Anybody installed a modified 3rd gear to a BWR hub?

[berlinonaut]

The spacer between the derailleur and frame is actually the tensioner and the frame?

I don't get exactly what you mean but try my best : The chain-tensioner has to come out a little bit more than stock. Therefore you need an additional (custom) spacer between the frame and the chain-tensioner/derailleur. You have to make it yourself as it has to be of a special shape, following the shape of the chain-tensioner. Two layers of an old credit card, cut with a scissor, work perfectly fine.

Are you dremelling the lip on the 12 tooth? I take it the standard 12 tooth that comes with the hub won't work?

The 12t is the second from the left in that picture:

To make the snap ring, that holds the cogs in place, fit you need to make the inner lower, slightly stepped down part of the 12t cog slightly bigger in circumference by dremeling. It is not much but if you fail to do so said ring won't stay in place. You could however replace it with something smaller so you would not need to dremel on the cog. Either a less thick snap ring or maybe even a cable-tie could work.
If you dremel too much or your donor-cassette has a 12t as the smallest cog it is very likely that the cog will brake apart after a short time of use.

I did not try to use the stock 12t or 16t. For one (if my memory is correct) they are slightly thicker than the ones from the donor cassette and space is the biggest issue with the conversion. Second I preferred to have cogs of the same type and age, thus I carefully selected a cassette that had all the cog-sizes I would need.
To create enough space for the three cogs on the driver you have to get rid of the black inner plastic protection ring that originally sits in between cogs and spokes.

Can a 17 or 18 tooth sprocket work in lieu of the 16?

17t usually works (but not on all rear frames), for 18t you usually have to file a little bit of material off the rear end of the chain-stay tube. It probably depends a little bit from the cogs you use - the higher their teeth are the more likely you get in trouble. Additionally there seems to be a minimal amount of variation in between Brompton rear frames - normally one would not recognize it but with this space-delicate operation you do. Seems to affect ti-frames more than steel frames.

 

[berlinonaut]

I'm dense so help me out. Does this 3 gear set up result in a 6% lower gear ratio on the low end compared to the stock 2 speed?

This depends on the cogs that you are using. If i.e. you choose 13/15/17 instead of the stock 12/16 you'll end up lower. Just play with ritzelrechner a bit and keep in mind that smaller than 12t does not work standardwise, bigger than 17t will probably create issues that require filing on the frame slightly and bigger than 18t will create even bigger issues. So you have to find your personal combination within this range in combination with one (or two) chainwheels on the front that you can choose more or less freely. I i.e. run my 2-speed with 12/16 in the back but a 60t chainwheel in the front.

 

[12boy]

Thanks for the info. Something to work on when I can. I have a SRF 3 wheel which the LBS says cannot take more than 1 sprocket. If I can work out 2 and have 13/17 I will have a fairly wide gear spread. Or I can get a BWR which will take the two and go so low I can climb a tree!

 

[bikegang]

Thank you. So the 12-15-17 mod would significantly reduce the gearing of a stock 2 speed? Do I understand that correctly?

Not really, it is only one tooth difference, but with one more sprocket the gaps are smaller and more subtle gear change ... 11-13-17 will be our next version. and this is the widest external cogs range you can get without frame end filing.

 

[Schwinnsta]

Is a three position shifter required or is just used to know which gear in?

 

[Schwinnsta]

Would someone please answer the above question.
Thanks

 

[ukoldschool]

I have. Works flawlessly. I did a writeup in another forum: https://www.bikeforums.net/20170010-post8.html

Thank you @berlinonaut! And thanks for the link to your thread, makes a lot of sense what you wrote there, its nice to see these mods in use in the real world

I think given your experience, the price of the kit from bikegang looks like great value, and it also gives me what I asked for, ie a lower gear (12) and a higher (17): https://bikegang.ecwid.com/#!/Bromp...ter-Kit-MiniMODs-x-Sturmey-Archer/p/115439619

Just need a new chain (which did you use by the way?) and to figure out a way to modify the standard left hand (new model version), from searching online it seems the internals work in the same way as the old version, so probably just a case of filing a slot into the right place to give a middle gear point:

Although I am still considering just buying a new right hand 3 speed standard housing and using the internals from that as they are not expensive... only worry is can I modify it as the cable comes in from a different place?

 

[berlinonaut]

Is a three position shifter required or is just used to know which gear in?

Would someone please answer the above question.
Thanks

How much are you willing to pay for an answer? The stock shifter has two stop positions for the two stock gears. It won't stick in the new third (middle) gear unless you modify it to do so. As with any derailleur you can alternatively use a friction shifter (like I did). What probably will not work is a stock three speed stop-shifter like it is used for three chainwheels on the front as the positions will probably not fit. It is easy to modify the old, pre 2017 Brompton 2-speed-shifter. I've no experience if it is the same with new post-2017 below-the-bar shifters.

 

[berlinonaut]

I think given your experience, the price of the kit from bikegang looks like great value, and it also gives me what I asked for, ie a lower gear (12) and a higher (17): https://bikegang.ecwid.com/#!/Bromp...ter-Kit-MiniMODs-x-Sturmey-Archer/p/115439619

It depends - for 25 Pounds you get the three cogs along with a spacer and a snapring (that you both already own on you six-speed) and a barrel nut.

You can buy the cogs separately, instead of buying a cassette and taking it apart, i.e. from here:

12t, 3,49€: https://www.rosebikes.de/shimano-xt-cs-m771-10-fach-12-zahne-ersatzritzel-483988
15t, 3€: https://www.rosebikes.de/shimano-xt-cs-m771-10-fach-15-zahne-ersatzritzel-483992
17t, 3,39€: https://www.rosebikes.de/shimano-xt-cs-m771-10-fach-17-zahne-ersatzritzel-574091

Loads of different cogs to choose from. Together less than 10€. You do not need the spacer and the snapring as you do already own them. You do not need the barrel nut if you already have the post-2017 Brompton shifters as along with them comes the new dogleg where the cable is clamped at the rear end (instead as in the shifter as before 2017). Even if you have the old shifters you'd only need the barrel nut if you exchange the 2-spped shifter for a different one instead of modifying it and even then I'd prefer buying the post-2017 dogleg (~6 Pounds) over the barrel nut as dealing with this nut at the rear end is a bit painful due to the spring inside the dogleg (having three hands, one of them very tiny, would be helpful. Unfortunately people tend to only have two hands). Thus the new version of the dogleg is much more easy to deal with than with the barrel nut.
Regarding the Bikegang kit it does not seem to include a spacer for the derailleur and the 12t cog looks unmodified. So I am not sure if this will fit and work out of the box w/o further modifications. Maybe if the snapring is a bit slimmer than the original one one can avoid the dremeling on the cog. So if you do not mind the extra money it may be a convenient solution but with some bad luck you still may have to do a bit of homework. So personally I'd ask BikeGang about these details before buying but your mileage may vary.

Just need a new chain (which did you use by the way?)

Don't remember brand and model. A ten-speed. Needed a new one anyway as the old one was worn. Maybe the factory one would have worked as well, maybe a 8-spreed or 9-speed would have worked. I just wanted to play it safe and therefore invested the extra money for a ten speed chain - was not too much.

 

[12boy]

While it is nice to have index shifting that exactly pops into the gear, a friction shifter that has the range will work fine. I only have a 2 speed I converted from a single, but I already had an SA 3 speed shifter which clicks in 3 positions. Although the clicks do not correspond to the shift points it works just fine.

 

[ukoldschool]

Thank you again @berlinonaut I will have a search for individual cogs

Do you happen to remember the chain length that you bought?

 

[ukoldschool]

Been giving this a lot of thought...

As a result I have compiled this sheet of the various gear ratios in a format that I can get my head around, which is in metres rolled per single full rotation of the chainring/pedals (I cant understand gear inches, I was born in the metric world..... ) Thought it was woth posting for info.

Im probably going to go for the final option (larger front chainring and 12/14/17 rear cog conversion) as it seems to give me what I seek which is a wider spread of gears in the mid range (but still retains ratios very close to the standard 6 speed), plus an extra top gear

 

[Kell]

You must have thighs like Robert Förstemann to get over the top of the standard 6-speed top gear AND need another one on top.

 

[ukoldschool]

You must have thighs like Robert Förstemann to get over the top of the standard 6-speed top gear AND need another one on top.

Sadly not, its more that it feels unsafe spinning out when downhill (something i'm sure you also get coming down the hill in Wycombe?), I would struggle to get it moving if I set off in 6th

 

[Kell]

I give up trying to keep up with it and enjoy the rest.

From experience, I start to feel like I'm spinning out at about 25/26 mph.

As it happens, last time I bought a new chainring, I thought I'd try the reduced gearing option, though I've yet to fit it. It will be interesting to see how it feels in use as I can (and do) get up to about 22mph on the flat in certain parts of my commute.