Bikes

Technologies That Made the Motorcycle What It Is

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for Cycle magazine and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

The five building blocks were in place by 1890—fatigue-resistant seamless drawn steel tube, the ball bearing, the roller chain and sprockets, the tension-spoked wire wheel, and John Dunlop’s pneumatic tire. In 1895 the “safety” bicycle, easily ridden by nonathletes, became a popular craze. With the availability of small piston internal combustion engines and suitable fuels, the motorcycle was inevitable.

Engines

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Because of the early worldwide distribution of small four-stroke engines made by Marquis de Dion and Georges Bouton in France (20,000 motor-tricycles sold by 1900), Bouton’s design was often the seed from which other makes grew. Their engines featured a cam-operated side exhaust valve with a concentric intake valve directly above it, stem upward, opened against a weak spring by intake vacuum (so-called “automatic inlet”).

A 1902 Verschaeve & Truffaut with a de Dion and Bouton engine.

A 1902 Verschaeve & Truffaut with a de Dion and Bouton engine. (Yesterdays Antique Motorcycles, CC BY-SA 4.0 , via Wikimedia Commons/)

Ignition

The earliest engines were barely more than proof-of-concept toys that could be made to run. Flame ignition, borrowed from 1860s stationary engines running on city gas, was quickly displaced by more convenient coil-and-dry-battery ignition (the myriad pockets of today’s Barbour jackets are today a style, but were originally put there to carry spare batteries, wire, small parts, and tools). From 1900 to 1903, ideas came together to make possible the much more reliable self-generating magneto ignition. The logo of the German Bosch company, which today provides electronics and IMUs to modern motorcycle makers, is a stylized cross-section of such a magneto.

A cross-section of an early 20th century magento. Bosch’s logo is derived from this shape.

A cross-section of an early 20th century magento. Bosch’s logo is derived from this shape. (Bosch/)

Frames

They were built as many bicycles are to this day—by the “tube-and-lug” method. At each point where two or more tubes must be joined, a cast, forged, or formed sheet-metal lug is provided, having sockets into which steel tubes can be slipped. A bicycle’s steering head and bottom bracket are its major lugs. Before insertion of a tube into a lug it was coated with “spelter,” a mixture of brazing metal powder and flux; this mixture melted at a lower temperature than the tubes and lugs. With all parts held in alignment by a jig, the loose assembly was placed in a furnace where temperature increased until the spelter liquefied to fill the joints.

A common early frame design was the “keystone,” in which the engine itself took the place of the bicycle’s bottom bracket, being bolted to the front downtube and seat tube with no structure under the engine. To brace the steering head and give the frame strength to stand with the engine removed, there were two top tubes, one above the other, with the flat fuel tank mounted between them. Such single-plane frames were adequate so long as engines made too little torque to distort them by chain pull, producing unintended steering and wobbling.

Gas Tanks

These were made by folding thin sheet metal into a flat rectangular shape like that of a cookie tin, and soldering broad overlaps to achieve fuel-tightness. Where the filler cap and fuel connections were required, formed or threaded inserts were soldered in. As you can expect, such construction was vulnerable to vibration. It was at first common to devote part of the fuel tank to oil, which was drip-fed to the engine (pumped recirculating oil systems did not become universal until the 1930s).

A 1932 Vincent HRD Python Sport.

A 1932 Vincent HRD Python Sport. (Jeff Allen/)

Style

Because early engines had their valves beside rather than above the cylinder, they were not tall. The usual result before World War I (1914–1918) was a low, rather long and slender machine whose look would change little until the coming of taller OHV engines, faster-steering short wheelbases, and “saddle” fuel tanks (those which fitted over the top frame tube). Saddle tanks of 1925 were press-formed in bulbous organic shapes rather than resembling long candy boxes or fireworks rockets. Important pioneers of the taller, shorter-wheelbase, sportier saddle-tank look were George Brough and rider-engineer Howard R. Davies (the “HRD” in the revered name Vincent HRD). Plated fuel tanks became possible only when welded as opposed to soldered tank construction was adopted.

Wheels

The motorcycle naturally borrowed that elegant balance between tension and compression, the wire-spoked wheel.

Brakes

In the early days stopping was less of a problem than going. Front brakes were rare and rear brakes were feeble; a brake block pressed into the vee of a dummy belt rim attached to the wheel, a contracting band around an inner drum, or even the caliper brake of the pedal bicycle was used.

Transmissions

Steam or electric power can exert torque from zero rpm, but the internal combustion piston engine (IC engine, or ICE) cannot make torque until it has been turned enough times to first fill its cylinder with fuel-air mixture and then compress and ignite it to begin running.

The earliest bikes had single-speed belt transmission from engine to rear wheel, but the inconvenience of having to restart the engine after every stop required what cars then already had: the “free engine clutch” which allowed the engine to continue running as the vehicle was braked to a stop.

A big problem was hills! The engine-to-rear-wheel ratio that gave a solid 25 mph on the level was too tall for climbing hills, so motorbikes kept their bicycle pedals, allowing an athletic rider to give “light pedal assistance.”

A 1915 Harley-Davidson three-speed transmission.

A 1915 Harley-Davidson three-speed transmission. (Harley-Davidson/)

The appeal of bikes in early days was that many more could afford them than could afford autos, and simplicity kept them cheap. For the 1911 Isle of Man TT races it was decided to force the issue in the interest of progress: pedaling gear would be banned, forcing builders to devise some form of variable gear ratio between engine and rear wheel. Easiest was variable-diameter belt pulleys, much like modern snowmobile drives. But the big winner at that 1911 TT was Indian, with a proper free engine clutch and a workmanlike two-speed gear transmission, finishing 1-2-3. Remember “American ingenuity”?

After that clear demonstration, the next question was: How many speeds? Three-speeds would serve through most of the 1920s, persisting even longer in the US. Four-speeds arrived in the 1930s with the realization that as top speeds rose and the nature of engines shifted from ox to greyhound, more gearbox speeds would be needed.

Suspension

You can see its evolution in prints of the earliest motor-bicycles. The first step was to reinforce the standard and quite flexible bicycle fork with struts to stop its constant breakage at higher motor-bicycle speeds. Even better might be to allow some yield in the form of spring-mounting the front wheel. An explosion of ideas resulted: leading and trailing pivoted links, Triumph’s early tilt-a-fork, sliding pillars. These quickly condensed into the girder fork, which would be definitively replaced by telescopic forks only after WWII (1939–1945). In it, the front wheel is bolted between a sloping pair of struts joined to each other to form a girder, and the girder is joined to a structure pivoting on the steering-head bearings by a parallelogram linkage of four pivoted links plus one or more suspension springs. The girder was fairly stiff, at least front to back, but overall precision of steering depended on how finely fitted the four links were to their pivots. Front wheel travel was limited, most often between 1 and 2 inches.

A 1935 Moto Guzzi Bicilindrica 500 with a girder fork; the Bicilindrica 500 Stanley Woods rode to win the 1935 500 TT had rear suspension.

A 1935 Moto Guzzi Bicilindrica 500 with a girder fork; the Bicilindrica 500 Stanley Woods rode to win the 1935 500 TT had rear suspension. (Moto Guzzi/)

At the rear, experience dictated no suspension at all. Attempts to provide comfort and road-holding by providing a sprung rear wheel clashed with builders’ lack of understanding of just how stiff such a system had to be. Although Indian offered sprung rear suspension as an option just before WWI, it worked too poorly to catch on. Therefore up until 1935, when the brilliant Stanley Woods won the 500 TT on a Guzzi with rear suspension, the old-timers insisted “Nothing steers like a rigid.” And it was true; a well-executed rigid frame could produce a bike that was acceptably stable against high-speed weave oscillations, but early “springer” designs could not.

Necessity

The motorcycle’s evolution was driven by immediate necessity more than by inventors. When shorter-wheelbase bikes felt more responsive and won races by their quicker maneuvering, longer wheelbases looked outdated and were dropped. When lower-powered side-valve engines were relegated to putt-putt commuters, and new OHV engines and bikes grew taller, older riders clung hard to their cherished long and low look while young Brooklands hotties embraced an emerging new look.

The great new Brooklands speedway, 18 miles southwest of London, had opened in 1907, and quickly became the prime testing ground of the British motor industry as well as the venue for both car and motorcycle racing events. As a gathering place for engineers it had no equal, and was in effect England’s practical university of motor transportation.

As the increasing chain pull from more powerful engines distorted spindly traditional frames, greater stiffness was the key to stability at higher speeds. Makers learned their lessons quickly by putting up-powered new engines into traditional frames: It did not and could not work. Bigger tubes and cradle construction banished the Edwardian look and Keystone construction faded.

Traditionalists disliked such changes, preferring bikes as they had been, weighing no more than 140 pounds, slender and delicate as bicycles. But nothing could stop the motorcycle’s rapid evolution, driven by so many riders, thinkers, and doers. We may have loved what the motorcycle was when we were 20 years old, but we can be sure everything will be different again, even almost unrecognizable, in 10 years, in 20, in a lifetime.

The motorcycle appeals to us because it is in human scale, weighing hundreds rather than thousands of pounds. It easily becomes an extension of our physical and emotional selves. Its evolution has been driven by practical people finding solutions to problems of the moment—and implementing them with their own hands.

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