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2 Chainrings on Brompton

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2 Chainrings on Brompton

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Old 04-14-18, 05:09 PM
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2 Chainrings on Brompton

My Brompton is now a grownup getting promoted to 12 speeds with an addition of second chain ring.







I accomplished the upgrade upon combining tidbits of advice from forum posters, @Winfried, @fietsbob, @berlinonaut and others, in these threads


Brompton Choice/Advice


[Brompton] Adding a front derailleur: Clamp?


and from other postings on the web. The goal was to have 50/34 rings without much sacrificing the benefits of Brompton: chainguard, compact folding size, low weight and ease of use. The final weight cost was: 486-414+120+103+45+29=369g. Given that some grip length got removed, it is presumably about 350g. I could lower this a bit, but then sacrificing performance. I post details below given that others might benefit from my experience just as I benefited from that of others. I will be still tweaking my setup but there are not going to be there any dramatic improvements it is already a finely working setup.






Left: Brompton w/2 rings, right: w/1 ring


For maintaing the folding size, the right-side folding pedal is essential.




34T implied 110BCD for the rings. A lightweight effective chainguard implied the search for a 50T ring meant to work with a chainguard. I chose Shimano Sora R3000-CG. I replaced the bolts affixing the guard to the ring with alu ones for bottle cages.





Left: Original drive crankset, right: new crankset



The 5-arm crank needed to be one where the crank fell in-between rather than over one of the chainring arms. I chose Sunrace FCR86. Unfortunately you cannot get arms separately, only a set with steel rings. As 34T ring I bought a Vuelta. My first try was Shimano Tiagra but it comes in steel while it is listed as alu. If I did it again, I might go with FSA Pro as made from harder alu than Vuelta.




As derailleur I took an old Shimano XTR FD-M900 bottom pull 34.9 clamp one. I carefully enlarged the clamp to 40mm. This is not trivial and will not work with every clamp. The issue is of the mounting bolt being able to reach across the gap in the clamp. You will find no clamp on cable stop for the Brompton 40mm seatpost and I soldered one on a hose clamp. The latter is likely to become my standard for cable stops.







My shifter is Sram X.9 friction Gripshift. I tried a couple of lighter ones, Sunrace TS-M10 and an older Sram. Sunrace turned out to be a disappointment in that it barely held the derailleur in the larger ring position, jumping off if slightly touched. The older Sram yielded uncomfortable grip.








Deformed left fender stay.



The Sunrace cranks sit farther out on the bottom bracket spindle than Brompton. At this point I did not change the non-drive crank and will look into shorter bottom brackets.


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Old 04-14-18, 06:50 PM
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Well done.
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Old 04-14-18, 08:38 PM
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Thanks for sharing that.... I was considering the extra chain ring too but with manual/finger shift. Very nicely done.
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Old 04-14-18, 09:36 PM
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Great job! Thanks for posting the procedure.
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Old 04-14-18, 10:17 PM
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For completeness a couple of more shots that were taking me over the limit of 10 in the first post:



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Old 04-15-18, 09:23 AM
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As a non-brompton owner, I just don't understand why they don't offer this. From pics, the fold is still remarkably tight. I'd be willing to bet there are people who decide against a brompton because they want more gears, not just a bigger spread, and for a bike manufacturer this seems like it would an easy option. Put it on a titanium model to offset the weight. I will never understand Brompton's reluctance to make changes that don't deviate from the basic frame. Even good things can be improved. Aging knees like smaller jumps in gearing =).
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Old 04-15-18, 09:36 AM
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Brompton has spawned a number of after-market companies, their winning compact fold makes them popular in Asia,
where the apartments are small in high rise towers..

their 6 speed is 2 cogs on a 3 speed , a TW company mixes parts to offer a 5 speed hub with 2 cogs there is your smaller jumps..
the rear cog switch half steps the internal gear wider spaced 3 speed...

Putting a 2 rings on the cranks spreads the range out, it can be done with a gear box crank, greasy finger shifting , or as shown , addinga front derailleur ans shift lever..

If a company cannot keep up with the world wide demand for the bikes they make, as it is, why make it more complicated?






...
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Old 04-15-18, 11:12 AM
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Was not aware that Brompton could not keep up with the demand for their bikes.
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Old 04-15-18, 11:38 AM
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To Meet Demand, the Investments were made.. They moved to a much larger factory building, for more space,

improved and increased the jigs & tooling to increase efficiency & output volume, and have hired more people, also.







...

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Old 04-15-18, 01:39 PM
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Originally Posted by fietsbob View Post
To Meet Demand, the Investments were made.. They moved to a much larger factory building, for more space,

improved and increased the jigs & tooling to increase efficiency & output volume, and have hired more people, also.
Too bad that their sales are stuck around 45.000 bikes a year for three or four years now. The old factory had a capacity of 50k bikes a year. So regarding current demand there was no need for a new factory, though they almost outrun their capacity. It was an investment in the future growth, fostered by the Brompton electric. They planned to sell 100k bikes a year in the early 2020ies. But for some years now there was no further growth. However: The new factory enables further growth, they have the space now to do more things inhouse now (like i.e. the painting, resulting in more special editions) and space for a r+d department, too.
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Old 04-15-18, 01:52 PM
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2_i, do you have a picture of the right side with the bike folded? I just realized that with the back wheel folded under, the front derailleur must be holding the chain out from/off of the chain ring?


Originally Posted by linberl View Post
As a non-brompton owner, I just don't understand why they don't offer this. From pics, the fold is still remarkably tight. I'd be willing to bet there are people who decide against a brompton because they want more gears, not just a bigger spread, and for a bike manufacturer this seems like it would an easy option. Put it on a titanium model to offset the weight. I will never understand Brompton's reluctance to make changes that don't deviate from the basic frame. Even good things can be improved. Aging knees like smaller jumps in gearing =).
I think the issue is the gear overlap and Brompton's odd IGH/derailleur system. The 34t front ring adds two extra good low low gears, but the other 4 gears mostly overlap with gears on the large ring (only ~6% diff). The shift steps will still likely be ~25% assuming one does not want to further complicate the already quirky Brompton double shifts into triple shifts for 'lumpy' ~6% and ~19% steps.

The double chain ring mod looks good for riding/touring in mountainous areas viewed as low range set of 6spds/~25% steps for long uphill slogs, and a high range of the same for the flats and downhills. But for the ultimate Brompton gearing, I think the OP should do the 3cog rear derailleur mod for a low/high range 9 speed with even-ish ~18% steps...
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Old 04-15-18, 02:09 PM
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Originally Posted by reppans View Post
I think the issue is the gear overlap and Brompton's odd IGH/derailleur system.
Indeed. You could achieve at least an interesting setup with tiny steps and short gears for some by using 12t and 17t sprockets on the back on the BWR and 36/44 on the front: Bicycle Gear Calculator
But it would be complicated to remember which of the then three shifters to shift when.


Originally Posted by reppans View Post
The double chain ring mod looks good for riding/touring in mountainous areas viewed as low range set of 6spds/~25% steps for long uphill slogs, and a high range of the same for the flats and downhills. But for the ultimate Brompton gearing, I think the OP should do the 3cog rear derailleur mod for a low/high range 9 speed with even-ish ~18% steps...
Jupp, that's my opinion, too. To add a second ring as a granny gear for steep hills is always an option. But three shifters for general riding are a bit of a pita if you ask me.
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Old 04-15-18, 02:39 PM
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Originally Posted by reppans View Post
2_i, do you have a picture of the right side with the bike folded? I just realized that with the back wheel folded under, the front derailleur must be holding the chain out from/off of the chain ring?
I hope the photo below provides enough detail of interest. The chain going through the derailleur does not cause problems in folding and unfolding. On rare occasions it gets mangled in unfolding but getting it on track is not much of a deal.



By now I switched from a Brompton BB, nominal 119mm (120 in my measurement), to 116 that can be shifted laterally. I will be next looking for 113 spindle with Brompton crank on the left. My estimate is that one could go with 110 if putting SunRace on the left too. From where I am right now, it looks that one could pile there even 3 rings without compromising the fold.

Originally Posted by reppans View Post
The double chain ring mod looks good for riding/touring in mountainous areas viewed as low range set of 6spds/~25% steps for long uphill slogs, and a high range of the same for the flats and downhills. But for the ultimate Brompton gearing, I think the OP should do the 3cog rear derailleur mod for a low/high range 9 speed with even-ish ~18% steps...
Yes, my interest in lower gears is in the context of riding Brompton in mountainous areas. 3 rear cogs and 3 front rings will have to wait - my next priority is to convert T-bag into a backpack
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Old 04-15-18, 03:23 PM
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Originally Posted by berlinonaut View Post
Jupp, that's my opinion, too. To add a second ring as a granny gear for steep hills is always an option. But three shifters for general riding are a bit of a pita if you ask me.
Well for clarity, I meant OP should do the 3cog /9spd mod next - for the ultimate 18 spd Brommie. I personally am also interested in doing the small chain ring mod first as I feel a greater need for the gearing range (esp low lows when lugging touring gear and/or riding in hill country) than I do for the closer ratios. I was even just interested in the greasy finger shifter, so it wouldn't be used much - but when I need it, I really need it.

Just wish there was a little more clearance for a simple bolt-on smaller chain ring mod - not sure I want to get into changing out bottom brackets etc.
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Old 04-15-18, 03:42 PM
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Originally Posted by 2_i View Post
I hope the photo below provides enough detail of interest. The chain going through the derailleur does not cause problems in folding and unfolding. On rare occasions it gets mangled in unfolding but getting it on track is not much of a deal.

Yes, my interest in lower gears is in the context of riding Brompton in mountainous areas. 3 rear cogs and 3 front rings will have to wait - my next priority is to convert T-bag into a backpack
Thanks, that helps and I like how the derailleur cage seems somewhat protected from bumps by the right pedal.

FWIW, here's one simply backpack mod that I've been using... just need a longer shoulder strap.

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Old 04-15-18, 04:21 PM
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Originally Posted by reppans View Post
FWIW, here's one simply backpack mod that I've been using... just need a longer shoulder strap.


Thanks, I already have more than this . I raise the T-bag higher and am now working on the bottom part of its back, with an extensive sewing task ahead of me. Obviously your arrangement makes the bottom irrelevant, but I am thinking about running between different modes of transport
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Old 04-16-18, 10:23 AM
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Thanks for this thread - it is something that I want to do - I believe that there is another similar thread on this topic
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Old 04-28-18, 09:04 AM
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Update












With more time to optimize my 2-ring Brompton, I evolved the setup towards that of Winfried in

[Brompton] Adding a front derailleur: Clamp?

Specifically, I switched to the Litepro Braze-On Adapter Clamp for Dahon K Frame. There is another one for 1/4 of the price, SP8 40mm Derailleur Adapter, that seems functionally equivalent, but it is still in the mail. I combined the clamp with Shimano Tiagra FD, following Winfried, and Sunrace M96 Thumb Shifter.

The problem in adapting clamp-on derailleurs to Brompton is in the fact that Brompton's seat tube does not point towards the BB axle yielding a mismatch between the angle of FD cage and chainring circumferences. The clamps above have fingers for mounting the FDs that solve that. In addition those clamps have 3 places where derailleur angles and elevation are adjusted, allowing to fine tune the derailleur placement far better than typical for clamp-on derailleurs on standard bikes. The end result is that it is far easier to get a good shifting. It was fine with my earlier setup but the range within which all worked fine was more narrow, The Litepro clamp was a tad too large for Brompton seatpost and I glued some tube rubber to its inside to make it fit, which was presumably a good idea anyway. There is an expected flex in the finger of Litepro holding the derailleur when shifter cable is pulled, but the finger then leans against the seatpost, so it is seemingly OK.

Regarding the shifter, I decided that I wanted to regain grip space. I actually like ergo/contour grips.

Finally, I shortened the BB, going to 113mm with titanium axle, shifted towards nondrive side. The latter is because I continue to use the original Brompton left side crank. If I swapped both cranks and go with symmetric BB, presumably 109mm would be optimal. However, the BB swapping is optional. You can live with the original Brompton BB.

My current weight change is:
  1. 486-414=72g (RH crankset)
  2. 54g (der mount)
  3. 87g (der)
  4. 10g (shifter cable)
  5. 45g (cable housing)
  6. 76g (shifter)
  7. 159-237=-78g (BB)
for the total of 266g (344g w/o BB swap). I think this project is now complete. The photos illustrate the final result.

Last edited by 2_i; 04-28-18 at 11:44 AM.
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Old 04-28-18, 10:12 AM
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Originally Posted by 2_i View Post











With more time to optimize my 2-ring Brompton, I evolved the setup towards that of Winfried in

[Brompton] Adding a front derailleur: Clamp?

Specifically, I switched to the Litepro Braze-On Adapter Clamp for Dahon K Frame. There is another one for 1/4 of the price, SP8 40mm Derailleur Adapter, that seems functionally equivalent, but it is still in the mail. I combined the clamp with Shimano Tiagra FD, following Wifried, and Sunrace M96 Thumb Shifter.

The problem in adapting clamp-on derailleurs to Brompton is in the fact that Brompton's seat tube does not point towards the BB axle yielding a mismatch between the angle of FD cage and chainring circumferences. The clamps above have fingers for mounting the FDs that solve that. In addition those clamps have 3 places where derailleur angles and elevation are adjusted, allowing to fine tune the derailleur placement far better than typical for clamp-on derailleurs on standard bikes. The end result is that it is far easier to get a good shifting. It was fine with my earlier setup but the range within which all worked fine was more narrow, The Litepro clamp was a tad too large for Brompton seatpost and I glued some tube rubber to its inside to make it fit, which was presumably a good idea anyway. There is an expected flex in the finger of Litepro holding the derailleur when shifter cable is pulled, but the finger then leans against the seatpost, so it is seemingly OK.

Regarding the shifter, I decided that I wanted to regain grip space. I actually like ergo/contour grips.

Finally, I shortened the BB, going to 113mm with titanium axle, shifted towards nondrive side. The latter is because I continue to use the original Brompton left side crank. If I swapped both cranks and go with symmetric BB, presumably 109mm would be optimal. However, the BB swapping is optional. You can live with the original Brompton BB.

My current weight change is:
  1. 486-414=72g (RH crankset)
  2. 54g (der mount)
  3. 87g (der)
  4. 10g (shifter cable)
  5. 45g (cable housing)
  6. 76g (shifter)
  7. 159-237=-78g (BB)
for the total of 266g (344g w/o BB swap). I think this project is now complete. The photos illustrate the final result.

Thanks that looks really good - does the routing of the cable from front derailleur to the changer on the bars just follow the other cables - can you remember what length the new cable was?

Did you have to alter the chain line at all with any spacers on the BB to push the inner chainring away from the frame ....... and did the Bike fold OK without the inner Chainring touching anything?
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Old 04-28-18, 10:37 AM
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What is the round thingy with 4 screws on the handlebar bridge?
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Old 04-28-18, 10:44 AM
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Originally Posted by BillN33 View Post
Thanks that looks really good - does the routing of the cable from front derailleur to the changer on the bars just follow the other cables - can you remember what length the new cable was?
From what I can determine now, the cable housing length is about 41"/105cm. I would add at least 1"/2cm for any mishaps - I already shortened it by 1cm or so due to one. Yes, it goes with other cables through the hoops. There is one sharp upturn to go up the seattube - difficult to make it milder.

Originally Posted by BillN33 View Post
Did you have to alter the chain line at all with any spacers on the BB to push the inner chainring away from the frame ....... and did the Bike fold OK without the inner Chainring touching anything?
See my earlier posts in this thread. The Sunrace cranks mount farther out on the axle than Brompton. My going with the shorter BB axle aims at pulling the Sunrace crankset in, not pushing it out. Yes I have a 2mm spacer on the RH side, but it is inside the BB cup to move the spindle to the left, rather than right, empasizing again. Yes the bike folds fine and neither the inner ring nor chain on the inner ring touches anything. I could presumably move the axle to the left by 1mm more without a problem, but I am not forced here to push it to the wire.
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Old 04-28-18, 10:51 AM
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Originally Posted by Schwinnsta View Post
What is the round thingy with 4 screws on the handlebar bridge?
That's a pad for attaching phone and batteries - can be moved between bike and car. If you take off the pad, you can attach camera or radio - another project .
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Old 04-28-18, 11:25 AM
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Originally Posted by 2_i View Post
From what I can determine now, the cable housing length is about 41"/105cm. I would add at least 1"/2cm for any mishaps - I already shortened it by 1cm or so due to one. Yes, it goes with other cables through the hoops. There is one sharp upturn to go up the seattube - difficult to make it milder.



See my earlier posts in this thread. The Sunrace cranks mount farther out on the axle than Brompton. My going with the shorter BB axle aims at pulling the Sunrace crankset in, not pushing it out. Yes I have a 2mm spacer on the RH side, but it is inside the BB cup to move the spindle to the left, rather than right, empasizing again. Yes the bike folds fine and neither the inner ring nor chain on the inner ring touches anything. I could presumably move the axle to the left by 1mm more without a problem, but I am not forced here to push it to the wire.
Thanks - I fitted the Shimano Hollowtech ll, (SM-BB-R60) BB - but other Shimano BB's also fit as they are the same size, (119mm) as the Brompton BB, (the newer JIS one) - and used Shimano Alfine cranks with a crank length of 170mm - 5 arm - PCD 130 with a Middleburn Outer Chainring - it fitted the Brompton just fine with no spacers........ and the Chainline with one Chainring was fine.
Looking at developing this based on your experience with the 105 compact changer and the Braze on bracket off ebay
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Old 04-28-18, 11:47 AM
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P.S. This Sunrace M96 Thumb Shifter is a joy in giving you freedom to adjust its orientation.
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Old 04-28-18, 02:09 PM
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I got a Steel , fabricated FD mount from CM Wasson, but only test fit it , never actually used it
went with the 2 speed geared crank which works better with my BSR, wide range from 17~80 with 15:54.. doubled the IGH advantage As Both can be shifted at once.. and all 6 gears are in a row , no redundancies.

As a practical swiss machinist's product, wont be a gram counters choice , but it will out live me..



..

Last edited by fietsbob; 04-30-18 at 08:56 AM.
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