Lowering the gearing on a Brompton
Re: Lowering the gearing on a Brompton
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.
Re: Lowering the gearing on a Brompton
Ah, now I understand. Thank you.
Re: Lowering the gearing on a Brompton
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
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
Re: Lowering the gearing on a Brompton
Discussion from last week: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..
viewtopic.php?t=151201
Jonathan
- Philip Benstead
- Posts: 1944
- Joined: 13 Jan 2007, 7:06pm
- Location: Victoria , London
Re: Lowering the gearing on a Brompton
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
Organizing events and representing cyclists' in southeast since 1988
Bikeability Instructor/Mechanic
- Philip Benstead
- Posts: 1944
- Joined: 13 Jan 2007, 7:06pm
- Location: Victoria , London
Re: Lowering the gearing on a Brompton
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
Organizing events and representing cyclists' in southeast since 1988
Bikeability Instructor/Mechanic
Re: Lowering the gearing on a Brompton
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.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?
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.
Re: Lowering the gearing on a Brompton
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!)
Bike fitting D.I.Y. .....http://wheel-easy.org.uk/wp-content/uploads/bike-set-up-2017a.pdf
Tracks in the Dales etc...http://www.flickr.com/photos/52358536@N06/collections/
Tracks in the Dales etc...http://www.flickr.com/photos/52358536@N06/collections/
Re: Lowering the gearing on a Brompton
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.
- Philip Benstead
- Posts: 1944
- Joined: 13 Jan 2007, 7:06pm
- Location: Victoria , London
Re: Lowering the gearing on a Brompton
Can we have some photos from the side on and from above once it is all done?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.
Philip Benstead | Life Member Former CTC Councillor/Trustee
Organizing events and representing cyclists' in southeast since 1988
Bikeability Instructor/Mechanic
Organizing events and representing cyclists' in southeast since 1988
Bikeability Instructor/Mechanic
Re: Lowering the gearing on a Brompton
Simplest and cheapest is sometimes the best!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.
Re: Lowering the gearing on a Brompton
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?
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?