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Technical


Talking Technical: Brakes - Part 2

In his first instalment, Dave Minton explained why your classic bike brakes don't work so well. Here he investigates how an expert can get the best from them...

In today's traffic you may consider your classic's original braking capability to be marginal. If you regularly force the pace of your machine to warrant that little extra, then there are other means of stopping hard, fast and faithful. Preferably another, larger diameter, brake. That should be the primary move.

However, for whatever reason, you may prefer to retain the original equipment, unsatisfactory though it may be. So what can be done to improve matters?

All there is to it...

On the assumption that the actual brake operating mechanism is in good order, because worn fulcrum bushes, for instance, will spoil any hope of improved brake power, there are four items to tackle:-

  • 1. The brake shoes / linings.
  • 2. The brake drum.
  • 3. The brake cam levers.
  • 4. The brake plate and spoke tensioning.

    Brake Linings

    It really is best to consult a specialist such as Ian Campbell (email classicbrake@aol.com). Different brakes require different treatment. Do not assume that a hard (racing) lining kit simply home-riveted to your shoes will do the trick. Nor will sending your shoes away for re-lining. A brake specialist must have the complete brake, which usually means the wheel as well.

    Ferodo stopped manufacturing AM4s when the advent of discs reduced their demand to a trickle. Coupled to this was the introduction of regulations restricting asbestos-based braking material. After diligent searching, Alan Campbell found an alternative. It arrives not as a shoe but as a flexible strip, barely recognisable as a brake lining. And this, you see, is the difference between an expert mechanic and a specialist brake engineer. For Alan not only recognised the potential of this curiously thick and soft strip of brake material in the raw, but had the means to process it into a proper motorcycle brake lining.

    It is also untrue that the old dodge of fitting one shoe with a touring lining and one with a racing lining will give you the best of both braking worlds - low and high speed. Racing linings should equal touring linings' low speed co-efficient of friction. But as very few tourists actually need racing linings' high temperature performance, the softer touring lining is amply adequate, cheaper, and easier on the drum.

    As tyres can go off overnight following high temperature work, so can brakes. Within an hour or two a racing machine will dust its drums with molecular-deep rust from condensation, especially in a damp atmosphere. This can grab harshly at first brake operation, particularly with hard linings.

    Thor's little brother chips in to remove the old rivets.

    The Brake Drum

    A fine grain cast iron drum or drum liner is essential. Second best is a drum or liner in cold rolled steel to Brinell 180 (a standard of hardness). The mating surface of the drum liner is absolutely vital. An aluminium drum or hub radiates heat best and is lighter but a cast iron drum runs a close second. Cast metal - iron and aluminium - resists heat distortion. The worst is pressed steel, most especially a pressed steel drum without a cast iron liner, and worse still in an unlinered pressed steel drum doubling up as a hub by carrying a spoke flange around it periphery. Such brakes are chosen strictly for their cheapness and usually defy any attempt to improve them. Pressed steel has a poor co-efficient of friction and, besides, it will probably distort from spoke tension when overheated. Sling it if you need more front brake power and something that will cope with modern traffic conditions.

    If such advice appears questionable then reflect that even Edward Turner, who was never a man to spend a penny when he could save one, equipped his pre-WW2 T100s with cast iron, ribbed front brakes, while the Speed Twins relied upon pressed steel items. Ditto Vincent with its iron-drummed Black Shadows and pressed steel drummed Rapids.

    All drums should be checked for truth (roundness). Alan would allow a maximum run-out of fractions of a millimetre for a front drum, but placed less importance on the rear. Out-of-round drums have to be skimmed until absolutely true. An owner with a distorted drum may not feel this as the usual juddering under braking; it could (and often does) manifest itself as brake squeal. The two may be closely-related faults of the same emanation but of different frequency. Loose dust in the drum is a less common alternative source of squeal.

    Brakes fade when hot simply because the shoe lining has passed its maximum operating temperature. Once beyond this its mu plummets. If your brakes are thus troubled then be thankful because it means your drum is not expanding by much and all you need to do is replace the linings. Most drum brakes, however, signal impending failure by fast increasing slop in the brake levers, generally front, as the drum expands beyond the reach of the shoes. About the only cure is improved cooling and you need specialist advice over this.

    Brake Cam Levers

    Lengthening these, as a means of loosening the hand pressure needed to achieve the same sort of brake pressure, should improve sensitivity but it is unlikely to measurably increase braking power. What it may also do is increase the likelihood of excessive lever free-play, or slop, at high temperatures.

    Changing brake shoe cam forms will not improve brake operation, at all. If you remain dissatisfied with the spongy feel of a brake following its refurbishment by an expert, then the fault lies in the operating mechanism. You may be surprised by inbuilt flexure in the train. Handlebar clamps, balance beams and brake plates can all bend, and cables can stretch and compress during use, as any honest Vincentee will admit. Most of these problems can be eliminated by reinforcement or exchange.

    The Brake Plate and Spoke Tensioning

    Debatably, while brake shoe material's high temperature efficiency may have declined a little in the past two decades, a means of redress may be found. Where once huge air scoops delighted us with their racy statement of speed, more recent knowledge has proved that they alone help minimally. The exit of hot air from a brake is more important than the entry of cool.

    Most scoops are incorrectly sited in areas of low pressure, ragged turbulence off the front tyre. And what does find its way into the brake drum then hangs around, hot and bothered. A well designed exit will create low pressure within the drum which will draw in cool air. In all probability a few expertly located holes in the brake plate, coupled to an exit bell-mouth will work best. But do consult a specialist before leaping in with the drill.

    Precise shoe operation is one of the keys to the whole business of good braking. The linkages of 2LS brakes need exceptionally close attention to ensure that activation of their shoes occurs with the sort of precision normally associated with twin contact breaker ignition timing. If one lining, or part of one lining, leads then the whole brake will feels spongy and unresponsive, as well as the overloaded section overheating and quickly passing this on.

    As for spoke tensioning; on wire wheels with spokes laced directly onto a flange around the brake drum itself, it is wise to have spoke tension and rim truth checked and corrected before attending to the drum itself. Although this is much less important with heavy grade cast aluminium drums, such as Norton's full width type which can withstand spoke tensioning without worthwhile distortion - but others are notoriously weak. Of the full width type perhaps none are so likely to follow the unequal tug of the spokes as BSA's full width brakes of the 1960s. A good brake specialist will advise you over the best wheelbuilder.

    Still Stymied?

    Ian Campbell of Classic Brake Services 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

    I made one of those Engineer's Clamps when I was an apprentice. Always wondered what they were for...

  • Rivetting stuff?


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