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Talking Technical: Brakes - Part 1 (Part 2 Here)

Are you baffled by brakes? Do your drums let you down? Dave Minton explains how and why, and how to fix it and why you should...

Brakes are small, simple devices, which when used to their maximum have to absorb more energy than their bike's complex engine produces in the same time-span. Ixion, writing in 1950's Motor Cycle Cavalcade about veteran, turn-of-the-century riding said; 'Fortunately, a good pair of hobnailed boots, firmly pressed against the road surface... acted as quite tolerable stoppers in average emergencies.'

The average spam-can dozer relies on his or her brakes to avoid trouble. The chances are that a stamp on their inherently stable vehicle's brake pedal will save their bacon. Squealing rubber and blue smoke may prove momentarily embarrassing but so what? And if the road is wet - well, shame about the splashed paintwork. Co-efficient of what friction?

All there is to it...

The average old bike rider knows full well that far from suffering fools gladly, motorcycles do not suffer fools at all. Any rider who on public roads relies on his of her bike's brakes as the principle hazard escape option will sooner rather than later discover the flaw in their survival philosophy. The brakes of a modern motorcycle will certainly stop the wheels turning but the locked wheels will not stop the bike.

By 1958 all the big Triumph twins were wearing a heavily-finned full-width front hub that incorporated a 7-inch stopperTherefore we could be justified in assuming that bike brakes are of secondary importance. Wrong. A classic British 650 parallel twin has enough engine power and roadholding in cunning hands over minor A and good B-roads to match the velocity of most modern machinery between here and there. Match it, yes, but you just try getting in front and staying there! Such ambitions are brave but foolhardy because skill and experience cannot provide more than a transient advantage to the drum-braked rider.

Disc brakes are hard to over-tax, save by mechanical defect or stupidity. But use even the best road-intended drum brake hard, thrice in quick succession, and you will be lucky not to find your right fist full of slack front brake lever.

While brakes may be small, simple devices, their operation involves some of the most complex physics associated with the inherently complicated business of bike dynamics. So we will sidestep the theoretical delights of moments and kinetic energy and force, and concentrate on the practicalities.

Back in the 70s, DLM could squeak to a halt from 60mph in just under 120ft on one of these. Scared? You should be!The best starting point is to measure the existing efficiency of your brakes. Let's start with an example of a BSA B50 which I used in the mid-1970s. Mainly on the strength of its 7-inch 2ls drum brake it stopped on average from 60mph to zero in 119ft. To discover the power of the deceleration involved we have to know the force of gravity created during deceleration. While it can be calculated, you would do better to accept the 60mph constant of 120.36. Why? Just do yourself a favour and accept its mathematical convenience!

We divide the constant by the distance (120.36 over 119 = 1.01G). The B50 weighed 336lb, I weighed with winter togs 192lb, a gross of 528lb.

Never mind the big air-scoop, the sheer size of the conical hub brake helped it dissipate heatCompare this to a more modern single cylinder machine which I have also ridden, the lamentably overlooked Yamaha SZR660. It weighed 395lb, I weighed 200lb, a total of 595lb. It stopped in 98ft. 120.36 over 98 equals an astonishing 1.2G.

But let's go back to the good old days. My 1939 grosses 573lb with me aboard, and from 60mph it stops in 181ft, a braking figure of 0.66G. Not much, eh? Maybe not, but it's roughly comparable to a 900lb (gross) current Harley's 168ft and 0.68G.

By curious coincidence, a deceleration force of one-gee equates pretty closely to that old favourite standard of brake testing journalists, now thankfully ignored, of 30ft from 30mph. It is typified by Motor Cycling's 29.5ft stopping distance from 30mph for its 1954 T110 road test. Yet for a machine capable of over 110mph it remains a pointless exercise!

Why then, our concentration on 60mph? because that is about the best speed from which the average rider can safely sudden-stop without planning ahead. Anything more finds the varying co-efficient of friction betwixt tyre and questionable surface becoming the dominant factor and lateral stability its demon partner.

So what do we learn from this about actual braking efficiency?

According to Ferodo's Table of Stopping Distance & Time, the B50 achieved 100% efficiency. The Harley 72%, the T100 66% and the SZR660 a remarkable 102%. This occurrence has three roots:-

  • 1. Ferodo's table was drawn up several years ago and since then the co-efficient of friction of tyres has improved beyond all past perception.
  • 2. The table was drawn up for cars and their best brake pad co-efficient maximises at about 0.36mu whereas motorcycle pads achieve 0.46mu (drum brake linings are similar).
  • 3. Motorcycle tyres are manufactured from a much softer polymer than car tyres and therefore also have a higher inbuilt mu.

    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.

    Drum, left, Disc, right. More to it than friction?

    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.

    The Norton full-width 8-inch brake, as seen here on a 1963 Dommi SS wins Capt Minton's approval, not least for its cooling ribbingMost 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.

    The Honda 2LS brake of the mid-60s was a stunner, and turns up on many a classic racer todayYamaha'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.

    Mind you, not every Honda braking innovation won true approval...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...

    Edward Turner didn't wait long to move on from the pre-war Speed Twin's pressed steel front brake

    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.

  • Phone 01663 732025

  • Drums ok?


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