Primer on Motorcycle Valve Trains

| 6/30/2011 2:05:02 PM

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 parilla high cam 
The arrowed line highlights the Parilla's
cam, placed high in the engine's left case.

Ever wonder why it matters where the camshaft on a four-stroke engine is located? Or why 1930s race bikes adopted overhead camshafts while most road bikes didn’t? And just why did Parrilla put his camshaft half way up the engine?

To get the best out of a four-stroke engine, the valves need to be located in the cylinder head. This offers the easiest path for intake and exhaust gases in and out of the combustion chamber. Early side-valve (or “flathead”) engines as used by Harley, Indian and the like, located their intake and exhaust valves in the cylinder, parallel to the piston. This forces the intake and exhaust gases to follow tortuous twists to get into and out of the combustion chamber, severely limiting flow and hence performance.

That’s all well and good, but with the valves in the cylinder head, why does it matter where the camshaft is? Any valve train — that is, the series of components that permit the operation of the valves — exhibits properties associated with moving metal parts: inertia, flexing, resonance, etc. In an overhead valve engine, a camshaft (often buried in the engine case next to the crankshaft) acts on cam followers, lifting them. The followers in turn act on pushrods, lifting them, which in turn actuate pivoted rockers, and these, finally, open the valves — which are typically returned closed by spring pressure. Every component in this train is subject to energy losses and wear, and can introduce unwanted mechanical characteristics.

The entire valve operating system has reciprocating mass, and therefore inertia. For this reason, the valve springs must be strong enough to overcome the inertia of the rockers, pushrods and cam followers to close the valves effectively. If the springs are not strong enough, the valves will float or bounce, wasting energy and limiting power. Long pushrods may also absorb some of the action of the camshaft by bending and resonating, also causing losses. Rockers introduce other losses: Because they move in an arc, they slide across the top of the valve stem, putting side loads on the valves, causing valve guide and valve stem wear.

In low-performance, low revving overhead valve engines of the type fitted to motorcycles in the Twenties, not much of this mattered. But as revs rose, valve train limitations became more important, especially the poor metallurgy of then available valve springs. The solution was to make the valve train as short and light as possible. Ideally, the cams would act directly on the valves as in an overhead cam design, eliminating the valve train completely. But overhead valve engines have advantages. They’re typically smaller, simpler and cheaper to build. Putting the camshafts in the cylinder head makes the engine physically larger and more complex, and some form of cam drive — shaft, gears, chains, belts — has to be arranged. Rockers also allow for easy valve adjustment, whereas overhead cam engines often require camshaft removal for valve clearance adjustment.

8/11/2011 6:26:16 PM

Despite the limited rpm, there is one big advantage to push rods. They keep the engine's center of gravity low. One reason an old British push-rod bike will always handle better than any Japanese OHC bike of the same period. I remember having to wrestle my brother's CB350 through corners I took effortlessly on my Trident.

7/7/2011 9:20:30 AM

I am anticipating a channge in racing bikes, at least, to the same valve systen used in F1 cars. Pneumatic, I believe, that has no rotating mass. Just a thought.

Tim Kern
7/7/2011 9:02:13 AM

Lovely bike, with a wonderful sound. Thanks for sharing with us!

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