Author Topic: The RA brakes  (Read 6965 times)

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Offline Doug

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The RA brakes
« on: 05 Jan 2006 at 03:33 »
It appears that the braking effect is generated by downward force of the 'caliper' exerting pressure against the splayed faces of the 'disc' - is that right? - how is the downward pressure developed and the reactionary force countered? - one would assume the force is generated by cable actuated leverage but that's not clearly shown? The 'disc' appears to take the form of a spoked wheel - how is it coupled to the road wheel? - is it on a common wheel hub with the road wheel or keyed to it in some way? In it's own way, it appears to have been a forerunner of the modern disc brake.


The RA brake, or to give its full title, the Research Association brake, really is not a disk brake. In a true disk brake a caliper pinches a rotor with an axial motion. The RA brake presses on the periphery of the rotating member, though this member looks like a disk. It is really not much different than a belt rim brake, it is just the brake shoe has the v-groove and the ‘belt rim’ the wedge profile.

The ‘disk’ is made of Ferrodo material, riveted to a light sheet metal rim. One side of this rim is clipped to the spokes of the left-hand side of the wheel. The other side has it own set of miniature spokes lacing it to the hub. There is a third spoke flange at the extreme left-hand end of the hub for this purpose. It is hard to see as it is scalloped away between spoke heads to save weight. The shoe is of cast aluminum to promote head disposition. 

Larger image

Sorry the photo is not sharper, but it was taken indoors from about 30 feet away, with a telephoto lens.

Actuation is by cable, it is just on the back side of the girder fork blade and partially obscured in this view, but you can see the clevis. It lifts up on the tail end of the brake arm, which is pivoted in the middle at the girder fork, and at the front end pushed down by a short connecting link to the shoe itself. There is a second, link immediately below the brake arm that gives a ‘parallel’ action to the shoe.

The RA brakes were reputed to be quite good for the day, but prone to grabbing. It is written some riders applied a small amount of oil to the friction material to tame them! 

The RA brake was patented by the British Motorcycle & Cycle-Car Research Association in 1922, so you can see why they shortened it to RA! It also became the designation of the Douglas 500cc model, though the 350cc version was perplexingly called the RW. Initially it was called the I.o.M. model, after there successes in the Island in 1923.  But the I.o.M. or TT model name became better associated with RA’s replacement, first seen in 1925. These 1925 racers were also the first models to have the Douglas servo-band drum brakes, so the RA brakes reigned only for two years.

How Douglas came to use this RA brake design, or who introduced it to them I have not heard. It was immediately preceded by a very similar design patented by Douglas themselves. The only difference being it was slightly smaller and the friction material was on the shoe rather than the disk!

« Last Edit: 05 Jan 2006 at 03:51 by Doug »