How Do Valve Mechanisms Work In A Motorcycle Engine?

Cycle World Technical Editor Kevin Cameron answers your engineering and mechanical questions

I've set up a little display to demonstrate the various ways in which valve mechanism work. The first is an automotive valve train. Automotive engines—these parts are from a 455ci Oldsmobile—often use flat tappets. The cam lobe rotates and pushes the tappet up. The pushrod, which has ball ends so misalignment can be tolerated, transmits the force from the cam lobe to a rocker arm, which pivots in the middle. The far end of the rocker arm pushes the valve open.

Bear in mind that we want the whole valve mechanism to be under the control of the camshaft at all times. The cam doesn’t push the valve open and then the valve spring snaps the valve shut; the valve would never be able to tolerate that kind of violent seating. Here’s what has to happen: The mechanism accelerates the valve off its seat, the valve reaches the top of its lift and stops. The spring, holding the valve against the cam profile, reaccelerates the valve back toward its seat. As the valve nears the seat, the cam slows the motion and the valve goes back onto its seat at a survivable speed of, say, 2 feet per second.

The inverted bucket tappet is a little cup. When this tappet is installed, the valve is in the cylinder head facing the piston with the stem upward. The bucket tappet fits down over the valve spring, retainer, and collets. The cam lobe—an overhead cam—pushes directly on the tappet, which opens the valve. This is a lot less mass than the pushrod-and-rocker system, which is why it needs stiffer springs than an overhead-valve system.

The finger follower is the easy road to success. This one is from a BMW S1000RR. The little arc-shaped piece bears against the valve-stem tip, and the cam lobe bears against the radius pad. A rod passes through all the finger followers in the cylinder head. The finger follower, being much lighter than the inverted bucket tappet, can operate at higher rpm. That's why followers are generally used in Formula 1 and MotoGP. It's also easy to extend the wear track to accommodate increased lift if you've made the cam lobe taller.

So that is my short review of various valve mechanisms: pushrod, tappet, rocker arm, and valve; an inverted-bucket tappet sits down over the springs and is fairly light; and the finger follower, the lightest of all.

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for Cycle magazine and, since 1992, for Cycle World. Kevin’s unparalleled experience and knowledge of the sport were—and continue to be—prompted by a lifetime of curiosity. His willingness to share that information with anyone who is willing to listen is likewise unique.

Kevin’s greatest strength lies in his ability to present complex subjects in simple terms with clarity and, often, humor. In this video series, shot in his home shop, Kevin draws upon his vast historical references to address modern-day questions. As Kevin has written, “Emotions bring us to engineering, but engineering then becomes a special way of confronting reality.”