MXA SPEED SECRETS: DESAXE PHILOSOPHY AND CRANK OFFSET

 In most old-school engines, the piston tracks the center line of the cylinder’s bore. But on most modern engines, the cylinder and piston are offset several millimeters towards the exhaust side of the cylinder.

In most old-school engines, the piston tracks the center line of the cylinder’s bore. It achieves this by being mounted directly in line with the center of the crank. In short, the center of the crankshaft, crank pin, wrist pin and cylinder bore line up on a vertical axis at top dead center. In recent years, there has been a move to have the piston offset radially to the crank (perpendicular to the rotational axis). In most cases, the cylinder and piston are offset several millimeters towards the exhaust side of the cylinder.

Engines with the cranks offset from the cylinder’s center line are called “desaxe” engines. “Désaxé,” the French word for “unbalanced,” describes an engine in which the cylinder is positioned with its exact center (the bore axis) slightly offset from the center line of the crankshaft. And while offset cranks are rarely discussed in motocross and are relatively new to motorcycles (mostly within the last 15 years), there is nothing truly new about desaxe engines. Offset cranks and cylinders have been around as long as internal combustion engines—even Henry Ford recognized the desaxe philosophy and implemented it in the Ford Flathead V8 engine in the 1930s.

The goal of offsetting cranks is to reduce friction, and friction is at its highest during the combustion stroke because the piston is thrust into the side of the cylinder with the most force. The engine designer targets a specific rpm range, the one that they assume riders use the most (for example, between 5000 rpm and 8000 rpm) and then chooses the number of millimeters of offset to match that target rpm range. If the piston is in line with the center of the crank (as on most older-model four-stroke engines), there is a moment during the stroke when the piston’s pin, big-end rod bearing, top-end bearing and crank center are aligned into a position of stalemate at top dead center. This alignment of all three bearing surfaces on a vertical axis is a momentary negation of leverage. When the crank is offset by 4mm, 8mm, 10mm or 12mm, the three bearings are never aligned at top dead center. In a conventional four-stroke engine, each of the strokes—intake, compression, power and exhaust—involves a nominal rotation of 180 degrees, totaling 720 degrees for the complete four-stroke combustion cycle. A desaxe engine adds to the duration of the two downward strokes (intake and power) and subtracts the same amount from the two upward strokes (compression and exhaust), with the total remaining 720 degrees. A typical desaxe engine will have strokes of 185, 175, 185 and 175 degrees, with the degrees as a percentage of offset distance to stroke length.

Yamaha chose 12mm offset for the YZ450F engine, but that doesn’t mean that 15mm ‘s of offset would be beteer.

The constant stopping and starting of the piston imposes stress on the crank, rod, and piston. Moving the piston off-center causes the piston to reach top dead center at a different time than the connecting rod, effectively spreading the shock loading over a greater number of crankshaft degrees. This reduction in reciprocal loading allows for higher rpm with less stress—or, more accurately, less stress at higher rpm. The reduction in side forces (thrust) means less friction. Since the connecting rod spends most of its time at an angle, the piston produces minor and major thrust. Major thrust is on the downward stroke during combustion, while minor thrust is the piston’s side load against the cylinder wall (due to the rod being angled in that direction). Major and minor thrust depend on which direction the piston is traveling. Offsetting the center line of the crank and piston makes the transition from major to minor thrust less sudden (and instead of a sudden reversal of rod angle, as with a centerline arrangement, the piston actually rolls from major to minor thrust). By offsetting the crank, the piston’s down force is directed in a straighter line from the piston through the connecting rod to the crankshaft. Instead of pushing the rod at an extreme angle, the rod’s relationship to the crank is less severe, thus more efficient on the power stroke.

Many mountain bike riders were attracted to Shimano’s Biopace chain rings. They were ovalized and made the most of the rider’s more powerful downward leg stroke and, on the flip side, used less energy on the rider’s weaker upstroke.

If you are having trouble seeing how an offset crank works in your mind’s eye, think about mountain bikes. Back in the 1990s, Shimano made oval chainrings to mount on mountain bike cranks. Called “Biopace,” these ovalized chainrings made the most of the rider’s more powerful downward leg stroke and, on the flip side, used less energy on the rider’s weaker upstroke. To some degree, that is what an offset crank does. With the piston moved over to the downward side of the crank stroke, the power of combustion is pushing down on it with advantageous leverage, and when the piston is on the upstroke, there is a reduction in decompression braking, allowing a four-stroke motorcycle to roll into corners with the throttle off without going chug, chug, chug.

As a rule of thumb, less crank offset affects higher rpm ranges, while more offset has its greatest effect at lower rpm. Thus, when Yamaha’s engineers chose a 12mm offset for the YZ450F engine, they were using test rider input and computer modeling to get the type of power they wanted. It should be noted that there is a point of diminishing returns. Yamaha chose 12mm, but that does not mean that 15mm would be even better. And, in fact, the Suzuki
RM-Z450 uses 0mm of crank offset.

“OFFSETTING THE CRANKSHAFT TO ONE SIDE MAKES IT POSSIBLE TO ACHIEVE BOTH LESS FRICTION ON THE UP-STROKE AND MORE POWER ON THE DOWN-STROKE.”

The concept is simple. In a traditional center-line-correct engine, the connecting rod sits at equal angles when it pushes the piston against the cylinder walls. There is a lot of friction in places where you don’t want friction. This friction reduces power output and requires stronger components, which increases reciprocating mass. This waste of energy is most grievous on the power stroke. Repositioning the cylinder/crank alignment allows the connecting rod and piston to move straight up and down during the power stroke. The result is that less force is pushing the piston sideways, and therefore there is less friction. The traditional idea of having the crank aligned dead center with the cylinder’s bore is long gone. Every motorcycle and automobile manufacturer experiments with moving the crank out of the centerline of the cylinder. This means combustion stroke has more leverage, less friction and maximum power.

How are cranks offset? It can be as simple as having the crank aligned with the center line of the cylinder and offsetting the piston with an eccentric wrist pin. In 2009, Yamaha wanted the benefits of offset cranks for the YZ450F, but didn’t want the cost of new cases. So, Yamaha used a 1mm-offset-piston wrist pin to move the centerline over 1mm. However, you can’t get 8mm, 10mm or 12mm movement from an offset wrist pin. They require more complex solutions, often requiring casting offset engine cases.

Ultimately the objective of a desaxe engine is to produce more power with less friction. Since the compression stroke occurs on one side of the crank’s rotation and the power stroke on the other, offsetting the crankshaft to one side makes it possible to achieve both less friction on the up-stroke and more power on the down-stroke. But, for now, let’s leave the desaxe philosophy before we are forced to wander into the mathematical theorem known as the Chebyshev rule of rotational motion.