Driving The Camshafts—Roller Chain, Silent Chain, Morse Hy-Vo Chain

In the summer of 1964, I was assembling a Honda 305cc twin and was about to complete the installation of its cam chain when I fumbled the master-link plate and it disappeared down the central cam-chain well. Fortunately, I was able to recover the lost plate (magnets, flashlight, embarrassment) but the experience underlined the value of riveted endless chains in the application of camshaft drives; no small parts to fumble and no possibility of shedding a master link in high-speed operation.

Because the four-stroke operating cycle requires two rotations of the crank and one rotation of the camshaft to complete, there must be a two-to-one drive between the crank and camshaft(s). Most modern four-stroke motorcycle engines have overhead camshafts, though a few pushrod-and-rocker designs are still with us for sentimental reasons. In pure racing engines, the usual cam drive is by a train of spur gears, but two pairs of bevel gears and a shaft have been used as well (Manx Norton, Ducati bevel-drive engines). Another drive is via cog belt and sprockets, which is quiet, easy to install, and requires no lubrication (later Ducatis). Yet the great majority of production motorcycle engines drive their cams with some form of metal chain and sprockets, which are inexpensive and adequately precise for routine applications.

Curiosity can be a terrible thing because it sentences the curious to go through life carrying unanswered questions they’d like resolved. One of mine was, why did AJS drive the cam chain of its 7R not directly from the crankshaft but from a sprocket driven from the crank by a two-to-one gear reduction? The answer is “chordal motion,” the slight up-and-down motion of the chain as it hits the tooth spaces of the sprocket, a motion that becomes greater the fewer the teeth there are on the sprocket. Why does this motion exist? If you think of the sprocket as just the center lines of its several tooth spaces, it becomes clear that the chain is not wrapping around something circular, as a flat belt does, but is actually wrapping around a polygon (which just means a many-sided figure) with the same number of sides as it has teeth. Now all becomes clear: The fewer the teeth, the “bumpier” the sprocket becomes and the greater the “chordal action,” the up-and-down shaking imparted by it by the sprocket. Why is chordal action harmful? Because it imposes extra peak loads at high frequency on the drive, shortening its life.

Further, this is why chain-and-sprocket manufacturers recommend in their catalog material that, for best results, neither sprocket in a drive should have fewer than 20 teeth. In my own experience, only Harley-Davidson has actually taken this advice; drive sprockets on its classic racebikes have historically been up in that range. I still have a 13-tooth front sprocket in my 250cc sprocket set.

All this tells me why so many engine makers—AJS, for example, in its 350cc 7R racer—employing a roller chain to drive overhead cam(s) use a two-to-one gear reduction from the crank to drive the bottom sprocket; it allows that sprocket to have twice as many teeth and so avoid exaggerated chordal action.

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It should be noted that all free runs of cam chains are damped against “bowstringing” by being in contact with plastic-faced Weller blades, one of which acts as a chain tensioner.

Motorcycle final drives are most often by roller chain, and people are surprised to find that the chains used in MotoGP are quite light. Again, there is a reason. Part of chain tension is that produced by the engine, reeling in chain for all it’s worth. But there’s another chunk of tension added by the simple centrifugal force of the chain itself. At 200 mph, the back wheel is turning 2,700 rpm, and with a 40-tooth rear sprocket and 100 links of chain, the chain as a whole is circulating at 1,100 rpm, imposing an extra centrifugal tension on the load it carries. The lighter the chain, the less becomes that extra tension.

Back in the 1990s, Superbike race teams were bothered by cams going out of timing as a result of chain wear during an event, so they began to make gear-drive conversions to eliminate the resulting power loss. It was fun to listen to race engines warming up, trying to distinguish those with gear drive from those with stock chain cam drive. Eventually the rules makers banned gear-drive kits, as they reasoned the stock chain was cheaper than a gear drive. But, of course, teams then had to change the chain at least for every event, and maybe on Saturday afternoon as well, making the result much more expensive in money and time than a gear drive. How do you change an endless cam chain? You split the crankcases, then lift the crankshaft enough to slip off the old chain and slip on the new one. Then you reassemble.

Now we come to silent chain and Morse Hy-Vo chain, which are both inverted-tooth chains rather than employing round rollers as final-drive chains do. “Silent” inverted-tooth chains date back at least to 1843, when such a drive was applied in place of noisier and more expensive gears in a ship’s propeller drive. Sprockets for inverted-tooth chains look more like gears than like conventional roller-chain sprockets.

Morse Hy-Vo chain goes a step further to actually mechanically reduce chordal action (Morse is a division of BorgWarner). Instead of its links being joined by round pins, they are joined by rolling contact rocker pins. These are like placing in kissing rolling contact a pair of chair rockers, only made small enough to fit into the holes in the chain’s link and guide plates. The rolling contact of these rocker pins is so designed that as the chain bends, its line of action (point of contact, through which chain tension passes) moves in the direction opposite to the normal chordal motion. The result is a drive better able to operate smoothly at high rpm, which is why today Morse Hy-Vo chain is so often chosen to drive the overhead cams of motorcycle engines. With reduced chordal action, such chain can better survive the tight wrap around the small drive sprocket on the crankshaft.