Motorcycle Final Drive Systems (Chain, Belt and Shaft)
By David Tong
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. I will discuss each of the three major 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 final drive types, the metal linked chain.
Chain drive has some admirable qualities. First off, no other 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 super bike.
I once owned a Hinckley Triumph Thunderbird and one of the things I noticed after riding it home was that the gearing seemed short (too low). That means that the engine was spinning too many revs at highway speeds. A simple change of the countershaft sprocket, the cheaper of the two versus the large rear sprocket, to a sprocket with one additional tooth dropped the engine speed roughly 300 RPM with a commensurate improvement in both noise and fuel economy at speed.
When decreasing the numeric final drive ratio in this way, you do lose some acceleration, but since most bikes have more than adequate power for reasonable traffic conditions, the trade-off seemed worthwhile and my fuel mileage went from 43mpg to 47mpg at 65mph. Nice in 1997 and really nice today with $4 gas, thanks to our messed-up national energy policy.
The larger countershaft sprocket also produced a larger chain wrap, because of the increase in diameter of that sprocket, which reduces wear on the chain. This really is the best way to provide taller gearing, because one has to decrease the rear wheel’s sprocket to create the same taller drive ratio and doing it that way reduces the chain wrap and decreases chain life.
Chains offer the most power efficient 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 drive train 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, leaning 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 degreasing 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. Final drive chains are typically found on less expensive motorcycles, racing bikes, dirt bikes, enduro bikes and sport bikes.
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 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 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 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 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.
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 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.
Final Thoughts and Opinions
One thing that the reader should understand is that a particular model of motorcycle will usually not offer final drive options. In the old days (think, late 1970's and early 1980's), Honda, Kawasaki, and Suzuki all made “sport,” and “cruiser” models of their bikes. The sporting model always had chain drive and these were cheaper. The cruisers nearly always had shaft drive, as they were marketed more toward riders interested in less everyday hassle and willing to pay a few hundred bucks more for that privilege. Today, chain drive still holds sway for those who like its gearing flexibility, its traditional appearance, or want to go racing.
Belt drive is, first and foremost, the province of Harley-Davidson, whose usually affluent owners like its nearly maintenance free regimen and smooth power transmission. Not having chain fling sure makes keeping all that chrome shiny a lot easier, too.
Shaft drive is as close to maintenance-free as any motorcycle drive system can be. With the exception of the very popular, belt drive Harley-Davidson touring models, you will seldom find dedicated sport-touring or luxury touring bikes without that alloy case in the rear wheel hub, despite the additional cost, weight and complexity.
Today's most popular police motorcycles are BMW’s R1200RTP, Honda’s ST1100 / ST1300 and Kawasaki’s Concours 1400. All three of these machines use the shaft for reliability over high mileages and severe use cycles. A stronger argument for shaft drive in police service applications could hardly be made.
I have owned motorcycles using each of these final drive systems and each has its good and not-so-good points. Generally, these are inherent in the style of bike one selects. The rider who buys a very high horsepower sport bike should expect it to be more maintenance intensive, so chain drive becomes less of an issue. Harley riders extol the virtues of their low maintenance, quiet, efficient and smooth belt drives. The mile-crusher or daily commuter who simply wants to ride is likely to appreciate the shaft.
Me? I’ve enjoyed all three systems and each has its place. Getting older means I’d rather ride than wrench, meaning that I may prefer a shaft most of the time, but my grin was no less wide riding the others!
Copyright 2013 by David Tong and/or chuckhawks.com. All rights reserved.