Author Topic: Brompton derailleur slipping  (Read 1894 times)

Brompton derailleur slipping
« on: September 02, 2018, 05:19:04 pm »
I'm a bit puzzled by a problem on my P6R.

I was on the way to work on Friday, and it suddenly started slipping in the small sprocket. Once it started, it did it consistently, several times on each pedal stroke. I had a quick look and didn't see anything, but found that I could put it on the large sprocket and still have three hub gears with no such problem, so did that for the rest of the day.

I've been adjusting the derailleur stops, using the Brompton instructions. It seems a lot better (one slip every ten or so pedal revolutions), but I can't clear the issue. I'm not entirely convinced that the root cause is in the derailleur adjustment, to be honest. For one thing, adjustment problems develop slowly - but this started quite abruptly and then was consistent. I even wondered about a broken sprocket (happened to me once on a Regina freewheel, decades ago), but it's pretty clear that it's not that. Watching carefully, it does seem that the large sprocket is a bit out of roundtrue - it moves from side to side a bit, like the ring on a cheap chainset.

What does the panel suggest? I've ruled out hub gear issues on the basis that it's correctly adjusted, and doesn't slip in the large sprocket, in any (hub) gear.

As an aside, I can't quite see how the Brompton derailleur works, in one key respect. On a conventional derailleur, you use cable tension to position the derailleur correctly, and the limit screws are just a backup to prevent the chain going off completely. On the Brompton, the over-ride spring means that cable tension is pretty-much outside your control. So, it's almost as though the chain itself keeps the derailleur correctly positioned, rather than the other way around. Of course, the cable and spring are limiting any side-to-side pull, but not, surely, setting it as precisely as in the conventional design?


  • Whimsy Rider
Re: Brompton derailleur slipping
« Reply #1 on: September 02, 2018, 07:33:39 pm »
A cracked small cog would behave pretty much like you've described. Have you taken the cogs off and given the small cog a good clean?
Wheel meet again, don't know where, don't know when...

Re: Brompton derailleur slipping
« Reply #2 on: September 03, 2018, 12:01:49 am »
Not yet. Need to set some time aside to do it. I have inspected it as carefully as possible without removing it. The fact that it slips much less often after adjustment is odd, if that is indeed the cause, but these things can be a bit unpredictable.

I wasn't clear whether the two sprockets were one unit (in which case only one cracking is not likely) or two separate cogs. I've taken a look at some product pages, and I can see now how it goes together.

It's not likely to be wear, by the way. I've looked for that, obviously, and I put a new chain on around early May, after the old one broke.

Re: Brompton derailleur slipping
« Reply #3 on: September 04, 2018, 05:00:13 am »
The sprockets are separate flat plates.

I’d suggest giving the upper jockey wheel a good clean.

Re: Brompton derailleur slipping
« Reply #4 on: September 04, 2018, 08:05:03 am »
Thanks. I'm going to try that. I did clean one part (but not that) while investigating before. Like LWaB, I thought that the original problem was more like a cracked sprocket, but the fact that I managed to reduce it by adjustment may suggest not. So I think a strip-down and clean is called for.

If I could have spotted the problem in advance, I could have laid in any spares - I'll want the bike again the next working day. Probably a job for the weekend now.


  • Professional Gobshite
  • Just a tart for retro kit . . .
    • John's Bikes
Re: Brompton derailleur slipping
« Reply #5 on: September 04, 2018, 11:38:53 am »
If this was a standard derailleur setup I would suggest excessive chain and/or sprocket wear, simply the small sprocket has worn to the point that it can't grip the chain.

Can't offer anything more due to a total lack of experience.

Well that's the more blunt way of putting it but as usual he's dead right.

Re: Brompton derailleur slipping
« Reply #6 on: November 05, 2018, 08:35:15 pm »
When I was about 129kg, I had a P6R with no issues

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Re: Brompton derailleur slipping
« Reply #7 on: November 05, 2018, 11:00:49 pm »
Bizarrely, this problem just went away by itself some weeks ago. I'm at a bit of a loss, because that kind of problem doesn't normally do that. But I'm taking something of an "If it ain't broke..." approach.