The Evolution of Motorcycle Design

The close of the 2^nd millennium has stirred a renewed interest in technological advances of the past. Innovations in transportation offer many marvels for consideration, from the wheel to the space shuttle.  Every form of modern transportation provides a unique opportunity for consideration.  For example, the motorcycle offers many intriguing aspects.  What defines a motorcycle?  From what did it evolve?  From what components is it constructed?  How has its design evolved over time?  Are there any patterns in the evolution of its design?  Can we model these patterns and predict future design changes?

                Merriam Webster defines a motorcycle as “a kind of very heavy bicycle that is run by a gasoline engine”.  World Book encyclopedia defines it as “a two- or three-wheeled vehicle powered by a gasoline engine mounted midway between the front and rear wheels.”  Whereas Webster’s definition seems insufficient, World Book’s seems too limiting.  Is a motorcycle simply a motorized bicycle?  This definition would include scooters and steam-powered velocipedes, and is therefore too broad.  What makes a motorcycle a motorcycle?  Clearly, a motorcycle is one type of machine that allows people to travel along the ground without exerting their own energy.  It has a motor separating it from a bicycle; it travels on two wheels with its operator seated somewhere between the two in an exposed, forward-facing, straddling position – all of which distinguish it from automobiles.  If the motorcyclist sat in a chair that trailed the two-wheeled vehicle, would it still be a motorcycle?  If a design allows a motorcyclist to sit with both of his feet planted squarely before him, as in a chair, would it still qualify as motorcycle?  The answers seem to be, “No”.  Then elements of form, not merely function, must be addressed to truly define a motorcycle.  A motorcycle is a two-wheeled vehicle that uses a gasoline-fueled internal combustion engine to propel it and its forward-facing operator along the ground.  The operator must sit straddling the body of the vehicle, somewhere between its two wheels, at least partially exposed to the surrounding atmosphere.

                A motorcycle incorporates many unique components into an intricate and beautifully effective whole.  The components of a motorcycle can be broken down into five main systems:  engine, transmission, wheel, structural, and control.  Using these five main systems design of the past and present can be more effectively compared and the complex whole can be more easily understood.  First, each system must be understood separately.  The engine system transforms chemical energy (within the chosen fuel, gasoline) into mechanical power.  This system includes the fuel storage and delivery, intake, exhaust, ignition, combustion chamber, and cooling subsystems.  The transmission system transmits power from the engine to the wheels.  This includes the clutch, gearbox, and both the primary and final drives.  The wheel system transfers mechanical power from the transmission into a force against the road surface.  The wheel system includes the wheels, the tires and the brakes.  The tires transmit the wheels’ force to the road surface (and vice versa).  The brakes interrupt the transfer of transmission torque to the road, and in this way, help determine the amount of force given to the road – justifying their inclusion into the wheel system.  The structural system holds everything together; it maintains the proper relation between components and supports the rider.  This system includes the frame (or chassis), the suspension and the seat.  The control system allows the operator to activate the motorcycle and to adjust certain vehicle parameters to achieve the desired performance.  The control system includes the starter, clutch lever, gear shift, brake lever, throttle control, and steering.

                Although many successful self-propelled two-wheeled vehicles precede it, Gottlieb Daimler’s Reitwagen is considered the first motorcycle.  Distinguishing factors lie in his choice for fuel and his structural layout.  Predecessors had used steam power, while experimenting with tricycle, high-wheel, and more exotic structural layouts.  He was the first to make use of an internal combustion engine, when in 1885 he mounted his experimental, gasoline-fueled single cylinder engine to what was essentially a wooden bicycle frame.

                The Reitwagen’s structural system was built upon a wooden version of the classic diamond-framed safety bicycle.  Two long wooden arms extended from the rear wheel’s hub, forward and upward above the front wheel.  Four wooden legs extended down from these arms, surrounding the engine, and bolted to a thin rectangular frame positioned below the engine.  It lacked rear suspension and had minimal front suspension that consisted of a simple metal fork.  The metal fork resembles an inverted goalpost, with the prongs of the fork straddling the front wheel, bolted to wooden arms that are bolted to each end of the hub.  The straight, round stem of the fork extended upward through a hole in front, where the two long wooden arms of the frame join.  The wooden seat was mounted to a vertical wooden pole that extended above the engine, and was covered with leather padding.

                The wheel system on this first motorcycle was crude and actually utilized four wheels.  Two small “stabilizers” joined the two larger main wheels, generally present in a bicycle.  The main wheels were simple wooden wagon wheels rimmed with iron and had wooden spokes.  The stabilizers were small wooden discs also rimmed with iron.  They rotated about separate metal axles, each of which was attached by a metal fork to the thin rectangular frame below the engine.  Its tires were the iron bands that wrapped around each wheel.  Braking controlled the rear wheel only.

        The engine system relied on one cylinder to burn gasoline fuel through a four-stroke process.  Fuel was stored in a small metal reservoir behind the seat.  Daimler utilized a surface carburetor, which relied upon the stream of intake air flowing over the surface of the fuel reservoir stimulating natural convection of fuel vapor into the air stream.  He used poppet valves to regulate inlet and exhaust.  As the piston moved downward, it created a vacuum that sucked the fuel and air mixture into the solitary cylinder (called “Suction Inlet”).  This vertical cylinder was mounted between the two main wheels and below the seat.  Once the cylinder had filled with the inlet mixture, the vacuum had been eliminated and the inlet valve closed.  Returning upward, the piston compressed the mixture forcing it into a metal tube connected to the cylinder.  The compressed mixture followed along this metal tube outward from the top of the cylinder until it contacted the sealed and heated end – igniting the mixture.  (This is called hot-tube ignition.)  The contained explosion forced the piston downward driving the crankshaft.  As the piston returned upward, the exhaust poppet valve mechanically opened allowing exhaust gases to escape to the rear of the seat.  Air naturally cooled the engine as flowed across the cylinder.  The engine produced about half a horsepower while running at 700-900 rpm and displacing about 267 cubic centimeters.

The transmission system used all of that power from the engine to drive the rear wheel.  Serving as the primary drive, a belt from the pulley mounted onto the crankshaft drove a countershaft pulley.  This countershaft pulley drove a gear built into the spokes of the rear wheel.  The Reitwagen did not have a gearbox, but it did have a clutch to disengage the engine and interrupt the transmission of power to the rear wheel.

The control system conveniently positioned many of today’s controls.  A handlebar allowed the driver to steer the motorcycle by turning the front wheel.  The handlebar shaft was clamped into a metal linkage that attached to front fork.  The clutch and rear brake were engaged simultaneously by turning a twist-grip on the right hand end of the handlebar.  Without a gearbox, there was no need for a shifting control.  Between the seat and handlebars, a drive belt tension lever allowed the motorcyclist to adjust the tensioning pulley to appropriately adjust the slack in the primary drive belt.  Throttle controls have not been described any of the sources consulted.  Starting the Reitwagen consisted of a simple push.

These five elemental motorcycle systems have gone through many changes over the last 115 years.  With the vast amount of technological development over this period, it is interesting to note that many of the elements present in the Reitwagen have persisted or won renewed appreciation in today’s motorcycles.

The structural system has come a long way.  As engine power increased, the need for stronger frames developed.  In the 1970s, designers began to realize the additional importance of chassis rigidity to vehicle handling.  Advances in material technology have allowed great increases in structural rigidity while reducing weight.  Aluminum and carbon composites have become the standards of today’s frames.  Early single and double cradle frame designs continued into the mid-1980s when they were replaced by the twin-spar design.  Seats remained simple bicycle style supports until well the 1950s, when the longer flat seat pad was introduced and quickly adopted as an industry standard.  Daimler’s tiny fuel reservoir, hidden behind the seat, quickly grew in size and importance.  Manufacturers experimented with the position of the fuel tank.  In 1909, Pierce ingeniously used their 3.5inch diameter frame tubing to store gasoline.  By the 1920s, however, positioning of the fuel tank between the seat and handlebar had become an industry standard.  Over the years, motorcyclists fell in love with the distinctive teardrop shape and demanded it from manufacturers – quickly creating another industry standard.  Early motorcycles had depended on the stream of flowing air to cool off the engine and therefore allowed the entire engine to remain exposed.  As driving speeds increased, motorcyclists began to notice the disadvantages of air flowing directly into their faces.  Although a small windshield had been used on a Brough Superior in 1926, they did not really take hold until they were re-invented by Craig Vetter in the late 1960s.  His windshield attachments, called fairings, quickly took hold of motorcycle design in the 1970s and 1980s.  During this time period, increasing use of liquid-coolant systems eliminated the need to expose the engine to the surround air.  Designers began to cover the engine with body panels called “cowlings”, attempting to increase styling and aerodynamics.  In the late 1980s and early 1990s, consumers’ interest in the more powerful and nostalgic look of “naked” motorcycles prompted designers to reduce their use of both fairings and cowlings.

 Around the turn of the 20^th century manufacturers began mounting springs between the seat and the chassis hinting at rear suspension.  In 1904 Peugeot became the first manufacturer to install a true rear suspension system.  They used a swing-arm, which consists fixes the horizontal position of the wheel relative to the chassis through a metal tube connected from the wheel hub horizontally to a pivot on the lower chassis.  Originally, one shock was mounted from the wheel hub and secured to the chassis just below the seat; this regulated the vertical position of the wheel relative to the chassis, isolated the wheel’s movement from that of the chassis and dampened the forces transmitted from road surface irregularities to the chassis.  Rear suspension technology progressed slowly until the late 1940s, with the only advances being Vincent’s introduction of a triangulated rear swing-arm in 1928, BMW’s introduction of the rear-plunger suspension in 1938, and Imme’s 1949 introduction of a cantilevered swing-arm.  Despite these achievements, most motorcycles built before 1950 offered nothing more than a sprung seat for rear suspension.  During the following ten years, the plunger and standard swing-arm became industry standards for rear suspension.  Over the last 40 years, developments in rear suspension have focused on increasing suspension travel, adjusting damping stiffness to match the amount of displacement, and refining the swing-arm design.  Suspension travel is the maximum distance that a wheel may be displaced, or that a shock may be compressed between its two mounting points.  Rising rate linkages replace the shock of swing-arms with a linkage providing increasing stiffness (damping force) as the wheel is increasingly displaced.  Most motorcycles today use a form of swing-arm rear suspension.

Front suspension started as undamped, one piece forks, such as the one used by Daimler.  In the 1910s and ‘20s, many designers added springs to their front forks, but the industry standard remained druid forks.  By the mid 1930s, spring forks had become the new industry standard.  In 1935, BMW introduced the telescopic front fork; in 1949, Harley-Davidson offered improved telescopic front forks with hydraulic damping.  The telescopic fork consists of two separate, concentric tubes whose relative movement is regulated by a shock.  The inner (narrower) tube mounts to the chassis, and slides within the (wider) outer tube that is mounted the wheel hub.  A derivative of the design called the “Upside-down” telescopic fork.  This design switched the position of the outer and inner tubes, mounting the inner tube to the hub and the outer to the chassis, increasing fork strength and providing more predictable handling when braking.  With a few refinements (including leading links and anti-dive damping), the basic telescopic fork design remains an industry standard.  The design does not isolate the steering column from large compressive braking forces; therefore, it jeopardizes stability and steering when braking.  In the late 1980s and ‘90s, manufacturers began experimenting with hub-centered steering.  This system uses a swing-arm to dampen braking forces and transmit what remains to the chassis rather than to the steering column, while a separate linkage to the wheel hub provides steering control.  Although this concept has produced multiple performance-boosting designs, its unique look has proven unappealing to most motorcyclists prompting manufacturers to retain the telescopic forks.  BMW’s has developed their own hub-centered steering design called the Telelever that both improves performance and captures the traditional look that consumers have demanded.

The wheel system has also seen great improvements, with maybe the greatest change occurring three years after Daimler constructed the Reitwagen.  In 1888 John Dunlop invented the pneumatic tire improving traction and providing some suspension.  Tires were continuously refined as material technology continued to improve, increasing traction and longevity.  In the 1970s, aluminum cast wheels replaced traditional wire wheels and allowed motorcycles to make use of tubeless tires.  Tubeless tires greatly reduced the chances of blowing a tire.  All motorcycles use tubeless pneumatic tires; most use cast wheels made of aluminum or sometimes manganese.  Designers experimented with various combinations of wheel size arrangements.  In the 1980s, for example, it 16 inch front wheels with 17 inch rear wheels became the fashion.  Design has since returned to using wheels of identical size.  Initially motorcycles only offered hand-operated brakes that controlled the driving wheel, but soon a brake for each wheel had become the standard.  One early brake design was the “Dummy-rim”, which relied upon a cantilevered fork to create friction against the dummy-rim, stopping the wheel.  In the 1920s, motorcycles began using internally expanding brakes leading to the drum brake design.  In a drum brake, two semicircular brake “shoes” (lying within the same plane) move outward against the circular drum that contains them.  The drum is mounted to the wheel hub and thereby transmits the friction force from the shoes to the wheel.  In the late 1960s, disc brakes were introduced.  In disc brakes, a caliper straddles a metal disc that is mounted to a wheel hub.  As hydraulic fluid is forced into the caliper unit, it forces a piston to compress high-friction brake pads against the metal disc at the wheel hub.  The disc brake improved the consistency of braking performance over the range of speeds and simplified maintenance.  Today, the large majority of motorcycles use disc brakes for the front wheel; a small majority uses a disc brake in the rear as well, while many use drum brakes.

Naturally the engine has changed greatly from Daimler’s model.  The float-feed spray carburetor replaced the surface-carburetor by the 1920s.  As motorcycle racing increased in importance, designers began installing one carburetor per cylinder – still the most commonly used setup today.  In 1984, BMW offered the first motorcycle with fuel injection.  This system offers improved performance by providing improved regulation of the fuel to air ratio in the inlet mixture, with greatly reduced dependence upon atmospheric conditions.  Fuel injection was not immediately adopted, however, because it increased cost, weight, and space requirements, while motorcycle designers sought to reduce each of these elements.   Additionally, fuel injection seemed to cause power surges when running at high rpm – creating a great risk of hydroplaning.  With continued refinements, more and more of this year’s models have chosen to use fuel injection, however the industry standard is still one carburetor per cylinder.  Daimler’s engine forced the inlet mixture into the “hot-tube” for ignition.  Electrical ignition was developed soon afterwards, increasing safety and comfort.  Electrical ignition was gradually refined from high-tension magneto with coils, to coil-less, to capacitive discharge.  Here electrical current is fed to the spark plug and disrupted at the right time to create a high voltage electrical spark to ignite the compressed mixture.  Today’s models continue to use electrical ignition with one spark plug per cylinder.

The valve train has evolved from Daimler’s one inlet and one exhaust poppet valve per cylinder design with suction inlet to today’s standard of two inlet and two exhaust valves per cylinder.  Rotary valves, driven by chain from the crankshaft, replaced the poppet valves.  Pushrod-operated valves followed.  The camshaft drove a pushrod upward tilting a “rocker” that forces the valve to open.  The desire for increased speed and efficiency prompted the development of the overhead camshaft system.  This system positions the camshaft above the valves allowing the cam lobes to push open the valves directly.  Today most motorcycles use double overhead camshafts to operate inlet and exhaust valves.

Although 2-stroke engines were developed soon after Daimler’s 4-stroke Reitwagen and provided a greater power-to-weight ratio, they have largely dropped out of production.  Increasingly stringent emissions regulations proved too much for the 2-stroke engine, as it needs to mix lubricating oil with the fuel resulting in emissions of burned oil.  They were banned from production and sale in the United Sates in the early 1980s, but a small number are still production as small displacement motorcycles around the world.  Motorcycles have predominantly used 4-strokes engines since they were first developed; today the 4-stroke is almost universal.

Early motorcycles used single cylinder engines and experimented with the position of the cylinder.  Designers experimented by placing the engine behind the seat, trailing the bicycle frame (as an add-on), and even within the wheel, but quickly settled into its current position below the seat.  It was quickly recognized that adding cylinders would provide a smoother performance and that an even number of cylinders could reduce vibrations by balancing each other.   By the 1920s, most motorcycles had two cylinder engines, although one-cylinder engines were still used greatly, and four cylinder engines had already made a strong showing.  Use of an even number of cylinders quickly became the standard.  Layout of the cylinders never settled to one standard setting, as it greatly affected engine performance characteristics and completely altered the packaging and hence styling of a motorcycle.  The “V” configuration became very popular as it seemed to fit so well into the diamond frame and provided some balance; even this configuration, however, had its varieties: the 45 degree, 90 degree, and the “L” derivative.  Vertical, horizontal, transverse, and opposing layouts were also tried.  BMW introduced its classic “Boxer” engine in 1923.  This layout positioned the cylinders directly opposite each other so that the pistons fired simultaneously outward, away from the centerline of the motorcycle, “like the fists of a boxer”.  Today most motorcycles have either 2 or 4 cylinders, but their layout varies greatly.

Exhaust systems have had many aesthetic refinements to maximize the look and styling.  Most motorcycles today have continued to funnel the exhaust from multiple cylinders through one common exhaust pipe, although many do use two.  Exhaust pipes almost universally channel gases to the rear of the vehicle.  Mufflers have been used universally for over 40 years to reduce the noise of escaping gases as they expand into the atmosphere.

            The transmission system has undergone some interesting developments.  The clutch has been refined but remained essentially the same.  Although Daimler’s motorcycle lacked a gearbox, they quickly became a standard component.  The number of gears has continued to increase reaching an average of five for today’s motorcycles.  Motorcycles have almost universally retained manual transmission, although Honda and Moto Guzzi have developed semi-automatic models.  The primary drive, used to transmit power from the crankshaft to the gearbox, used both belts and chains.  In the 1950s, designers completely eliminated the primary drive by mounting the gearbox onto the crankshaft, creating a “unitized” transmission.  Today almost all motorcycles use unitized transmissions (except for some Harley-Davidson and Moto Guzzi models).  Within the first 25 years in the life of the motorcycle, all of the method used for final drive today had already been developed: belt, chain, gearing, and shaft drive.  The final drive is the component that transmits power from the gearbox to the rear axle.  Belts were used first, providing a simple and clean solution for transmitting power.  They are not very durable, however, and they are difficult, time-consuming and costly to replace.  Chains have proven to be more durable, but require more frequent, messier maintenance.  Shaft drive operates cleanly and required less maintenance; however, it adds to the unsprung weight of the motorcycle, reducing handling performance.  More “shockingly”, the shaft drive’s rigid structure transmitted impact energy from contact with the road surface, back to the chassis and to the driver.  It causes strange motion, called “shaft-jacking”, in which the rear end moves up when it should move down, and vice versa.  Bevel gearing offered all of the disadvantages of the shaft drive with increased weight and maintenance concerns.  Additionally, the chain and belt provide look and styling preferred by most motorcyclists today.  Today’s motorcycles continue to use the belt, chain and shaft drive methods, but chain drive remains the most prevalent.

With the exception of the starter, motorcycle controls have remained essentially unchanged since the 1920s.  Daimler’s motorcycle and many others of the period relied upon a physical push to turn the wheels and drive the crankshaft, starting the engine.  By the 1900, pedals had become a common method of starting the engine.  Looking for easier and faster ways to get going, Alfred Scott introduced the kickstart on his 333cc motorcycle in 1909, which became an industry standard in only two years.  The next major simplification was the electric starter, but few could produce a reliable system until Honda did so with their 1965 models.  Today’s electric starters are very similar to that which Honda produced 35 years ago.  The throttle, clutch, gear shifter, and brake levers settled into standardized locations long ago.  The throttle is controlled by a twist-grip control on the right end of the handlebar.  The front brake lever is the lever that extends from the right end of the handlebar.  The clutch lever extends from the left end of the handlebar.  The rear brake lever is positioned near the right footpeg, and the gear shift lever is positioned near the left footpeg.  All motorcycles since for the past 90 years have had headlights, and all motorcycles built in the last 25 years have had turn signals and taillights.  Motorcycles have carried batteries since they first used headlights, but with today’s increased use of electronics to monitor engine performance and to provide convenient controls, the battery has become more important.  Originally the crankshaft drove a generator that provided direct current to recharge the battery as the motorcycle ran.  Today, and for more than 50 years, a motorcycle crankshaft drives an alternator that provides alternating current to recharge the battery.

So what is today’s standard motorcycle?  As mentioned above, controls are essentially identical for all motorcycles manufactured today.  Modern motorcycles have also converged to an essentially standard wheel system consisting of tubeless pneumatic tires mounted onto cast aluminum wheels, with a separate disc brake controlling the front wheel and rear wheels.  This is where the similarities essentially end.  Modern motorcycles have greatly differentiated their engine, transmission and suspension systems to correspond to the styling and performance characteristic that most closely matches their motorcycle type.  There are six main types of motorcycles today:  the standard street bike, sport bikes, dirt bikes, dual sports, custom cruisers and touring motorcycles.  These motorcycle categories offer significant differences between motorcycles because their purposes prompt their designs to satisfy different performance requirements.

The standard (street) motorcycle most closely resembles the Reitwagen’s goal to provide basic transportation.  The standard motorcycle today is has a V-twin single overhead camshaft operating 2 valves and one carburetor per cylinder, while relying upon air-cooling.  It has a unitized transmission with a chain final drive, hydraulic upside-down telescopic front forks, and a rear swing-arm.

The Racing competitions that quickly sprang up at the turn of the century and continued to draw great interest from many motorcyclists have helped to create the sport bike category of motorcycles that aim for maximum speed.  These motorcycles tend to have inline 4 cylinder engines, with a dual overhead camshaft operating 4 valves per fuel-injected cylinder.  Sport bikes use unitized transmissions with chain final drives and typically 6 different gear ratios.  Their engines are water-cooled and fully enclosed by aerodynamic cowlings.  These bikes have forward-leaning seats with fairings.  They tend use single-shock cantilevered swing-arm for the rear suspension and telescopic front forks.  Seeking increased power, sport bikes often make use of large megaphone exhaust pipes. 

Dirt bikes are designed strictly for off-road use; therefore, they must be rugged and they must adapt well to irregular surfaces.  They are the mavericks of motorcycles displaying many striking differences especially in the structural system.  Dirt bikes require greatly increased ground clearance and attempt to maximize suspension travel for the rear swing-arm.  Front suspension is often handled by extra long upside-down telescopic forks.  The fuel tank is reduced greatly in size and weight; some dirt bikes even utilize the frame for fuel storage.  Most designs call for an erect seating position on a thin and flat seat, positioned closer to the handlebar; some opt for a standing riding position and completely eliminate the seat.  Extra-sensitive disc brakes control the rear and front wheels, and are generally protected by plastic shields.  A large fender high above the front wheel shields the rider from splashing mud.  Both wheels are spoked.  Tires are very thick and knobby to provide better traction.  Traditionally dirt bikes have been 2-stroke one-cylinder engines with 2 valves per cylinder operated by a single overhead camshaft, since these bikes are not meant for use on public roads.  Today some models have begun to use 4-stroke engines.  The cylinder is often laid horizontally to maximize ground clearance.  Even the exhaust pipe is generally reduced in size to minimize weight.

Dual sport bikes are a combination of the dirt bike and standard motorcycle.  These bikes greatly resemble dirt bikes in appearance, structure and styling but offer the cleaner emissions and standard road equipment (such as turn signals, headlights, and taillights) of street legal vehicles.  They use long telescopic front forks and single-shock rear suspension to increase suspension travel and to provide greater ground clearance.  A more normal size fuel tank returns to its standard position between the seat and handlebar, while a thin seat accommodates an erect but seated driver.  The wheel system greatly resembles the dirt bike with knobby tires mounted onto metal-spoked wheels.  A disc brake controls the front wheel, but unlike the dirt bike, drum brakes generally control the rear wheel.  The engine system consists of an air-cooled, carbureted, 4-stroke cylinder with 2 valves operated by a single overhead camshaft.  The generally 6-speed unitized transmission uses a chain for the final drive.

Custom Cruisers, or “choppers”, provide personalized styling and high speed for short distance pleasure rides.  This is the type of motorcycle most often associated with Harley-Davidson, such as the “Easy Rider’ driven by “Captain America” in the 1969 film Easy Rider.  Cruisers emphasize aesthetics, speed and power, and often choose older technology to achieve a retro-look.  These bikes proudly display the classic teardrop fuel tank and an elaborate (often high and curved) handlebar with a long seat.  They generally rely upon an extended front fork without any rear suspension for the low, long frame.  They generally use a disc brake up front and a drum brake in the rear.  The engine system is a throwback air-cooled V-twin (2 cylinders in the V configuration).  Although a single overhead camshaft is often used to operate the 2 valves in each cylinder, some Moto Guzzi and most Harley-Davidson models use the pushrod and rocker system.  The transmission system also exhibits older technology as many of these bikes have a primary drive, and use belts to transmit power; most however still use a unitized transmission with chain final drive.  They generally offer five gear ratios.

The largest motorcycles constitute the touring class.  These motorcycles were designed for long-distance pleasure riding, and therefore offer more comfort and cargo considerations.  Touring motorcycles generally have long wheelbase (like the cruisers) with a huge, comfortable and elaborate seat and very large fuel tanks.  Panniers and luggage are frequently attached to the rear of the seat.  Although the engine is generally left exposed, full-size fairings are standard.  Touring motorcycles use standard tires and wheels, controlled by a disc brakes in both the front and rear.  The standard engine system here consists of a liquid-cooled V-4 engine with two valves per cylinder controlled by a double overhead camshaft.  These engines are generally carbureted with one carburetor per cylinder.  The unitized, 5-speed transmission often uses a shaft final drive.

The evolution of motorcycle design has not evolved along a well-defined one-way path.  For the most part, motorcycle designers and engineers have not followed a direct sequence of refinement to improve motorcycles by developing components far superior to their predecessors.  A few examples will show this distinction.  The starting mechanism for the motorcycle did follow a one-way path of refinement.  Originally a push of the motorcycle started its engine.  This soon gave way to the safer and easier “kickstart”, which was soon followed by the electric starter.  Each new technique (or component) made the task easier, safer, faster, and more convenient.  Once the kickstart was introduced, it completely replaced the push start; just as the electric starter completely replaced the kickstart once it was proven reliable.  This complete replacement affirms the superiority of the newer design.  A large number of motorcycle components fail to follow this logical progression.  For example, Gottlieb Daimler used a belt for the final drive in his Reitwagen.  Designers quickly saw the weaknesses of belt drive and introduced the chain drive.  Shortly after that, unsatisfied designers experimented with a geared final drive, which in turn was followed by shaft drive.  When the chain was introduced, the manufacturers did not completely stop using belts.  After the shaft drive was introduced, manufacturers continued to use the chain and belt for power transmission.  Each of these methods for transmitting power to the rear wheel had been introduced by 1903, and more than 90 years later, all three methods are still in use.  There was no one-way progression, but a divergence instead.  A similar argument can be made for carburetion (versus fuel injection), the cooling system (air versus liquid), the “optimal” number of cylinders in an engine, the “optimal” cylinder layout, and the “optimal” number of valves per cylinder.  Another interesting aspect is the way in which many designs proven to boost performance were ignored in favor of the older inferior ones.  For example, hub-centered steering improves safety and handling by isolating steering from braking forces, but has been largely ignored in favor of its predecessor the telescopic fork.  In fact, many design changes have been guided by the desire to perfect styling or the need to satisfy new emissions, displacement, or other legal restrictions.  Motorcycle design has also displayed a circular life cycle for many components or designs, consisting of introduction, popularity, and decline.  For example, Daimler introduced belt drive in 1885.  The belt dominated motorcycle designs in the very early 1900s, then almost disappeared in the 1960s and 1970s.  Today it has returned especially within the design of Harley-Davidson cruisers.  Another example, is the “naked” look.  Initially, motorcycle design relied on the natural stream of air to cool the engine; therefore, requiring the cylinders to be exposed to the air (and into view).  By the 1970s, development of liquid cooling systems and a desire for improved aerodynamics prompted designers to cover the engine with cowlings.  This look quickly took hold of the motorcycle industry, until consumers began to desire the “naked” look once again in the late 1980s.

Motorcycle design today is very similar to that of its 115 year-old ancestor, the Reitwagen.  The vehicle is still composed of the same fundamental five systems, without adding many additional functions.  Design has definitely been improved, although it has not followed a straightforward path of continued refinement.  Instead motorcycle design has diverged to satisfy multiple unique combinations of functional and aesthetic specifications.  Today, motorcycle design has evolved into a collection of almost personalized machines.

Bibliography

The Ultimate Motorcycle Book,  Hugo Wilson.  Darling Kindersley, New York, 1993.

The Encyclopedia of Motorcycles:  The Complete Book of Motorcycles and Their Riders, Roland Brown.  Anness

Publishing, New York, 1996.

Historic Motorcycles,  David Burgess Wise.  Hamlyn Publishing, New York, 1973.

The Complete Idiot’s Guide to Motorcycles.  Peterson Publishing, New York, 1998.

Bicycling Magazine's Complete Guide to Bicycle Maintenance and Repair : Including Road Bikes and Mountain

Bikes.  Rodale, New York, 1994.

The World Book Encyclopedia, “Motorcycle,” World Book, New York, 1988.

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