Cycling UK Forum

I have a ten year old Brompton 3 speed (M3) with a 44T chainring which I would like to use for touring in hilly places. In its lowest gear the bike travels 338cm for one pedal revolution which, for me, is much too high a gear for touring in hills. My touring bike does about 190cm for one pedal revolution in lowest gear and copes well with steep hills. I'd like to get a similar lowest gear for my Brompton but estimate that I would need a 25T chainring which would not give high enough gears on the flat. One solution would be to fit a double chainring with say 25T and 38T rings. I am happy to change manually between chainrings. Is this possible and if so what components do I need? I want to use off the shelf components if they exist and do it without any drilling or compromising the folding mechanism. I looked at a related thread on this forum but it did not address this question and I have been unable to find anything on the web. I don't think that there is room to increase the size of the rear sprocket to give me a low enough gear. Any help gratefully received. I can be contacted on clivegm@gmail.com

Googling "brompton + double chainring" throws up lots of hits. This goes into a lot of detail. A quick skim suggests that with your 10-year-old chainset it may not be too difficult to bolt on another ring.

An alternative is the Schlumpf mountain drive (2-speed bottom bracket) but that sounds like megabucks.

As in LollyKat's link, this chainset http://www.spacycles.co.uk/products.php?plid=m2b0s109p2188 and an extra chainring.
At Spa, we used to bolt the small ring to the "wrong" side of the big ring......much easier for the manual change!
86 BCD chainrings go down to 28 teeth from memory.

Thanks for the swift and helpful replies. I must work on my Google searching!

Better to have the inner chainring as the smallest because chainline will be better..You are likely to need a slightly longer bottom bracket but I have heard of stories of people putting on an extra washer or two on the right side if you have a bolt to hold the chainwheel to the axle ..Washer pushes the chainwheel out a few millimetres..saves getting a new bottom bracket..
Not sure a good idea myself but ok to experiment and buy a longer bottom bracket later..
Check on the fold though because a new double may work really well but may not fold without inner ring hitting the folding rear..
I think if you go below say a 32 or 34 teeth chainwheel the chain tensioner will not cope and it will cause problems when riding and folding..

531colin wrote:As in LollyKat's link, this chainset http://www.spacycles.co.uk/products.php?plid=m2b0s109p2188 and an extra chainring.
At Spa, we used to bolt the small ring to the "wrong" side of the big ring......much easier for the manual change!
86 BCD chainrings go down to 28 teeth from memory.

Confirmed 86 BCD chainrings DO take a 28-tooth ring as the minimum size.

OFF Topic :
In extremis, that ST55 chainset MIGHT be just what I need to replace my ageing Stronglight 80 ( which is long-time out of production ).
With 40-t outer and a 28-t ring inner plus spacer & bolts it would be an almost exact match - with the rather big downside that once the outer is worn out the unified crank and ring would have to be completely replaced.

Kinetics in Glasgow do various kits (or will build for you) to put a more capable gear hub in a Brom.

http://www.kinetics-online.co.uk/folding-bikes/brompton/

Ben will also do a Schlumpf mountain/speed drive for the front if you prefer that avenue. A friend has an 8/Schlumpf and has been very happy with it and the way it's made is leetle friend a much more do-anything machine.

Pete.

Not quite on theme but almost. I have a 3 x 2 Brompton with a 44t chainring which I have now changed to 40t. I have not altered the chain. In theory I should remove 2 links to bring it to 96 instead of the standard 98 for the 44t.On the basis of being lazy anyway and if it ain't broke don't fix it I have left the chain alone. Will 2 links make any practical difference anyway? On a short test run it seemed ok. The gears this gives suits me fine and the system proposed by the OP seems a bit too far down .

clivecycle wrote:.... In its lowest gear the bike travels 338cm for one pedal revolution which, for me, is much too high a gear for touring in hills. My touring bike does about 190cm for one pedal revolution in lowest gear and copes well with steep hills....

those are 42.4" and 23.8" gears respectively.

I agree that a twin sprocket at the rear, or a double chainset would be fairly easy modifications that would give you the lower gears you desire.

cheers

531colin wrote:.....At Spa, we used to bolt the small ring to the "wrong" side of the big ring.....

Do you remember what kind of chainset bolts you used?
(I bought stronglight double ones (from Spa) but they seem too narrow).

SA_SA_SA wrote:

531colin wrote:.....At Spa, we used to bolt the small ring to the "wrong" side of the big ring.....

Do you remember what kind of chainset bolts you used?
(I bought stronglight double ones (from Spa) but they seem too narrow).

No, I'm afraid not....but my guess is that John Pocklington who owns Spa would remember....he certainly used to know all the bottom bracket lengths, and all the spoke lengths, which I find quite staggering.

SA_SA_SA wrote:

531colin wrote:.....At Spa, we used to bolt the small ring to the "wrong" side of the big ring.....

Do you remember what kind of chainset bolts you used?
(I bought stronglight double ones (from Spa) but they seem too narrow).

I have a set of bolts and spaces to turn one of those into a triple chainset - if I can find them....