Technical innovations continue to change the world of motorcycles, and a lesser-known but fascinating concept is the offset crankshaft in four-stroke engines. Where in the past the piston, connecting rod and crankshaft were always exactly in line, manufacturers now often opt for a small deviation. But why?
This principle is called “desaxation”, which literally means “off-center”. It means that the cylinder is not positioned exactly above the crankshaft, but is slightly offset forward or backward. This results in less internal friction and more efficient power transmission. Although this technique was already used in some car engines in the 30s, it has only become common practice in motocross and enduro motorcycles in the last 15 years.
The reason behind this change is simple: better performance. When the crankshaft is slightly offset from the cylinder, the force exerted by the piston is better transferred to the connecting rod. This reduces the side pressure on the cylinder wall, which reduces friction and reduces wear. The result? An engine that runs smoother and uses its power more efficiently.
How much offset is optimal depends on the intended riding behavior. Bikes with a smaller offset benefit mainly at high revs, while a larger offset is beneficial at low revs. For example, Yamaha has opted for a 450mm offset for their YZ12F, while Honda and Kawasaki remain more modest with 4,5mm and 8,5mm respectively. On the other hand, Suzuki still uses a traditional, non-offset configuration.
A Suzuki block without an offset crankshaft.
A conventional four-stroke engine goes through a full cycle in 720 degrees of crankshaft rotation, divided into four strokes: intake, compression, power and exhaust. An engine with an offset crankshaft subtly changes the ratio of these strokes: the downward strokes (intake and power) are slightly longer, while the upward strokes (compression and exhaust) are slightly shorter. This has a beneficial effect on power output.
There are several ways to implement this offset technique. Some manufacturers opt for an eccentrically placed piston pin, while others change the entire cylinder position in the engine block. Yamaha experimented with a 2009mm offset piston pin in 1 as a relatively simple solution without modifying the crankcase, but larger displacements require structural changes to the engine design.
Yamaha YZ450F with 12mm offset
In summary, the desaxation technique offers a clever way to improve the efficiency and performance of four-stroke engines. Although not every brand uses the same approach, it is an innovation that is increasingly being used. A small detail in engine design can make a big difference on the track!
Photos: Honda, Hot Rod and Yamaha Racing
Tekst: Danny Hermans
