Harley Davidson model "F" drum brake

*The Harley Davidson drum brake
The model "F" HD was one of the first motorbikes with a drum brake. A single drum was used on the rear wheel apparently because at the time a front brake was considered dangerous (nowadays we are used to almost perfect road surfaces but at the beginning of the century most of the roads were not surfaced at all and were very slippery even when it was not raining).Although the drum brake configuration applied by HD must have given a considerable improvement in braking efficiency when compared to the brakes then in use, judged by current standards it is not very effective, in particular when a sidecar is attached.


*Brake problems
The Model "F" drum brake has a foot-operated expanding inner brake band (see Figure 1), on some models combined with a foot- or hand-operated outer band brake (see pic 1).The drum itself is made out of steel plate, drawn in one-piece configuration for the early ones, later ones are of two-piece construction in which the drum is welded to the backplate. The material of the drums is rather thin (about 2 mm) which may be one of the reasons why they are not very long-lasting.The early brake drums are mounted on the hub by means of a toothed connection with rectangular teeth and a retaining ring screwed onto the hub. On later models the teeth are rounded-off (see pic 4).


If the bike is to pass the German TÜV (corresponding to the British MOT), then to be approved for road use the bike must have two independently controlled brakes, which must be effective too.However, looking at the original brake construction (see pic 1) it is immediately apparent that the higher the force on the actuating mechanism the higher the risk that the narrow and thin band supporting the brake material will distort with consequential reduction in brake efficiency. The original brake construction, provided the linings are in perfect condition, may suffice for solo-work with an experienced rider, but with a sidecar adaptations are certainly necessary to improve braking.

Some restorers provide a front brake to improve braking but, apart from detracting from the original appearance, the early front forks are not really capable of resisting the braking forces. Therefore, when a HD "F" model is regularly used on the road there is no other alternative but to improve the efficiency of the rear brake.This can be achieved by using current brake shoes which permit higher brake forces to be applied effectively but consequently, to avoid distortion of the original drum, a more substantial brake drum is then needed.A further problem is that original drums are very difficult to find (this situation might of course change) and moreover are rusted and/or worn (see pic 1 and 2) so that reclaiming and adapting them is normally out of the question. 



*Brake drum conversion considerations
In view of the problems referred to above a new drum construction based on a new or existing drum seemed the only way to arrive at a satisfactory solution.When looking for a replacement, it was the toothed connection to the wheel hub that in particular needed thorough consideration. The narrow teeth are small in height and are moreover provided on a small diameter of the hub, so that the resulting surface pressure on each tooth is high; for reasons of safety only steel can in fact be used in this area. However neither steel drums of the size needed nor cast iron ones (with enough material at the area of mounting the drum to the hub, so that an adaptation could be made) were found, mainly because the original drum has a concave backplate to give clearance to the spokes; all the available ones had flat rear sides and would therefore stick out too much when adapted to the HD hub.

There was therefore no other solution than to manufacture new brake drums myself.Cast iron was preferred because of its better properties concerning braking, and a small batch (10) of drums (I still have some available!) of the form approaching the original ones was cast (pic 3). The rough castings were machined on a lathe to the form shown in drawing 1.The toothed connecting part of the original brake drum was still in order and a large ring was cut out of the back of the original drum comprising the toothed hole (see pic 4 and 5). This ring was mounted in a recess machined in the back of the cast drum (see pic 6) and a connection was provided by means of hexagon socket head screws (the purist may use "home" made bolts) and an extra mounting ring (see pic 7 and 8); details of the construction may be seen in drawing 1. In this manner in fact an additional coupling was made between the iron brake drum and the steel ring at a larger diameter with larger load bearing surfaces which easily cope with the reaction forces during braking. Of course machining must be accurate to give a perfect fit since any play here would soon prove to be fatal (both to the brake and to the driver!).


*Brake shoes
Brake shoes can be found in all sorts and sizes at autojumbles and some (probably Japanese ones...) were found with the same outer diameter as the original inner band brake but of slightly larger width (see pic 9).When using brake shoes of larger width than the original brake band further adaptations become necessary, essentially a widening of the rear frame fork because the total hub width becomes larger. This results in further work ensuring alignment of the rear chain sprockets and in most cases an adjustment of the offset of the wheel rim with respect to the hub. In view of this extra work, which normally entails a complete stripping of the bike to provide the necessary alignment aids, the use of wider brake shoes is advised only when a complete restoration is undertaken, otherwise brake shoes with the same width as the original band brake should be selected.It is advisable in any case to check for the alignment of the sprocket wheels and rim because in each of the bikes I have restored there was a substantial deviation from the ideal line giving cause to noise and high sprocket wear. 

The new brake shoes were mounted on the original brake plate, ensuring perfect concentricity with the drum. Concentricity is essential and the best way to obtain proper lining up is to mount the new drum to the hub and, with the brake shoes provisionally mounted on the brake plate, to actuate the brake so that the linings are pressed against the inner surface of the drum and then to fix the turning point(s) of the brake shoes to the brake plate. Obviously, the inner diameter of the drum must be suitably adapted to the outer diameter of the brake shoes so that the whole surface of the brake lining is in contact with the drum when the brake is actuated!

A new outer band brake was made of a strip of spring (or stainless) steel and a strip of ferodo brake lining material glued to it.The original brake material had been rivetted to the strip (see pic 10). However, as well as causing distortion of the brake band, also making it very difficult to obtain proper contact over its entire surface, it proves impossible to give the brake band the required constant circular form; this is essential for the brake band to grip and for a constant clearance over its circumference with respect to the drum. Therefore, adhesively bonding of the lining is preferred. The strip and brake lining were wrapped around the drum, the latter being used as a support, with 1 mm carton strip inserted between the brake lining material and the drum to provide for the necessary clearance in the finished state and the assembly was baked in the kitchen oven for 1 hour at 180 C, following the adhesive manufacturer's instructions (and wife's permission).

(Clutch linings can also be bonded to their supports instead of being rivetted thereto, resulting in a much smoother and more progressive action of the clutch!)
Warning!!


The outer band brake is pulled back from the drum by means of 2 or 3 (later models) small springs acting against guiding hooks fixed to the brake plate (see pic 2 and original brake bands in pic 10). Originally, pins having the spring on them were rivetted to the brake band but, to avoid any distortion and provide easier assembly of the brake band to the brake plate, small nuts may be spot welded to the band to which the springs are mounted by means of screws. Another preferred mounting alternative of the spring supporting pins is the use of special nuts which are applied in pop-rivet manner to the brake band and which are obtainable from the fastener specialist (see pic 9, completed brake arrangement).

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