No more crank calls please

by Simon Hargreaves

Do Yamaha’s new MT-09 and MT-07 really use R1-style crossplane technology?

Lots of people get cross when Yamaha claim their new MT-09’s 850cc inline triple and MT-07’s 689cc parallel twin are crossplane engines. How can they be? They don’t have enough pistons.

The crossplane idea works like this: in a conventional inline four the pistons move in pairs, two up, two down. When the middle pair are at top dead centre, the piston either side is at bottom dead centre. They fire evenly, every 180°, 1-2-4-3. This layout has perfect primary balance because piston pairs move in opposition, but secondary vibration is created by a difference in inertia between the upward and downward strokes. It gets worse as revs increase, which is why some seemingly smooth inline fours give you numb fingers on the motorway, and some run a balancer to prevent it. Or big bar end counterweights.

In a crossplane crank four, the pistons are arranged if the nearest is at tdc, the middle two will be halfway through their strokes and the end piston will be at bdc (viewed end-on, they’re 90° around the crank, hence ‘crossplane’). They fire unevenly, at 270°, 180°, 90°, 180°, 1-3-2-4 (coincidentally, the same as Honda’s VFR800 V4).

The disadvantage of a standard inline four, say Yamaha, is that the pure, linear combustion torque, created by pistons pushing the crank, is interfered with by a ‘noisy’ inertial torque created by change in piston inertia as they stop and start up and down the bore. The result is it’s harder for the rider to modulate throttle at the outer reaches of traction. In other words, the bike is harder to control. The crossplane crank is easier to use on the edge because it harmonises combustion and inertial torques. So while the engine sounds grufty, like a V4, the force arrives at the rear wheel in a linear flow.

Which is fair enough. The idea works, proven by WSB, BSB and four MotoGP world titles. So why not apply the idea to an inline triple or parallel twin?

Because, on the face of it, you need four pistons to make a crossplane the same way you need four points to make a cross. But Yamaha say the MT-09 and MT-07 use crossplane philosophy, which is semantically correct. The MT-09’s crankpins are equally at 120° and provide a similar blending of combustion and inertial torque. The fact Triumph triples have done the same for years isn’t Yamaha’s fault. And the MT-07 copies the crank layout of Yamaha’s 1996 TRX and TDM850; both had 270° cranks, or half a crossplane – neither piston at tdc or bdc at the same time (unlike 180° or 360° twins). Again, the principle holds true and combustion and inertial torques are harmonised. And yes, Triumph have had 270° parallel twins for years.

So while it’s cheeky to say the MT-09 and MT-07 engines are crossplane, it’s also, technically, true. And it’ll be even more true next year when Yamaha bring out the MT-10, with a crossplane R1 engine in a naked roadster...

pittsy
pittsy's picture
Offline
Joined: 06/08/2011

Hi Simon. I had myself very much in mind when I saw your header. Sorry mate, I couldn't resist! :D

Haven't yamaha verbally moved things around so much that, to my mind, they have forgotten which way is up and which way is down? They now say, effectively, that the new twin and triple are trying to behave like a cross plane four. But surely it is the other way around?

Their twin is trying its best to behave like a ducati anyway! Maybe, instead of saying they are applying cross plane philosophy, they should say they are applying ducati philosophy! :D

(I'm only having a freindly dig in yamahas ribs.)

Simon wrote : "In a crossplane crank four, the pistons are arranged if the nearest is at tdc, the middle two will be halfway through their strokes and the end piston will be at bdc....."

Halfway through their strokes? This was an aspect which was discussed at the time, when yamaha released the cross plane r1. I'm not sure that 90 degrees around from tdc represents 50 percent stroke. I found the attached link really useful.

http://www.epi-eng.com/piston_engine_technology/piston_motion_basics.htm

Cheers Simon.

Edit: my comment assumes that the cross plane crank pins are 90 degrees apart (a cross!). Maybe they're not.