# Brompton Double Chainring



## BromptonChrispy (6 Aug 2018)

Thanks to the contributors who helped me with a simple but very effective modification, particularly
Fab Foodie, The Doctor, and Pale Rider.

As I had no "bits-box chain-sets" I bought a new Stronglight 50/34 with 170mm cranks - same as the Brompton stock on my 2014 S6L with JIS taper BB. 

Cheapest new (read "non scrap useable") I found was from Cycle Clinic £70, free quick delivery though.
https://thecycleclinic.co.uk/produc...mpact-double-chainset-9-10-speed-square-taper 

Straight trouble free swap. 

Using an online calculator, I now have 2 lower gears:
Standard 50tooth with SWR hub gears - 33.1", 40.7", 52", 63.8, 81.3", 100".
Using 34 tooth, (greasy fingers* method) - *22.5", 27.7"* 35.3", 43.4", 55.3", 68". 

n.b.my 2017 Genesis tourer, triple ringer only goes down to 25.6". *bit of old black cloth works wonders
The chain length is as was - stock. I can fold the bike with the chain on the 34T without it coming off if I'm careful but have to get the pedals in their correct fold position beforehand as the chain fouls itself (as it were ) on the tensioner. Practice is the key to this.

Interestingly, the pedals seem slightly wider which helps my clumsy heel-catching of the Eezy-wheels.

Anyway my ride along a towpath with some short steep cobbly ramps was brill this morning, and my "low-tech rear triangle anti rattle device" (home made bungy loop 8mm diameter cord) also works a treat.

Hope this is of some use and thanks again to the OP!


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## TheDoctor (6 Aug 2018)

Chain's a bit shiny! And liking the green frame.


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## Pale Rider (6 Aug 2018)

It being a bicycle - and a Brompton - I would have expected more niggles and complications.

Tidy job, nicely done.


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## gom (6 Aug 2018)

Looks nice.
Many years ago I put a 52/26 combination on mine. It's always looked a bit odd...




Works, but I'm not sure about this:


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## BromptonChrispy (6 Aug 2018)

TheDoctor said:


> Chain's a bit shiny! And liking the green frame.


Good maintenance regime! The frame colour was what was available on the day from Cycle Heaven of York. I didn't order it specially, I just wasn't leaving the shop without one having tried it! Thanks for the encouragement


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## BromptonFan (12 Aug 2018)

BromptonChrispy said:


> Thanks to the contributors who helped me with a simple but very effective modification, particularly
> Fab Foodie, The Doctor, and Pale Rider.
> 
> As I had no "bits-box chain-sets" I bought a new Stronglight 50/34 with 170mm cranks - same as the Brompton stock on my 2014 S6L with JIS taper BB.
> ...




What does the cord around the seat post do?


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## BromptonChrispy (16 Aug 2018)

Hi there, its a bungee loop..it stops the rear triangle rattling when I ride on the cobble bits of towpath etc where I live. The hook rattles on the catch as it loads and offloads over the bumps. I hate rattles on my bikes.......Just loop it round the from wheel - seat post - wheel and the bungee keeps it all rattle free. Simply remove to fold. If I’m riding on roads it’s not really necessary. 8mm diameter cord, eBay, about 99p. Loop diameter - bit smaller than the big chainring.


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## TheDoctor (16 Aug 2018)

52/26?!? Wow.
I thought the 50/30 I had on an old Carlton was a big jump. I take it you can climb brick walls?


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## gom (18 Aug 2020)

Got me up Monsal Head from the valley, but I don’t think anyone noticed


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## 12boy (19 Aug 2020)

What BCD is the crank? I tried a 24 to go with my 58/38 combo with a 110 BCD crank made for a triple chainring but the 24 wouldn't let it fold. Otherwise it worked fine. 58 26 would be ideal for me.... I rarely use the 38 but for serious grades, especially with a headwind, 26 teeth would be great. It looks like you have drilled out the crankarms for the 26 with 78 BCD spacing.


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## gom (20 Aug 2020)

BCD = 80mm.
It's a SPECIALITES T.A, bought from Spa Cycles 10-15 years ago. It did not work with the original bottom bracket, so I gave the chainset & bike to a local shop and asked them to fit a BB that worked. No problem with folding now. I think the original BB meant the new rings were too close, which it looks like would block folding, but I'm not sure I could even get them fully seated on the spindle.

Have thought about trying to fit a front changer, as it used to be possible to source something. No idea if any options for this are still available.


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## Fab Foodie (20 Aug 2020)

gom said:


> BCD = 80mm.
> It's a SPECIALITES T.A, bought from Spa Cycles 10-15 years ago. It did not work with the original bottom bracket, so I gave the chainset & bike to a local shop and asked them to fit a BB that worked. No problem with folding now. I think the original BB meant the new rings were too close, which it looks like would block folding, but I'm not sure I could even get them fully seated on the spindle.
> 
> Have thought about trying to fit a front changer, as it used to be possible to source something. No idea if any options for this are still available.


It can be done but you need to cut-off the rear if rve derailleur cage.


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## Schwinnsta (20 Aug 2020)

What is rve?


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## rualexander (21 Aug 2020)

Fab Foodie said:


> It can be done but you need to cut-off the rear if rve derailleur cage.


I didn't have to cut off anything when I fitted a front derailleur to my Brompton


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## Fab Foodie (21 Aug 2020)

rualexander said:


> I didn't have to cut off anything when I fitted a front derailleur to my Brompton


Ok that’s good news, the only one I’ve seen needed to do that (it worked just fine). 
Do you have any photos/details?


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## rualexander (21 Aug 2020)

Fab Foodie said:


> Ok that’s good news, the only one I’ve seen needed to do that (it worked just fine).
> Do you have any photos/details?



Info on this thread here https://www.cyclechat.net/threads/front-derailleur-on-a-bromptopn.248908/post-5620100

https://www.cyclechat.net/threads/front-derailleur-on-a-bromptopn.248908/post-5619038


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