The improved roadways of the 1930s allowed motorcycles to maintain consistent high speeds, but the rapid deterioration of roller bearing big ends were a major stumbling block. At Royal Enfield’s experimental department, a breakthrough happened in the form of floating bush big end bearings. Tested to the limit at the 1935 Isle of Man Senior TT race, the floating bush ends proved their mettle and became an integral part our engines for years to come.
In the 1930s, motorcycle manufacturers predominantly used roller bearing big ends. However, improved roads meant motorcycles could maintain prolonged high engine speeds and this brought into focus the unacceptably rapid wear of these bearings.
In 1934, the Royal Enfield experimental department, led by chief engineer Tony Wilson Jones, tested plain floating bush big end bearings. Working in partnership with the Glacier Metal Co., real-world trials were carried out in two specially prepared 500cc LF Bullet racing engines, complete with bronze 4-valve cylinder heads and straight-through racing pipes.
Both bikes were entered in the Isle of Man Senior TT race. During practice, rider Cecil Barrow crashed and injured his arm too severely to compete and the other competitor, Jack Booker, was forced to retire in the fifth lap.
The following year, however, Barrow rode the same machine with a much better outcome. Running 12th by the end of the first lap, he’d moved up to 7th place by the end of the fourth. Overtaken close to the finish line, the Bullet finished in a highly commendable 8th place, averaging 73.94 mph. Incredibly, Barrow rode without a twist-grip throttle, preferring instead an outmoded lever throttle. He is thought to have been the last TT entrant to use this type of throttle control.
1934 LF 500 Bullet Isle of Man TT racer.
Paving the Way
Although this was to prove Royal Enfield’s final work’s outing on the revered TT circuit, it did prove the reliability of floating bush big ends and pave the way for the future adoption of in its engines.
In 1949, a technical paper by Tony Wilson Jones explained the shortcomings of the roller bearing big end and advantages of the plain floating bush that Royal Enfield adopted.
In summary, big end bearings operate under very different conditions from an ordinary journal bearing, for example, a mainshaft bearing. The most obvious difference is that the crankpin on which the big end bearing runs is not rotating about its own centre but about a point some 1 1/2” to 2” away from it. The result is that the rotating parts of the bearings and the eye of the connecting rod are subject to centrifugal force, thus imposing a load on the inner face of the crankpin, which increases rapidly as the speed of the engine rises.
Furthermore, the intensity and direction of load on the big end bearing varies considerably from point to point in the cycle of operation and depends to a large extent on the speed and throttle opening at which the engine is running.
Finally, when the engine is running at a uniform speed, the rotation of the big end bearing is not uniform but is constantly accelerating and decelerating during each revolution.
Under these conditions, the roller bearing is far from being frictionless. On the contrary, there is heavy friction between the rollers and cage and also a lot of skating between the rollers and the inner and outer races. Experience has shown that to keep such a bearing even reasonably cool, a lot of oil must be pumped through it to carry the heat away and to lubricate the skidding services.
By contrast, not only has the plain floating bush bearing negligible inertia, so that it responds immediately to the acceleration and deceleration imposed on it, but the oil film that separates the inner and outer races is constantly being renewed, providing, of course, that oil is being fed to it in sufficient quantities.
