6 December 2011
May I comment on the use of the term ‘disc brake’ when referring to the Research Association (RA) brakes used by Douglas in the 1920s. Most readers will be familiar with the belt rim type of brake, which existed in two forms – those in which the block was pushed inwards from the outside and those in which the block was pushed outwards from the inside.

In both cases the operating force is applied radially to the axis of rotation and exactly the same situation applies to the RA brake, which is effectively like the second type described turned inside out.

Now consider a disc brake. In this the operating force is applied parallel to the axis of rotation and braking is achieved via a clamping action as opposed to a wedging action. The extreme example of this is the front brake used on the Honda 50cc racing twin, in which the braking surface was the edge of the wheel rim, just as in the calliper brakes common on bicycles.

The disc brake as we know it did not become common until after the Second World War, being developed primarily to cater for the higher landing speeds of jet-powered aircraft. Early users in the automotive field were the V16 BRM and the C-type Jaguar. I therefore suggest that it is wrong to refer to the RA type brake as a disc brake. It would be interesting to know what the terminology for these brakes was prior to the Second World War. Am I the only one in step (again!)?

Mike Jackson via email

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