Fouled Plugs: Keep Your Belt On!

Fouled Plugs: Keep Your Belt On!
Photo by: Philip Myrtorp

I’ll apologize before your eyes glaze over—this piece is about a technical subject. Specifically, why would manufacturers design engines that use timing belts as opposed to chains or gears? You likely already know that four-stroke engines have valves that are opened by camshafts. The cams turn at a 1:2 ratio with respect to the crankshaft, and there are three main methods for transferring the rotational energy of the crankshaft to the timing drive—gears, chains, or toothed belts. There are new technologies on the horizon, but to my knowledge, none have been implemented on motorcycles. Gear-driven systems are pretty rare on motorcycles, so I’ll exclude them.

The timing drive’s tolerance must be maintained over the life of the engine or significant engine damage will occur. This is why regular adjustment of valve clearances is necessary and engines are equipped with chain or belt tensioners. However, sudden failures are not uncommon and if any system is more likely to fail catastrophically, it’s the belt drive. So why in the bent valves would a manufacturer choose a belt system when a properly designed and maintained gear or chain system will usually last the life of an engine?

On Belts

Alright, fine, belt drive systems do have some advantages. Let’s dig into that. There’s a list of factors to consider when designing an engine, including frictional loss, durability, maintenance, timing accuracy over the life of the engine, dynamic behavior, weight, packaging, a quality known as Noise, Vibration, and Harshness (NVH), and, usually the biggest influencer, production cost.