Crossplane what? Pull the other one…

Yamaha have made much of saying the engine in their new MT-09 ‘new generation sports bike’ shares a ‘crossplane philosophy’ with its growling R1 and M1. But is this just marketing hype? Well, yes and no. Strictly speaking, a triple can’t be ‘crossplane’; the name comes from the cross shape of the crank when viewed end-on. A triple with 120 degree crank spacing is more of a ‘Y’ shape than an ‘X’ shape. But what the triple does do is exploit the inertial torque cancellation effect as achieved by the crossplane in a four. The torque from the engine is the sum of inertial and combustion torque. The inertial force of the piston/rod assemblies is at its greatest at the dead centre positions where the assembly stops then starts again. Obviously, if all the pistons stop and start simultaneously, the inertial torque would be maximised. A triple (with 120 degree crank spacing) never has two or more pistons stopping/starting at the same time, which is the benefit of going to a crossplane arrangement in a four cylinder engine. The benefits Yamaha claims about ‘clean torque’ is inherent to triples with 120 degree crank spacing – and they’re as true of the Triumph Street Triple as they are of the MT-09 and new XSR900. So how significant are the benefits of having low inertial torque? The real benefit of an engine which cancels out some or all of its inertial torque is in throttle control. If it isn’t cancelled out, inertial torque increases with increasing rpm following a square law, which makes the engine difficult for the rider to control as its effect is constantly varying with change in rpm. By cancelling or minimising its effect, the linear combustion torque – provided by the explosion of the fuel/air mixture – becomes much more dominant. The disadvantage is that you trade inertial torque cancellation for balance – which requires a balance shaft, adding cost, weight and friction (lost bhp) to counteract this.