YouTube enables us to hear classic motorcycles running, such as Indian’s fabled eight-valve boardtrack racers. What is unusual about the sounds of such machines is that rather than producing a pleasant throaty rumble, they mostly clatter.
This tells us how far modern engines have come in the sophistication of their operation. Self-adjusting hydraulic tappets have taken the place of the constant clicking of manually adjusted valve tappets. The simple shapes of early pistons made it impossible to avoid the “piston slap” that occurs as rod angularity changes direction at top and bottom center, causing the piston to “change sides” in its cylinder—making a clearly audible sound. American motorcycle engines through the 1930s had a hard time sustaining speeds of over 50 mph without real danger of piston scoring or seizure. Increasing the piston’s clearance reduced its ability to cool by contact with the cylinder wall, and its clattering might bend or break the piston’s skirts. Reducing the clearance brought danger of seizure.
Modern pistons are “cam-ground,” meaning they are deliberately made oval so that they are in close contact with the cylinder wall only on their thrust surfaces; this reduces friction. They may also be made of materials whose heat expansion is lower than with the simpler alloys of early times. Wrist pins are offset so that as a piston decelerates at the end of each stroke, its inertia causes it to tilt slightly, making a quieter two-stage transition from thrust to non-thrust cylinder walls and back again.
Modern pistons are deliberately made oval to reduce friction by only contacting the thrust surfaces; this in turn does away with piston slap and the noise associated with that slap.
Courtesy of Ducati
Timing gears (for camshaft drives) are difficult to manufacture accurately because some deformation may occur when the steel is hardened by heat-treating. Tooth profiles can be ground after hardening, but this adds expense. Some backlash is required between gears to allow room for an oil film, but controlling that backlash also costs money—and the more lash there is, the louder becomes the clicking that occurs as cam events constantly reverse the torque on the gears. This has tempted modern designers to abandon gears in favor of silent chains in cam drives. Another technique is to use so-called “scissors gears.” One gear of a pair (for example, in gears by which an engine drives its clutch and gearbox) is made as a stack of two—one thick, the other thinner and spring-loaded such that the teeth of the pair are slightly driven apart, like scissors opening. When engaged with the mating gear, this has the effect of taking up any backlash that is present, greatly quieting operation.
Engine valves weigh a fair amount—from 20–25 grams for one from a multi-valve 750-4 to more like 110 grams or more for one from a large-displacement two-valve. Their seating velocity is carefully limited to avoid valve seat recession, but in early engines there was still quite a clatter and the many slight valve impacts per second set the head itself into vibration, radiating the sound to our ears.
The sounds made by modern engines are closely studied by NVH specialists (Noise, Vibration, and Harshness), who identify each source and whether it is being amplified by sympathetic vibrations excited in crankcase, engine covers, or other parts. In many cases, clutch covers have sound dampers inside them. To assure smooth engagement and release, clutches must have some clearance between clutch disc driving tangs and the slots they fit into in the surrounding clutch basket. At low rpm, when crank speed fluctuates quite a bit at each cylinder firing, a chatter is generated between clutch tangs and slots (you can hear this most clearly from a racing engine with an exposed dry clutch). The sound damper inside a wet clutch case acts like civilizing an empty room by filling it with upholstered furniture—the soft material damps out sound, making the room quieter. Crankcases are given curved shapes that stiffen them against acting as “loudspeakers.” In the case of air-cooled engines, the ringing of cooling fins can not only tickle the ear but has often caused fins to fatigue and break off. For these reasons fins may be joined by cast-in vertical members that discourage ringing, or rubber dampers may be molded to allow insertion between fins to stop noise.
Sprockets look round but are polygons; this creates as rising and falling of the chain that creates noise. The more teeth on the sprocket, the lesser the effect.
When C.B. Franklin designed the original Indian Scout of 1919–’20, he gave it a quiet-running primary drive consisting of three helical gears, their teeth angled enough to assure that more than one tooth carried the load at all times. This reduced the noise that can occur as one tooth pair gives up the load and it passes to the next pair. It was possible to use gears in that engine because the gearbox was bolted to the crankcase and not to the chassis—an early example of modern unit construction.
Anyone who has coasted on a motorcycle with a stopped engine has heard the “zhhh” noise a drive chain makes, generated as each roller hits the sprockets. Sprockets look round but in action they are polygons, causing a rising and falling of the chain that becomes larger as the number of sprocket teeth is made smaller. Some manufacturers place rubber damping material close to the front sprocket to quiet the “landings” of chain rollers.
It has taken years of study and experiment to turn the clatter of a 1920s motorcycle engine into the much-reduced mechanical noise signature of modern machines.