VALVE TRAINS

Motorcycle News (UK) - - This Week -

There are three main types of mo­tor­cy­cle valve train oper­a­tion – desmod­romic, bucket and shim, and fin­ger rock­ers (fin­ger fol­low­ers).

The desmod­romic sys­tem is syn­ony­mous with Du­cati, but inline-four sports­bike man­u­fac­tur­ers are split on us­ing ei­ther the bucket and shim sys­tem, or a fin­ger rocker sys­tem – as used on the new Suzuki GSX-R1000.

In a bucket and shim sys­tem the valve’s stem is cov­ered by a metal bucket. When the cam’s lobe turns it pushes on this bucket, which opens the valve against the valve spring.

In a fin­ger rocker sys­tem, the cam’s lobe in­stead ac­ti­vates a rocker arm, which pushes di­rectly on to the top of the valve’s stem (through a shim) to open the valve. In the­ory, a rocker sys­tem per­forms bet­ter as it is about 50% lighter than a bucket and shim sys­tem and has less float, mak­ing it more ac­cu­rate at high revs and there­fore bet­ter suited to a sports­bike en­gine. By re­duc­ing the weight, you can re­duce the force needed to open and close the valve, and have a faster revving en­gine with re­duced fric­tion and in­creased valve re­sponse.

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