The Technical Development of the Modern Motorcycle

Are motorcycles better than ever? Certainly the parts that make them up are. Lets explore the improvements to component design, and what they've meant to modern motorcycle design.

When I was a young I learned that when engines older than myself were disassembled, you could expect to find a wear ridge at the top of each cylinder. This ridge was several tenths of a millimeters high – often enough to prevent the piston and rod from being pushed out the top of the cylinder. It was then necessary to use a ridge reamer – a kind of crude manual boring device – to cut down the ridge enough to allow the piston rings to pop over it as the piston was pushed up.

Today, this ridge is greatly reduced. Why? Today, wear is slowed dramatically by the presence of anti-wear additives. In the late 50s this took the form of tricresyl phosphate, but today it is 'zinc' – a mouth-filler abbreviated to 'ZDDP'. Such additives form a metallic phosphide on the ring and/or cylinder surfaces. This low-friction layer sacrificially protects the metal beneath fro contact wear, and it re-formed from additive in the oil if scraped away.
The reason the ridge was there at the top of a worn cylinder was that as the piston ring nears TDC, it slows to a stop, then re-accelerates, offering an opportunity for the oil film beneath it to be squeezed out – especially when high temperature drastically reduced oil viscosity. Without the presence of additives, lubrication would then break down, allowing metal-to-metal contact, local welding, and the plucking of wear particles from both surfaces.

Another contributor to reduced wear has been the use of materials of differing hardness. Increasingly, cylinder surfaces are hard coatings deposited directly on light aluminum, replacing the traditional iron cylinder liner. When iron is still used as a cylinder material, the wear faces of rings may be rendered hard by chromium plating or by molybdenum filling. This process began during WWII, when traditional cast iron piston rings wore at a frightening rate in aircraft engines. The reason? High temperatures thinned lubricant films, permitting metal-to-metal contact. The practice of hard chrome-plating at least top piston rings thereafter became widespread.

Ball and roller bearings were widely adopted, first, to make possible the 'safety bicycle' of the 1890s, and automatic machines for their manufacture were soon developed. Yet even with improved bearing steels of the later 20s, one commentator remarked that: 'such were the defect levels in these materials that the failure of ball bearings began in the first seconds of their use.' Stress in such bearings concentrates just below their contact surfaces, and increases as the radius of ball or roller decreases. It was for this reason that quite large rollers were originally used in connecting-rod-big-end bearings.

The big improvement came when, in the late 50s, special steels began to be re-melted in vacuum, allowing impurities to boil out. Such extremely clean steels improved the life of rolling bearings.

Heavy con-rd roller-and-cage assemblies limited maximum rpm, for as the rod swings forward and back, its swing alternately adds to and subtracts from big-end bearing instantaneous rpm. At some high rpm the result was combined rolling and skidding of the rollers – a recipe for overheating, lubricant boiling, and seizure. Rollers shrank in diameter, and higher manufacturing precision made it possible in the late 60s to use a single row of wide but slender rollers in place of the traditional multiple rows of narrow, large-diameter ones.

In similar fashion, every part on your motorcycle has a long history of evolutionary change – even lowly bolts and studs. In early days, fasteners broke constantly, driving research into how to cheaply produce strong steels of high fatigue strength. And it came to pass. Lighter, higher-performing machines stress their parts harder, so there can be no end to this evolutionary developing process.
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