The Evolution of the Telescopic Front Fork
There was a lot of opposition to the telescopic fork when it appeared – people found it too whobbely during braking and if its sliding joints were made loose enough not to bind as it flexed, riders could feel the lost motion.I used to hear a lot about how exemplary were the Girdraulic forks on postwar Vincents. “Those forks are perfect,” I was told, “so long as all the link bushings are properly finished with a planishing broach.” I'll make a note of it.
In a classic contest in 1939 Walter Rusk on the on the complex and heavy AJS V4 raced even-Steven with Dorino Serafini on the Gilera-4 at the Ulster Grand Prix – until one of the four links of his girder fork failed. A big development of the later '30s was increased braking power – to complement the growing power of supercharged engines. And braking power of exerted very great leverage over the links of a girder fork, located as they were up at the steering-head.
Almost grudgingly, the telescopic fork was adopted – with a momentary diversion into Earles and other leading-link forks in the 1950s.
Think about what happens as you steer a motorcycle. You exert a steer torque on the handlebars, but because the fork is flexible in twist , the top of the fork 'steers' but that heavy gyroscope of a front wheel tends to keep right on as it is pointed. Only with reluctance does it follow your attempt to steer. Now the tire begins to steer off in the new direction, but because its rubber structure is flexible, the rim moves less promptly than the tire. Now the spokes feel the pull of the new direction and, flexing, they begin to pull the hub, axle, and fork bottoms with them, each after the other.
All these delays were additive, so the rider would feel the result like trying to steer a big car. One can then understand why Gilera, for 1956, decided to build the fork crowns of its four-cylinder 500cc racer into a single very stiff box structure of sheet steel. This took a lot of the fork twist out of that motorcycle's steering, making its response more prompt.
Today if you walk the paddock of a World Superbike or MotoGP race you will see milled-from-solid lower fork crowns with three pinch-bolts for each fork tube – getting very serious about eliminating twist. Lower crowns of 40 years ago looked like pieces of plate but the latest ones more resemble a pair of shorts, so long it their engagement with each fork tube. Fork tubes of the late 1930s and through the 1950s were slender because of the preoccupation with weight, but by 1966 MV were putting 35mm tubes on their racers, and 10 years later Suzuki were ready to make the modest leap from 36 to 37mm.
A couple of years ago when Valentino Rossi was fighting a losing battle against front wheel chatter, his Yamaha M1 MotoGP bike was given 50mm tubes, It was about time, too, for chassis and swingarms had in the interim gained huge stiffness and the same was required for fork tubes. No other part of chassis structure is cantilevered so far from its point of support – in this case the steering-head.
A related part whose stiffness has greatly increased is the front axle. In my apprenticeship as a callow youth I became accustomed to tiny solid front axles of 14 or 15mm diameter. The same material made into a tube, gains greatly in stiffness because now all the material near the center simply bends, but when the same material is made into a tube, it must be stretched or compressed if the axle is to bend. For insight into this, imagine the reduction in bending stiffness if the material is a commercial aircraft's fuselage were made into a solid bar. The fuselage may be 5 to 6 meters in diameter, but the bar would be tiny – like 0.4 meter. Solid axles crept up to 17mm in the early 1970s but their weight and structural inefficiency discouraged further increase.
The late John Britten, who so famously said: “I prefer if possible to work from first principles,” naturally gave his creation tubular axles. In 500 Grand Prix racing of the 1980s large tubular axles became firmly established and today a solid axle has become an anachronism. As the only connection between the otherwise indepentent sliders of a male-slider or 'up-side-down' fork, the front axle's stiffness is critical to the task of keeping the front wheel always in the plane of the machine – not tilting to the sides under stress, knocking the brake pads back and delaying steering.
This slow, detailed evolution never ceases...
