chagzuki

I thought it'd be useful to collate info on swapping out the Brompton chainset in one thread; anyone who's done this can share their experiences here. (This isn't an upgrade that I've undertaken, but I'd like information readily available such that I know what choices there are).
Because of the nature of the Brompton fold making any changes to the stock parts can lead to problems, and with the chainset there's a bunch of things that need to be taken into consideration; chainline, Q-factor, frames being scratched from colliding parts, etc.

Whether there's any benefit to changing the chainset or BB is debatable, the stock parts are very light and it's not easy to find any replacements that reduce weight. However, many people report the stock chainset/BB combo to be flexy and to break easily, plus the chainring isn't replacable. If the teeth are worn of if you decide to switch to a different size ring it requires switching the drive-side crank. And people may have spare chainsets handy and wonder if they'll be compatible.
Many people use a double chainring setup with or without front dérailleur, or a Schlumpf system (Jur?).

So far I only have stats for the Brompton 44T chainset:
44T: Left: 190g, Right: 288g, Chainguard: 57g, Total: 535g
*** Bottom Bracket plus bolts: 279g
Total: 814g

The bottom bracket is a square taper ISO type, spindle length 119mm, made by ***. It appears to be model L66BSA-EV119/23AE which sells for as little as 10 EUR in Germany(!). Doesn't seem so easy to find in the UK, though SJS do some *** bottom brackets (cheaper than the official Brompton one) as do Spa cycles.

Stock cranks are 170mm.

I've seen the claim made online that the Brompton chainset is a rebadged Stronglight 55S chainset, but there seem to be some differences in the hole patterns and the Stronglight version only appears to go up to 46T. Here's a pic of the Stronglight 55S:

So far I have no info on whether this is the same weight as the Brompton crankset. It can be found at a significantly cheaper price (particularly in the US), so if it is the same there's no sense in buying the Brompton version.
It seems as though the same chainset is also referred to as the ST55 on various websites.

By my measurements the Brompton chainset Q-factor is about 160mm, I'm under the impression this coincides with Stronglight cranks in general.
Shimano tend to be 145.5 for road double and 154.5mm road triple chainsets. This would indicate that using the Shimano double configuration (as below with octalink 105) would lead to a smaller clearance between crank arm and rear frame when folded under (Shimano crank arms are generally narrower which counters the reduced Q-factor somewhat in this respect). Using cranks longer than 170mm might, with this reduced clearance, lead to collision as the rear frame gets wider towards the rear wheel. Plus there would be a slight increase in risk of heel-strike (might be an issue if one is using skate wheels).


A detailed thread in French on the issues in installing an Shimano Octalink 105 chainset:
https://www.bromptonforum.net/t4606-l...n-mono-plateau


More here:
https://www.bikeforums.net/showthread...-Brompton-Mods
Again using the 105 chainset, indicating that some bottom bracket spacers on the drive are necessary.


Cool mods using a Shimano Hollowtech 2 setup plus FD:
https://www.bikeforums.net/showthread...himano-nexus-8



I'll add any relevant information/stats posted in this thread to this post, as it evolves.


Can anyone confirm which spindle length Shimano UN54 BB fits the Brompton chainset? As I understand it using a JIS type spindle should mean taking about 4mm off the length? The UN54 or an equivalent older model can sometimes be found very cheaply, good to have an alternative.
According to this thread (hard to figure out via google translate (which is better than my French)) there's only a 0.3mm difference when installing an ISO crank on a JIS BB, which would mean that a 118mm UN54 (or equivalent) BB would produce the correct chainline:
https://www.bromptonforum.net/t3122p2...er-de-pedalier

fietsbob

The chainring is swaged onto the Crank arm, of the standard Stronglight made for Brompton cranksets.
So a change to a crank which bolts on a chainring will handle more torque,
say adding a second chainring for mountain climbing.

the swaging connection has come loose on the stock cranks,
like putting a 30t, 86bcd chainring

Hilly west coast,
I fitted a Schlumpf Mountain Drive crankset, 54t, 130 bolt circle.
the BSR has a 15t (3/32 wide from the Mk3, Sachs 2by3 hub)

Florian's company makes a Brompton Specific Model.

neither crankarm is connected directly , the right one turns faster
than the chainring , in low range.

the reduction gear torque strut has a knob that sits above
the BB /rear pivot portion of the frame.

the internal reduction gear has a big drop, so the next lower gear,
from the low gear in the hub, direct range, is low range, High Gear.

so a wide range 6 speed .. [pretty = %, 33, 41 at the double shift]
the planetary low is a 21.6't' ..

with a 13t cog a 47t chainring will do similar .. ~18" ~ 80" range..

chagzuki

I finally got round to trying a Shimano octalink setup and things worked out perfectly. Unlike in the article mentioned above I opted for a spacing based on a triple BB, i.e. 118.5mm, and put the chainring on the inside. Chainline is perfect and Q-factor should be around 154mm, perhaps a shade narrower than the Brompton crank. Clearance over the rear triangle is fine with 172.5mm cranks.
The bike is dusty and the chainring an odd colour, so the pics don't look fantastic but they should give a clear indication of the fit.
This spacing should work on earlier square taper Shimano cranks from the 5 years or so before they switched to the octalink type as in that period their chainline/q-factor dimensions stabilized. A square taper system would be preferable in terms of BB choice; road Octalink BBs are expensive now and the Ultegra range has been discontinued. The 105 BB is overpriced.








I've been led to believe that a Sugino XD2 with either a 110mm or 113mm BB would fit the Brompton. This seems like a good option, it's a nice-looking Crankset and people might have spares left over from Dahon bikes or other builds. However I'm not sure what the resultant q-factor would be other than that it would fit the bike.

jur

I will have stuff to add to this thread in not too long.

14R 

Campagnolo Centaur. Poor man's FD is the foot.

chagzuki

Looks nice. Could you give some details about the chainline and the bottom bracket spindle length you've opted for (edit, actually this is an external BB crank, isn't it?), plus how crank arm clearance compares to that of the original? And it'd make for a better organized thread if added details are edited into original posts so all info is easily accessible.

I've been looking at older square taper Campagnolo Record and Chorus cranks and it looks as though chainline and Q-factor ought to be near enough equivalent to shimano cranks using an ISO taper BB of 111mm and chainring on inside of crank, however, availability of suitable chainrings (135 BCD) for single ring setups is questionable.

kamtsa

Great thread, will follow it closely. Hopefully it will include a recipe (with specific part numbers) for a two chainring configuration. I have a 6sp 44t but would like to have a second, lower chainring for steeper hills I cannot climb now.

Also, I presume that the mods above do not require an irrevocable modification of the frame. If so, please mention it in the description.

chagzuki

It appears that the 2013 Brompton will feature a new chainset with 130 BCD removable chainring mounted on the inside of the spider(!).

14R 

New Brompton, same chainring:

jur

Titanium seatpost?

amf14

Titanium crankset!

https://www.facebook.com/photo.php?f...levant_count=1

fietsbob

Who makes those?

spambait11

Tried a 118mm ISIS bottom bracket today (60T chainring, 165mm cranks, 15T cog), and my chainline is off. Probably should have used a 108mm bottom bracket instead, but I can't be bothered to change it now.




Permanently set up as a single speed.




BTW, 60T is way too big and affects the fold: the chainring teeth smash into the plastic hook used to hold the fork onto the chainstay.

chagzuki

What chainset is that, spambait11?

Abeillaud

A quick reply.

I'm the author of this article. Sorry for you, it's french. But very informative, argued, very detailed and didactic as possible with many sources. It took me a long time. It's worth taking the time to read. There are technical files of the mounting. I mentioned all the useful features and success criteria. When speaking about technical aspects, it's important to be as complete as possible. If not, risk of misunderstanding.

Here is an example of mounting (presented as #1 and #2 ) with translation for the readers of Bikeforums.net. This is the "technical file" (summary) with accurate measurements.

What you could (aslo) read in french in our article:

Fiche technique du montage #1 (Résumé)

In green, success. In red, to be improved.

Frame Brompton Mark 4, year 2009, with original steel rear frame (main frame # 285337)
Rear hub: Brompton Wide Range BWR, 2 sprockets, unmodified.
Type of axle of the bottom bracket: Octalink V1
Bottom bracket: Shimano BB 7700. Dimension: BSA 68 mm, lenght of the axel: 109,5 mm. Category "semi ajustable" [définition gived in french in this article]
Crankset: Shimano FC 7700
Position of the chainring: outside
Spot facing ("lamage") 1,5 mm à 2,0 mm detph on the spider
With reduction of the original chainring bolts flush to the surface of the spider)
Spacer on the BB (thickness): 1,5 mm drive side (on the right of the bike)
Space between the spider and the rear frame when folded: 1,5 mm
Chain line: [46,3 +- 0.5] mm, almost perfect. [définition of perfect chain line described in french in this article).
Q-Factor: [146,5 + - 0.5] mm
Symmetry of the Q-Factor (delta): + 5 mm (excessive value if used with clipless pedals). [définition and calcul of the symmetry mentionned also]

Conclusion of the mounting #1: mount successfully, except the delta Q-Factor.
Must add an additional ring on the BB to reduce the delta (if ring 2 mm, new delta 1 mm), therefore in the spot facing of the spider will not be useful, but + 2 mm line chain, or the solution of the washer on the axle of the right pedal (thickness of the washer on the pedal axle to 'cancel' the value of delta,= 0), or use the two schemes to avoid changing too much the chain line and to reduce too the screwed f the right pedal (as a solution, a ring of 1 mm thicker on the BB and a washer of 2 mm to achieve a new delta of +1 mm ).

What you could aslo read in french in our article:

Fiche technique du montage #2 (Résumé)

Main frame Mark 4, year 2007 , with original titanium rear frame (# main frame: ?)
Rear hub: Brompton 2-speeds hub (year 2008), 2 sprockets, unmodified.
Type of axle of the bottom bracket: Octalink V1
Bottom bracket: Shimano BB 7700. Dimension: BSA 68 mm, lenght of the axel: 109,5 mm. Category "semi ajustable" [définition gived in french in this article]
Crankset: Shimano FC 6500 Ultegra
Position of the chainring: outside
Spot facing ("lamage") NO
With reduction of the original chainring bolts (but short bolts, you can find it for some BGP, sold for fixie)
Spacer on the BB (thickness): 2,5 mm drive side (on the right of the bike)
Space between the spider and the rear frame when folded: ~1 mm (all's goo)
Chain line:: [47,7 +- 0.5] mm , almost perfect".
Q-Factor: [146,8 + - 0.5] mm
Symmetry of the Q-Factor (delta):73,9 mm (left crank to center of the tube)- 72,9 mm (right crank to center of the tube) = + 1 mm (symmetry almost perfect)

Conclusion of the mounting #2: mount successfully, including the delta Q-Factor.
Note that the delta of Q-Factor is 1 mm instead of + 5 mm. A case identical, what can explain this difference? The space between the spider and the rear frame is both 1 mm. The same range cranksets using the same axis system (FC 6500 and FC 7700) according to the logic should not play. The main frame is the cause of this difference? This is possible because no other parts can have an impact on this measure.

Do you have measure the symmetry of the Q-Factor on your Brompton?

The mounting with a new BB-7000 and almost new and complete FC 7700 costed for me 75 euros!
The mounting with a new BB-7700 and a new and complete FC-6500 costed 85 euros.

You have to search in the big old new stock or in second hand market for the crankset.
We have already made 5 mounting with a crankset and BB in Octalink V1 (we explain that bearing internal frame is better to adjust the chain line on a Brompton and even taht there is also a category entirely ajustable as certain old Campa Record BB, but hard to find now).

Picture of the FC-7700: https://farm8.staticflickr.com/7014/6...9caa8e05_o.jpg
As the Campa Record old serie, according to our taste and our knowledge, we think the best better aesthetics for the B (classical and refined).
Note that the BB-7700 is an exceptional quality and is the master piece of our mounting. But you have to mount it perfectly (follow the instructions) if not premature aging.
To save ~30 euros, you can chose a 105 or ultegra Octalink V1 BB, it's just a sealed cartridge in this case, but better than the original BB mounted till now by Brompton.


Olivier from Belgium
Abeillaud [Bromptonforum.net]
https://www.bromptonforum.net/u809

chagzuki

Hi Abeillaud, nice to see you here. That was an impressively detailed article you wrote, quite frighteningly so in fact. I was considering trying to get hold of a Campagnolo Record crankset but I figured the limited availability of suitable chainrings be too restrictive. I'm sure it would look nice.

Interestingly, from what I've seen the 2013 Brompton crankset will not be particularly classical aesthetically.
Edit: oops, looking again actually it resembles the Record crankset, in a somewhat chunkier guise.

Abeillaud

My post was also written for Chen Mun (from Singapour) to give a public answer in english to his private questions.


Olivier
who doesn't write in english as he would like to do it.

___________
Le meilleur article pour "bien commencer en Brompton" et découvrir les richesses de ce forum [Written by a friend, surely one of the best article for the Brompton's users, sorry yet in french]
Mon Brompton World Championship 2011... Récits de rencontre bromptonesque... Mark I #264: Sanierungsprojekt... Mark I #426: tribute to Andrew Ritchie and Julian Vereker... and our discussion about Mark I.
Mark IV:tuning's projects... Vous pouvez me retrouver sur le chemin des glaces...

Abeillaud

Yes it looks a little like the Campagnolo Record: https://fbcdn-sphotos-f-a.akamaihd.n...88184886_n.jpg
But certainly without the same quality. A campa Record with the big chainring in the inner position: https://farm4.static.flickr.com/3663/...580c280345.jpg (source: https://www.bromptonforum.net/t3165-c...ier-campagnolo )
For the look, if possible, the chainring with a outside position is more aesthetical.
Note that the integration of the chainring on the spider is not a total success for the new crankset of Brompton. And we know nothing about the BB that will be installed on the Brompton bicycle from 2013.

Positive point: for the first time with this Brompton's "innovation", that will be mounted in the factory from 2013, we are no longer forced to throw the right part of the crankset when the chainring is too damaged. Few Brompton bicycle 's owners have (and use) more than one chainring. but I think it will be possible to change the size of the chainring (BCD 130 mm, 5 bolts I guest) without having several cranksets.

I suppose that the Brompton's crank will be always delivered in the (standard) lenght of 170 mm. So there are yet a lot of advantages to think "alternatives". And certainly, when you can keep the same budget for these other "alternatives" parts. And more, it's funny to tune ourselves our "B" sharing our discovers and tricks between friends and fans.

alesi

Hi Oliver,

I'm the guy from Singapore who wrote you the email. Thank you so much for answering. Incidentally, I have asked the tech guy from Brompton factory, his reply was that JIS bb may be the replacement although nothing is confirm for now. Going the JIS way may a wiser choice in my opinion due to many option to choose from.

chagzuki

Great pictures. The Record looks lovely indeed and the Brompton chainset very nice also, I'd not seen that pic before. I wonder what the BB spindle length will be, hopefully I'll have a ti BB that'll fit.

Abeillaud

Yes, not so easy to follow, even in french, this discussion about the compatibility JIS/ISO between a crankset and a bottom bracket.

What I could translate for you? Perhaps the experiences of Fredo.

Fredo wrote (source: his original french-speaking post) that with a crankset Tiagra (JIS axle), there was not a problem on a ISO axle.

I translate his last posts (24th March 2010) - I hope that it will be more understable than the translation of a boot.

First post:

"I measured several combinations of mounting:
I mixed the original bottom bracket with, in the first case, the original Brompton's crankset and, in the second case, a Shimano Tiagra crankset. I repeated the operation [with the both cranksets] with a bottom bracket Shimano BB51.

[His observations:]

The axis is fitted on the crank to a depth of:

On the Brompton's axis [ISO]:

Brompton bracket [ISO]: 16.4mm
Tiagra crankset [JIS]: 17.6mm

With the Shimano axis [JIS]:

Brompton crankset [ISO]: 15.3mm
Shimano Tiagra crankset [JIS]: 15.6mm

Note, the "input" [not found the right term fot my translatation] of the square on the crank Brompton is chamfered, not the Shimano. So the "contact" is longer (+ / - 1 mm) with the crank Shimano)

The difference of the "fitting" is very small in fact, between 0.3mm and 1.2mm (considering the same bottom bracket).

The best value is obtained with a mixed mounting: 17.6

I tried to measure the size of the square on the end of the axis but a chamfered axis of the Shimano prevented me from taking the measurement axis end ...

Note: I tightened the cranks on the axis so as regularly as possible but I have not used a torque wrench: these values ​​are not measures "lab" but reflect a certain reality."

And in the second post:

Fredo wrote:

"I mention the conclusions of "Berthoud":

JIS crank on a ISO crankset, clamping is possible, there is even a margin of a few milllimeters.

ISO crank on JIS axis: the "loss" of penetration is only 0.3mm compared to a fitting JIS / JIS."

And my answer about the UN-54:

I used a UN-54 (Shimano Tappered square) of 118 mm (of course BSA 68 mm). Not a problem with the original Bromton's crankset (model 2009) to fix it on the axis and the chain line was correct (visual observation). Sorry, I did not measure the chain line and Q-factor (+ delta) of this mount

Parenthesis: anyway, It's possible that the delta Q-factor is not good originally, today, on a Brompton. Of course with flat pedals, no matter. And so, we know also that a part of the production (before the Mark IV - pre-2005-, if well understood) of the Brompton bicycles, that the frame was not exactly "straight" looking it on a measuring table. I have 2 Mark I and perhaps soon an early Mark II, I'm curious to verify this point, the bikes are yet in (very) good condition and not modified.

To return to the UN-54: not really a great bottom bracket. I prefer other BB if I have to chose a sealed cartridge for less than 25 euros.

Abeillaud

You are lucky, Stéphane who realized this mounting is a friend and the answer, coming soon.

Abeillaud

Here it is:

"I do not remember. It's been 3 years, I sold this stuff.
It was an axis of the Record's serie. Adjustable. I do not know the length."

spambait11

The cranks are from a company called FMF, which makes BMX cranks - the cheapest way to get 165mm cranks and smaller without breaking the bank. The chainring is a single speed 110BCD chainring from Vuelta.

I may change the length of the bottom bracket once I get my new wheelset. We'll see if the rear cog can be adjusted to match the chainline or not.

A.Danger

I'm really benefitting from this thread, thanks to all. I want to change out the BB and cranks on my Brompton. The goals is aesthetics, naturally but most importantly: weight reduction. What is the classiest-looking, but functionally lightweight combo? Currently running 6 sp (3 sp Sram hub + 2 sprockets). I've seen Zipp carbon, but it's not so pretty. Any suggestions? Thank you!