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Old 11-02-13, 12:19 PM   #1
seniordesign
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Opinions on a retrofit design

Hello, I'm a senior mechanical engineering student considering designing a retrofit kit for Brompton 2-speed folding bicycles that would allow the user to have kickback shifting ability. Shifting would feel similar to that of a kickback hub, but the bike modifications wouldn't add any additional mass to the bike. The retrofit would include removing the index shifter and associated cable, and I've been considering adding a coaster brake to remove the need for the rear brake cable. Is this something that anyone would be interested in, what are your positive/negative thoughts? I greatly appreciate any feedback
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Old 11-02-13, 12:32 PM   #2
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Talk to your faculty adviser , you are paying them . In general It's a DNF .

and you must not be grasping some really basic mechanical principals.
should have gotten that by the freshman year. what did they teach you?

N.1) the thing wont fold any more, ..

Buy a Bike Friday Tikit instead , the BB is part of the rear section.
and in their seasons Tikit and Belt drive versions, you can pull the rear axle Back.
by their dropout design.

and will work with both drive and return sides of the chain, somewhat tensioned ,
which is required for a coaster brake .

Last edited by fietsbob; 11-02-13 at 12:37 PM.
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Old 11-03-13, 12:06 PM   #3
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Talk to your faculty adviser , you are paying them . In general It's a DNF .

and you must not be grasping some really basic mechanical principals.
should have gotten that by the freshman year. what did they teach you?

N.1) the thing wont fold any more, ..

Buy a Bike Friday Tikit instead , the BB is part of the rear section.
and in their seasons Tikit and Belt drive versions, you can pull the rear axle Back.
by their dropout design.

and will work with both drive and return sides of the chain, somewhat tensioned ,
which is required for a coaster brake .
Can you clean this up please? I have no idea what you are saying.
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Old 11-03-13, 12:45 PM   #4
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You are barking up the wrong tree.

proposing to ruin a well designed folding bike, by destroying it's ability to fold.
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Old 11-03-13, 12:47 PM   #5
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Can you clean this up please? I have no idea what you are saying.
haha.

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Old 11-03-13, 01:13 PM   #6
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by destroying it's ability to fold.
This is not my intention. My design is basically a modification of the jockey axle that will result in a smooth gear change upon a predefined reverse rotation of the crankshaft by the user. The derailleur cable will no longer be necessary but other than that I'm not modifying any functions of the bike.
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Old 11-03-13, 01:53 PM   #7
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Sorry Grasshopper.. Need chain slack to fold the rear , and the pivot for the fold is behind the BB.

I had to accept the Rohloff chain tensioner on my Bike Friday Pocket LLama
because it too has a fold pivot between the BB and the rear wheel.

Show Us all your working Prototype.
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Old 11-03-13, 02:16 PM   #8
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Need chain slack to fold the rear , and the pivot for the fold is behind the BB.
I don't see how this applies. Here is the Brompton derailleur:


The part outlined in red moves the jockey between the two speeds. I will be removing this and modifying parts of the black plastic assembly without affecting the operation of the chain tension arm.
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File Type: jpg brompton-chain-tensioner-assembly.jpg (75.4 KB, 9 views)
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File Type: jpg bboption17.jpg (54.0 KB, 11 views)
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Old 11-03-13, 02:32 PM   #9
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I have the 3 speed M3L, then the chain tensioner pulleys dont have sideways float.
[shift fork shoves a pulley sideways on the 3 by 2]
given I dont need to go beween 2 cogs I have a bit shorter chainn my Brompton.

4" behind the BB shell is where the rear section fold axis is ,, the rear hub axle is 12-9/16 behind that

show how you will fold that with a tight chain, lad.

Have you bought the Brompton bike and tested this for function? lets see it.
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Old 11-03-13, 02:49 PM   #10
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Are you saying that the chain tension will increase if I increase the jockey diameter to accommodate my design? I'm not sure yet if that is necessary, but if it is would the addition of a few extra chain links really be that bad? I have not built a prototype as of now, i'm trying to gather some sense of what experienced bike users think of the idea before moving forward. I'll be sure to post it once it is finished.
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Old 11-03-13, 04:17 PM   #11
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no Im saying the chain tension line for backpedaling is the bottom half.
It has to be a straight line just like the top

the force applied to panic stop a coaster brake,will break most any Idler used
deviating from that straight line.

Last edited by fietsbob; 11-04-13 at 06:20 PM.
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Old 11-03-13, 04:53 PM   #12
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seniordesign, closely watch some videos of the Brompton bike folding procedure on YouTube. Watch what the chain tensioner does. You need the tensioner in order to accommodate the fold. However, as fietsbob has been pointing out; you really are NOT going to want to use a coaster brake hub (or kick back shifting hub) on any bike that is using a chain tensioner, as it's very ineffective and dangerous. Hence some of the earlier comments in this thread regarding the bottom bracket being behind or in front of the fold.

Regardless of the folding part of the equation, go test this yourself before moving forward in this project with any simple coaster brake bike. (Pick up a $5 garage sale kids junker bike even!) Add a tensioner (slap on a rear derailleur even just to act as a tensioner) and then {CAREFULLY** ride the bike forward and use the brakes. Watch what happens (and doesn't happen).

With all due respect.... please start experimenting with these bicycle engineering basics before you attempt to re-engineer them!

(A better mouse trap is not one that catches the mouse and also destroys the whole room.)
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Old 11-03-13, 05:23 PM   #13
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I'm sure I've seen successful kickback and coaster hub Brompton builds on this forum.
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Old 11-03-13, 09:31 PM   #14
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I'm sure I've seen successful kickback and coaster hub Brompton builds on this forum.
here's one anecdote: http://www.bikeforums.net/archive/in.../t-721778.html
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Will the coaster brake work with the chain tensioner?
Yes it does because I have tried it. Its not perfect but works ok. The fold is not compromised and the coaster brake works fine. I am told it is better to use the tensioner on the two speed changer whereas I used the one from my three speed.
I appreciate you pointing this out though, I'm going to investigate using the model I'm using for testing.
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Old 11-03-13, 09:47 PM   #15
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Good to know! Thanks for the link.
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Old 11-04-13, 11:18 AM   #16
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now if we could engineer a gadget which would eliminate the kickback on some internal hubs ....

ohh did I say that ?
Seems to me that some engineering is going the wrong way, making something , nobody needs or wants, maybe makes the bike dangerous....

:-) Just sayin
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Old 11-04-13, 02:38 PM   #17
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now if we could engineer a gadget which would eliminate the kickback on some internal hubs ....

ohh did I say that ?
Seems to me that some engineering is going the wrong way, making something , nobody needs or wants, maybe makes the bike dangerous....

:-) Just sayin
Because you wouldn't buy a kickback hub that implies that nobody needs or wants it? Thanks for your speculation but i'm looking for more constructive feedback.
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Old 11-04-13, 02:58 PM   #18
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Because you wouldn't buy a kickback hub that implies that nobody needs or wants it? Thanks for your speculation but i'm looking for more constructive feedback.
I've ridden both Sachs and S/A kickback hubs with coaster brake... what I found annoying was the use in city riding where you actuate the rear brake frequently, and consequently shift between high and low (whether you want to or not) and sometimes leaving yourself in high gear from a stop sign ( or slow down) when you would have preferred low range .. for the past few years I have been running the Automatic version (with coaster brake) of the Duomatic hub, and like it enormously ... using the two cog version of the Brompton, if you wanted to use kickback shifting, and appeal to a possibly wider audience, I'd retain the use of the caliper brake..
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Old 11-04-13, 03:17 PM   #19
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Because you wouldn't buy a kickback hub that implies that nobody needs or wants it? Thanks for your speculation but i'm looking for more constructive feedback.
unfortunately its not speculation, but knowledge what bikes I am selling and what bikes I am not selling. That is somewhat very different than your speculation that Bromptonriders want something like this.
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Old 11-04-13, 05:43 PM   #20
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using the two cog version of the Brompton, if you wanted to use kickback shifting, and appeal to a possibly wider audience, I'd retain the use of the caliper brake..
I think I agree with you at this point, the coaster brake was honestly a secondary part of this idea I didn't realize it would be such a point of contention. Your concerns about the kickback are exactly what I'm looking for, thanks.

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unfortunately its not speculation, but knowledge what bikes I am selling and what bikes I am not selling. That is somewhat very different than your speculation that Bromptonriders want something like this.
I apologize, I didn't realize you were speaking for a wider audience. I'm in the tough position of trying to look at this complex market and 'solve a problem' in a limited amount of time, there is really no way for me not to look like i'm barging in here proclaiming that I've improved the Brompton after a few days of research. I'd just like to get this made for some design and prototyping experience, if the market niche i'm in is tiny that's alright. I really appreciate any feedback that helps me determine if the design is viable.
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