Lessons Learned, Challenges Overcome, Part 1

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Bike designers and riders alike had it a lot tougher in the 1950s than we do today. (Harley-Davidson/)

By looking at some of the problems motorcycle designers faced in the 1950s, and seeing how they overcame them, we can gain valuable insight and appreciation for the bikes we ride today. Here’s a partial list of their challenges.

1. Manufacturing, especially in Britain, remained primitive and not well organized. Bikes were more often “crafted” than manufactured.
2. Most crankshafts spun on rolling bearings of limited life.
3. Valve springs could break without warning.
4. Built-up crankshafts could shift at their joints, imposing serious rpm limits
5. Valve trains were inherently rpm-limited, a characteristic made memorable by the phrase, “My bike may be slow, but at least it’s unreliable.”
6. Lubricants were short-lived, sludging and gumming easily, especially in air-cooled engines.
7. Although auto electrical systems worked well, bike electrics acted as if they were still in the prototype stage. Solution? Sacrifice in the temple of Lucas, Prince of Darkness!
8. Vibration was a constant battle. As one rider put it, “Can’t get some part off your BSA? Go for a ride and it’ll fall off.”
9. Oil leakage of embarrassing degree, provoking the ironic observation, “If it’s not leaking, it’s out of oil.”
10. Inadequate cooling. Iron cylinders and heads were suitable for waffling along country roads, but as modern restorers have learned, vintage engines whose cooling can’t keep up with modern traffic quickly reveal their limits.

Tradition and Old-World Craftsmanship

England’s response to calamity has always been to get back to normal as soon as possible. Although mass production was a necessity in World War II, in 1950s’ England, motorcycles were still at least partly built by bench assembly. Why are we still charmed by handcraft?

What it comes down to is the warm human element: mistakes. For romantics, the master craftsperson measures twice and then cuts once—perfectly. In reality, he or she was working as fast as possible, driven by the company’s need to trim back the labor bill for each bike. Machine tools were often well past their best. Automatic tooling? No way! Stockholders need dividends. Respected British industries were crippled by a permanent Cold War between labor and management: “If you get more, that means I get less.” Nobody wants less.

In the US, Harley-Davidson and Indian continued to elaborate on what had worked best after World War I: substantial machines on large-section tires that doubled as suspension, powered by low-revving flathead or pushrod engines with three-speed transmissions, well suited to surviving on pre-interstate farm roads. Production was too small to fund much innovation. In 1966, a Mr. Fink who interviewed me at Harley explained that, “We like to hire young men out of high-school drafting programs and put them to work on easy stuff. After about ten years they’ve understood how we like to do things and they’re ready to take on small changes and updates.”

Crankshafts, From Complex to Simple

While the first bike engines turned on bronze bushings as per normal railroad practice, it’s rolling bearings that survive best on too little or overheated oil. Between the 1880s and 1910, automated ball- and roller-bearing production grew progressively. Millions of balls, rollers, and races were being produced every week, but rolling bearings had definite and limited fatigue lives. The intense pressures between the rolling elements and raceways produced high shear stress just below the surface, eventually creating cracks. But at low revs and light duty such bearings could last a long time.

The harder you push design, the sooner failure arrives. In the early 1950s, Mercedes had to lower its all-roller 300SLR’s redline to finish 24-hour races. Without that precaution, the bearings fatigued out. In that same period, Italy’s high-revving (13,000 rpm) factory racing 125 singles needed a fresh big-end bearing for every weekend. (Chris Carr, during his time on factory Harley XR750s, rode bikes with the same maintenance requirement.)

From about 1957 on, the ball bearings in military jet engines were made from special, vacuum-remelted steels. The process boiled away mineral oxide inclusions (referred to as “stringers”) and helped make them much more fatigue resistant. By 1964 major bearing manufacturers were able to offer standard ball bearings with six times the previous fatigue life, and it has improved from there.

Abandoning multipiece, pressed-together crankshaft construction and adopting one-piece forged-steel cranks spinning on pump-lubricated plain journal bearings with split-and-bolted con-rods was even more important in achieving long engine life. That began with Honda’s 1969 CB750, a design inspired by automotive practice.

Making Valve Springs Last

When we bend a can lid back and forth to break it off, or do the same with a wire coat hanger, we learn that applying stress repeatedly changes metals. Valve springs accumulate large numbers of stress cycles, and spring failures remained an irritant. Such a problem, in fact, that Mercedes-Benz, Ducati, and other makers developed springless valve trains employing a second cam lobe and a separate rocker to close each valve. Mercedes called its “Zed-Drive,” and Ducati continues with its “desmo” system to this day.

Valve springs from a Ducati Multistrada V4. Sacrilege? (Ducati/)

Because conventional helical-coil valve springs could also support traveling deflection waves (as when you tweak a stretched Slinky toy), their accumulated stress cycles could be far greater than just the number of valve openings and closings. To eliminate these extra bounce cycles, racing engines in the 1950s often used “hairpin” springs, a design resembling the spring on a household clothespin. Since they are much less able to “ring” as helical springs do, they lasted longer. Later, friction damping was used to suppress wave action in springs.

S&W developed very superior helical springs that used vacuum-remelted spring wire, shot-peened to place the wire surfaces in compression. As soon as they appeared, hairpin springs fell out of use.

Racing is a very tech-greedy application for valve springs. As designers pushed maximum wire-fiber stress levels higher and higher (in Harley’s VR1000 Superbike racer it was reportedly 140,000 psi), the life of even the very best springs became short. In MotoGP, by 2004 teams were changing valve springs every night. Finally, in the most demanding applications, it was time to replace metal springs with compressed gas—fatigue cracks cannot form in a gas! All the bikes (except the Ducatis) on MotoGP starting grids now employ pneumatic valve springs.

Stronger, More Durable Crankshafts

Three-time world champion Dani Pedrosa aboard the Honda RC181. (Honda/)

In 1950, Harley and Indian crankshafts were seven-piece constructions: two main shafts, two flywheels, one crankpin, and a fork-and-blade connecting-rod pair running on caged rollers. The pieces were pressed or drawn together and retained by thin nuts; the whole assembly was then placed on a fixture equipped with dial gauges to indicate degree of straightness. A skilled person with a copper hammer consulted the gauges, decided in which direction the error lay, and with measured blows brought everything into alignment.

The cranks in British twins were generally bolted together from two or three elements, while single-cylinder cranks retained six-piece pressed-and-nutted construction.

Yes, such cranks were kept in alignment by nothing but the friction in the tapered or pressed joints. Yes, the joints slipped if more were asked of them than they could give. Honda’s mighty RC181 500 GP bike of 1966-67 was said to have suffered such indignities. Pressed-together roller cranks soldier on to this day in Suzuki GS-based Pro Stock Motorcycle drag engines.

By the 1950s, crankpins in the classic big single-cylinder British race engines (Manx Nortons, Matchless G50s, AJS 7Rs) were beginning to break at low hours. As tuners extracted more power and peak revs crept up from 6500 to 6800 and then to 7000 rpm, the combination of heavy flywheels pressed onto a crankpin of modest diameter acted more and more like a giant low-frequency tuning fork. Vibratory stress cycles accumulated until, just as with valve springs, a crack appeared and began to propagate. Bang—the crankpin would break. This is why the last big single to win the Senior TT at the Isle of Man (1961), prepared by Bill Lacey and ridden by Mike Hailwood, had a special one-piece crank with a split-and-bolted Jaguar connecting rod.

Nearly every bike engine made today has a one-piece forged-steel crankshaft.

This is part one of a two-part story. Part two will appear later this week.

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[Guest]

This has the hallmarks of Kevin.

[billy sugger]

I'll read it it when I can't sleep 

[MikeHorton]

Sadly I've been bored enough to click the full article link. To my shock and horror this is indeed penned by the legendary Kevin Cameron. I think he needs a pseudonym. Maybe Philip Tuttleworth? Or Edna Tayle as I've no clue what he's on about most the time