Saturday, 2 January 2016

Bob McIntyre’s design principles: When a brake pedal is so much more

The 1958 season razorblade frames were technically innovative in so many ways. These Bellshill built frames have been described previously on this blog, please see the link below for earlier posts:

  Razorblade frames

The simplicity in design and attention to detail throughout the bike is staggering, even when observed 50+ years later and is a lasting tribute to all involved.

Oulton Park 7th April 1958 - 500cc race

Take the humble brake pedal for example. 

As all will know, the objective of this component is to apply the rear brake whilst sat astride and racing the motorcycle. However for the racer it is also important for him to know that for any given pressure applied to the pedal there will be a repeatable and constant braking effect on the machine. The Potts rear brake plate is of a fully floating design and anchored to the main frame via a torque arm. This arm is adjustable in length, in order that a true parallelogram with the swinging arm be maintained at all times and allows for any rear wheel adjustment necessary.

Desmo floating rear brake

 As with the hugely reduced engine and gearbox mountings used on this frame (4 as against 11 on a standard Manx), the team were very economical in the use of fixings. A single stepped clevis pin bolted through the frame bracket, acts as a common pivot to both torque arm and brake pedal. This pin is drilled and a grease nipple provided to ensure that these pivots remain relatively frictionless. An ‘ear’ on the brake pedal acts against the footrest to provide a stop.

Now the clever bit.

In a break from tradition, Bob decided to anchor the Bowden inner cable and move the outer. Convention sees the outer wire-wound sheath located in cable stops at either end, the inner cable being actuated to apply the brake. On the razorblade frames the inner cable is anchored to the main frame and a pivoting outer cable end-stop is attached to the brake pedal. This allows the outer cable to deform upwards in a curve as the brake is applied.

Razorblade rear brake pedal

Why would he do this you ask?

Because the self-straightening effect of the wire-wound outer sheath acts as a return spring to the brake pedal and hence is no longer required. With this simple arrangement the pedal always returns against the stop without the need for a return spring.

So there you have it, one thinking man’s answer to a perenial problem.

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