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.
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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