Here comes the piston of a single-cylinder internal combustion engine, rising toward TDC on its compression stroke. Because combustion spreads by turbulent shredding, mixing, and transport of the flame originating at the spark plug’s gap, it takes time to attain peak combustion pressure. Ignition is therefore set to occur some 35–40 degrees before TDC in order to reach peak pressure just as the piston begins significant downward motion on its power stroke at about 11 degrees after TDC.

During the power stroke, torque exerted on the crankshaft is the product of combustion pressure times bore area, times the effective crank arm, which is the lateral offset of the crankpin from a vertical plane through the crankshaft axis.

Combine shock with a great number of repetitions and you have a recipe for fatigue failure.

– Kevin Cameron

At the moment of peak pressure this crankpin offset is quite small, so the resulting torque is also small. From the point of peak pressure, cylinder pressure falls rapidly as the trapped volume above the piston expands in driving the piston, so maximum torque is reached later—some 30 degrees ATDC, when the effective crank arm has grown significantly but the cylinder pressure has not yet fallen too much.

This is interesting because it reveals that torque rises from zero at TDC (where the effective crank arm is zero) to its peak value in 30 degrees, which is just over 8 percent of one revolution of the crank. This sudden rise in pressure resulted in Dudley’s Handbook of Practical Gear Design and Manufacture classifying the output of a single-cylinder engine as “medium shock” (section 13-2). Electric motors and turbines deliver their power uniformly and without shock, while the category “heavy shock” applies to ore crushers and single-cylinder compressors.

Combine shock with a great number of repetitions and you have a recipe for fatigue failure. A tiny bit of material damage from each cycle can become serious business in a million cycles. An hour at 6,000 rpm on a rare and precious Ducati Supermono delivers 180,000 shocks. As Charles Lindbergh crossed the Atlantic in 1927, his nine-cylinder Wright J-5C engine droning at 1,425 rpm, its crankshaft, and the metal propeller it drove, endured roughly 13,000,000 shocks.

I spent some time in 1963 riding an AJS 500 single and can testify, even though there was a large cam-and-saddle torsional shock absorber built into its engine sprocket, it was propelled by a succession of firm shoves. No wonder, then, that Edward Turner’s Triumph Speed Twin of 1937 was hailed as progress: propulsion doubled in smoothness (halved in harshness?).

The same pursuit of smooth propulsion drove the 1920s fascination with grand luxury autos whose slow-turning engines had eight, 12, or even 16 cylinders. Creamy smooth.

To this day, the drivelines of auto and bike engines are protected from the peak values of combustion shock by being driven through springs or rubber elements, usually located in the clutch. They allow the crankshaft to advance as crankshafts do—in a series of short, sharp accelerations—while sending a considerably smoothed torque, shorn of damaging peaks, to the gearbox. Without such protection, gears and their shafts would have to be significantly larger and heavier in order to deliver the desired life.

This is quite analogous to the effects of engine vibration on chassis. By reducing engine shaking forces with balance shafts or other means, much lighter chassis and other cycle parts can survive to honored old age. Race mechanics in the romantic “era of raw vibration” soon learned where the chassis, exhaust pipes, and mounting brackets of particular models would crack.