Hi, need a little help here from the Brompton experts. I have just upgraded my Brompton 2-speed to a 6-speed. The 3-speed BWR hub is working fine, but I can't get the 2-speed derailleur to run smoothly in the lower gear (larger sprocket). The shifting from the small cog to the big cog works ok, but during pedalling it feels rough. I could feel vibrations through my feet. It doesn't happen in the small sprocket though.

I have already tried adjusting the derailleur, and even tried out a newish one, with no difference. It was working well before I upgraded to 6-speed. Does anybody have the same experience, and if so, how did you solve the problem? Any suggestion will be most appreciated. Thanks.

Did you swap out the chain as well? The new system may need a different length and or width chain. Could also be a chain line situation. Have no experience with Bromptons, but changing chainrings and sprockets/ cassettes can effect chain requirements.

Unfortunately I can't give specific help... but what I would do in your situation, is first establish, is the roughness felt in all BWR gears? Sometimes geared hubs may impart a rough feeling especially more so in some gears. If that draws a blank, backpedal or pedal on some sort of back wheel off the ground setup and try to spot what rubs to find the culprit. The only one I can think of is the derailer actuator rubbing against the chain but you have already eliminated that, I understand.

I have a Brompton six speed with the Stram T3 instead of Sturmey-Archer hub and of course dual cogs and derailleur system and I can tell you that its prone to skipping and some roughness if the cogs are dirty or the chain is dirty and requires some attention to that area of the drivetrain but I don't think this is the case with your newly installed system. Clownbike might be right that a slightly narrower chain might be required.

where did you get the cogs ? where they from Brompton ? IF so did the vendor help at all in the situation or solutions?

I haven't tried swapping out the chain yet. It's the same chain that is on the 2-speed system before I upgraded, so the width shouldn't be an issue. May try using a different length to see what happens.

Thanks for the suggestions. Yes, the roughness is felt in all BWR gears when the chain is on the large sprocket, but not at all when it is on the small sprocket. The funny thing is I don't feel the roughness when backpedaling (in the large sprocket).

The chain is the original one used on the SL-2, with the two gears. Isn't that already the correct width? I ordered the BWR and new cogs online from a Brompton supplier. I haven't asked them for advice yet, as they are only a parts supplier.

Anybody here has a six-speed BWR?

Have you tried adjusting the limit screws on the thing that pushes the chain over? Is the chain rubbing against it?

Let me put it this way...an 8 speed derailleur system uses a different chain than a 9 speed, etc. And every bike I've ever purchased had a too long chain, which leads to chain slap and possibly funky gear changes. When changing chainrings and cogs/cassettes, the chain length should be adjusted to fit the combo.

Best thing to do is pick up Park Tool's Big Blue Book. It will help you understand bicycle engineering and how it relates to maintenance and modification. Which comes in handy even if you're not going to be performing the actual work.

Odd, blissfully ignorant in the early 60's I ran an 1/8" chain 3 cogs in a 3 speed hub,
with a triple chainring set, and never noticed an issue, 40~ 50 years ago.

Yes, I have. There's an instructional video on how to do this here: https://www.youtube.com/watch?v=TwkTWiPuXYY. I have also followed the instruction manual https://www.brompton.co.uk/extranet/d...gtrigdr_ds.pdf.

Both the 6-speed system and the 2-speed system use the same 2-speed derailleur with two gear cogs. The only difference is the hub. So I don't understand why chain width is an issue here. I have searched online and have found no mention of the need to change the chain. I think I'll try it out anyway.


Actually, I'm not a newbie; been doing my own bike maintenance for years. I'm very familiar with road and mtb derailleur systems. The Park Tool's Big Blue Book doesn't say anything about Brompton's unique 2-spd derailleur design. :-)

I wonder if the chainline is a little different with the BWR hub, causing the teeth to rub against the chain in one case.

I've read posts about the 1/8 chain plus derailleur combination being noisy, the reason being the chain isn't laterally flexible enough.

Ah, it was here:
https://www.foldsoc.co.uk/bromptont6.html

I'm pretty sure the original chain on the SL2 is 3/32".

Yes, I thought about that too. But if that is the case, what can be done to solve the problem, other than adjusting the limit screws of the derailleur?

Oops, I didn't read the post properly.