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Is there a Brompton style frame for 20 inch (406) wheels?

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Is there a Brompton style frame for 20 inch (406) wheels?

Old 02-11-22, 02:25 PM
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Originally Posted by Jipe
Since Birdy III there is a totally new concept of tensioner dedicated to the tensioning of the chain while the bike is folded, the derailleur or a Shimano or a Rohloff chain tensioner being used to tension the chain while riding.

This new concept is a parallelogram type arm attached around the bottom bracket box of the frame. It works perfectly.

The same concept could work on a Brompton with rear derailleur.
Thanks! This tensioner looks really good. The price is high though. In fact, that tensioner could be even desired with what I have now, as the Brompton one works at the edge of what it can do already in my current setup. After agonizing for a while, it will presumably end badly, i.e., spending money without a fully compelling reason .
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Old 02-12-22, 04:15 AM
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Originally Posted by 2_i
There is a thread here on the primary development. The essential part is a triple crankset 50-34-20. A Suntour derailleur is mounted on a Litepro adapter, both highly tweaked. It is operated by a SunRace shifter. The drivetrain modification fits into the regular Brompton fold. The added low gears are needed, in my case, for winter and for a mountainous surrounding I take my bike to.


In what way do you think your bike allowed you to expand the gear range better compared than Brompton? (I admire your bike anyway, of course .) How good is the standard derailleur in gathering slack, when you have the two rings in the front? When I tried to put a standard derailleur onto Brompton, I found its tensioner to work marginally in the needed gathering of slack, compared to the standard Brompton tensioner. I tried chain catcher, as in Birdy, but it was largely leading to nowhere. Humbled, I went back to the Brompton pusher arrangement
Thanks for the link to your triple chain wheel thread. Very interesting. Congratulations on your tenacity getting it to work successfully.

I didn't really think my bike would expand the gear range better than the Brompton. I didn't develop the bike that way. What I was aiming for was to combine what I considered to be the best features of a Brompton that I previously owned with the best features of a 20" wheeled Dahon I also owned. Being happy with the gear range on the Dahon, I simply wanted to use those wheels with a Brompton chainset for the first build. That set up was lacking a bit at the top end, so in the second build I used an 11-36 T cassette with 43/61 chain rings which yielded a range of 22-102 gear inches which was acceptably close to my target of 22-110 inches.

The standard derailleur was completely inadequate in gathering the slack during folding - no surprise there. I mounted the derailleur on a pivoting plate which by means of a tension spring took up the slack during folding very well. I should mention that this set up still isn't ideal because I had to use a long cage derailleur to accommodate the large tooth range so the bottom jockey wheel has taken a few hits and debris is collected by the chain very easily. Nothing broken or worn out so far though .
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Old 02-12-22, 06:11 AM
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I have the same problem to extend the 8-32 cassette range on my Birdy on which I do not want a double chainring: medium cage derailleur go up to 32t or 34t max for the latest 11s Shimano Ultegra RD-8000gs and 105 RD-7000gs. But I read that these derailleur can go up to 36t.
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Old 02-12-22, 02:02 PM
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Originally Posted by Jonesandrew
The standard derailleur was completely inadequate in gathering the slack during folding - no surprise there. I mounted the derailleur on a pivoting plate which by means of a tension spring took up the slack during folding very well. I should mention that this set up still isn't ideal because I had to use a long cage derailleur to accommodate the large tooth range so the bottom jockey wheel has taken a few hits and debris is collected by the chain very easily. Nothing broken or worn out so far though .
The Brompton tensioner struggles in winter too in my setup, due to its low position during riding. Basically, in the original Brompton setup, that tensioner is not very active during riding, as the slack changes then very little. It mostly comes into play during folding. However, in my current setup, the tensioner must gather a lot of slack when I shift to the smallest ring and it positions itself near the ground. When I ride into a pile of snow, it is easy for the chain to go off the tensioner. Usually you can still ride, just the chain starts skipping - I learnt to recognize this as a sign that the chain went off the tensioner. Come to think of it, I may need to come up with some brace around the tensioner wheel, somewhat reminiscent of the Birdy chain catcher, to prevent the chain from going off.
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Old 03-04-22, 09:51 AM
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Originally Posted by Jipe
The Birdy catcher, a kind of extension of the derailleur cage, wasn't working well neither on the Birdy. The chain was often falling of the chainring when unfolding the bike.

Since Birdy III there is a totally new concept of tensioner dedicated to the tensioning of the chain while the bike is folded, the derailleur or a Shimano or a Rohloff chain tensioner being used to tension the chain while riding.

This new concept is a parallelogram type arm attached around the bottom bracket box of the frame. It works perfectly.

The same concept could work on a Brompton with rear derailleur.
What about the Kamoya Birdy Cheing, that seems to be similar in concept to the Birdy III BB tensioner? Is there any experience with the Kamoya among the owners of earlier Birdy models? I ordered that Birdy III tensioner, but it may be stuck in the mail for months. I expect that it may need to be modified to work with my Brompton and I may also go with some construction of my own, using the original Birdy tensioner only as an inspiration, hence my poking around.
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Old 03-04-22, 04:37 PM
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Where did you bought the Birdy III chain tensioner ?
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Old 03-04-22, 07:31 PM
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Originally Posted by Jipe
Where did you bought the Birdy III chain tensioner ?
Bikegang - that seemed the safest option.
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Old 03-08-22, 08:27 PM
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Originally Posted by Jipe
Since Birdy III there is a totally new concept of tensioner dedicated to the tensioning of the chain while the bike is folded, the derailleur or a Shimano or a Rohloff chain tensioner being used to tension the chain while riding.

This new concept is a parallelogram type arm attached around the bottom bracket box of the frame. It works perfectly.

The same concept could work on a Brompton with rear derailleur.
The Birdy III tensioner came in and it fails to mount over the Brompton BB shell by a wide margin - not enough space. I thought that there could be interference of the mounting clamp with the brazing material flowing out, but, if nothing special is done, there is even interference with the bridge to the triangle. I suspect that there could be some options there, such as to attach the parallelogram arms to some BB mount adapter, of the type that got popular now, rather than to the clamp. Overall this would be turning itself into a longer-term project, nothing quick, and maybe not worth the hassle relative to the expected outcome.

Last edited by 2_i; 03-08-22 at 11:19 PM.
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Old 03-09-22, 12:38 AM
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Originally Posted by 2_i
The Birdy III tensioner came in and it fails to mount over the Brompton BB shell by a wide margin - not enough space. I thought that there could be interference of the mounting clamp with the brazing material flowing out, but, if nothing special is done, there is even interference with the bridge to the triangle. I suspect that there could be some options there, such as to attach the parallelogram arms to some BB mount adapter, of the type that got popular now, rather than to the clamp. Overall this would be turning itself into a longer-term project, nothing quick, and maybe not worth the hassle relative to the expected outcome.
Very interesting information, thanks for trying and sharing experience.

Could it work when changing the bottom bracket for one with external bearings and mounting the tensionner on this external bearing ?

Anyway, the bottom bracket box of the Birdy is different, made to let space to mount the tensionner besides an external bearing shell.
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Old 03-09-22, 05:53 AM
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Originally Posted by 2_i
The Brompton tensioner struggles in winter too in my setup, due to its low position during riding.
But do you really need an 818% range? Wouldn't a double chainring be good enough, wthout that issue with the tensioner?

A six-speeder + 50/34 chairing = 443%

And a Nexus 8 + 50/34 + 16T cog + = 452%

Hub Gear Calculator
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Old 03-09-22, 07:48 AM
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Originally Posted by Winfried
But do you really need an 818% range? Wouldn't a double chainring be good enough, wthout that issue with the tensioner?

A six-speeder + 50/34 chairing = 443%

And a Nexus 8 + 50/34 + 16T cog + = 452%

Hub Gear Calculator
Yes, I absolutely need the low gears. I even changed from 50/30+/24 to 50/34/20, in my memory, because I needed still lower gears. I might do with 22T as the smallest ring presumably though. I need the lowest gears for mountains and for winter. At times in winter I would ride exclusively on the 20T in front.

Originally Posted by Jipe
Very interesting information, thanks for trying and sharing experience.

Could it work when changing the bottom bracket for one with external bearings and mounting the tensionner on this external bearing ?

Anyway, the bottom bracket box of the Birdy is different, made to let space to mount the tensionner besides an external bearing shell.
I think one could use a BB in Brompton with the lip of the BB cup filed down to a diameter that matches that of the shell + spacer(s) that also match the shell diameter + Dremel removal of brazing material. However, I found another product on AliExpress that goes under the lip of the BB cup, more akin to the Kamoya solution without the spring mechanism. It is on order and I will report on how it works.

However, overall, I think the problem has 3 levels:

A. Gathering moderate amount of extra slack for folded bike when one either puts the standard derailleur in the rear or adds a second ring in the front.

B. Gathering large amount of extra slack for folded bike when one puts either 3 rings in the front or a combination of 2 rings and a standard derailleur in the rear.

C. Gathering extra slack for a bicycle in riding, to eliminate tensioner or derailleur extended so that it rides close to the ground.

I think that A can be solved with some contraption along the lines of what is commercially available. I see no easy solutions for B. The most important for me is C, as I repeatedly drop the chain when riding the Brompton into a pile of snow when I am on the smallest ring in the front. Solving C would require an extra tensioner/idler with a wheel on the inner side of the chain, not outer. My interest in the potential solutions to A/B has been in their potential conversion for use in C. At present I see no solution for C, either.
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Old 03-09-22, 10:49 AM
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If you want a Brompton with a large range of gear inches, the simple solution is to put a Rohloff on the Brompton with a 54x13 transmission, it gives 19” to 102” (1.54m to 8.11m), that's what I have, with 1m54 I can climb any hill with it and with 8m11 I can really ride fast.

And if keeping the original Brompton titanium rear triangle + some titanium parts on the Rohloff (13t cog, replacing original steel bolts...), the weight is almost the same as a BWR 6s (150g more).
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Old 03-09-22, 12:32 PM
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Originally Posted by Jipe
If you want a Brompton with a large range of gear inches, the simple solution is to put a Rohloff on the Brompton with a 54x13 transmission, it gives 19 to 102 (1.54m to 8.11m), that's what I have, with 1m54 I can climb any hill with it and with 8m11 I can really ride fast.

And if keeping the original Brompton titanium rear triangle + some titanium parts on the Rohloff (13t cog, replacing original steel bolts...), the weight is almost the same as a BWR 6s (150g more).
1.54m would be the 4th gear from the bottom in my current setup. I need the second from the bottom to get on my way to the street from my mountainous work location, followed briefly by the first, to conquer a sudden increase in the slope midway . This is not exceptional for the situation there. If Schlumpf/ATF finally came out with a non-chamfer version of their mountain drive, I might jump. In the meantime, I think I see an opening to progress on B and C above, once the ordered part comes in. In tests where I intentionally rode into a pile of snow, I noticed that even a modest reduction of slack had a noticeable positive impact on the chain staying put.
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Old 03-10-22, 01:16 PM
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Originally Posted by 2_i
If Schlumpf/ATF finally came out with a non-chamfer version of their mountain drive, I might jump.
Apparently, it's gone but was available at some point

https://www.bikegang.co.uk/bikegangh...e-HM-p86744203

https://www.bikegang.co.uk/bikegangh...wan-p132002807
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Old 03-10-22, 01:30 PM
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Originally Posted by Winfried
Thanks, I emailed Bikegang about it in recent past, but have yet to get a response. There was some confusing contradictory info on their site about not shipping Schlumpf/ATF to US/EU. I guess this might have to do with limits on competition, but I saw no other non-chamfer offers, at least for the Mountain Drive and US.
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