Sports Cycle

Final Drive Systems: Chains, Shaft or Belt ?

You now have a fair idea about what all that commotion taking place down below your legs is all about, but really, it means nothing if you have no way of transmitting that commotion to the back wheel of your bike.

As with many other aspects of motorcycle technology, the three final-drive systems most commonly used today - chain, shaft, and belt - had all been tried before the European squabbling that started World War I got out of hand.

In Chains

In the early years of the sport, the crude belt system mentioned earlier was employed, but by the end of World War I, most manufacturers had settled on a chain-drive system (although a few opted for drive shafts). The simplicity and ease of repair of a chain-driven system made it a logical choice back then.

In this system, a sprocket mounted to an output shaft in the transmission is connected to a sprocket attached to the rear wheel of the motorcycle by the drive chain. Although such a setup requires periodic maintenance (the rear wheel needs to be adjusted to take up the slack created in the chain as it stretches over time), the benefits of this system make it popular to this day.

Unlike shafts, chains absorb much of the impact created during acceleration (which is the main reason they stretch over time). Shafts, which are much more rigid than chains, feed that energy back into the frame, causing all kinds of funky motion that a rider feels. This motion is called shaft jacking, and while it’s not a serious problem in most real-world riding conditions, it detracts from a bike’s handling.

Probably the biggest drawback of chain drives is that they are messy. In addition to needing to be tightened, they need to be cleaned and lubricated on a regular basis. The solutions used to do this are sticky and greasy, and they tend to fly off chains and onto a rider’s jacket and clothes.

Getting the Shaft

Shaft-drive final-drive systems transmit the power to the rear wheel via a drive shaft. The chassis refers to the combined frame and suspension on a motorcycle.

Shaft drives do affect handling: They transmit the energy a chain absorbs directly to the chassis (the combined frame and suspension), causing the rear of the motorcycle to move up when it should move down and vice versa. Still, shaft drives are popular because they are so convenient and low-maintenance, especially when traveling. It’s hard enough packing a motorcycle for an extended trip without having to put cans of lube and cleaning solvent in your duffel bag next to your shirts and underwear. Because of this, the most popular touring and sport-touring bikes have shaft drives.

A couple of clever companies (BMW and Moto Guzzi) have all but eliminated the antics associated with shaft systems by using creative rear-suspension geometry. By arranging suspension components in a parallelogram-type position, the rear suspensions on these bikes feed the shaft-jacking motion into the frame along a horizontal axis rather than a vertical axis, converting the up-and-down motion into a forwardand - backward motion. The rider hardly notices this sort of movement.

While the shaft’s acrobatics can be overcome by clever designers, another drawback of shaft systems cannot be overcome so easily: their weight. Shaft drives are heavy, and that weight is unsprung, meaning that it is not supported by the suspension. Unsprung weight tends to decrease the bike’s capability to switch directions more than the same amount of weight carried by the suspension. Because it is more directly connected to the road than weight supported by the suspension, unsprung weight has a disproportionate (and negative) effect on handling.

The drawbacks of both shaft and chain systems led some companies to explore an old alternative: belt drives.

Belt It Out

Although belt drives were abandoned early on, by the beginning of the 1980s, advances in materials had once again made them a viable alternative. Harley-Davidson was the first to reintroduce the concept, equipping special-edition bikes (like its original Sturgis model) with both primary and final drive belts. The primary belt system never caught on, but the final drive belt did, and today every Harley made comes standard with such a belt. Kawasaki also experimented with belts on some of its small displacement bikes.

The advantages of belts over chains are that belts are much cleaner, requiring neither messy lube nor cleaning solvents. While they do need periodic adjustments, they need such attention less frequently than chains. The advantages of a belt system over a shaft is that belts, like chains, do not transmit unwanted motion into the chassis. They also do not come with the unsprung-weight penalty of a shaft system.

The disadvantages of belt systems, at least in the case of Harley-Davidson’s application, is that they are true buggers to replace when they do go bad, especially on the big twins. Except for certain models, most big twins require that you partially disassemble the frame in order to replace a belt. And to replace a belt on all big twins, you must remove the primary drive and clutch assembly, which is no small job.

This disadvantage has been negated on many of the newer bikes using belts. Victory uses a removable subframe spa to ease belt replacement, as does the Buell Firebolt. Such convenient features makes belt drive increasingly attractive. Even BMW uses a belt on its latest rendition of its F650GS Single.