What Are The Identifiers Of A Modern Motorcycle Engine Design?

A high-mounted clutch, a one-piece valve cover, and the location of the camshaft chain tensioner...

What are some of the things that have changed in more recent sportbike engines? A vertically stacked gearbox enables an engine to be made shorter from front to back. Placing the clutch shaft above the output shaft puts the clutch rather high on the engine. I often find myself doing a double take when I look at pictures of some of the most recent engines because the clutch is so high.

Narrower valve angles are another feature of modern engines. In the old days, the valves were at a large angle. A four-cylinder engine, for example, had a cover over the intake cam and a separate one over the exhaust cam with a space down the middle in which you could see where the spark plugs were located. With engines where the angle between the intake and exhaust valves has been narrowed from as much as 100 degrees to 25 or even less to make the combustion chamber flatter, a single cover can be used.

A cam-chain tensioner is only seen on engines with overhead cams that are driven by chain. Plastic dampers prevent the free runs of the chain from vibrating and destroying themselves. The tensioner—this one from a Honda CBR600RR is hydraulically operated; some use springs—presses the shoe against the chain. The fact that this one is on the back of the engine indicates a forward-rotating engine.

The first time I saw Yamaha’s YZR-M1 MotoGP engine, I noticed the cam-chain tensioner was on the front, a dead giveaway the engine was backward rotating. Why would Yamaha do that? You have two big gyroscopes on a motorcycle that want to keep on spinning. When you steer the front wheel and try to lean the motorcycle over, those big gyros—the wheels, tires, rims, and brake discs—resist that with a precession force. So does the crankshaft. If we rotate the crank backward, its precession force will cancel some of the precession force from the wheels and the motorcycle will roll over more quickly. For a long time, Yamaha had the only reverse-rotating engine in MotoGP. Now others are doing it too.

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