Author Topic: B29 Servo Action Brake Question  (Read 492 times)

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Offline cycarmark

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B29 Servo Action Brake Question
« on: 21 Aug 2019 at 00:27 »
I finally got a new kickstart gear for the B29 and was able to start the bike and take it for a short ride, here are the observations of a first ride on a Douglas:

The engine is a little hard to start, but runs nicely and is very smooth.  Much more so than my Triumph Twin or any of the singles (two and four stroke) that I have.

The gearbox is a little clunky but works well and is pretty quiet.

The primary chain rattles on the chaincase and will need some tweaking to get it quieter.

The brakes are horrible, which brings me to my question:  They are supposed to have a servo action, so I took off the front wheel to look at it, and it appears that the direction of the wheel rotation for the servo action would be with the bike going backwards.  Attached is a photo of the brake band hanging from the front fork, looking at it, and then the picture in the manual showing wheel rotation for the servo action, the bike would indeed need to go backwards.  Any thoughts on this are really appreciated, this one has me confused!

Offline eddie

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Re: B29 Servo Action Brake Question
« Reply #1 on: 21 Aug 2019 at 06:52 »
Depending on the model, some bikes had the brake on the LH side, and some on the right, so the brake bands were available for both clockwise and anti-clockwise rotation.
  Your brakeband can be converted to the opposite hand quite easily by knocking out the pivot pins from the bobbin. Turn the bobbin over (so that the boss is on the other side), then fit new pivot pins.

  Regards,
                 Eddie.

Offline Aero

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Re: B29 Servo Action Brake Question
« Reply #2 on: 21 Aug 2019 at 08:09 »
I had the same issue on my Aero, so turned the bobbin only to realise on refitting that the brake arm would then have to rotate in the opposite direction for the brake to expand. So basically you are stuck with what you have already. It seems some models are servo action & others aren't by design.

Offline cycarmark

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Re: B29 Servo Action Brake Question
« Reply #3 on: 21 Aug 2019 at 16:17 »
This morning I checked the rear brake band an it is indeed the opposite hand from the front one, so it does have the servo action.  I am glad Aero mentioned his experience with flipping the bobbin, it would not work on my front wheel as the brake arm is going in the wrong direction for the reversed bobbin.  Swapping the bobbin position would have worked on the rear as the brake arm is rotating in the proper direction.  At least when I had them both apart I was able to make some adjustments and the front brake has now improved to poor and the rear satisfactory.  Does the LDMC have replacement brake bands?  The rear one is looking somewhat worn.

Offline Doug

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Re: B29 Servo Action Brake Question
« Reply #4 on: 22 Aug 2019 at 17:17 »
The notorious Douglas 'servo' band brakes topic has come up before, as you can imagine. For the servo action to work the bobbin has to rotate in the same direction as the wheel. As the brake band engages the drum, it is pulled around with it and contributes torque to the bobbin. The 350 EW started out this way, but somewhere around 1929 Douglas reversed the brake lever from pointing rearward to pointing forward to defeat the servo-action.  Speculation is the front brake was prone to grabbing and possibly causing the dreaded front wheel skid.  For whatever reason, they left it anti-servo from then on.

The 600 EW had a forward facing brake arm from its introduction in 1927, so it was non-servo right from the start. As was the OC (1926); while it had the brake drum on the opposite side (right), the forward facing lever still rotates the bobbin in a direction opposite the brake drum. That is, traveling forward, when you most frequently require retardation!

Some rear brakes were still servo, unless the lever points down, then it too is anti-servo.

A slight advantage is also gained by having the end of the band directly attached to the bobbin as the leading edge, and the end attached via the link as the trailing edge. The trailing edge is trying to push the bobbin 'off', and the link causes that reaction to push on the bobbin at a smaller effective radius than the fixed end.

Article on the servo band brake.
https://www.douglasmotorcycles.net/index.php?topic=338.msg969#msg969

Relining 350EW brakes.
https://www.douglasmotorcycles.net/index.php?topic=2287.msg8267#msg8267

General comments.
https://www.douglasmotorcycles.net/index.php?topic=889.msg2907#msg2907
https://www.douglasmotorcycles.net/index.php?topic=1927.msg7072#msg7072

Brake arm and shaft.
https://www.douglasmotorcycles.net/index.php?topic=2440.msg8823#msg8823
https://www.douglasmotorcycles.net/index.php?topic=3431.msg12530#msg12530

-Doug

 

Offline cycarmark

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Re: B29 Servo Action Brake Question
« Reply #5 on: 24 Aug 2019 at 15:42 »
Thanks for the links, especially the one for the re-lining.  If I cannot find anyone in the US to do the work, I may consider sending it over.  There used to be a guy here in town who did brake and clutch relining, and had a building filled with old brake shoes and clutches, some to be done, some completed.  He did the brake shoes for my BSA M20, and when I went to pick them up, he quickly went to the appropriate pile and pulled them out, it was pretty amazing, he seemed to know where everything was.

 

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