About Motorcycle Engine Cam Drive Systems

Why gears, chains, or belts?

A 1958 Benelli 248 grand prix racer geared cam drive system.Benelli

Reader Orange Toast (register to comment and we can use your name rather than a color assigned to a random object) writes to ask why Honda in its new CBR1000 “Triple-R” didn’t carry on with compact, rigid spur gears all the way from crankshaft to camshafts, as is normal practice in pure racing engines.

The terrible fact is that aluminum, of which engines are mostly made, expands with heat about three times more than does the steel of which gears are made. Therefore as a cold engine warms up to operating temperature, tooth-to-tooth gear backlash increases in gear cam drives. That makes noise and in our present world vehicle manufacturers must meet mandatory sound limits.

A way around this is to mount the gears in a steel frame, keeping their backlash constant, but the engine’s temperature expansion has to be dealt with somewhere. Gear backlash in the final mesh can be limited by employing a scissors gear. This is a gear that is actually two half-thickness gears, carrying springs that cause their teeth to spread out like so many little scissors, thereby taking up the clearance and (mostly) eliminating the noise. A 100-percent gear cam drive works well but every gear and its bearings must be precisely located and lubricated.

Layout of the 1985 VFR1000R’s camshaft gear drive.Honda

Another scheme was adopted by Norton and Velocette in their classic racing singles—to drive the cam(s) by a vertical shaft with a pair of right-angle bevel gears at top and bottom. The vertical shaft could then be equipped either with a spline or Oldham coupler that would neatly and soundlessly compensate for engine expansion during warm-up.

Kawasaki’s W800 is a modern (though undeniably retro) motorcycle that employs bevel gears and a shaft to drive its single overhead camshaft.Kawasaki

Alas, the experience with shaft and bevels on multi-cylinder engines has been that shaft torsional wind-up can be a problem. Ducati OHC singles and the bevel-drive twins originally used this shaft-and-bevels drive, but the company adopted toothed belts as soon as they were capable of taking over the job. Why? Because a proper job of setting bevel gear clearances and tooth contact allegedly took a skilled tech seven hours of fiddle-faddle with dial gauges and Prussian blue.

Toothed belts were a blessing for Ducati production bikes because they cut costs and were quiet and reliable. That gradually changed in Ducati’s Superbike race engines, in which the load on belts increased every year. To remain competitive, valve duration had to be reduced in the interest of midrange acceleration, and valve lift had to be increased to compensate for the reduction in open time. Those two changes added up to ever-increasing valve accelerations, which in turn required harder yanking on the belts. When the race team found itself having to change the belts daily, the design of the final giant Panigale twin was switched to conventional steel chains.

Ducati uses toothed rubber to drive the camshafts in many of its modern engines (821 Testastretta shown here) but has gone to chain drive in its highest-performing models.Ducati

In earlier times, cams were driven by actual roller chains—as for instance in AJS’s pre-WWII R7 and R10 racing singles. They evidently ran into cam-chain problems just as Honda did in developing the 2020 CBR1000 “triple-R,” and adopted the same remedy—using a gear drive from crank to a slower-turning gear, to which the bottom cam sprocket was attached. This, by allowing the number of teeth on the bottom sprocket to be increased, greatly reduced chain vibration, allowing the drive to be made stable with “Weller blades”—rubber-faced spring-steel chain guides.

Today cam drive by silent chain is the majority solution.

Harkening back to the 1970 Daytona, Honda entered four of its new CB750-based racers whose single overhead cam (SOHC) was driven by roller chain straight from the crank. Workable in a production bike, at racing rpm the roller chain displayed destructive dynamics. During practice week, rubber from the distressed chain guides was appearing in the engine oil, and in the 200-miler only that conservative engine manager Dick Mann was still running at the end—in first place. When his engine was re-started later after the finish, its cam drive failed as well.

Today cam drive by silent chain is the majority solution. It’s compact, inexpensive, and quiet. During the early years of World Superbike, teams running four-cylinder engines developed gear-drive kits to avoid changes in valve timing from the chain stretch that then occurred during a race. I used to listen to see which engines had gear drives—they did make a distinctive noise. Eventually class rules required that bikes race with the production cam drive. Today, each World Superbike rider is allocated seven engines per season, forcing factories to find ways to make silent chain cam drive adequately accurate and reliable.

BMW’s S 1000 RR superbike uses a chain to drive a gear that in turn drives both camshaft gears.BMW Motorrad

Back in the 1950s, NSU, in its production 250 “Max” single, used eccentrics and rods to drive its SOHC, and before WWII W.O. Bentley gave one of his auto engines a cam drive consisting of three such eccentric-driven rods, timed at 120 degrees. In the NSU only two rods were used, “quartered” (timed at 90 degrees to each other) like locomotive rods to prevent any possibility of the drive becoming stuck on center.

I find gears aesthetically pleasing in both appearance and in the sounds they make. Their use in MotoGP and other pure-racing applications is dictated by their long-lasting timing accuracy.

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