Timing gears

If we look at a production engine from a roadcar, the chances are high that the cams will be driven by a toothed belt, whether the engine is overhead camshaft or overhead valve (pushrod). If we look at an engine from a motorcycle, the camshaft drive is often internal and the drive to the camshaft from the crank is taken care of by a special type of chain. The fact that there are discrete timing elements on both of these components – as in teeth on the belt and links on the chain – means that there is, or should be, no possibility for the relationship between crankshaft and camshaft to alter once set. Clearly, variable valve timing (VVT) systems deliberately alter cam timing, but this is very carefully controlled. Cam belts are an inexpensive and quiet solution. Chains are relatively noisy and require more complex tensioning and guidance than a typical belt.

In a bespoke race engine, you will quite likely find that the camshaft drive is via a series of gears, which are generally straight-cut spur gears. While helical gears are generally quieter in operation than spur gears, they generate axial thrust and require thrust bearings to cope with more load. Friction and wear are considerations in choosing straight-cut spur gears over the helical type.

Compared to a belt or a chain, these can represent a significant weight disadvantage, so why use them? There are several reasons, chief among which is precision. The improvement in precision is aided by the fact that gears are very much stiffer than either belts or chains. All cam drives are elastic systems, and they have to cope with rapidly varying loads. With lower stiffness, the elastic deflections are greater and precision is reduced accordingly; with a system lacking in stiffness we cannot be sure of the exact relationship between crankshaft angle and camshaft angle. This means that, for an engine equipped with chains or belts, there has to be an extra allowance for elastic deflections in the system compared to an engine with gear-driven cams.

This allowance might be that the timing using standard cams is more conservative than we would like to use or, where bespoke cam profiles are used, these may not be as aggressive as the engineers would like. Aggressive opening profiles, with high rates of acceleration, cause higher loads which then exacerbate the deflection in any cam drive. However, gears are less affected than either chains or belts in this regard.

A definite advantage of gears over belts or chains is the lack of tensioner and the fact that there is no periodic adjustment or maintenance. There are many race engine failures due to poor adjustment of chain tensioners or incorrectly functioning tensioners.

In high-speed engines, gear drives seem to be favoured. The last chain drive designed into a Formula One engine was during the mid-to-late 1990s by Cosworth. More recently, Aprilia caused some controversy by changing from chain-driven to gear-driven cams in the middle of the 2010 season, a move which was legal within the rules but which wasn’t welcomed by other teams, who could see a very fast bike becoming even faster. The race kit parts and engine modifications to convert the Aprilia engine were reputed to be very expensive. However, Ducati is known to have a ‘hybrid’ chain and gear drive system for its desmodromic valvetrain on its twin-cylinder superbike engine.

Written by Wayne Ward