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.