What is a 'mu' ? Simply the symbol of co-efficient of friction. To give it familiar perspective; an ordinary unworn tyre rolling over a clean dry road at 56mph manages 0.8mu. At 81mph it achieves .075mu. At the same speed through 1mm a rainwater the co-efficient of friction falls to a mere 0.2mu, and if the tyre is worn then it's an insubstantial 0.1mu. If brake mu quotient was forced higher than it is then the temperatures involved would rocket and thus lower braking efficiency all over again.

The most significant lesson here is that, precisely as with acceleration and cornering, science can lag behind practice because science, or more accurately, physics, is restricted to grappling with the known or published facts. A minor advance here, a minute shift there, can contribute apparently disproportionately to a major step in performance advance. Thus it may be possible for you, through current specialist experience, to actually exceed the original design limitations and performance of your old bike's anchors.

The common assumption of disc brakes' wide adoption for their superior power is incorrect. As with coil ignition over magneto, disc brakes were comparatively cheap, they favoured cast wheel construction, they were lighter so racers preferred their weaker gyroscopic effect and lower unsprung weight, and their reaction to overheating not only occurred almost beyond reasonable use but when it did it had a positive rather than a negative effect on brake lever clearance. Under duress the expanding disc reduced clearance, unlike a drum which increases it - often terrifyingly.

While a modern average disc brake pad offers a 0.33mu compared to an AM4's 0.42mu, the disc will hold its performance up to 800-degrees Celsius while the AM4 falls off at half that temperature.

Most of us, being classic machine owners, are faced with a variety of drum brake problems. They issue principally from a need for antiquated and often worn drum brake mechanicals to cope with modern speeds. Do not imagine that all you need to do is to fit 'racing' linings. Drum brakes need the attention of an expert if their efficiency is to be maintained. If their efficiency is to be improved then they must have the close attention of a specialist brake engineer.

One of the country's finest is Alan Campbell, who operated from a small, highly specialised motorcycle drum brake surgery in the Peak District in the UK. Alan retired at the end of the 1990s and his son, Ian, took over the reins, benefiting from his father's mass of hands-on experience with Ferodo. Alan used to divide his time between lathes, milling machines and baking ovens - and finishing off a handsome 1954 'bolt-up' Dominator (a model with offset brakes which Alan
prefers). He now has plenty of time to ride the
twisting lanes of his beloved Peaks.

One of the most popular brakes in Alan's experience, certainly the most massive and strongest of the drums, was the 10-inch duplex 4ls brake as employed on the great old TZ Yamaha GP machinery. This fascinating period of motorcycling's history should be remembered as that in which Japanese engineers appeared oblivious to the exploitation of friction as their prime means of disciplining the power they crammed into their amazing engines. Their tyres, their dampers and their brakes were perfect examples of the paradoxical conclusion of monomania.

Yamaha's own linings were, to quote; 'awful, just awful.' And Ferodo's own famous AM4s; 'too good by miles.' To give riders the sensitivity and power they needed in combination for safe racing, Alan had to re-engineer the brakes for essential AM4 compatibility. What he did was to fit all four of the shoes with leading tips of Ferodo's ultra-hard RM2 lining. This reduced the effect of the shoes' servo-action as they were 'lifted' into the drum by its rotation, simultaneously increasing brake power, but progressively.

However simple this task sounds now, it was not so simple then. AM4s will maintain their efficiency of 0.42mu up to 450-degrees C. Discovering the optimum compromise of AM4 and RM2, so that one did not seriously depress the other's performance, required painstaking development - especially as the Japanese followed a different design policy to our own.

British drum brake makers ensured that their brake shoes conformed faithfully to the radius of their drums. Japanese manufacturers relied upon grinding the linings, once bonded to their shoes, to the drum radius. In Alan's opinion it was a bad habit which could cause performance vagaries. So what he did (and which still works) was to true Japanese shoes to their drum's radius before fitting the linings, as per British practice.

Nor did Alan much like bonded linings, especially aluminium ones. Most are satisfactory but he found that about 5% have been made untrustworthy by either current or previous corrosion. Alan would normally rivet all linings into place.

Ferodo's famous AM4 lining has been out of production for more than a decade. A few precious examples occasionally arise from someone's secret treasure chest but you cannot rely on that. After experimenting with linings from other makers, Alan returned to Ferodo's huge list of alternatives. He settled on a lining from an industrial range which is hand-cut and turned to produce an viable working alternative.

You don't have to go quite that far to improve the stopping capability of your classic. Part Two will explain more...

People To Speak To

Ian Campbell has continued his father's business, Classic Brake Services, and offers a service for most motorcycles using drum brakes; re-lining brake shoes with modern asbestos-free linings in a range of compounds to suit all applications from vintage road runs to the fastest international classic racers. CBS can provide linings in standard thicknesses or oversize. All linings are drilled and riveted to the shoes using top quality semi-tubular copper rivets. Oversize linings are machined on the backplate to drum diameter. Drum skimming can also be arranged.