Swings and pins

I refer to the article on the BSA Lightning Clubman in OBA 100 and comments about the complex rear brake setup: (p87), this ‘was used, apparently purely to permit the stop light switch to remain in the standard position’.

I believe a more important consideration for this setup design was to keep the braking action consistent when the rear suspension moves up and down. If the front link rod pin is in a position inline with the swing arm spindle, then no matter how much the suspension moves, the distance from the front link rod pin to any part of the rear brake remains the same. If the pin is a large distance from the swing arm spindle then this distance will vary with suspension movement, the rider will feel a pulsing of the brake pedal and even braking force will be hard to maintain.

The photo of the New Imperial on page 35 ( OBA 100) makes this clear. The New Imperial has its front pin slightly ahead of the swing arm spindle. Slightly ahead or behind the spindle is not as detrimental to the best geometric outcome than if the pin is above or below the spindle. This is a design feature of many machines of the era, that had swing arm suspension, brake backing plate anchored on the swing arm and simple rod operation of the rear brake. For example Manx Nortons of 1951 to 1962 had the pin very close to inline, the 1969 Velocette Thruxton had the pin at the same level but about 20mm behind and the 1964 Triumph TR6 had the pin slightly above and behind.

From the photo, (bottom right, p 86 – also shown above), it can be seen that the link rod front clevis pin on its extra lever, is much closer to the swing arm spindle position, than if it had been connected to the crank of the new brake pedal.

It appears that an even closer position could have been achieved if the extra lever was pivoted on the unused spigot at the left of the photo. BSA guru Doug Fraser informed me that this spigot is for the footpeg on the base models and that the new lever pivots at the same point that was used for the base model’s brake pedal. This being used for the new lever was therefore an obvious choice and of course made the brake light switch positioning easier.