Where Did Those Middleweight Twin-Cylinder Motorcycles Come From?

A few months ago, I compared today's class of 250cc to 400cc twins with the classic Honda CB350, 250cc Suzuki X6, and Yamaha RD350 of the 1960s and '70s. I was surprised to find that weights, engine power, and top speeds have not changed in four or five decades.

Why not? The peak power of big sportbikes has trebled since 1973 and quadrupled since 1966. Why haven’t middleweights stayed in step? They haven’t because the role of such motorcycles—to achieve a “sweet spot” amid the tuggings of contradictory requirements—hasn’t changed. People want bikes that are a bit sporty, inexpensive, capable of carrying a passenger. Not too much vibration, manageable performance, good gas mileage, and good looks.

Where did these bikes come from and how have they evolved into what we’ve seen?

Both before and after World War II, Germany was the center of two-stroke development, helped in no small degree by research undertaken in the then-fast-expanding diesel engine business. At first, the power of two-stroke motorcycle engines was limited by their use of pistons with deflector domes. Deflectors, by absorbing extra heat from combustion, severely limited engine power (the more power they made, the quicker they seized). But when Adolf Schnürle, developing marine two-stroke diesels, came up with a scavenge scheme that allowed use of a flat-topped piston of minimum surface area, DKW bought into his patent. This large reduction in piston heat load allowed DKW to successfully develop larger-bore engines such as its SB-series singles, beginning in 1933, with bores increasing from 60mm to 68, then to 72mm. Could DKW do even more? Yes, the SB 500 of 1935—now an electric-start twin—made a useful 15 hp.

This trend resumed after WWII, beginning with a 125cc and then a 250cc single, expanding into the RT 350 of 1954. In Czechoslovakia, Jawa produced similar basic two-stroke bikes. Adler, in Germany, introduced 200cc and 250cc parallel twins in 1951 and 1952, then the MB250 a year later.

RELATED: Do You Appreciate Motorcycling’s New 300cc Lightweights?

Understand that the goal at this time was to get workable transportation into production quickly after a war. Two-strokes, having the fewest moving parts, were a speedy solution. If saving fuel and preventing global warming had been important human goals in the first half of the 20th century, could two disastrous and wasteful world wars have taken place?

In England during the late 1940s, the need for hard currency forced the export of roughly 70 percent of that country’s production of larger twin-cylinder bikes to markets like the US and Argentina. Critics insisted that basic two-wheel transportation could find a large British domestic market, as imported Italian scooters were selling well there. Ariel, for years a producer of solid but hardly sporting four-stroke singles, decided to bet the farm on following the German lead. It designed a line of future-styled two-stroke twins with weather protection, carrier boxes, and pressed-steel structure. In retrospect, we can see that these bikes were effectively designed by social critics and not by engineers and stylists in touch with the market. The critics asked, “Why can’t motorcycles be more like cars? Clean, quiet, and cozy?” But an important aspect of the motorcycle’s appeal was its “not-car-ness,” as shown by the failure of Velocette’s critic mobile, the putt-putt LE, and by the fact that Ariel soon had to supplement the Leader with sportier-looking variants.

Why pressed steel, when for so long motorcycles had been built with tubular steel chassis? Cars are pressed steel! The Germans had demonstrated the low production cost of pressed bike chassis for years and had squeezed the idea for every pfennig as the German motorcycle market collapsed circa 1955. Why the collapse? Same as in the US in 1914: the arrival of cheap, mass-produced cars.

Ariel’s Leader enjoyed some degree of sales success, but it was selling into the falling side of the postwar bike wave. The new Ariels arrived just after England’s year of peak motorcycle registrations, 1959.

Meanwhile in Japan, that nation’s 200 small motorbike producers bit into one another in Darwinian fashion until just five or six remained. They lost no time in saturating the market, then offering bigger 150s, 200s, and 250s to see if riders had a taste for upgrades. Turning from WWII propeller manufacturing to something they could sell, Yamaha engineers had a close look at the Adler MB250 twin, then went their own way with the air-cooled YDS series. Honda, beginning with pressed-steel chassis and styling reminiscent of German NSUs, aggressively researched the US market and found that Americans were pretty sure that real motorcycles have tubular steel chassis and fenders that don’t look like iris petals.

Tohatsu and Suzuki also produced two-stroke twins. Like Yamaha, they saw the transition from a single to a twin as best packaged between two vertically split crankcase halves. Singles had been this way since before 1900, why change now? Adler made a twin crank by drawing two single cranks together with a drawbolt system, but Yamaha simplified this by pressing its cranks together over a center block containing main bearings and a seal.

Honda made the leap early from the motorcycle-inspired vertically split crankcase to the automotive horizontal split, even though NSU had produced mainly vertically split engines. This speeded assembly by allowing line workers to plop down the crank and gearbox shafts into the upper case, then slide the lower case over the case studs and do up the fasteners. Yamaha persisted for a time with the vertical split, assembling parts into one side, closing the cases with substantial force, then adding a bolted-on shift mechanism under the gearbox.

Tying design to production so that more people could afford the product was one of Japan’s greatest contributions to motorcycle design. Bench assembly (“old-world craftsmanship”) sounds romantic only until you realize that the bike you’re considering contains 2,000 person-hours of work at $30 an hour.

Suzuki saw the value of horizontal-split assembly early; its 250cc and 500cc two-stroke parallel twins were built this way. Yamaha would arrive at the same conclusion with its R-series 350cc twin in 1967.

Meanwhile, Honda, after making a great number of pressed-steel bikes with dated-looking flared fenders and leading-link forks, hit upon a successful style that would last through the 1970s, giving its bikes tubular frames, telescopic forks, racebike fenders, and mostly pleasing looks that reminded some of us of all the 1960s Grand Prix races it had won.

Yamaha did some kind of production review around 1970, cutting tooling and assembly costs by building its 250cc and 350cc two-stroke twins on common chassis and crankcases in a somewhat modular system. Away went terrible vertically split cases and bolted-on gear selectors.

The result of all this cut-and-try was the excellent middleweight twins of the later 1960s and the 1970s—the Suzuki 250cc X6, Yamaha 250cc and 350cc RDs, and Honda’s four-stroke CB350. Perfectly timed too, for US motorcycle registrations doubled between 1965 and 1970, and then doubled again from 1970 to 1975. Thus these important Japanese bikes hit the rising side of the wave. Do you suppose they planned it that way?