27-Speed Brompton
 

This is a continuation of the threads: 3 Chainrings on Brompton and Brompton Choice/Advice. The 27 speeds result from 3-speed BWR x 3 cogs in the rear x 3 rings in the front. Update details follow.

3 Chainrings
Litepro adapter for Birdy came in and it is the best current off-the-shelf option for mounting a triple derailleur on Brompton. It is close in design to the 3D print product of @tomtomtom123 in the 3Chainrings on Brompton thread. It puts the derailleur cage under the band around the seatpost, so there is less interference than in the case of the Litepro Dahon adapters, allowing the cage to get close to the seatpost, As the arms of the derailleur swings there are some deformities tied to the limit screws protruding from the back and potentially pushing against the band and you can take care of this by dremeling the band a bit on one hand and the deformities on the other. As the derailleur bolts on right by the adapter’s band, rather than on an arm, as in the Dahon adapters, the adapter does not deform during shifting.

To get the right angle for the derailleur I had to use a couple of asymmetric Sram washers that push the cost up. You can get by presumably with one washer or making some substitute yourself. Just like with the Dahon mounts the Birdy mount’s diameter is a tad too large for Brompton. I put a rubber strap underneath the mount to protect the post, so I do not really care.

An alternative properly working mount for Brompton is the one by C.M. Wasson, that I got from @fietsbob and that alas is no longer there in the market. Since it is hard to find even photos of the Wasson mount and I was taking the mount off, I attach a photo for reference.

I continue to use my modified Suntour derailleur, but you could potentially get by with IRD Alpina and no modifications. My crankset is 110/58 with 50-32-20 rings, but you could possibly get by with 110/74.

3 Cogs
I explored 2 routes: that of @berlinonout in the Choice/Advice thread and @bikegang e.g. here. The rundown is basically as follows. @berlinonout suggests using 10-speed cogs and spacer in 12-14-16 combination. To make them fit you need to file down the 12 cog’s built-in spacer a bit, kick out the spoke protector and move out the rear derailleur out a bit with a washer.
@bikegang’s cogs and spacer are 11-speed and they fit into the available space originally meant for 2 cogs without changes. An essential part of the @bikegang’s set is the retaining ring, thinner than the original, ensuring that everything fits into space. None of the cog combinations that @bikegang offers is 12-14-16, but e.g. 12-14-17 allowing to push out the gear range.

My take is as follows. Once you took care of the gear range with chainrings, you only need to worry about gear spacing in selecting the cogs and the @berlinonout’s 12-14-16 is optimal. In addition, 10-speed pieces allow you to go with a lower-speed chain. I tested that in emergency I could even use 9-speed and maybe lower – important if things go awry at a location where you can get only very basic components. So I went with the @berlinonout’s solution, but combined it with a thinner retaining ring so that I would not need to file down the 12t cog. Your options are either to negotiate the thinner ring with bikegang or file down the original Brompton’s retaining ring to make it flatter or source a thinner ring from somewhere else.

@berlinonout suggests making an extra notch in the original 2-speed shifter to make it shift over the 3 positions. In my opinion this is not practical – you will likely just damage the shifter. If your Brompton is pre-2017, you then need to reverse the shift cable’s direction so that its stop ends up on the side of the shifter, when going with a standard shifter such as SunRace. There are barrel nuts offered in the market for the purpose, but the most economic solution is just to get derailleur anchorage for the new Brompton shifter that relies on the standard shift cable orientation. As this new anchorage is a hell to set up, though, as already mentioned in another thread, you may just as well fall onto the easier barrel nut.

OK, I will presumably rest now as far as the speeds are concerned. The photos follow. Thanks to everybody for posting their pieces of knowledge – without these I would not even think of any serious gear conversion. Come to think, the range of cable tension where the chain sits comfortably on the middle cog is very narrow, so there is a room for improvement there :).

https://cimg2.ibsrv.net/gimg/bikefor...10e478d38d.jpg

27-Speed Brompton

https://cimg8.ibsrv.net/gimg/bikefor...734817eb8b.jpg

Close Up

https://cimg1.ibsrv.net/gimg/bikefor...aed79b7806.jpg

Litepro Mount

https://cimg2.ibsrv.net/gimg/bikefor...a39ff52d46.jpg

C.M. Wasson Mount

https://cimg6.ibsrv.net/gimg/bikefor...5cc5f6d832.jpg

Cogset

https://cimg7.ibsrv.net/gimg/bikefor...2976013b83.jpg

Shifters

https://cimg0.ibsrv.net/gimg/bikefor...2500ce0a9e.jpg

Detail

Fantastic work. I just don't understand how you shift the rear three sprockets. Isn't the Brompton just equipped with a chain tensioner and not a derailleur? Can you elaborate.

Have you run a gear calculator to see what your spread is and if there are any overlaps?

By the way, I'm a big fan of "the more gears the better." I've been running an 81 speed Cannondale for the past five years and I'm about to unveil a 54 speed Dahon.

Thanks! :D Brompton actually has a rear derailleur. It is quite modest. Basically there are tweezers under the chainstay that move the upper pulley left and right, bringing it under the left or right cog, depending on the pull in the shift cable. The functionality is like that of a typical rear derailleur - it just does not quite look like the typical one. In the modification you make the tweezers rest in one of the three rather than two positions, after adding the third cog. You set the pull of the shift cable with a standard 3-speed indexed shifter. For 3 positions you do not need about the amount of pull in the cable as the extreme positions of the derailleur are set with limit screws.

This will have to wait as I am off to work travel in the morning. There are overlaps, but it is good too. I think that numerically my gears are better than in my main bike. The latter is justified as my vicinity is relatively flat and the folder may be taken to quite extreme places. However, operating these 3 shifters can be confusing. I got it under control with 2 cogs, but with 3 there will be some learning curve.

Wow! This is impressive. I'll be biting my fingers concerning the Dahon, but it would be good to learn details on the Cannondale in the meantime. I had 36 speeds on my Trek, but then went down to 32.

2 I, in both cases I'm using a SRAM Dual Drive rear hub which is set up as a 3 speed IGH and it carries a 9 speed cassette. The Cannondale came equipped with the Dual Drive and I added the triple crankset and FD.

My Dahon has a Shimano 105 double crankset (53X42) and the rear cassette is a12-25. I was looking on ebay for a replacement set of wheels for my Dahon Speed 7 and stumbled on to the Dual Drive. I made a lowball offer and the seller accepted. :D

Since most of my riding on the Dahon is when I pack it in a suitcase and travel to warm places in the winter (i.e. the beach/coastal areas which tend to be flat) I prefer the tight gearing with more high end. However, there is the occasional bridge spanning shipping lanes, when a low gear is appreciated. Also the Dual Drive high gear will turn me into a rocket :p when I have a stiff tail wind!

No need to bite your fingers about my Dahon. I bought it for a cool $100 new at TJMaxx several years ago, very much on a whim for the travel purpose mentioned above. I knew absolutely nothing about folding bikes at the time. I realize there are significantly better folding bikes out there. Maybe one day I'll own a different one, but for now my Dahon serves my needs. I'll create a thread about it in the next week or so.

I really like what you are doing with your Brompton and feel as if you are conquering its weakness at a very reasonable cost (as compared to converting it to a Rohloff or Shimano IGH rear wheel). I'm still having a difficult time picturing the "tweezer" RD. Is there anyway to provide better pictures of this part, especially when the chain is shifted across your three rear sprockets? Thanks!

I did a 3 by 3 by 3 speed on a JC Higgins in the 1950s , Then, it was a snap ring triple cog cluster machined out of 1 piece of steel.. on an AW3..

Triple crank, cottered type.. Huret Alvit D's 3 shift levers on the down tube..

I'm curious. Do you know the angle of the Litepro adapter for the Birdy?
Also, this adapter also appears to have the derailleur mounted 6-8mm further away from the seat tube than the Litepro versions for Dahon. But is there enough slack in the pulley to allow the cage to get flush against the seat tube?
This one also appears to be more stable.
Do you still have the same bending problem with the adapter for Dahon?

Great post! Small error: I did not suggest making an extra notch - I mentioned it as one possiblity, especially if you want to safe on cost. I am using a Sunrace thumbshifter on my build (and pictured that as well) as I like this better and the newer Brompton dogleg instead of your way with the barrel nut as it is cheap and works very well. Did not find it difficult to set up.

OK, makes sense. On a full size bike I hesitate going into planetary gears for efficiency issues. On a folder you need to meet so many expectations that you must be open to more compromises.

I am away from the bike and will remain so for one more day. The photos will need to come thereafter. As to the costs, one might say that they pile up as I try out different directions. However without big individual commitments I can roll individual steps back and whatever is left I may move around our other bikes or sell back on Ebay.

The set up seems to be more rigid than even with the Wasson mount. Indeed the derailleur ends up farther away from the seatpost, but I compensate for it a bit by rotating the mounting point backwards, shortening the lateral displacement. Also what matters is how close the cage can get to the seatpost rather than just the mounting point. Finally, for Brompton there is no advantage of getting overly close to the seatpost with the cage. For the cage close to the seatpost you need to get with the smallest ring close to the seatpost and then the ring begins to interfere with folding the rear triangle under the frame.

Again, the angle will need to wait until I get back to the bike.

Thanks! You wrote though

I know that you used Sunrace, but just wrote that cutting the original shifter to arrive at the desired effect is not practical.

As to the cable reversal, I went through both the barrel nut and new anchorage. At this moment it looks as if the barrel nut worked better, but this may just reflect how much testing I did with each setup. In any case, I have problems with the shifting to the middle cog being consistent. I am not sure where the problem lies, whether my cogset is tight enough, whether the cable gets caught somewhere, whether there is some latency in the derailleur itself. I will need to go over every item and I also consider increasing the lever arm for the shift cable in the derailleur.

well, I don't really need the angle. You don't need to measure it if you don't know.
Can you slacken the pulley enough to get the cage/parallelogram to touch the seatpost tube? On a Dahon, you need the cage as close to the frame as possible?

No, I don't think I can. On Brompton my goal was to have the the bottom inner tip of the cage nearly touch the rear triangle for a loose cable. With this I could get everything right all at once, unobstructed folding and cage staying clear of the chain when it is at its closest practical distance from the frame. Importantly the chain quit rubbing against the bottom of the cage in the lowest gear which I had to a small degree with Wasson. It is possible that I would have arrived at that with Wasson, if I used two Sram washers, but I will not be presumably going back and forth between the mounts.

tomtomtom, I don't want to derail (sic) 2 i's thread by down-grading it with Dahon pictures, but this is the one picture I have of my FD with my double. I'm using the Litepro FD mount which can barely be seen. If you'd like I can take additional pictures, but I should probably start a new thread.
http://i125.photobucket.com/albums/p...pshmsulrks.jpg

Very nice alignment with the chainring! Is the Litepro mount holding its shape for you? In my triple configuration, the finger to which the derailleur attaches was bending over. In the double configuration I do not really know because I switched to another cheaper, but more rigid, mount for Dahon, that can be also found on Ebay.

How many unique gear ratios did you achieve? How many are more than 2% different from all the others?

I'm not certain I you are asking me this about my 54 speed Dahon or 2 I about his 27 speed Brompton, but I'll assume it is about his Brompton. I'll start a thread about my Dahon soon and post some gear charts and answer this in its own thread.

My Litepro FD mount is holding up very well. I don't notice any bending at all.

Obviously, I do not plan to try out every possible combination at any given moment. Rather I will be setting a broad range with the chainrings, then a more refined range with BWR and the fine-tuned gear will come from the cogs. The overall range is 50/20 (rings) * 1.57/.64 (BWR) * 16/12 (cogs) = 8.18. The fine-tune ratio is about sqrt(16/12)=1.154. With this my effective number of gears is about log(8.18)/log(1.154)+1 =16. I.e. if you wanted to arrive at a similar choice of gears with a hub, you would go after 16 speeds.

My B'ton I went Double planetary .. 54t MD 15t hub BSR ..double shift 43.4 to 30.8 can go at any speed , bogged on a rise for example ..

6, 5, 4 .. double shift
throwing hub lever from low to high and tapping the mountain drive shift button shuttling though the BB, into low range..
3, 2, 1 (17.4)
The 57.9 is the fine pootling along gear..

(sheldon's chart does not transfer well.. given my computer skills )

2.5:1, reduction gear low, a 54t acts like a 21.6t as cranks turn faster than the chainring.. through the gearbox..





...

Here is a photo of the Brompton rear derailleur. I marked the bolt that is an axle for the square with the tweezers that grab the upper pulley. I also marked the limit screws to make it easier to understand how the derailleur works. The shift cable is attached to a corner of the square from above. I am impressed by the Brompton's simple, inexpensive and creative solutions in their details. It is easy to design, making incremental improvements in solutions tested and adopted across the industry. In many details Brompton people pound alone and those contributing creativity into the designs evidently communicate with those responsible for production.

https://cimg2.ibsrv.net/gimg/bikefor...db6064b227.jpg

Thanks! I did not know that his site had the hub option. Small correction is that my cogs are 12-14-16 following @berlinonaut, not 12-14-17 of @bikegang. 16t gives a better gear spacing.

In the after market in Asia the fork pulley flange pusher gets a roller sleeve bearings on both sides and both pulleys got sealed bearings..

M3L was easy to just put 2 Tacx pulleys on ..

Thank you for the additional picture. Very helpful. There are many ways to make derailleurs work and great to see another variant.

OK, I appear to have solved the shift to middle cog problem and the main culprit turns out to be pretty embarrassing: clogged up derailleur pulleys and especially the upper one. After cleaning up the tensioner in an ultrasonic cleaner down to bare plastic the problem is completely gone.

On the way I also found that the axle bolt for the pusher was loose and that the coupler between the shift cables from the derailleur and shifter was sticking. That new anchorage basically runs a second shift cable from derailleur, with an orientation reverse to that from the shifter, and couples them. For the moment, to solve this I just went back to the old anchorage.

Finally I tightened cogs on the cassette, going with a spacer from a 9-speed cassette and 12t cog also from 9-speed and just filing them down a bit. I.e. the news is that you can use 9-speed pieces on the Brompton cassette just as long as you use a thin retaining ring.

I still have some unease in the drivetrain (momentary skating) when running at the highest gear ratios, but this is somewhat normal when ironing out the wrinkles. An update to the gear chart from @pastorbobnlnh/Sheldon Brown is below, I incorrectly claimed that my middle ring is 34t - it is 32t. (It was 34 on the way, hence confusion.) I use distance traveled because I understand what it means. However, Sheldon's calculator gives too few digits for the lowest gears. In terms of practicality there are 15 distinct gears here.

https://cimg7.ibsrv.net/gimg/bikefor...7b1023e391.png


Chart for 27-Speed Brompton

Ahhhhh! A clean transmission is always appreciated by changing gears! :D Glad this solved many of your shifting issues.

I wonder if the "skating" could be solved by a new chain? You don't mention what brand or speed chain you are currently using (at least I couldn't find this with the search function). I did see where you speculate that it would be best to stick to a 9 speed or lower chain. It would seem to me that if you are using 10 and 11 speed sprockets, a matched chain would be essential to better shifting performance. I'm with you that I wouldn't want to shell out the $$$ for an 11 speed chain. But consider trying a new 9 or 10 speed SRAM or KMC chain.

Or, remove and clean your current chain. This is how I clean my chains: Remove and wipe thoroughly with a WD40 soaked rag. Next, I scrub mine in hot soapy Dawn dish detergent, followed by 1/2 hour in the ultrasonic cleaner with a Branson MC-3/hot water degreaser solution. After a complete rinsing in hot water it goes in the oven at 200F for at least a 1/2 hour. As soon as I remove it from the oven I lube each link with warm ChainL oil.

I went ahead and deleted my post with the Sheldon Brown Gear Calculator Chart since it was incorrect. Best not to confuse folks with the wrong information. :thumb:

I went then back and corrected the middle ring size, 34->32. Apologies for the initial incorrect info.

I am usually bad as far as cleanliness of the drivetrain is concerned and may be this was a lesson that should be absorbed.

Following @berlinonaut I got myself a 10-speed chain for this project, i.e. the issue is not the width of the chain. This has been my first venture into 10-speed components and I am glad to see how inexpensive they have become.

I am not sure where the skating comes from. It could be e.g. the chain getting pushed over by the front derailleur onto the chainguard or interaction between the pulley wheels that close to each other and lift the chain to go between cogs. Sometimes such things emerge only when the chain is operated under tension and are not easy to see on the stand. It may need to wait until the weekend when I can work on the bike during daylight.

I also use an ultrasonic cleaner, but put Simple Green there. I will check what Branson MC-3 is. I use my own mixture for lubing, competing with ChainL ;) and providing a protection under the rough weather conditions we commonly have.

My thought . because of the molded single piece, flanges & toothed pulley design, the chain oil dirt combined solids, packed in, have no place to go..

Wondering if a few holes thru the flanges .. a part of some of the CNC after market pulleys . gives some of that inevitable accumulation a way to sluff off and out..

3 speed , its no issue , backing disc gets a bit grimy , but it does not rotate in unison with the lower pulley ..







....

The side holes are there but indeed the flanges and plastic, rather than metal, likely contribute to the grime accumulation. When I looked at the aftermarket of pulleys and tensioners for Brompton, it seemed that you needed to pay a lot of money for a dubious gain. However, I may have another look.

back of the envelope design ...
 

A scheme to add the flanges to a good commercially available sealed bearing pulley , like Tacx , sandwiched between them popped in my mind ,,
the flanges need not be part of the pulley to work as far as I can grasp the mechanics involved..

brass or oil bronze tubing can join the 3 pieces and slide along the pin in the chain tensioner.. as required.. & separate to do the occasional cleaning..

pulley can even turn faster than the flanges.. fixed to the bushing sleeve the bearing rolls around ..





.....

The "skating" when riding with the largest ring turned out to be the residual problem with poor shifting to the middle cog from the smallest cog :( that I hoped to have solved with cleaning of the pulleys. (The latter helped but evidently not fully.) The problem that developed could be best illustrated with the diagram describing shifts: 12t -(pathologically hard)-> 14t -(pathologically easy)->16t and 12t <-(normal)- 14t <-(normal)- 16t. If tension in the shift cable was too low, the derailleur was failing to move the chain from 12t to 14t. This seemed tied to an insufficient stiffness in the plastic derailleur yielding hiccups in the derailleur operation. If I increased tension in the cable, the derailleur immediately started to overshift, jumping from 12t to 16t for the middle shifter position and then falling back to 14t, and so on, yielding the "skating".

I could not find a middle-ground cable tension that would solve the problem and I went with a nuclear option. Given that I had cassette pieces for all kinds of speeds, I reduced the gap between 12t and 14t, by using 10-sp 12t cog with built in spacer, and I increased the gap between 14t and 16t, by using a 9sp spacer there w/o filing. In my estimate one gap was reduced by just 0.15mm and the other increased by that amount. This nuclear option took care of the pathologies (or basically went along with them) and all shifts fine now for all gears in all combinations. In the back of my mind I worry though that, if 0.15mm matters, then with temperature changes or something, and the derailleur arm softening or stiffening, the problem might come back. If you do not try, though, you will never know :D.

The bike is now all ready for a trip in 2 weeks from now, where my survival crucially depends on this bike - my lodging location will be inaccessible by public transport for much of the week. The bike better perform there.

@2 I, where are you traveling to? Sounds like an interesting adventure. For the three rear sprockets and the Brompton RD, is the shifter friction or indexed? I've been assuming friction.

It is just Germany, but I will be thrown into fields, with public transport there only to carry people to work and back and the nearest shop 20min away by bike, not even on the way of the public transport. There might be snow there, so I ponder taking along a studded tire for the Brompton, but I will check the forecast right before my travel. The shifter is an indexed SunRace M90. I have also a friction version of this shifter, that largely uses the same components. Friction can be easier when you are free to concentrate on the shifting, but I want to be ready when the surrounding is rough and I have to concentrate on making it through.