Désaxé engines

Kawasaki’s new ZX-10R Ninja is bristling with state-of-the-art and new technology, but like so many ideas, some can trace their roots back a very long way. The Ninja’s engine uses a désaxé layout, which means the centre line of the cylinders is offset, and rather than crossing through the centre line of the crank as on most engines, it passes 2mm in front of it, on the exhaust side.

Horex VR6Just like the crossplane crankshaft in Yamaha’s R1, this is an ‘innovation’ which was being used by the Victorians in their steam engines before the first petrol engine was ever thought of. Where the crossplane is designed to produce a more constant crankshaft speed, désaxé engines aim at giving the pistons an easier life.

During the power stroke, the cylinder pressure is forcing the piston down the bore, but the conrod is at an angle to it. This means the piston is being forced hard against the side of the bore, which every engine builder will know about as there’s always a lot more wear on one side of the bore and piston than the other. By shifting the cylinder to one side relative to the crank, the conrod’s angle with the piston is reduced during the downward power stroke, and the sidethrust is reduced with it.

This has two useful effects: one is a reduction in friction, improving the efficiency of the motor. The second is that because the peak forces on the pistons are smaller, they can be lighter, which in turn allows for lighter conrods and smaller bearings, also reducing internal friction.

Désaxé is often written De Saxé or something similar with claims made that the idea was invented by a Frenchman of this name. This is not the case, and it’s unlikely as one meaning of désaxé is a deranged person – roughly translated it means nutter! The other meaning is off-centre, which describes this cylinder arrangement.

Hesketh 1200 VulcanThere are other consequences. A small increase in the stroke compared with the crank throw has to be accounted for at the design stage. The power stroke takes longer, which is good for a high revving engine as it allows more time for efficient combustion, but it also makes controlling vibration more difficult as the secondary out-of-balance forces become more complex and asymmetric. The 1970s Hesketh V-twin used a désaxé design which was largely responsible for that bike’s excessive vibration, and the new VW VR6-derived Horex engine is also désaxé, although for packaging reasons rather than to deal with piston sidethrust.

The Ariel Leader two-stroke achieved désaxé advantages but by using gudgeon pins offset in the pistons – the cylinders were still centred directly above the crank.