Lowering the gearing on a Brompton

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2_i
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Re: Lowering the gearing on a Brompton

Post by 2_i »

The main slack that the Brompton tensioner needs to take is tied to the folding, with the triangle going underneath the rest of the bike, and not any cog differences. The folding yields a lot of slack since you move the points that stretch the chain close together. You realize how much extra slack is produced in practice when you try to put any standard tensioner on Brompton and find that it struggles compared to the original Brompton tensioner. However, that Brompton tensioner has significant excess capacity on top. When I put 20T chainring on Brompton, in addition to 50T, I found that the Brompton's tensioner took off nearly all that additional 30 link slack completely in riding and nearly completely in folding when the chain was on 20T - I needed to add a small hook for catching the chain and this took care of that folding issue. If I put any standard tensioner onto the Brompton, I basically could only fold with the chain on 50T.
nirakaro
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Re: Lowering the gearing on a Brompton

Post by nirakaro »

Ah, now I understand. Thank you.
Nigel
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Re: Lowering the gearing on a Brompton

Post by Nigel »

Another, expensive, option is to fit a Schlumpf bottom bracket. There are different versions available, which will give different gear combinations, much as a twin front ring setup.

A friend of mine had one on a Brompton 6-speed best part of 20 years ago. He was able to ride up alpine passes with it, etc..


- Nigel
Jdsk
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Re: Lowering the gearing on a Brompton

Post by Jdsk »

Nigel wrote: 29 May 2022, 9:07pm Another, expensive, option is to fit a Schlumpf bottom bracket. There are different versions available, which will give different gear combinations, much as a twin front ring setup.

A friend of mine had one on a Brompton 6-speed best part of 20 years ago. He was able to ride up alpine passes with it, etc..
Discussion from last week:
viewtopic.php?t=151201

Jonathan
nirakaro
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Re: Lowering the gearing on a Brompton

Post by nirakaro »

531colin wrote: 23 May 2022, 9:39am When Brompton were prepared to deal with Spa we regularly used to fit an additional smaller chainring on the crank side of the existing chainring, to be changed by hand.
Would I be right in thinking that to do that you have to lose the chainguard?
2_i
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Re: Lowering the gearing on a Brompton

Post by 2_i »

nirakaro wrote: 30 May 2022, 11:46amWould I be right in thinking that to do that you have to lose the chainguard?
1 vs 3 rings w/chainguard:

Image
Image
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Philip Benstead
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Re: Lowering the gearing on a Brompton

Post by Philip Benstead »

2_i wrote: 30 May 2022, 9:38pm
nirakaro wrote: 30 May 2022, 11:46amWould I be right in thinking that to do that you have to lose the chainguard?
1 vs 3 rings w/chainguard:

Image
Image
Can we see a full frontal side on view?
Philip Benstead | Life Member Former CTC Councillor/Trustee
Organizing events and representing cyclists' in southeast since 1988
Bikeability Instructor/Mechanic
2_i
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Re: Lowering the gearing on a Brompton

Post by 2_i »

Philip Benstead wrote: 30 May 2022, 9:41pm Can we see a full frontal side on view?
Sure

Image
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Philip Benstead
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Re: Lowering the gearing on a Brompton

Post by Philip Benstead »

2_i wrote: 30 May 2022, 11:32pm
Philip Benstead wrote: 30 May 2022, 9:41pm Can we see a full frontal side on view?
Sure

Image
Thanks

Questions

1 did you use a original brompton chainset?
2 If yes how did you cope with spacing for the other chainrings?
3 what is that piece of metal on your bottom jockey wheel?
Philip Benstead | Life Member Former CTC Councillor/Trustee
Organizing events and representing cyclists' in southeast since 1988
Bikeability Instructor/Mechanic
2_i
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Re: Lowering the gearing on a Brompton

Post by 2_i »

Philip Benstead wrote: 31 May 2022, 9:52am Questions

1 did you use a original brompton chainset?
2 If yes how did you cope with spacing for the other chainrings?
3 what is that piece of metal on your bottom jockey wheel?
1. On the left I use the original Brompton crank. On the right it is some Shimano that unusually went down to 20T. I may check details if necessary. TA Zephyr was another option. Obviously, many people do not need to go this low. BB continues to be 119mm, although it is not the original Brompton at this stage.

3. It is a hook preventing chain drop when riding into deep snow. When you ride on the small ring in front, which is the case practically all the time in rough winter, the tensioner arm points down and the jockey wheel accumulates snow when you ride into it. The hook prevents the chain climbing up onto the accumulating snow.
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531colin
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Re: Lowering the gearing on a Brompton

Post by 531colin »

nirakaro wrote: 30 May 2022, 11:46am
531colin wrote: 23 May 2022, 9:39am When Brompton were prepared to deal with Spa we regularly used to fit an additional smaller chainring on the crank side of the existing chainring, to be changed by hand.
Would I be right in thinking that to do that you have to lose the chainguard?
Yes, you would lose the chainguard if you fit an extra chainring on the crank side of the existing ring.
(But I can't remember whether the bikes had a chainguard then....its a long time ago!)
nirakaro
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Re: Lowering the gearing on a Brompton

Post by nirakaro »

Thanks indeed for all that helpful discussion. So, what I've done, is buy a pair of new 104 bcd cranks, and replaced the 50t Brompton chainset with 44t and 30t chainrings that I already had. This gives me – if I've done my sums right – a well-spaced range of 19 to 85 gear inches, which suits my purposes well. I've put the rings on normally, not 'inside-out', and all seems to ride, and fold, well. Now I'm just working on inventing a chain-guard.
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Philip Benstead
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Re: Lowering the gearing on a Brompton

Post by Philip Benstead »

nirakaro wrote: 2 Jun 2022, 1:05pm Thanks indeed for all that helpful discussion. So, what I've done, is buy a pair of new 104 bcd cranks, and replaced the 50t Brompton chainset with 44t and 30t chainrings that I already had. This gives me – if I've done my sums right – a well-spaced range of 19 to 85 gear inches, which suits my purposes well. I've put the rings on normally, not 'inside-out', and all seems to ride, and fold, well. Now I'm just working on inventing a chain-guard.
Can we have some photos from the side on and from above once it is all done?
Philip Benstead | Life Member Former CTC Councillor/Trustee
Organizing events and representing cyclists' in southeast since 1988
Bikeability Instructor/Mechanic
djnotts
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Re: Lowering the gearing on a Brompton

Post by djnotts »

nirakaro wrote: 2 Jun 2022, 1:05pm Thanks indeed for all that helpful discussion. So, what I've done, is buy a pair of new 104 bcd cranks, and replaced the 50t Brompton chainset with 44t and 30t chainrings that I already had. This gives me – if I've done my sums right – a well-spaced range of 19 to 85 gear inches, which suits my purposes well. I've put the rings on normally, not 'inside-out', and all seems to ride, and fold, well. Now I'm just working on inventing a chain-guard.
Simplest and cheapest is sometimes the best!
nirakaro
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Joined: 22 Dec 2007, 2:01am

Re: Lowering the gearing on a Brompton

Post by nirakaro »

Eventually a few photos as requested.
The only issue I still have is that the drive is quite noisy as the chainline's now all to cock. I've reversed the chainrings to small-outside/large-inside so that the usual large ring is better aligned, and I'll experiment with a shorter bottom bracket. Am I right in thinking that Bromptons take a standard 68mm bottom bracket? JIS or ISO?
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