Motorcycle Final Drive Systems
The modern motorcycle is a remarkably reliable device. Very few bikes sold these days have notable flaws and this includes how they transmit power to their rear wheels. We will discuss each of the three major motorcycle final drive systems to allow the reader to decide which is best.
The earliest motorcycles used leather belts on large diameter pulleys for power drive. These were soon replaced, within the first decade of the 20th Century, with the oldest of the motorcycle final drive types, the metal linked chain.
Well, this is one of the most common topics of discussion among bikers, but the majority of them always prefer a chain drive as it’s the most common system as well as the best compromise among the all three. In this article, we are going to explain in brief the pros and cons of all the three drive systems and why chain drive is preferred over the belt and shaft drive systems.
Chain drive has some admirable qualities. First off, no other motorcycle final drive type allows for as much flexibility in gearing changes. While this may matter most to the professional racer, who may need to adjust gear ratios per the speed potential of a particular racetrack, taller gearing can be beneficial to the long distance road rider and lower gearing can be beneficial to the city commuter.
Secondly, the chain is the most adaptable and ubiquitous of any of the three systems. It is found on tiny 50cc mopeds all the way up to a Suzuki Hayabusa or Ducati superbike.
Chains offer the most power efficient motorcycle final drive possible, in that they consume less power in parasitic losses than the other two major drive systems. The chain is also narrower than a Kevlar belt, which can also mean that wide rear tires, such as those used on road racing bikes or big cruisers, won’t require any kind of drivetrain offset to allow for the fitting of large rubber.
Chains do not create any kind of torque rise by the rear wheel extending, as do older shaft drive systems when power is applied. This means a more consistent chassis attitude and a generally more comfortable ride quality and better handling.
A modern, rubber O-ringed chain uses those seals to essentially contain the heavy grease lubrication the factory installed by hot dipping the chain before packaging it for sale. Lubrication of the rollers and plates is key to increasing chain life. A well-maintained chain of this type can last up to 25,000 miles before it and its sprockets must all be replaced as a set. If a penurious owner does not replace the sprockets, premature chain wear will occur, because the chain will be encircling worn gear teeth and chain stretch is the result. You can expect little change from $250 if you replace all three parts with a high-quality O-ring type chain.
Therein is the rub of the chain, no pun intended. The owner of a chain drive bike possesses the most time consumptive system in terms of maintenance. Chain drive requires a very well aligned rear wheel to ensure that the chain is running in a straight line. Chains must be lubricated on their INSIDE run because the lubrication also cushions the steel sprockets from premature metal-on-metal wear.
Use of only properly hardened steel sprockets is usually recommended for street bike applications, as materials such as nylon or aluminum, so cool for racing and light weight, do not tolerate any abuse and often fail at inopportune times, leaving one stranded. Also, fixing even a flat tubeless tire requires complete removal of the rear wheel and dealing with a grease and dirt laden chain is not my idea of fun.
Chains are usually supplied as “endless” units on new bikes from their makers, while replacement chains can have their pins staked by the shop that installs it, or by using a master link that is removable for off the bike cleaning and servicing of the chain. These spring clips can fail and cause the entire chain to become one nasty rotating projectile and usually locking the rear wheel, but simply de-greasing that master link and dabbing on some high-temperature silicon sealant pretty much precludes this from happening. (It has happened twice to me, both times locking the rear wheel at freeway speeds and causing a long, harrowing skid to a stop. -Editor)
Finally, chains run dirty. “Chain fling” throws grease and dirt over all nearby surfaces, including the rider’s leg, rear wheel, swing arm and the interior of the countershaft sprocket cover, among other places. Motorcycle final drive chains are typically found on less expensive motorcycles, racing bikes, dirt bikes, enduro bikes and sports bikes.
Cost of Maintenance – Medium
Cost of Replacement – Low
Maintenance Efforts – Very High
In some ways, the modern Kevlar-reinforced belt is a throwback to the original leather belts used on the seminal Benz cycle of the late 1880’s. Most famously used by Harley-Davidson since the late 1980s, belt drive has much to recommend it. Other manufacturers who have used modern belt motorcycle final drive include Suzuki, Buell, Yamaha and BMW. Yamaha’s large Road Star tourer and cruiser mimics the H-D Electra-Glide and Road King in its engine architecture and motorcycle final drive system. BMW uses belt drives for several of their 650cc and 800cc single and twin cylinder street bikes and has for the past 17 years or so. At the other end of the money spectrum, many scooter manufacturers also use them with engine sizes as large as 700cc.
Belt drive is quieter than chain drives or shaft drives; indeed, they are the quietest of any of the drive systems. It is also highly efficient, very close to being as efficient as chain drive and considerably more efficient than shaft drive. Lacking any need for lubrication, periodic maintenance consists of occasional inspection for removal of excess dirt.
Belts run much cleaner than chains. Think of the very similar belts used under the hood of any automobile and how most people can essentially ignore them until a replacement is called for. A belt is generally quite a bit lighter than a chain itself, though the sprockets needed to drive it are generally heavier and larger than their chain drive counterparts.
Like chain drive, belts do not create any kind of torque rise by the rear wheel extending, as do shaft drive systems when power is applied. This means a more consistent chassis attitude and a generally more comfortable ride quality and better handling. The most popular touring bikes in America, Harley-Davidsons, use belt drive exclusively.
A properly inspected and tension maintained belt will typically outlast a chain by at least a factor of two or three, 40,000+ miles as a minimum. (H-D belts may go twice that far.) This means that most owners will never have to replace a drive belt. It is recommended to replace the belt pulleys every other belt change because they do slowly wear. (Of course, by the time a motorcycle has 80,000 to 160,000 miles on the clock, more than the drive pulleys will probably need replacement! -Editor)
Belt drive pulleys are generally more limited in selection for gearing changes than chain drive sprockets, although belt drive is more flexible in this regard than shaft drive. Toothed pulleys are more expensive to manufacture than the flat sprockets used by chains and consequently cost more to the consumer.
Finally, a belt drive is possibly not as durable as a chain when subjected to very high horsepower engines or if drag raced. Synthetic rubber reinforced with Kevlar is very strong stuff, but those little drive teeth can and will shear under abuse, though you’d have to be pretty Neanderthal in your street riding to have an issue with this.
Of the three systems, the modern belt motorcycle final drive is smoother, quieter and less expensive over time than a chain. Belt drive is second only to shaft drive in terms of durability and much lighter than a drive shaft.
Cost of Maintenance – Low
Cost of Replacement – High
Maintenance Efforts – Nil
The use of an automotive style drive shaft, internally spinning in an oil bath within one leg of the rear swing arm, has been around for a long time. In 1923, BMW introduced the first of its long line of shaft drive, opposed twin bikes. While the shafts on those early bikes were exposed, by the 1940s they ran in oil.
All BMW and most Japanese touring and sport touring motorcycles, as well as many of the Japanese cruisers, use shaft drive. Except for their dedicated sport bikes, all four of the Japanese manufacturers equip most of their liter-plus motorcycles with shaft drive, which accounts for the majority of shaft drive bikes on the street.
Given some care in sealing up the swing arm and the motorcycle final drive unit, shaft drives are the cleanest of all the drive systems. There simply is no chain fling or external wear, as with the other two methods. All one will see with a shaft is the dirt flung by the rear wheel on the interior of the fender and the top front of the swing arm.
Another neat feature of the shaft is that, due to the rear drive being fixed to the swing arm, there is no need for rear wheel alignment to be performed by the owner during tires changes or repairs. Indeed, on all current BMWs using shafts, the swing arm is single-sided and removal of the rear wheel is as simple as turning out four lug nuts with the included tool kit angle wrench, same as a car. No muss, no fuss, no access problem, no need to align an axle through a swing arm, or dealing with multiple washers or fasteners.
The routine maintenance of a shaft drive includes only the purchase of one copper crush washer for the drain plug at the bottom of the drive case, removing the fill and drain plugs, replacing (generally) less than two pints of your favorite gear oil and replacing the two threaded plugs. Then you are back on the road.
There are a few downsides to shaft drive. First off, they are much heavier than other drive systems, degrading acceleration, handling and ride quality while increasing braking distance. While the drive case itself is cast aluminum, the ring and pinion helical gears, the drive shaft and its U-joints and the larger swing arm to contain and mount all that stuff are substantially heavier than other drive systems, resulting in higher unsprung weight.
Second, older shaft drive systems lacked modern torque canceling positioning arms and old BMWs, Moto-Guzzis, Gold Wings and Yamahas had a characteristic known as “shaft-jacking” effect. When one applies power to the wheel, the fixed pinion gear rotates in line with the end of the swing arm, but the ring gear that is 90-degrees from the pinion geometrically reacts to power application by causing a perceptible rise to the rider’s seat.
Conversely, when power is chopped suddenly, the bike’s rear attitude drops just as quickly. If one is riding at a fast clip, exploring the limits of ground clearance and the rider is ham-fisted about cutting power, one could have hard parts levering the wheel to the outside of the turn, causing a crash. This is an “in extremis” case, but it is something old hands riding those older shaft bikes understand. Nowadays, things like the torque canceling Paralever used by BMW and similar copies of the system used by Moto-Guzzi and Kawasaki on their Concours 1400cc sport-tourer, largely eliminate the chassis attitude issues of the older bikes.
Shaft drives are also more expensive to buy on a new bike, as well as more complex than a chain or belt. They are typically slightly noisier than a belt, but quieter than a chain. Unlike a chain or belt drive, easy gearing changes are impractical. When the shaft unit finally needs a rebuild, usually at mileages near or exceeding 100,000 miles, they are the most expensive, due to gear replacement and possibly u-joint and main bearing replacement. However, most new motorcycle buyers will never put enough miles on their shaft drive motorcycles to require rebuilding the drive system.
Cost of Maintenance – Very Low
Cost of Replacement – Very High
Maintenance Efforts – Nil