SOHC or DOHC, which is better and how do they Work

A cam is one of the most basic engineering devices devised by man. It's not quite at the primitive level of the wheel, the screw, or the lever, but it has been around for some time.

At its most simple, a cam is an offset part of a shaft (camshaft) which pushes against something when the shaft turns. This makes it perfect for converting rotating motion into linear, reciprocating motion – exactly what you need to operate the poppet – style valves used in motorcycle engines.

In a modern motorcycle engine, the cam generally takes the form of an 'egg-shaped' lobe formed onto a rotating shaft called the camshaft. Depending on the engine design, there may be one lobe for each valve in the engine, or one lobe can operate more than one valve at a time. The cams can operate the valves directly, or via a pivoting rocker arm, pushing the valve open against a spring, which then closes the valve as the cam lobe turns past the valve or rocker.

The most common camshaft layouts today are the SOHC – Single Overhead Cam - and DOHC – Double Overhead Cam. The SOHC single-cylinder engine, the SOHC generally operates one inlet and one exhaust valve, for the DOHC layout it generally operated two-inlet and two-exhaust valves per cylinder. Having DOHC in the engine aids performance sporty motorcycles in a number of ways; it allows a shorter, more direct load path between the cam and the valve, reduces the mass of the valve train, and makes the components stiffer. This both improves engine pickup and raises the maximum rev limit. The SOHC camshaft layout has as advantage that it performance is more efficient at the lower revving range. With current highly efficient fuel-injection systems we likely to see the SOHC system making a comeback.

The shape of a cam lobe, mostly called the 'nose', defines how the valve it operates opens. A sharply-angled ramp will open the valve faster and a flatter nose will hold the valve open for longer, which is good for high rpm power. The height of the nose above the base circle of the shaft defines how far it opens the valve. So, broadly, a sharply-profiled nose with a large tip and a high peak will work better at high-rpm.

The position of the lobe on the camshaft also defines when, in the engine cycle, the valve will open. An inlet valve needs to open a bit before the piston is at top dead center to give time for the air/fuel mix to get into the combustion chamber. At high revs, the valve needs to open sooner, since there is less time to get the mixture into the combustion chamber.

There are various ways to drive your camshafts. The most common is a chain: a narrow drive chain runs from a sprocket on the crankshaft up inside the engine to the cylinder head, turning the camshaft via another sprocket. A tensioner device keeps the chain taut, and maintains the timing of the valve openings. Because it's inside the engine, the chain gets a perfect supply of lubrication and can last a very long time. These cam-chains are used in almost all Japanese motorcycles.

Similar to a chain is a belt drive system – as used on the famous Ducati's V-Twin engines. Here, a Kevlar – reinforced toothed rubber belt runs from the crankshaft to pulleys on the camshaft ends. Belts are normally mounted outside the engine and are a service item which needs to be replaced at recommended intervals.

Shaft and gear drives are also used, although they are generally more expensive and complex. Honda has used gear driven camshafts in various motorcycles: RC51, RC45 and the NC30. The system uses narrow, large gear wheels mounted parallel to the cylinder, taking drive from the crankshaft up to the camshaft. The advantages are perfect cam timing control, and an almost endless service life.