Brompton 2 to 3 Speed Conversion

I'm now actively exploring this idea and will be happy to share ideas and progress, but have one question at this stage: presumably a switch to a 9- or 10-speed chain is necessary? I ask because I accidentally tried to put a 9-speed chain on my B at an earlier point, and the Brompton shifter mechanism didn't like it. It sulked and refused to change sprockets. Anyone have any thoughts on this?

PS: My previous post on this shows up as from a guest, user89861. But I've now signed up to the site properly.

When you stack three cogs on the Brompton cassette you need the narrow width of the 10 speed cogs (sprockets) and chain for everything to fit and shift properly. The existing Brompton chain tensioner and pivoting shifter mechanism will work with no modifications with a 10 speed drive train. Ten speed stuff is readily available; International Racing Design (IRD) carries a nice 10 speed cassette in their 12-23 combination that have the necessary sprockets to experiment with.

You were using a slightly narrower chain (9 speed) to shift on a 6/7/8 speed cog spacing, so I see why you didn't have crisp shifting.

The conversion is proceeding OK so far, though I'm a little concerned about retaining the sprockets. You mentioned earlier (#12) that you'd had problems with the garden wire retention method. I've tried various things but that's the only one that works so far, but I'm naturally concerned about its longevity. Was it difficult to file away enough of the smallest sprocket to fit the Brompton circlip? And what tool did you use for this?

Another question: somewhere you mentioned that you'd had to fit a home-made washer on the outside to enable the shifter to move further out. I've been wondering whether it would be possible to file away partially at the wings of the chain pusher wing plate to extend the shifting range. Do you think that would work?

post #12 was from the original author to this article, not me.

I used a Dremel type of high speed hand held grinder to remove some material from the small cog. I sort-of chamfered it to allow the thickness of the original Brompton circlip to be installed, but not before I ground some material from the O.D. and a bit of the thickness of the circlip. I do not think grinding this clip compromises its strength. It is made of hardened spring steel, and its only purpose is to take up the space in the groove in the cassette body to keep the sprockets in place. Fitting the circlip is a trial and error application, but if you go (grind) slowly it will eventually fit and work fine.

I only shimmed the plastic chain tension mechanism out about 1 mm using two pieces cut from a plastic spiral notebook cover. No other modifications were necessary. You will have to adjust the two set screws on the chain pusher too. As far as removing material from the chain pusher wings: that may help a small bit, but this part is not very thick to begin with, so filing may weaken it. I have been riding with this set up regularly for the past few months, and it performs flawless.

Thanks very much for that, I think I'll have a go at chamfering the 12T sprocket (I've given up trying to use an 11T one for the reasons you outlined above), as I have some dremel tools that should work OK. But I also thought I'd have a go at milling the side of the circlip (using an angle grinder), making it slightly towards a D in cross-section. But should I install it on the wheel with the flat section facing outwards, or inwards towards the 12T sprocket - or does it not matter which way it goes in?

Thanks in advance.

I definitely had to remove some material from the circlip, mostly from the O.D.- if I could remember correctly. I don't think it matters how it is installed. My sprockets wiggle just a wee bit, but this does not affect the shifting. I am continually amazed how well this set up works. I want to hear about your success with this modification when it is completed:)

I didn't realise that this topic had been revived! Good to see other people have taken the ball and are running with it.

I'm considering taking the plunge and trying this mod for my Brompton. I'm currently putting my 6-speed on a diet (the first step being to switch to a 2-speed rear wheel) and was considering this as a way to get an extra ratio back without the weight of the hub. I'm quite mechanically minded but don't have much experience with bike mechanics. One question I had was, as I have a 3-speed shifter for the hub part of my current 6-speed, could I use that to shift the now 3-speed chain pusher, or am I way off with that idea?

I think it's the other lever that you need to mod, i.e. the 2 speed one.

You need to use the left 2-speed shifter, and modify it as indicated by the original author of this a post. I however have had better luck using any type of thumb lever friction front derailleur shifter. You really don’t need to index shift three cogs, plus friction shifting allows you to “trim” or adjust the shift so the chain rides on the center of the cog quietly.

I haven't tried it, but this https://bikegang.ecwid.com/?3vOM2tpoyAUSFIWU-Te2WMunXOCIAUaWhSyxRHLzwBUfmyTqrZ6dOUI9E#!/Brompton-2-to-3-Speed-Sprocket-Cog-Thumb-Shifter-Kit-MiniMODs-x-Sturmey-Archer/p/115439619/category=9803294 looks like a cheap and simple way to try this mod

Most of those parts are available individually on AliExpress and similar.

Usually for less money.

For ref.

Just seen this via @coventry_eagle's for sale post. Think going to do on my S2L as have all the parts kicking around and want a slightly wider gear range

@maynardeames can hook you ip with the Bikegang kit: https://www.ebay.co.uk/itm/Brompton-2-to-3-Speed-Sprocket-Cog-Thumb-Shifter-Kit-MiniMODs-x-Sturmey-Archer/192894987960?_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20160908105057%26meid%3Dcffe75a04f64411b84d3050dbf3150b4%26pid%3D100675%26rk%3D5%26rkt%3D15%26sd%3D263974939843%26itm%3D192894987960&_trksid=p2481888.c100675.m4236&_trkparms=pageci%3A16ef5800-7c6b-11e9-966c-74dbd180f780%7Cparentrq%3Adea4bee916a0a9ca2a533bbdffb4885f%7Ciid%3A1

Cheers, thats good to know. I have a 10 speed cassette and thumbshifter in the shed. Will have a look over the weekend.

Is there an indexed trigger shifter that would work? I’m thinking along the lines of one of the SRAM or Shimano left-hand 3x 8/9/10/11 shifters. Will one of the SRAM pull ratios pull the correct amount to index correctly at the back? Left- hand 3x shifters are properly out of fashion and you can pick up even high end ones for buttons.

Useful list of common pull ratios, spacing etc here: https://www.artscyclery.com/science-behind-the-magic/science-behind-the-magic-drivetrain-compatibility/

I have one of these
https://www.sjscycles.co.uk/gear-shifters/sunrace-m90-friction-thumbshifter-left-hand/
As supplied to me by @maynardeames
I've yet to fit it. Or start the conversion. I'm such a slacker.
Also, as I understand it, if you have access to a Dremel (other die grinders are available) it's possible covert a 2 speed shifter, into a 3 speed, by giving it an extra notch.

Old style shifter yes

Old style meaning pre-Nov 2014?

Pre 2017

Ta.

I'll state this again. Friction shifting three rear cogs is a piece of cake. There is a little bit of skill in riding a bicycle and this is one handy skill people should learn. The modern no brain indexing has made us spoiled.

Has anyone tried modifying the post 2017 2speed +/- shifter to have a third position?

Either introducing middle catch point (à la the older 2 speed shifter cutout method), or removing the return spring (assuming its a spring) to turn it into a friction shift between the three cogs?

As @gp700 asked.

Anyone know if the new style shifter is usable on three speed or does it need replacing?

After replacing the two cogs with a set of 12-14-16 on my 2019 Brompton I just modified the (new style) shifter to a friction type. I removed all the springs and index parts from the inside and thigtend the central screw in a way that the shifter stays in the selected position. Works so far for me.

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