Parallel-Twins and Sportbike Testing

Answers

By Tim Carrithers, Photography by Triumph

Got a question for answers? Send it to mcmail@sorc.com

Parallel Universe

Q Parallel-twins are usually made with 180-degree cranks. I suppose this allows the engine to be smoother as inertial forces are in balance. The firing order works out to 540 and 180 degrees, though, so that must mean some strain on the transmission and chain plus a rocking vibration from the offset of the two cylinders. Originally, most twins used 360-degree cranks, providing an even firing order every rotation. Lately, Triumph has gone with 270-degree cranks, which gives a firing order of 270/450 degrees, striking a compromise between rocking couple and inertial forces. A twin with a 180-degree crank gives the engine an off-kilter rhythm, but apparently it's not all that pleasant to listen to. I wonder why no one has tried using a 315-degree crank on a parallel-twin, giving a firing interval of 315/405 degrees, just like a Harley? You would get that great sound and the firing order would be semi-evenly spaced for less strain on the drive train.
Jeremy Barnes
Via E-mail

A Simon Warburton, Product Manager for Triumph Motorcycles Ltd. at the home office in Hinckley, has forgotten more about the pros and cons of various parallel-twin crankshaft configurations than most mortals will understand in a lifetime, so listen up. "If the engine speed is high," he says, "a balance shaft will be required to cancel out rocking couple. A 180-degree crank offers the most uneven firing interval: 180 and 540 degrees. It also gives the lowest pumping losses, as displacement in the crankcase remains roughly constant throughout the cycle. The 360-degree crank gives even firing, but as both pistons move together offers the same mechanical balance as a single (i.e. not very good). Again, balance shafts are required. The 360-degree firing interval has the advantage that only one ignition system is required, with a wasted spark.

"The 270-degree crank has a firing interval of 270 and 450 degrees, the same as a 90-degree V-twin. Therefore, this sounds and feels like a 90-degree V-twin. It's a compromise between the 180 and 360 configurations: better primary balance than the 360 and a lower rocking couple than the 180, but it still requires balance shafts at high engine speeds. "Another benefit is that as one piston is stationary, the other is moving. This gives a dynamic benefit whereby the crank speed fluctuates less during a cycle. Yamaha uses this effect on the crossplane R1 crank. On both the 360 and 180 cranks, both pistons are stationary at the same time."

So? Most such adventures in atypical crank timing stall at the cocktail napkin sketch stage because they come with more drawbacks than benefits. Harley's crankshaft design is based on the hallowed 45-degree angle between its V-twin's cylinders, and we all know that works. Your hypothetical 315-degree crank would trade the superior primary balance of a 180-degree arrangement for a more engaging exhaust cadence and not much else. The 270-degree crank in Triumph's T-16 parallel-twin requires a stronger and subsequently heavier supporting structure, along with a pair of gear-driven balance shafts, to cancel the ensuing vibration. Coolness, you see, comes at a price. The 315 might sound or even feel better than a conventionally timed parallel twin, but odds are it would be palpably worse at almost everything else.

FAQ

Q I have often wondered why you spend so much time testing sportbikes on the racetrack. Most of us buy motorcycles that will never see a track. I understand that racing may be the intended purpose of a sportbike, and that aspect needs to be addressed, but why use most of the article?

A Racing, as the overworked adage goes, improves the breed. And the racetrack teaches you things that can't be learned anywhere else. Our standard testing regimen touches all the practical bases-comfort, convenience, miles per gallon, basic handling manners and such-on public pavement, the same way most readers would. Track testing has always been an enlightening part of sportbike testing. Now it's an essential one.

Inspecting all four corners of a 90-horse Suzuki GSX-R1100's performance envelope anywhere but the racetrack was irrational back in 1987. Wringing out a 2011 Kawasaki ZX-10R with twice that much power without cornerworkers and an ambulance standing by would be downright insane-not to mention irresponsible and just plain stupid-which is why the manufacturers routinely hold sportbike press intros at racetracks nowadays.

At the end of the day, we test bikes as their designers intended them to be ridden. Putting a skilled rider on a racetrack is the only way to separate fact from brochure fiction

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