Brompton 3 or 6 gears

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berlinonaut

Veteran
Location
Berlin Germany
Updated information: I got hold of the spec. of the sprockets and smaller parts for the various hubs via a Brompton technical data sheet. Looks like that (inner to outer):

SRAM 3-speed
0,85mm dust cap
2,2mm spacer
3,0 or 3,1mm sprocket
2,2mm spacer
circlip (to hold the parts in place)
________________________________
total 8,05 - 8,15mm

SRAM 6-speed
0,85mm dust cap
2,0mm 15t sprocket
2,2mm spacer
3,0mm 13t sprocket
circlip
________________________________
total 8,05mm

S/A 3-speed (Alloy shell -> SRF-3/BSR), not applicable for the older steel shell 3-AW
2,1mm dust cap
no spacer
3,0 or 3,1mm sprocket
no spacer
circlip
________________________________
total 5,1-5,2mm

The circlip (QRSPRCLIP) is identical for all hubs as is the 13t sprocket (QSPR13DR) for SRAM and S/A-3-speed (not BWR hubs). The 3-speed versions can however take a couple of different 13t or 14t sprockets as well (all being 3,0 or 3,1mm of width), on the SRAM 6-speed it has to be the 3,0mm wide part number QSPR13DR (which has a flange that has to be mounted inwards on the 6-speed) . The 15t on the SRAM 6-speed (QSPR15DR) has an arrow that has to face outwards.

I measured the available space on a 2008 SRF-3 (BSR) hub and ended up with shortly below 5mm of space on the driver from the inner limit to the beginning of the notch that the circlip sits in.

So the BSR runs short by 3mm and even if one would omit the dustcap (assuming that this would work) one would still run 2mm short. So with the stock conversion parts/sprockets intended for the SRAM hub it defitively does not work to convert the SRF-3/BSR to two sprockets. A very comitted worker could maybe try to slim down the 13t sprocket from 3,0mm to 2,0mm and try to use a massively slimmer circlip (along with omitting the dustcap). Maybe two sprockets would physically fit - don't know. A lot of tinkering anyway. If it would not only physically fit but also work I cannot tell.

Obviously the SRAM offers - as assumed - way more space on the driver than the BSR, thus a straightforward, simple conversion is not possible and I doubt that the tinkering way would work flawlessy (but love to be proven wrong). If - for whatever reason - you want six speeds of a BSR swapping to the driver of the BWR along with some other parts seems the way to go (which offers the convenience of a far wider range of sprocket-sizes to choose from as well as way easier possiblities to get hold of those). Alternatively go for a used SRAM wheel - the are typically not very expensive.
 
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kais01

Regular
Location
Sweden
thanks for the thourough walkthrough berlinonaut. but writing that was most definitely a bigger effort than making the conversion work regardless:smile:

key seems to be to use a thinner dustcap and grind the cirklip narrower on the side which supports the cogs, something i have performed a number of times. takes about two minutes.

when i get home from vacation in a few days i will check the dimensions in the kit.
 

CSG

New Member
Location
Idaho
I don't know a thing about the new models but when I bought my M6L in 2011, and living in Idaho, getting the six-speed (three really with a low and high option) with the 12% reduction made great sense. I'm not a heavy user of the bike but it rides in the back of my Land Cruiser under the philosophy of "always have a bike". It's been trouble free for eight years so far and running on the original Brompton Kevlar tires.

I almost never use the lowest gears but they are there when I need them.
 
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Kell

Veteran
In Edinburgh the 6-speed with reduction is fab and well worth it. You can also get on the straights up to 20mph without totally spinning out so it’s very useful.

I bought the reduced chainring last time I swapped it out - as a trial run to help my knees on the climb out of my train station.

But, IMHO, it's compromised the speed of the bike too much elsewhere. 22mph is now where I top out (I prefer a lower cadence) and rather than being just low enough to bring each additional higher gear into play on my usual stretches (i.e. being able to use 5th where I would normally use 4th), it's effectively cut a couple of mph off every other stage of my commute as I'm still using the same gears, pedalling at a similar rpm and therefore going slower.

Next time I order a new chainring, I'll be going back to the 50T and just have to lump it on the hill.
 

Ming the Merciless

There is no mercy
Location
Inside my skull
I don't know a thing about the new models but when I bought my M6L in 2011, and living in Idaho, getting the six-speed (three really with a low and high option) with the 12% reduction made great sense. I'm not a heavy user of the bike but it rides in the back of my Land Cruiser under the philosophy of "always have a bike". It's been trouble free for eight years so far and running on the original Brompton Kevlar tires.

I almost never use the lowest gears but they are there when I need them.

I have the philosphy of "always ride a bike". Can't you fit the land cruiser in the front brompton bag in case of problems?
 

Handlebar Moustache

Well-Known Member
Location
Scotlandia
I bought the reduced chainring last time I swapped it out - as a trial run to help my knees on the climb out of my train station.

But, IMHO, it's compromised the speed of the bike too much elsewhere. 22mph is now where I top out (I prefer a lower cadence) and rather than being just low enough to bring each additional higher gear into play on my usual stretches (i.e. being able to use 5th where I would normally use 4th), it's effectively cut a couple of mph off every other stage of my commute as I'm still using the same gears, pedalling at a similar rpm and therefore going slower.

Next time I order a new chainring, I'll be going back to the 50T and just have to lump it on the hill.

Fair enough - our mileage may differ ;-)
 

Tenkaykev

Guru
Location
Poole
In Edinburgh the 6-speed with reduction is fab and well worth it. You can also get on the straights up to 20mph without totally spinning out so it’s very useful.

I just copied and pasted your post into a message to our daughter who lives in Porty.
She's just got a 6 speed Brompton after having a trial ride on mine whilst visiting a few weeks ago.

I've offered to take a 44 up with me when we visit later this month.
 

Handlebar Moustache

Well-Known Member
Location
Scotlandia
I just copied and pasted your post into a message to our daughter who lives in Porty.
She's just got a 6 speed Brompton after having a trial ride on mine whilst visiting a few weeks ago.

I've offered to take a 44 up with me when we visit later this month.

I’m not the fastest but I am always confident of going anywhere on my B - even up Dundas / Victoria Street
 

12boy

Guru
Location
Casper WY USA
For quite a while I have gone with a 4 speed......58 and 38 tooth chain rings and 12 and 17 sprockets. Pretty good most of the time but steep grades plus a 15 mph headwind was taxing. Since I am using a 118 mm Shimano BB and a 110 BCD crank arm it was easy to add a 24 tooth chain ring which I will test further today. These chain rings require manual shifting which requires stopping but only takes 10 seconds, and the bottom two are rarely used. The 58/17 combo is good for 5% grades of a 1/2 mile or less. This set up weighs much less than a 3 speed IGH but doesn't provide a higher gear than 77 gear inches. Still, it is a simple, clean set up that is cheap and easy. A 58/38 chain ring can also be done easily with the 130 BCD crank that comes with newer Brompton, but I don't think less than 38 is available. So, Kell, add a chain ring if you want to keep your high and low options. From your video clip of going to work at at 40 plus mph, you could go with the 50 to work and switch to the smaller to go home. That assumes the ride in is mostly downhill.

Since wrting this earlier I did discoverer that it rode fine but the 24 tooth chainring hit the mainstay when folding so I took it off. I can replace the 17 tooth sprocket with an 18 and replace the 38 tooth chain ring with a 34 but I may wait a while on that although it gets me to about 30 gear inches which might be a little better.
 

berlinonaut

Veteran
Location
Berlin Germany
I bought the reduced chainring last time I swapped it out - as a trial run to help my knees on the climb out of my train station.

But, IMHO, it's compromised the speed of the bike too much elsewhere. 22mph is now where I top out (I prefer a lower cadence) and rather than being just low enough to bring each additional higher gear into play on my usual stretches (i.e. being able to use 5th where I would normally use 4th), it's effectively cut a couple of mph off every other stage of my commute as I'm still using the same gears, pedalling at a similar rpm and therefore going slower.

Next time I order a new chainring, I'll be going back to the 50T and just have to lump it on the hill.

If you spin out at 22 mph with a 44t chainring and a BWR with stock 13t/16t this equals a cadence of ~83 while 22 mph equal about 35 kph according to ritzelrechner. Both pretty impressive in my opinion. I do not reach cadence of more than 80 for more than a short distance most of the time (in the flat) and consider around or slightly above 75 to be my favorite. I would personally not consider 35 kph to slow on a Brompton - my usual travel speed is around 28 kph with the 2-speed with 60/12-16. I run my BWR with the 44t chainwheel but considered this too low as well - this was one of the reasons to modify it to 44/12-14-16. Way better for my taste, offering sufficient speed in the flat and sufficient hill abilities at the same time. Still a tad shorter than the 50t chainwheel (which I consider to be too long anyway) but with a much wider spread and longer than the stock 44t.
 
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Kell

Veteran
I must admit, I hardly used 6th on the 50T chainwheel, but I did use it.

83 does sound a little low, but I've not done the maths (and wouldn't have known where to start), but I do find it harder to spin quickly on a Brompton as opposed to my road bike. I've got a couple of those Sufferfest videos I use on the Turbo Trainer and at times you're spinning at 110RPM, which I feel would be almost impossible on the Brompton. Maybe it's the shorter cranks, or the position you're in on the bike.

I did look down this morning as I was descending a very steep hill and noticed that it was about 25 mph when it felt unsafe and I couldn't keep up - certainly not with the 44mph top speed that I get down there.
 

Kell

Veteran
Tell a lie, this morning it was 47mph.

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