It seems to me that this could be the start of a never ending story. Let’s jump into chapter one of the serial called “perfect gear changes with my Brompton”. First the basics – the Brompton gearing consists of two major parts:
- a rear hub with three gears (called “Brompton Wide Range”, BWR in the 6-speed variant) and
- two small sprockets to double the available options of the rear hub
If you ride a 2-speed Brompton you don’t have the gears of the rear hub but still the two sprockets. If you are running a 3-speed Brompton you got only the gearing of the rear hub (called “Brompton Standard Range”, BSR) and no sprockets. There are older 5-speed models out there too.
The gear box in the hubs gives me no problems at all. Maybe it’s a little bit on the heavy side but it is trouble free so far.
This post deals with the other option: the derailleur to shift from one sprocket to the other.
Instead of a derailleur you may have seen on a current mountain or road bike – the Brompton uses something called chain pusher. That’s an archaic piece of engineering. To realize a crisp “state-of-the-art” shifting experience is out of scope with this solution.
You see two stop screws on the chain pusher. The left one stops the movement of the pusher in the direction of the high gear (away from you), the right screw stops the movement for the lower gear (towards you). Just make sure they don’t block the movement of the pusher to early. Second: the pusher should have max an occasional contact to the chain. There should be no rubbing pressure while the idler (the small black wheel) rotates.
I don’t want to run into Brompton bashing with this post, but the gear trigger (here with the cap removed) seems to fit on a shoddily made bike from the superstore but not on a high price Brompton. At least it works. Just don’t expect a crisp shifting from this part.
You see the derailleur gear cable fixed to the trigger. You can move the bearing down to increase the tension on the cable. The Brompton manual states “cable adjustment should seldom be necessary”. In contrast IMHO this is a good point to start. Try it out. Very easy to do. Open the trigger, move the bearing, try.
Your cable should be undamaged and clean. Mine has to be replaced soon because the wire starts to break.
Back to the back – if you remove the chain pusher – you will look on the derailleur chain pusher wing plate. Clean it – nothing more you could do here.
Here you have a look on the bottom side of the derailleur chain pusher – this side will run against the wing plate. Some dust is usually there but nothing that affects the function. Just check it.
If the chain-pusher does not move freely try slackening the M3 screw slightly. Here I nearly removed the screw – that’s too much – the screw shouldn’t be to tight. Put some Loctite on the thread to secure the screw. Try out to move the pusher – you should feel no resistance.
This M3 screw connects the chain pusher to the dog leg assembly. Check the screw – if the screw is to tight your shifting will suffer.
Don’t know how to check the dog leg assembly or improve it. This construction may be one possible source of problems with the shifting performance. The pusher doesn’t move back smooth and the bucking seems to be connected to this part of the gearing train. I will dive into it on a later.
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13 responses to “Brompton shifting problems”
A New Cycle Computer
I, too, was a bit dismayed by the appearance of the shifters when I picked up my new B, but, more than 1200 miles/1 921 km later I have no complaints; mine seem to shift very well, both precisely and easily.
My B’s gear indicator cable did fall out, but it’s likely that was due to faulty replacement by a mechanic — and my own failure to check it regularly.
So while there may be some reason for keeping an eye on things, at least in my experience, there’s been no trouble. I’ll be curious to see how you fare as time goes on.
It works – but it’s no sharp “click” like with a current Shimpanso derailleur. The shifting performance of the 3gear hub is fine. My problem are the two sprockets… I’ll try a new cable and pusher next…
Great description of the parts….thanks. Very helpful pix. On my second 2 speed brompton now. Agricultural shifting, luckily shifting seldom used!
After a particularly rough ride through the pasture checking cows, my 6 speed Brompton started making a rubbing/bumping noise when on the inner chainwheel. After cleaning grass seed, etc. from the various sprockets (which couldn’t have helped), it appeared that the chain pusher was allowing the chain to go too far that direction. I found your information here and adjusted the chain pusher screw, which fixed the problem. Thank you for the clear explanation and good pictures. I love my Brompton and have had no other problems. I find the shifting easy and smooth, even in cross country conditions. Hi ho Brompie, away!
thanks for the nice comment ;)
hi do you know where i can get a complete set of this? wish to convert 3 speed to 6 speeds
just ask you local Brompton dealer – they should be able to help you
My derailleur has suddenly stuck in low gear – large sprocket. It should be in this position when the cable is tensioned, yet it remains there when the cable is released. The cable is not stuck, and I’ve even removed the M3 screw to find that the chain pusher moves freely but is not biased by spring tension. It looks like there is a spring somewhere to pull the assembly into high gear when the cable is released. Is it in the chain pusher? Ironically, I have replaced this once (it never worked properly because it was stiff from new) but I don’t remember if it had a spring, and I haven’t managed to find any references to one. Many thanks.
hmm – hard to tell without touching your bike. The tension is build by the part called “dog leg assembly”. If the derailleur chain pusher is moving freely I would look there.
I’ve had the identical problem with a 2017 6 speed. The issue is caused by road grit getting between the two plastic derailliur tension cogs and the stems on which they are mounted. (an indication is if stiff to back pedal). A really poor designed part which so close to the road surface, should have a seal to prevent dirt from the road getting between the surfaces but that said, its fairly straightforward to strip and clean them (15mm spanner and small hex key) after which both parts should spin and importantly slide in and out easily.
thanks Paul for your help!
Thank you very much for your great pictures and your web page! It helped me a lot to adjust my gears on my Brompton…in contrast to the Brompton manual!
I’ve got shifting problems in wet weather since day one on my 2015 Brompton. They seem due to the tension cogs not sliding correctly on the plastic axles, as Paul V mentions. But I never got these problems before on my old 2003 Brompton, although the derailleur doesn’t seem to have changed much in the years (I can’t directly compare, the old one got stolen…). I’ve thoroughly cleaned the cogs a few times but shifting problems always recur, even with minimal dirt. Any trick (besides WD40) appreciated.