Servo brake

wurly

Esteemed Pedelecer
Aug 2, 2008
501
9
Yeovil, Somerset
Hi everyone.
I had a slight brake problem with my electric folder. (For those that didn't know, i bought it from ebay and fitted my own battery).
When ever i applied the rear brake it would lock on and send me snaking down the road like Evel Kneival.....
When it warmed up a bit, the brake seemed ok but there was obviously somthing wrong. The chrome cover would also move quite a bit. At a quick glance i assumed it was some sort of band around a drum that was pulled tight.
I decided enough was enough and stripped the whole assembly down where i found a servo drum brake hiding there. It's a bit of a pain to get the wheel/hub out, so i thought i'd clean it and reset everything so i wouldn't have to dismantle it ever again.


I took some pictures as i went.



It also occurred to me that there must be quite a few ebikes with this design rear brake, but how many people know how they work or know how to adjust them?
My bike has never had the rear adjusted from new. How do i know this?
The adjustment screw has not been moved.....look at the gap in the first picture.



As you can see the insides were a bit mucky, the shiney part of the drum is where i wiped it clean. It seems like the linings were not actually 'bedded in' very well and obviously not working as it should.

The mechanism works like this, the big triangular shaped cam pushes the first lining toward the drum surface and the first lining also pushes the second lining via the round adjustment cam (to the right).



This picture will show that only the second lining is fixed, the actuator cam pushes the first lining (and the second lining) expanding outwards to wards the drum.

 

wurly

Esteemed Pedelecer
Aug 2, 2008
501
9
Yeovil, Somerset
I further dismantled it, and cleaned it up. Applied a little grease to the mechanism and re assembled it.





Put the brake and hub back on the bike and adjusted the slack on the second lining.
This is the reason why the brakes would lock. As the first lining touched the drum surface it would push the second lining outwards with such force the wheel locked up. The design is very similar to car drum brakes, hence the 'servo' name. The moving force on the first(primary/leading) lining acts as extra force applied to the second(trailing) lining. Adjustment is critical to the drum working(or not).




Adjustment screw where it should be.....
 
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wurly

Esteemed Pedelecer
Aug 2, 2008
501
9
Yeovil, Somerset
A quick test ride down the hill and i'm a happy ebiker.






Disclaimer: This is posting purely for those who have an interest in how things work and want to fix or maintain things. I wouldn't have spent my time writing and uploading pictures for any other reasons. If jus one person reads it and finds it slightly interesting , then great. I'm not interested in adolation, rewards or knighthoods.
I had a meccano set as a child.
 
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Synthman

Esteemed Pedelecer
Aug 31, 2010
417
0
Oxford
My bike has that on the rear. I've had similar problems as you, the brake grabs a lot for the first few minutes of a journey. I would open mine up but there's issues with tightening the wheel nuts afterwards.
 

grldtnr

Esteemed Pedelecer
Sep 22, 2012
627
288
south east Essex
The 'works' of that brake seem similar t the Sturmey hub brake, I have experience of these from my time as a bike mech for the Royal mail ,it is important to put just a dab of hub gear grease on the pivotiing surface of the cams to keep them lubed, the other problem I used t find was on the cheaper drum brakes used on the mail trolleys, there was a design defect,in that the wrong kind of metal was used in the brake pivot, this reacted with dissimilar metals used in the brake shoes ,corroded, then seized solid, only answer was to junk the brake and fit a new one.
 

neptune

Esteemed Pedelecer
Jan 30, 2012
1,743
353
Boston lincs
I found that to be a very interesting and educational article. Nice photos, too. The one thing I am not clear about is that adjusting screw, with its coil spring to stop it turning under vibration. I assume that it adjusts the gap between the two shoes. Regarding the bracket that this screw screws into, is it fixed to the backplate, or can the bracket slide a limited distance on the backplate?
The maintenance of drum brakes has always been an art in itself. On mechanical brakes a s opposed to hydraulics, they would often seize up on the cam spindle. The problem was that the spindle does not turn many degrees. Too little grease, and they seize. Too much and it contaminates the linings. The answer is/was little and often.
@Synthman. What is the issue or problem with the tightening of wheelnuts?
 

wurly

Esteemed Pedelecer
Aug 2, 2008
501
9
Yeovil, Somerset
Yes, the adjustment screw is for setting the gap for both shoes as they are joined at that point. The freeplay at the lever end is taken up by the spring on the cable (you can see it in the first picture).
I think it's always nice to know how the insides work so when you adjust the screw you have some idea of what happens, which i why i photographed it to save someone else the hassle of dismantling theirs (synthman):).
I like drum brakes actually. Most of my old cars have them, once set correctly, they work very well.
 
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Deano

Finding my (electric) wheels
Mar 3, 2012
13
0
Does anyone know where I can get new brake shoes, who is the manufacturer. Very good post BTW.
 

Brutus

Just Joined
Feb 26, 2014
2
0
66
Wurly. you instruct in your post `Put the brake and hub back on the bike and adjusted the slack on the second lining.` Is the adjustment made with the brake on? How can you tell weather you have taken up the slack on the second lining when you can not see the linings? Sorry if I`m being thick.
 

wurly

Esteemed Pedelecer
Aug 2, 2008
501
9
Yeovil, Somerset
This is an old thread. I had to read it all to remind myself how they worked.
You adjust it with the brake off. Spin the wheel and listen or feel if the linings are touching and then back it off a bit. If the wheel is dragging then it's too close. When you apply the brakes the first lining will touch the drum and push the second lining which(if adjusted correctly will already be fairly close to the drum) so both linings touch the drum almost together.
 
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Brutus

Just Joined
Feb 26, 2014
2
0
66
Wurly, thank you for taking the time to reply. I`ll will check my brakes this weekend.
 

stronc

Just Joined
Aug 29, 2018
3
0
Hi has anyone ever tried to refurb he motor for one of these. The one bearing is behind the hub brake and it doesn't want to unscrew. anyone ever take the brake drum off if so how
 

wheeliepete

Esteemed Pedelecer
Feb 28, 2016
2,047
755
60
Devon
I've removed these drums with a heavy duty chain wrench, normal R/H thread, so lefty loosey. You don't have to remove it to change the bearing, undo the axle nut, remove the motor core, then drift the bearing out of the casing.
 

stronc

Just Joined
Aug 29, 2018
3
0
thanks for the reply, the one side with the sprocket has water damage bearing and that I can drift out ok, the other one in the main hub has a shoulder on the inside and a threaded insert inside the band brake, so unless I remove that the bearing will not come out. I tried the shank of two drills in a vice to go in the brake band centre holes, but it just bent them, so was surprised its on so tight. I'll have to take some photos to explain.
Got two bikes cheap (cyclamatic) but they have been badly abused, spokes broken, motor connector cut off, pedal bottom brackets bearings disintegrated.
I managed to get new cheap custom spokes made, new cable/connectors, bottom bracket cups with seals, so just have to refurb the motor. fun project to work on, beats watching the tv, just wanted to make sure I wasn't going to break the motor hub trying to get the band off, as the internals look ok.
will put some photo's of it up to explain
 

wheeliepete

Esteemed Pedelecer
Feb 28, 2016
2,047
755
60
Devon
Does this look like your motor? Undo the nut, pull the motor core out from the other side. This will expose the inner bearing race. Drift out the bearing from the outside. The large thread that the brake hub screws onto is part of the motor housing.

 

stronc

Just Joined
Aug 29, 2018
3
0
Yes that's it, finally got the brake band off and yes you are correct it looks like the threaded section is part of the hub. I did as you said and yes success the bearing is out, thanks for every ones help, just have to wait for the new bearings to be delivered
 

orlbuzz

Just Joined
Dec 23, 2021
2
0
Wurly,
Greeting from the other side of the pond. I have searched extensively for a video or description of how to adjust these brakes. All of the ones that I have seen show or describe how to adjust the cable. I have this brake on a used Razor ecosmart Metro scooter I recently purchased. I wanted to have someone else's opinion of how to adjust the shoes. I recently had the rear wheel off to change the sprocket
to a larger size and decided to adjust the drum brake while I had it off. I did it a little differently than how you suggested. I, like Brutus, questioned how it could be done assembled, so this is what I did. I used an original brake adjustment tool, not as intended, but as a thickness gauge. (It seemed appropriate and it was in my toolbox!) I held the brake on with the tool inserted with one hand between the "drum" and the lining and adjusted the screw until the gap on the other lining was the same as the gap created by the brake adjustment tool.
I now have great brakes. When they seem to need adjustment again, I will try your method. It would be a lot easier than taking the wheel off!
I too am simply curious and like to how things work.
Thank you Wurly!

PS: Deano, "electricscooterparts.com" in the USA has the parts. Item # BRK-S90S-1 is for the linings, and Item # BRK-S9010 is for the complete assembly. You might consider the complete assembly if your "drum" portion is worn.
 

orlbuzz

Just Joined
Dec 23, 2021
2
0
Wurly,
Greeting from the other side of the pond. I have searched extensively for a video or description of how to adjust these brakes. All of the ones that I have seen show or describe how to adjust the cable. I have this brake on a used Razor ecosmart Metro scooter I recently purchased. I wanted to have someone else's opinion of how to adjust the shoes. I recently had the rear wheel off to change the sprocket
to a larger size and decided to adjust the drum brake while I had it off. I did it a little differently than how you suggested. I, like Brutus, questioned how it could be done assembled, so this is what I did. I used an original brake adjustment tool, not as intended, but as a thickness gauge. (It seemed appropriate and it was in my toolbox!) I held the brake on with the tool inserted with one hand between the "drum" and the lining and adjusted the screw until the gap on the other lining was the same as the gap created by the brake adjustment tool.
I now have great brakes. When they seem to need adjustment again, I will try your method. It would be a lot easier than taking the wheel off!
I too am simply curious and like to how things work.
Thank you Wurly!

PS: Deano, "electricscooterparts.com" in the USA has the parts. Item # BRK-S90S-1 is for the linings, and Item # BRK-S9010 is for the complete assembly. You might consider the complete assembly if your "drum" portion is worn.
I wanted to edit this, but it may be better to leave an addendum. I didn't grease my brake two days ago, and I needed to take the rear wheel out for another problem today, so I took the brake apart again. I see now how Wurly adjusted the screw with the brake assembled. The cup the adjustment screw rides against moves off the end of the screw when the brake is applied, so one can get an idea of when the second shoe is moving.
I realize this is a very old thread, but as I stated, I had seen no one else discussing this brake. I wanted a second opinion. I added my thoughts in case in one else like me was looking for information on this type of brake.
Happy Holidays!