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Shapes on Motorcycles Tell a Story


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Fifty years ago, a motorcycle engine’s appearance told a story and excited our curiosity, just as did the steam railroad locomotives of my boyhood. We could see functional cooling fins, carburetors, and purposefully angled cam-boxes on bike engines, just as I could see the locomotive’s giant steam cylinders and the whirling and back-and-forthing motions of its long driving rods. Chuck Berry described Johnny B. Goode “strummin’ with the rhythm that the drivers made.” Shapes told stories, revealed functions, stimulated curiosity.

Motorbike engines have evolved to be more powerful and efficient. They are also lighter and more durable. But the shape of them has become less important. Most motors are hidden behind a plastic cover, but when uncovered they are tall lumps of black epoxy painted metal. It is difficult to find the excitement of a clear purpose.

I was excited in 1959 when a friend of my dad opened the hood of his Jaguar 3.4 “saloon,” revealing the angled cam-boxes of its DOHC six-cylinder engine. As Honda’s campaign to win all the classes in FIM Grand Prix roadracing began, spy photos of its succession of world-beating multicylinder engines appeared in magazines, richly clothed in cooling fins, with angled cam-boxes just like Jaguar’s, and with multiple carburetor bells jutting.

There’s a purpose to all those fins, but there is a also a inherent beauty.

There’s a purpose to all those fins, but there is a also a inherent beauty. (Douglas MacRae/)

After I became involved in racing two-stroke Yamahas later, I stared for long seconds, enjoying their message, at the dense, mass of fins that covered heads and cylinders. Making power from heat requires effective cooling, whether we’re mowing the grass or accelerating into orbit.

Today, we all know the fins still exist. They’ve just been moved, made thinner, more numerous, and more organized into radiators and oil coolers, tucked behind the front wheel. Honda engineer Shoichiro irimajiri explained to me that in order to cool down the hot area over the combustion chambers for his 20,000 rpm Honda GP motors from the mid-1960s intake and exhaust vales had to be swung wide enough to create a large maze between them. The cylinders had to be angled forward in order to encourage air flow through those cooling fins.

The needs of efficient combustion have since brought valve included angles down from the 60–80 degrees of 70 years ago to more like 20 degrees now—made possible by the ability of liquid coolants to extract heat from areas inaccessible to air. There is no longer two cam-boxes each with their own cover. Now there is one box that contains both cams. This robs the heads of today’s engines of speech. They are now black boxes.

Included valve angles have decreased dramatically, and now a single less interesting cover houses the entire valve train.

Included angle of valves has been drastically reduced, and a single less appealing cover now houses the whole valve train. (Ducati/)

There is beauty in the engines of today, but you have to look hard for it. Each engine has an intake airbox that looks like the best Samsonite luggage. When you remove the top, your eyes will be delighted to see the multiple intake bellmouths. Fuel injectors are also visible, as are a stepper motor and sensors.

In modern motorcycle engines, you will find the beauty and grace of parts that have been honed by years of service in high-acceleration environments. The organic, flowing pistons are the most prominent. Closely coupled are the connecting rods—all the ugly shapes broke, crossing them off the list of possibilities. Both fluid flow and acceleration shape the valves.

There’s hope for engine exteriors too. Ducati engineers inherit a tradition of beautiful shaped machines. The close-fitting enclosures of the cam-drive gear trains of Guzzi’s late-1950s liquid-cooled 500cc V8 and those of Ducati’s latest MotoGP V4 are visual celebrations of function that are also entirely functional.

I am still fascinated.

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