Reading between the centrelines

by Simon Hargreaves

Why the désaxé layout has become the latest fashion in engine design

Question: what has Yamaha’s new MT-09 inline triple got in common with their 2010 YZF450F motocross single, Honda’s MSX125 monkey bike and CBR250R, Kawasaki’s ZX-10R, Triumph’s Tiger 800, 1200 Explorer and Trophy, the Horex VR6, a 1979 Hesketh V-twin, a 1932 Ford V8 and an eighteenth century steam engine?
Answer: they all use the désaxé principle, in which the vertical centreline of the crankshaft is offset to the centreline of the pistons (désaxé is French for off-centre).
To picture it, imagine a side-on engine cut in half. Draw a line from the centre of the crankshaft up through the cylinders. On most bike engines with the piston at top dead centre, the line intersects the con rod and through the middle of the piston.
In a désaxé engine the crankshaft is set back a few mm (or cylinders forwards) so the line from the crankshaft no longer passes through the centre of the piston but to the rear of it.

During the combustion stroke in a conventional engine, the piston is pushed into the cylinder wall by sideways leverage from the con rod (now at an angle to the centreline). This creates friction. The higher you rev, the worse it gets. Exponentially. And friction is bad for performance and – particularly these days – fuel economy.
By offsetting the cylinders – a case of building the engine that way, without extra materials or complex design – the piston’s side thrust against the bore is reduced. How much offset depends on crank and con rod geometry, but can be anything from 4mm to 14mm (obviously the worst side thrust is during the combustion stroke, which is why the cylinders are forward of the crank). Honda’s Insight hybrid uses a 14mm désaxé layout, which they say reduces side thrust forces by 3%.
But there are other benefits too: piston rock, the moment when the thrust load moves from one side of the piston to the other at top dead centre, is reduced, making the engine quieter. Because of a change in the piston’s motion, the time it spends close to top dead centre is increased, giving more time to keep valves open, get more mixture in, burn it efficiently, and get it out. Average piston speed is lowered which reduces stress, permitting lighter pistons, shorter skirts (always good), and less pressure on piston rings. And the désaxé principle holds true for any configuration of engine, from single to twin, V-twin to V4, inline four to V6.
In short, you get a lot of advantages and no drawbacks (well, two: friction on the compression and exhaust stroke increases, and vibration can cause problems on longer stroke engines).
So why has is taken bike engineers so long to use it, when cars have had it for years?
Because they haven’t needed to. The benefits are only worthwhile when extracting small gains at extremes of engine efficiency – be they in performance or fuel economy. On modern bike engines the hunt for horsepower is over and the new challenge for engineers is improving fuel consumption.
If nothing else, the proliferation of désaxé layouts shows how seriously they’re taking it.

pittsy
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Interesting article.

I wonder if the desaxe layout increases torque in another way too? One might imagine the force from the piston is transmitted into the crank at an angle conducive to good mechanical advantage with the piston nearer tdc, where the gas pressure is higher?

My thinking here is that tdc of crank pin no longer coincides with tdc of piston. Tdc of piston would occur after tdc of crank pin, no? In fact, thinking about it, the "dc" in "tdc" no longer applies to the piston, surely? Just "t" ! Or how about atos? Absolute top of stroke. ;)

Wonder how it affects ignition timing?