About Motorbike Engine Cam Drive Techniques

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

Reader Orange Toast (register to remark and we are able to use your name slightly than a coloration assigned to a random object) writes to ask why Honda in its new CBR1000 “Triple-R” didn’t keep on with compact, inflexible spur gears all the best way from crankshaft to camshafts, as is regular apply in pure racing engines.

The horrible reality is that aluminum, of which engines are principally made, expands with warmth about thrice greater than does the metal of which gears are made. Subsequently as a chilly engine warms as much as working temperature, tooth-to-tooth gear backlash will increase in gear cam drives. That makes noise and in our current world car producers should meet obligatory sound limits.

A approach round that is to mount the gears in a metal body, protecting their backlash fixed, however the engine’s temperature growth must be handled someplace. Gear backlash within the remaining mesh could be restricted by using a scissors gear. This can be a gear that’s truly two half-thickness gears, carrying springs that trigger their tooth to unfold out like so many little scissors, thereby taking over the clearance and (principally) eliminating the noise. A 100-percent gear cam drive works nicely however each gear and its bearings should be exactly positioned and lubricated.

Layout of the 1985 VFR1000R’s camshaft gear drive.

Structure of the 1985 VFR1000R’s camshaft gear drive. (Honda /)

One other scheme was adopted by Norton and Velocette of their basic racing singles—to drive the cam(s) by a vertical shaft with a pair of right-angle bevel gears at prime and backside. The vertical shaft might then be outfitted both with a spline or Oldham coupler that might neatly and soundlessly compensate for engine growth throughout 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’s W800 is a contemporary (although undeniably retro) motorbike that employs bevel gears and a shaft to drive its single overhead camshaft. (Kawasaki /)

Alas, the expertise with shaft and bevels on multi-cylinder engines has been that shaft torsional wind-up is usually a drawback. Ducati OHC singles and the bevel-drive twins initially used this shaft-and-bevels drive, however the firm adopted toothed belts as quickly as they had been able to taking up the job. Why? As a result of a correct job of setting bevel gear clearances and tooth contact allegedly took a talented tech seven hours of fiddle-faddle with dial gauges and Prussian blue.

Toothed belts had been a blessing for Ducati manufacturing bikes as a result of they reduce prices and had been quiet and dependable. That steadily modified in Ducati’s Superbike race engines, by which the load on belts elevated yearly. To stay aggressive, valve period needed to be decreased within the curiosity of midrange acceleration, and valve carry needed to be elevated to compensate for the discount in open time. These two modifications added as much as ever-increasing valve accelerations, which in flip required more durable yanking on the belts. When the race staff discovered itself having to vary the belts day by day, the design of the ultimate big Panigale twin was switched to traditional metal 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 makes use of toothed rubber to drive the camshafts in lots of its trendy engines (821 Testastretta proven right here) however has gone to chain drive in its highest-performing fashions. (Ducati /)

In earlier occasions, cams had been pushed by precise curler chains—as for example in AJS’s pre-WWII R7 and R10 racing singles. They evidently bumped into cam-chain issues simply as Honda did in growing the 2020 CBR1000 “triple-R,” and adopted the identical treatment—utilizing a gear drive from crank to a slower-turning gear, to which the underside cam sprocket was connected. This, by permitting the variety of tooth on the underside sprocket to be elevated, enormously decreased chain vibration, permitting the drive to be made secure with “Weller blades”—rubber-faced spring-steel chain guides.

Harkening again to the 1970 Daytona, Honda entered 4 of its new CB750-based racers whose single overhead cam (SOHC) was pushed by curler chain straight from the crank. Workable in a manufacturing bike, at racing rpm the curler chain displayed damaging dynamics. Throughout apply week, rubber from the distressed chain guides was showing within the engine oil, and within the 200-miler solely that conservative engine supervisor Dick Mann was nonetheless working on the finish—in first place. When his engine was re-started later after the end, its cam drive failed as nicely.

Right this moment cam drive by silent chain is almost all answer. It’s compact, cheap, and quiet. Through the early years of World Superbike, groups working four-cylinder engines developed gear-drive kits to keep away from modifications in valve timing from the chain stretch that then occurred throughout a race. I used to hearken to see which engines had gear drives—they did make a particular noise. Finally class guidelines required that bikes race with the manufacturing cam drive. Right this moment, every World Superbike rider is allotted seven engines per season, forcing factories to search out methods to make silent chain cam drive adequately correct and dependable.

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

BMW’s S 1000 RR superbike makes use of a sequence to drive a gear that in flip drives each camshaft gears. (BMW Motorrad/)

Again within the 1950s, NSU, in its manufacturing 250 “Max” single, used eccentrics and rods to drive its SOHC, and earlier than WWII W.O. Bentley gave one in every of his auto engines a cam drive consisting of three such eccentric-driven rods, timed at 120 levels. Within the NSU solely two rods had been used, “quartered” (timed at 90 levels to one another) like locomotive rods to forestall any chance of the drive turning into caught on middle.

I discover gears aesthetically pleasing in each look and within the sounds they make. Their use in MotoGP and different pure-racing functions is dictated by their long-lasting timing accuracy.

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