MotoGP – the chain

In the cutting-edge world of MotoGP, coverage of technical developments tends to focus on game-changing technologies relating to the chassis or powertrain. However, less glamorous components are also subject to constant evolution, including the humble chain. So what differentiates a factory Honda or Kawasaki chain from one found on a lowly CB125 road bike.

‘Roller’ chains have been the universally favoured means of transmitting drive on motorcycles for many years. While belt and shaft drives occasionally feature on road-going machinery, the chain is still the most practical means of transferring drive.

Both belts and shafts have their advantages. Belt drives are very clean, quiet and relatively inexpensive to produce; however, the belts are not nearly as strong as chains. To make them as strong, they are made wider, and a belt running on a modern-day superbike would need to be many inches wide, making it completely impractical from a packaging perspective. Shaft drives also provide a very ‘clean’ package, but are far too heavy and complex and with high parasitic losses to be considered for modern racing machinery. The chain though has the advantage of being lightweight, easy to package and the ability to withstand the power levels produced by the most powerful bike engines.

Roller chains [Fig. 1] used in motorcycle applications consists of a series of short cylindrical rollers held together by side links. There are two types of link alternating in the roller chain – inner links and outer links. The inner links have two inner plates held together by two sleeves or bushings upon which rotate two rollers, while the outer links consist of two outer plates held together by pins passing through the bushings of the inner links.

Despite appearing to be a very simple mechanism, the latest generation of racing chains have seen each of these parts optimised to the nth degree to ensure they provide the greatest possible strength for minimum weight. To put these improvements into perspective, a 1970s vintage GP racer produced in the region of 130 hp and would generally use a 530-size chain weighing about 2.5 lb per yard. Modern MotoGP racers produce more than 200 hp but run a 520-size chain that weighs only 2.3 lb per yard. The chains have also become more mechanically efficient, sapping less power through frictional losses.

While much of this improvement has been down to gradual development of parts such as the bushing materials, there have been a few key advances in recent years. One such advance is in the sealing mechanism used between the inner and outer links. Before riveting in the factory, the internal parts of the chain are filled with chain grease by vacuum. The sealing rings then have two purposes – to keep the internal lubrication in, and the dirt out, thus vastly improving the durability of the chain.

The load-bearing pins and bushings that enable a chain to bend over a sprocket have precious little oil to keep them happy. As if that wasn’t enough, high centrifugal forces that occur when the chain turns around the drive sprocket throws the oil out. The single biggest factor in chain wear is loss of lubricant, and the advent of O-ring sealed chains enabled the chain to keep its oil inside and stay lubricated where it counts. For most chains, O-rings are still the favoured sealing medium, but in high-performance applications a new X-ring design has taken over.

The biggest problem with O-rings is that they create a large amount of friction between the sealing surfaces, a less than ideal situation. An X-ring seal is exactly as it sounds: a seal with an X-shaped cross-section. When clamped between the two side plates, only two points of the ring make contact, providing a good seal but with greatly reduced friction. The improvement is significant, with some manufacturers claiming a reduction of up to 40% in friction over O-ring sealed chains.

Clearly this is only one improvement to what is a surprisingly complex component, and many factors – not least of which is correct installation and lubrication – are of critical importance to ensuring a chain lasts a race distance. But it goes to show that even the most humble of components still presents engineers with a neverending scope for performance improvement.

Fig. 1 - Layout of a roller chain

1. Outer plate
2. Inner plate
3. Pin
4. Bushing
5. Roller

Written by Lawrence Butcher