Technologies That Revolutionized Modern Motorcycling, Chapter 4

[Hugh Janus]

Two-stroke racers like Kenny Roberts' YZR500 were constantly under development, helping set the tone for modern four-strokes even as they won championship titles. (Cycle World Archives/)

While some sour-grapes traditionalists call the two-stroke domination of motorcycle racing in the years 1975-2001 “the lost era,” 21st century four-stroke bikes owe much of their light weight, excellent brakes, and responsive handling to that period. It was the low R&D cost of boosting two-stroke power that created the “100 hp crisis” of 1970s roadracing. The resulting state of desperation forced engineers to consider alternatives to the status quo—the wide-section tire, engineered suspension damping curves, and much stiffer chassis that have become the solid foundation of the modern four-stroke motorcycle.

Honda says it developed the Gold Wing to be a next level sport bike. Its four-cylinder engine was water-cooled—no fins! (Honda Motor Co./)

Liquid-Cooling

Although Honda’s flat-four-powered Gold Wing of 1976 has now become a pure touring bike, it had been planned as the next level of sporting motorcycle, incorporating engine smoothness, water-cooling, and shaft drive. It was the users themselves who made a tourer of it. Why was the Wing water-cooled? The AJS company, reaching for the next level of performance in the mid-1930s, built a 500cc air-cooled V-4 engine. Try as the engineers might, they could not cool its rear cylinders with the hot air streaming back off the front jugs. Overheating caused the rear pair of cylinders to knock, so the engineers reduced their compression ratio, losing so much power in the process that the resulting bike was hardly faster than the company’s 350cc single! For air-cooling to work best, every cylinder and head must have its own supply of fresh, cool air. On the Gold Wing flat-four that would have required ducted fan cooling, but market research showed that motorcyclists scorned fan cooling as only for ho-hum scooters, golf carts, or Vee-Dubs. So water-cooled it had to be.

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Having made that step away from the limitations of air-cooling—which was mirrored in the design of the V-4 oval-piston NR500 GP engine—it was natural for Honda to next water-cool its 1980s production V-4s, the Sabre and Interceptor. Because water is 830 times more dense than air, problems like pulling heat out of the hot exhaust bridges of V-4 engines were easy. This made it possible to greatly increase torque by raising compression ratio. Compare the 1-liter air-cooled engines of the late ’70s and early ’80s to those of today and what do you find? Typical power was 80 hp at 8,500–9,500 rpm, on 8.5- to 10-to-1 compression. Compare that to today’s liquid-cooled V-4 engines at 2-1/2 times more power from the same displacement, given at a peak near 14,000 rpm, with 12- to 13.5-to-1 compression. The “thermal barrier” had imposed a practical limit to air-cooled power. Water-cooling broke down that barrier.

(While hot-rodders and racers did get a lot more than 80 hp from those engines, the results were not sufficiently reliable to be sold to the general public under warranty.)

By the 1980s, radial construction had become the go-to tire technology in racing. (Metzeler/)

The Radial Tire Revolution

Riders on streets and highways were still well-served by traditional bias-ply tire construction in 1980 but on the track it was clear that the “trellising” of crisscrossing bias tire cord plies, flexing against each other, was generating heat that in racing quickly led to rubber fatigue and loss of grip. By 1981-82, riders were coming to the start grid with tires that weren’t even scuffed in, in the hopes of getting maybe one or two extra good laps from them before they “went off” (tire life at peak properties was then about 10 laps!). Marco Lucchinelli won his 1981 World 500 Championship mainly by thoughtful tire strategy, choosing to cruise back in seventh place or so while the leaders destroyed their tires, then easily picking them off one by one later in the event. In 1984, Michelin fielded its first semi-radial-ply motorcycle racing tires, and American rider Freddie Spencer was a major player in their testing and development. As with any fundamental innovation, every tire maker had to adopt the superior technology or find itself “on the wrong side of history.”

Extending Spark Plug Life

Spark plugs had to be periodically regapped because high spark current eroded the gap. To stop this and also to reduce radio frequency interference from spark plug wires, the use of either resistor plugs (plugs with an internal gap) or carbon rather than metal plug wire became more common. With high resistance in place, only the capacitive part of the spark energy jumped the gap. This, by reducing spark duration, made spark plugs go for longer distances with little gap growth (I’ve never even seen the plugs in my car’s engine).

Anti-Dive Systems—They Just Didn’t Work

Suspension travel just kept growing in the MX arena (where it’s now more than 12 inches), but on pavement and when combined with powerful disc brakes and the increased grip of slick tires, it created new problems. Motocrossers needed ever-greater travel to soften jump landings and the effects of the harsh terrain, but on pavement, long travel resulted in abrupt brake dive and loss of steering rake, and, on occasion, reduced stability. Long travel combined with high-flow compression valves had given riders new confidence over rough pavement and transitions. But there was nothing attractive about brake dive, which brought instability and even sudden lifting of the rear wheel.

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This was at first tackled around 1980 by anti-dive systems—some of which limited dive by increasing fork compression damping, and some by using caliper reaction force acting through mechanical linkage as a “jack” to oppose dive. No sooner was the hardware in place (it appeared on several production bikes) than it was discovered that (a) mechanical anti-dives hopped and juddered on rough surfaces and (b) that braking distances were shorter with dive. The latter results from the reduction in center-of-mass height that takes place in dive (the higher the CG, the easier it is to lift the rear wheel with the brake). Anti-dive systems disappeared as quickly as they had come, replaced by more sophisticated use of compression damping.

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Most of this action took place in racing, for in production bikes hydraulic fork dampers remained simple, with a large fixed orifice to “control” dive and a one-way valve to place most damping on rebound (hold the front brake, compress the fork, and note that it goes down easily but comes up more slowly—this is rebound damping). Early long-travel roadrace bikes of the mid-1970s had 5–6 inches of wheel travel at both ends, but this soon dwindled away in the 1980s to 4.7, 4.5, and 4.1 inches, as growing control over compression damping permitted.

External adjusters are commonplace on production bikes thanks to the development of cartridge dampers. (Öhlins USA /)

Adjustable Suspension

At this point in racing we began to see new adjustments implemented at the top of each fork tube and on the rear shock. In addition to external preload adjusters (which appeared in 1974 on Yamaha’s TZ750A) there was now a “clicker” by which to adjust low-speed rebound damping. This was made possible in forks by replacing the “damper-rod” dampers of the 1970s with self-contained “cartridge” dampers that could be removed for adjustment without full disassembly of the fork legs, and for which external adjustment was practicable. Even though racing adopts new features for performance reasons, what the public sees is cool new action—mechanics adjusting clickers and changing preload. Soon everybody wants it. Some of the first generation of such things on production bikes had little or no effect on damping, but their owners could now enjoy twiddling their clickers.

Conventional rear dampers of the 1950s and ’60s were of twin-tube construction. The damper piston moved back and forth in an oil-filled inner cylinder, communicating via a bottom valve with an outer cylinder that was only half-filled with damper oil. The volume of air above the oil in the outer cylinder was necessary because as the damper rod enters the shock body on compression, something springy must accommodate its extra volume (oil being essentially incompressible). In rapid action, it was possible for some of that air to be entrained in the damper oil, making it “springy” and rendering damping inconsistent.

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The de Carbon system changed this to a damper cylinder filled with oil, connected to a gas/oil accumulator. One form separated gas and oil by a moving piston, another by a flexible diaphragm. Not only did this prevent entrainment of springy air into the damping fluid, it also allowed the oil in the damper cylinder to be pressurized. Imagine a conventional twin-tube damper as a bike moves over a rough surface. Rapid motions of the piston in the inner cylinder require oil to promptly follow it, but the only pressure available for this is the mostly atmospheric pressure above the oil in the outer cylinder. If the piston moves faster than this pressure can keep up with, what results is “cavitation”; the oil, unable to keep up with the piston, is pulled apart—it cavitates. When the cavity collapses, a sudden impact results that affects tire grip. With the de Carbon accumulator it was a simple matter to pressurize the system to whatever level was found to prevent cavitation. Yamaha acquired a patent for a form of single-shock rear suspension from the Belgian engineer Lucien Tilkens, and it became an instant killer advantage in motocross.

Pressurized modern rear suspension designs like this Marzocchi monoshock came about as an answer to cavitation. (Marzocchi/)

On single rear shocks of the 1980s and later, the cylindrical object connected to the damper by a flexible hose is a piston accumulator whose gas volume is pressurized to about 15 atmospheres. Today, a different arrangement of internal functions requires the accumulator to be a part of the shock body, making the combination look like a futuristic zap gun.

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