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Folding Bikes Discuss the unique features and issues of folding bikes. Also a great place to learn what folding bike will work best for your needs.

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Old 08-12-07, 11:52 AM   #1
jasong
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15t sprocket (cog) for modern Brompton 3spd?

Is there enough clearance for a 15t cog on modern (mine's 2007) 3spd sturmey archer bromptons?

I searched a bit, but from the ~10 year old archives that said only a 14t would work. There is a tight clearance between the frame and cog.

The Brompton manual says that this should work, but this breaks the data from the folding society archives :

==========
REAR SPROCKETS: for geared hub (to ISO 10230): non-derailleur, " x 13T x 3mm thick (but for lower gearing, use a 14T or 15T sprocket): derailleur, special Brompton sprockets, 13T and 15T only.
==========

Any easy source for the part in the US? I'm not familiar with their interchangeability with other more common parts (ie BMX).
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Old 08-12-07, 02:02 PM   #2
LWaB
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Any standard hub gear cog will fit on the splines.
I'm not sure about cog size on a modern Brommie, our two are '98 and '99. I think they changed the chain tensioner a few years back to give room for a slightly larger cog.
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Old 08-14-07, 10:05 PM   #3
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LWab, I can't seem to identify what are the standard hub gear cog that will be compatible. Do you have suggestions? From the little reading I've done, this seems to be a possible mod on the newer Bromptons that have more clearance with the tensioner and possibly chain stays.
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Old 08-15-07, 03:26 AM   #4
EvilV
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Quote:
Originally Posted by LWaB View Post
Any standard hub gear cog will fit on the splines.
I'm not sure about cog size on a modern Brommie, our two are '98 and '99. I think they changed the chain tensioner a few years back to give room for a slightly larger cog.
I think the main limitation will be the clearance between the cog and the chain tensioner. It has a fixed jocky wheel that runs with a small clearance. You need the advice of a specialist here maybe. You can join the Brompton Talk email list on Yahoo for that kind of help if no one pops up here with the answer. There are bike dealers on there who monitor that list and come in with sound advice.

http://groups.yahoo.com/group/BromptonTalk/

EDIT:

Ah - I just checked and it appears that this very question has just been asked. Jasong obviously got there without help.
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Old 08-31-07, 09:05 PM   #5
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I think the main limitation will be the clearance between the cog and the chain tensioner. It has a fixed jocky wheel that runs with a small clearance. You need the advice of a specialist
Anyone following this thread might also consider a simple modification in adding a standard tapered crankset to the Brompton. With a 39/53 crankset and doing the front derailling yourself, you can get a cheap 6 speed Brompton that will have the reduced initial lower gearing and also be quite fast in the bigger ring.

It's a $25 modification (or less if you find an old larger crankset) that you can do in 10 minutes (you may need a longer chain).
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