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Servo or Servile Brakes?

by Doug Kephart

So just what is servo action anyway? Servo brakes use a portion of the reaction from trying to retard the brake drum to further assist the application of the brake. Specifically in the Douglas servo expanding band brake used from about 1925 to the War, there is a tendency for the brake band to be pulled around with the drum when the brake is applied.

Both ends of the band are attached to the bobbin, one end directly and the other via a link. Rotation of the bobbin (1) pushes the ends of the band (2) apart, expanding it, and so bringing the friction material (3) in contact with the brake drum. One end, or the leading edge (4), tends to pull the bobbin further around, the other end, or trailing edge (5), pushes it back. If things were left at that, there would be no servo action.

But the other end of the band is connected to the bobbin by a link (6). This allows the designer to change the tangency point through which the reaction loads are fed back into the bobbin. So it can be arranged that the trailing edge can not exert quite as much push on the bobbin as the leading edge can exert pull. The difference in torque is indicated by (7) and (8). For the Douglas servo action to work, the bobbin has to rotate (9) in the same direction as the wheel (10).

Douglas motorcycle servo brakes

I thought I was rather clever to work out this universality, but I see it is printed in some of the handbooks too. This seems to be the case when it first appeared on the 350cc EW, but somewhere along the way this concept was discarded. Going by catalog illustrations (usual disclaimers), by 1929 the operating lever for the front brake of the 350cc is pointing forward. When lifted in operation it would cause the bobbin to rotate in a direction opposite to the wheel. (It would not matter which side of the wheel the brake was on.)

So any front brake with the lever pointing forward, is in fact not capable of servo action. All rear brakes with the lever pointing up (as many are that I know of) are capable of servo action. I think Douglas was well aware of this disparity, they always mention subtly in the literature that the servo band brake provides an "…exceedingly low pedal pressure". I am aware that back then a powerful front brake was considered dangerous and just a prelude to a skid. But also they should have been aware that if any brake needed servo assistance, one operated by hand would need it more than one operated by boot.

I do not have any pictures that show a definitive early 600 EW to tell which way its front brake lever points. However a November 1927 Motor Cycling road test of the 1928 600EW (F28 model) specifically states "The front brake control is now fitted the opposite way to that previously employed, and gives a genuine servo effect." Huh? The F28 model has the lever pointing to the front, and so is non-servo!

So why did they change the direction of the front brake lever? Was the brake too powerful for the lightweight front forks? I would not think so relative to the overall braking ability. The servo action should not have been all that much. Douglas states in their instruction manuals it is "…rather less than that necessary to overcome the effort of the various return springs." If this were not the case then once applied the brakes would 'lock on' until forward motion stopped. The rear wheel was servo acting, yet it was not prone to lock on that I have heard. It is true it had one additional return spring, but I would hardly think the brake design was so finely poised that it was the deciding factor. My guess is the brake tended to come on suddenly.

Once the clearance was taken up and greater resistance met it probably settled down into a nice progressive action so lauded in the testimonials. This tendency to grab would be less acceptable behavior in a front brake than the rear. As typical with many of Douglas's changes, they were made first on the big twins before trickling down to the smaller models.

Would anyone that has ridden a 350cc EW and say, a A31, care to write in their comparison of the two front brakes? They would need to be two machines in as equal condition and performance as possible to give equal footing to the test. Which is better, levers to the fore, or levers aft?

To confuse matters they also state in a Care and Maintenance book for the 350cc EW (printed after changes to the oiling system) "It will be found that the front brake will lock if it applied when the machine is moved backwards. Do not be tempted to reverse the action of the brakes or trouble will occur. They are servo acting and infinitely more powerful as the speed of the wheel increases." Well it sounds to me the servo action in that case was set to energize when the wheel is rotating the wrong way! This manual is undated, but I think it is still before they pointed the brake arm to the front. So I do not know what to make of the above statement.

One problem with servo brakes is the more effective the servo action is in assisting application of the brakes in one direction, the more effective it is in releasing them going the opposite direction. Douglas mentions this in their patent; "As under certain circumstances the braking effect might be too fierce, or, alternately, ineffective in holding the vehicle against backward movement…" The need to balance the design for adequate fore and aft braking action depending on the application was recognized. Not an overwhelming requirement in a motorcycle, but the patent was intended to cover automotive vehicles as well.

My father had a chance in the sixties to test drive a friend's 1928 six and a half liter Hispano Suiza (series B) that had been shortened into a 'hill climb' car. Hissos had a brake drum on the rear side of the transmission that when applied boosted the pedal pressure applied to the four road wheel brakes; or a servo boost action. My father described the brakes on that car the best mechanical brakes he even tried, and better than some hydraulic ones too. But if that car started to coast backwards down a hill, you could not push down on the brake pedal hard enough, even with both feet. To stop the car in reverse one had to in addition use the handbrake.

Even a twin leading shoe brake suffers from this. The twin leading edges of the brakes shoes that so aid forward retardation turn into twin trailing edges when the wheel is turning the other way. My 1957 Velocette Venom with its Tickle pattern front twin leading stopper requires a noticeably harder squeeze on the front brake lever to hold the bike from drifting back on an incline than when pulling up to a normal stop.

Back to the subject of patents, there is the mystery of Douglas's patent 256,758 Improvement in Vehicle Brakes. This was applied for July 10, 1925, and granted August 6, 1926 to Douglas Motors Ltd. and Cyril George Pullin. The illustration for this shows the brake band and bobbin in the off and on positions, typical of what you see in the Care and Maintenance booklets. The problem is the rotation of the brake drum is shown going the wrong way! The end of the brake band attached directly to the bobbin is shown as the trailing edge, the opposite that shown in every service manual I have seen so far. At first I thought it was just an error in the illustration, but on reading the claim there is no mistake. That is the way they wanted to depict it, and they describe it so in great detail and care. Sorry, that is not a servo brake! So what is it, a red herring to confuse would be patent infringements? Or did the person writing up the patent just misunderstand and get it backwards?

One item added to the patent claim that I have not seen done in practice is to provide multiple holes in the bobbin to rivet the link to, so allowing a variation in the effective servo action. Practically this would have been undesirable, the closely spaced holes would weaken the flanges of the bobbin. It also would have been unnecessary once the proper amount of desirable servo was determined. The other portion of the claim is transferring of the reaction loads directly into the frame of the vehicle, via the spindle of the brake operation arm. This is why the brake backing plate can be so lightly constructed.

Probably the best thing to improve the performance of the band brake is to get the entire friction lining in equal contact with the drum when the brake is applied. This means careful adjustment of the set screws on the backing plate to produce a minimum, and uniform, clearance between the brake drum and the friction material. The band needs to be nice and circular too. To some extent this can be jockeyed around with the set screws. Minor deviations can be corrected by sanding the lining to compensate, though with the cost of friction material these days it offends the Scottish parsimony of clan Douglas. But I suppose it is better to sand off two thirds the life of the linings and have good brakes for those few remaining miles.

An irregular band will conform to the shape of the drum given you apply the brake hard enough. But if you rely on this phenomenon to obtain full contact, you will have spongy brakes. Also you will need to run more clearance between the friction material and the drum to prevent the high spots from dragging. Consequently it will require more travel of the brake mechanism before the brake is fully on. So if the band is not very circular it needs to be corrected before the lining material is applied. Do not use the attachment of the lining to force the band into a shape it does not want to be. You will create unequal stresses in the band, which will result in unequal pressure being applied to the drum over the length of the band. Some areas will not end up doing their share of the work, others too much with consequently accelerated wear.

I put molded lining material the VMCC was selling at one time on the brake bands for my F28 Douglas. This was a graphite gray material with white filaments, sort of the texture of linoleum flooring. I mounted the backing plate, with brake band in place, in the lathe (not in the good lathe!) The set screws and bobbin were adjusted to slightly hold the brake in the 'on' position and locked in place. Then a light skim cut taken at an eight-inch diameter across the linings. Further adjustments were made; increasing the amount the brake was 'on' until the same eight-inch skim cut just cleaned up the entire surface. The material in question machined rather well, surprisingly. Each set screw was then backed off the same slight amount for the running clearance. Then when the brake was applied and the band expanded, it formed a perfect eight-inch diameter match to the brake drum. Of course you need to measure the brake drum and verify its diameter, and turn the linings to suit. If you can fully use the advertised twenty-five square inches of braking surface, you have little need of servo action.

The latest batch of lining I purchased is a green woven material (from Safetec.) I doubt I would be able to successfully turn this woven material as described above. It could be ground instead, as was done on many old brake shoe machines. But the dust would be horrendous. As well as dangerous if it were an older material that still had asbestos. I am sure even the new asbestos free stuff could be quite a pulmonary irritant.

One issue that recently came up was that of bonding the lining to the brake band. The lining in this case was molded. It was felt by the supplier that since it lacked the internal reinforcing of the woven material it may tear out where countersunk for the rivets. So it was both glued and riveted on, but this turned out to be a mistake. It was glued in a curve that did not match the brake drum, the band being in its relaxed state at the time. When flexed it cracked through the lining. It probably would have worked if glued up while sitting in the brake drum, so as to minimize the range it needs to be flexed through.

© 2002 D. Kephart

 

 


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