be the same again. Canny old Kel.

Yamaha engineer Masao Furusawa says the 90-degree crank is useful at full lean in turns, when the bike has little extra grip and the rider begins to first feed power. With a 180-degree crank, all pistons stop at either top or bottom dead center every 180 degrees of rotation. If the engine is turning 10,000 rpm at the bottom of its powerband, piston peak velocity is about 65 feet per second, so the engine’s approximately 3 pounds of pistons are trading some serious energy back and forth with the crank 333 times per second. The crank responds with an equally rapid rpm flutter, which is transmitted to the rear tire. The energy in each of these pulses is equal to the weight of a service automatic pistol falling seven stories. The rider is trying to feed power from full lean, but this crank-speed flutter is messing with traction in a rapid series of snatchy yanks, making the back tire “feel squirrelly,” so he waits. Riders on other makes aren’t waiting—they are gone.

Now, look at what happens in a 90-degree V-Twin. When one piston is at maximum velocity, the other piston is stopped, and 180 degrees later, that situation reverses. These pistons are trading energy, not with the crankshaft but with each other. As a result, the crank rotates more smoothly, and the rear tire is undisturbed by piston stop-and-start. The same is true of any Vee engine—V-Four, V-Five, V-Six. It is also true of a 120-degree Triple.

The main attraction of a 90-degree V-engine is its perfect primary balance. But that large Vee angle requires the crankshaft to be far back in the chassis, making it hard to get adequate weight on the front wheel. For this reason, many makers have chosen 60 degrees (previous Aprilia V-Twins, for example) or the KTM’s 75-degree Vee. These do require one or more balance shafts, but the overall compromise is attractive, and opening the Vee a bit from 60 allows more room for optimal intake setups. Aprilia’s V-Four has a 65-degree Vee.

But by rearranging an inline-Four’s crankpins at 90 degrees instead of 180, the same benefits result: When piston #1 is stopped at top or bottom center, piston #2 is near maximum velocity, and the same goes for #3 and #4. Suddenly, the engine is smooth. Our peerless star rider turns the throttle and the squirrelly feeling is gone. And so is he—down the next straight.

Because so few of us “street-quality” riders spend time trying to throttle up from full lean, this neither helps nor hinders—it just gives the engine a unique sound that suggests the excitement of the distant battle.

Confusion arises when journalists ride all the Superbike racers and report that the BMW clearly has the most power, while winning no races. Yet in the MasterBike competition, the new German was the fastest of the liter bikes. The explanation is that the European makers, particularly BMW and Ducati, have been the most aggressive at adopting and promoting electronic rider aids on production sportbikes. The Japanese makers have been cautious, unwilling also to make such systems a selling point.

And BMW power? It’s there for anyone who wants to push on toward larger bore and shorter stroke. Make an engine rev reliably and give it the valve area to fill itself at such speeds, and it will reward you with 175-180 rear-wheel horsepower as it has with the BMW. Their engineers know how engines work. What is taking them time to “get” is motorcycle racing experience and the intimate details of racing electronics. If the R&D; money keeps coming, they will learn what the others know—and maybe then some. For the moment, they are using more of what they know on their production bikes than is their competition—an interesting situation that has paid dividends in the showroom.