"CHUMMY BRAKES?" by Peter Booth
Click here to comment on this article



I have a set of English 1930 Workshop Repair Manuals which delve into the fundamentals of the mechanics and electrics of cars of that era, and this rang a bell. Were Sir Herbert or Stanley aware of this? Do the cams we use in our original restorations compensate for this fact?

If not it could go some way toward explaining some of the Chummy's short coming in the braking department.

The most favoured cam is the centrally pivoted rectangular type with radiused ends operating between flat and approximately parallel faces formed on the free ends of the shoes, on account of its simplicity, ease of manufacture, and effectiveness.

There is, however, one important point in connection with this type of cam often overlooked. Owing to the difference existing between the distance from the centre of the brake shoe pivot to the point of contact between the outer half of the cam and the shoe face, and the corresponding distance between the point of contact of the inner portion of the cam and the face of the other shoe, rotation of a symmetrical cam does not expand both shoes to the same extent.

The shoe operated by the inner portion of the cam will be moved a greater distance than the shoe operated by the outer portion of the cam, as will be seen from the accompanying diagram. The remedy for this is to machine the cam corners with a larger radius at the outer end than at the inner end, or dispose the cam slightly out of centre on the end of the brake camshaft to such an extent that both shoes are expanded as near as possible a like extent within the normal limits of rotation of the cam.

To the mechanic this is a point of the highest importance, since replacement of an offset or specially radiused cam the wrong way round after dismantling will result in serious brake inefficiency, whose source is extremely difficult to trace.

Peter Booth


Fig. 1 - The above diagram clearly demonstrates the action of a symmetrical expanding cam and the manner in which it moves the two shoes through varying angles, the angle A being considerably less than the angle B.

Fig. 2 - In the two-shoe arrangement due to Perrot , here shown, the tendency of the left-hand shoe to be carried round by the drum is made use of to apply the right-hand shoe with greater force through the medium of a floating hinge connecting the two. As explained in the text, this arrangement is uni-directional.