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M/C Mechanics - HOW2:

Overhaul a Drum Brake / Replace Drum Brake Shoes

The internal expanding drum brake is one of the most elegant braking arrangements ever conceived. Often maligned as being 'old fashioned' or 'weedy', they are by modern standards not as 'sophisticated' or ultimately as powerful as the modern hydraulic disk brake, BUT where fitted as standard, they usually do the job pretty well, if in good condition and properly adjusted, some-times, better, And they are DEAD easy to work on, and usually very 'cheap'

This brake is from the rear of Donna's CB125 Super-Dream, its pretty conventional, a 4" diameter 'single leading shoe' drum brake, operated by a rod direct from the brake pedal. It's still a commonly used arrangement for lightweight motorcycles today, and was common on both front and rear of motorcycles for many years.

Variations on the arrangement exist, and more sophisticated 'Twin-Leading Shoe' drums were once used on higher performance motorcycles of the 1950's, 60's & 70's though these fell out of favour to hydraulic disks.

But anyway, lets get on with it.& the first job is to remove the wheel, you want to service the brake in. So quick mention of 'torque restraints'.

First pic is the front brake on my Montesa Cota, which has a tie bar between the fork leg and the brake back plate to stop the back plate rotating with the wheel when the brake is applied. This is the same sort of arrangement conventionally used on rear brakes, between the brake back plate and the swing-arm, as shown in the last pic, on Donna's CB125. To remove the brake assembly these tie bars need to be unbolted to allow the back-plate (& sometimes the wheel) to be removed from the bike. Two middle pictures are of my DT125, which has 'lug' restraints, the brake back-plate located by a slot locating on a lug on the fork leg, for the front brake, on the swing arm for the rear brake. These should allow the back plate to just slide out with the wheel when the wheel spindle is removed.

When removing the wheel, pay attension to what is connected to it! One thing that will have to be unfastened is the brake cable. The inner cable is conventionally attached via nipple to the brake actuation arm and needs to be unclipped from its cup. Here on the Yamaha DT, the outer-cable is seated in a slotted adjustor on the brake back-plate, and after unclipping the inner, the adjuster can be twisted to line up the slots and let the cable be 'popped' out. On this bike, the Speedo-Drive is also housed in the brake back-plate, and the Speedo cable will need to be unclipped. On other bikes, the outer brake cable may be located elsewhere. For instance on the Montesa its located on a lug on the mudguard stay. Similarly the Speedo-drive is a separate part, in a 'spacer' on the front wheel spindle. Arrangements vary, and while common that speedo-s are driven off the front wheel, not always so. Many old Brit-Bikes have rear wheel driven speedo's with similar arrangements.

So on with stripping down the brake assembly

With the brake back plate removed from the wheel & bike, Here Dona uses a pair of pliers to remove a split pin from the brake-shoe pivot to allow the securing washer to be removed, and allow her to carefully lever the shoes off the back-plate.

The Actuation arm can then be unbolted from the brake-cam shaft and the shaft withdrawn from the brake back-plate. These parts can now be cleaned. (in the case of My DT, I took the opportunity to have the back-plate powder coated. Donna, painted hers with high-temperature 'matt' black BBQ paint to match the front brake caliper on her Super-Dream)

With the back-plate cleaned and if desired painted, the actuation cam-shaft, and the bush it sits in in the back-plate need to be inspected for signs of wear, but can then be greased and re-assembled. Pay particular attention to greasing (with brake grease or Coppa-Slip) to the shaft & bush, the cam-faces and the brake shoe pivot.

This is where many people struggle, trying to fit new brake shoes, getting the shoes on the cam and pivots and connected by their return springs. The trick is to pair the shoes together, ON their springs first. In this case I have clocked up! Note that end closest, top shoe has circle to mate to shoe-pivot, while lower has 'flat' to bear on cam! Take HEED, pivot cup to pivot cup, flat to flat!

Then with the shoes still 'side by side' they can be dropped onto the back-plate with the cam & pivot between them, and levered easily into place, simply pressing them apart like opening a book. Hard part DONE. Shoes, if retained by washer or split-pin can now be locked in place, and the brake actuation arm re-fitted to the cam-spindle, the brake is 90% overhauled.

The drum itself requires little attention, other than to clean, and possibly de-glaze the iron lining with a little medium grit sand-paper, checking that the wear indicator groves are still present and the lining not worn away. If the brake 'judders' the drum may be out of round, in which case you will probably need to take it to a machine-shop or engineers to be checked and possibly trued.

Otherwise, oil the cable and pivot of the handle-bar lever, put the back plate back in the drum, and put the wheel back on the bike. JOB DONE!

 

 

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