How do engineers decide how to send power to the rear wheel of a particular motorcycle?
Is it a cost issue? Is there any reason at all? Why do so many people hold diehard opinions about their particular favorite method of final drive? Which one is best? Man, that’s a lot of questions.
Final drive is simply the last step in the process of transferring engine power to a bike’s rear wheel. Early motorcycles that didn't have a clutch or gearbox drove the rear wheel directly by way of a tensioned leather belt running from the engine's output shaft. Early riders would “slip” the belt in order to get the machine up to speed before applying maximum tension, like a crude sort of clutch. Some also chose to slip the belt at stops to avoid re-starting the motorcycle each time they came to a halt. Clever, right?
Final drive today is different, because the rear wheel does not receive its power directly from the engine, but from the gearbox. Primary and final drive are separate stages of power transmission. Let’s examine the three kinds of final drive in use today.
Perhaps the most familiar final drive type is the chain. Many, many motorcycles have had chains linking their transmissions to their rear sprockets, and with good reason: chains are economical (read: cheap), durable, strong, and offer a lot of design flexibility. Chains are used on many sport bikes because they have a very high strength-to-weight ratio. They are also beloved by racers due to the ease in manipulating a motorcycle’s gear ratio: simply swap a sprocket to gear shorter for a tight track or taller for one with a long straightaway. Chains offer the least amount of parasitic loss of the three popular drivetrain styles. They also are easy to mend and work on, relative to belts or shafts.
It’s not all rosy, though. Chains can and do break, and probably need the most maintenance, in the form of cleaning, lubrication and adjustment. Chains tend to make a mess of rider and bike by flinging their lubricant about. They also require frequent readjustment of the rear wheel to maintain proper tension, because the pins and plates that compose a chain wear, giving the appearance of the chain “stretching.” The sprockets the chains run on are just as susceptible to wear, and usually need to be replaced along with the chain several times during the life of a bike.
Next up in terms of prevalence is the belt drive. Belt-driven bikes offer some advantages to riders who want low maintenance and low noise, so you’ll often see them on bikes geared toward long-haul mileage and commuting machines. Belts need next to no adjustment over the course of their lives, and they run super-quiet. In general, belts need no lubrication, so they also tend to be neat and tidy for both bike and rider. They’re not cheap at all, but if you amortize the cost of one over the service life it gives, belts are actually a really good value. In many cases, they are good for decades of riding. With no real maintenance required, you kinda just keep riding until it goes kaput.
Of course, that’s kind of one of the downers. When a belt snaps on the road, as infrequently as it happens, it’s usually not fixable on the road, and it's not even easy to fix in a shop. The belt, unlike a chain, cannot be broken, so some fairly major disassembly of the bike is usually required for installation. It involves removing a wheel at a bare minimum, and usually other parts, such as the swingarm. Belts also suck up a little more horsepower than a chain.
The third option you’ll find on bikes is the driveshaft. Often used on engines where the crankshaft runs parallel with the direction of travel, “shafties,” as they are known to their supporters, transmit power to the rear wheel via a spinning steel rod. Because they are typically fully enclosed in oil, they are incredibly long-lasting. Servicing the units, which is done fairly infrequently, usually means a change of the fluid in the differential housing. This is arguably the toughest method of final drive. Breaking a driveshaft or related componentry is very rare. Shafties are expensive, heavy and complicated, compared to chains and belts, but their bulletproof nature and infrequent maintenance requirements mean many riders will pay a premium to have them.
Of course, like all the other methods of turning gasoline into great big smoky burnouts, driveshafts are not without fault. Changing the gear ratio is very difficult to do, leaving the tire size as the only viable way to manipulate the bike’s gearing. The shaft is heavy, and does rob an appreciable amount of horsepower. Even though shafties are tough cookies, if you do manage to break something, it’s usually an expensive and difficult repair. Finally, a shaft can cause what's often call a "jacking effect," in which acceleration and deceleration forces cause the bike to rise or fall on the rear suspension, potentially affecting handling. Most shaft-drive bikes today have mechanisms to eliminate this effect.
Final word on final drive
So which one of our three forms of final drive is best? Well, like all the other things in life, it depends on what you’re doing! If you’ve got a rocket of a bike or you do a lot of your own wrenching, it’s hard to beat the strength, light weight and simplicity of a chain. There's a reason virtually all sport bikes use them.
For a super-nice bike with a beautiful finish you might want to keep extra-clean but still gets miles piled on, a belt might be the way to go. There's a reason Harley-Davidson went to belts. They want you to hear the "potato-potato" of the engine, not the whine of a chain.
If you want a low-maintenance commuter bike or you plan to cross several time zones and don't want to stop to adjust or lubricate a chain, a shaft-drive bike can shine. There's a reason bikes like the Honda Gold Wing, BMW K 1600 GT and Kawasaki Concours have shafts spinning their rear wheels.
Like so many other things we hash out in "Why things are the way they are," “best” just relates to what a given rider might prioritize. Each of these methods of power transfer is absolutely perfect for at least a few applications.