Author Topic: Brompton derailleur  (Read 2914 times)

Brompton derailleur
« on: June 20, 2013, 12:44:19 pm »
The parts appear to be moving freely, but it is failing to change gear - adjusting the cable means it either fails to go into 1st or fails to go into 2nd.

What part is likely to need changing? The cable is new.
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that's not science, it's semantics.

Biggsy

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Re: Brompton derailleur
« Reply #1 on: June 20, 2013, 01:28:33 pm »
There are little screws to adjust on the mech.  They work in a different way from a normal derailleur's, but have the same end result.  See the Brompton manual (online if you don't have a paper copy).

Give it a few drops of oil as well.  I think it's unlikely to need anything replaced.
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Zipperhead

  • The cyclist formerly known as Big Helga
Re: Brompton derailleur
« Reply #2 on: June 21, 2013, 12:41:37 am »
Mine got very unhappy about changing like that, eventually it wouldn't change unless you got off and pushed it manually. I tried lubrication, adjustment, new cable etc. None of these made any difference.

After a bout of head scratching/swearing I replaced the chain and the cogs* and that fixed it and it changes beautifully again.

* - actually I changed the chain and that's when I found out just how knackered the cogs were. The new chain kept slipping on them and they were visibly beyond the point where you might describe them as knackered.
Our son does know who Boz Scaggs is, we've done ok as parents.

Re: Brompton derailleur
« Reply #3 on: June 21, 2013, 02:36:50 pm »
There are little screws to adjust on the mech.  They work in a different way from a normal derailleur's, but, but have the same end result.  See the Brompton manual (online if you don't have a paper copy).

Give it a few drops of oil as well.  I think it's unlikely to need anything replaced.

Thanks, that fixed it - they're very well hidden, aren't they? :)

I'm still a little unclear how changing the derailleur endstop has made it work, when adjusting the cable could put it in either gear so it wasn't hitting the endstops.

Zips, that's a good point, and in the past has worked for me. I regularly spend a few months in single speed before getting round to changing stuff. Recently did the cables, chain and sprockets and was surprised not to get the gears back.

Oh, and within a kilometre of fixing this, the folding pedal snapped.  :( This is proving a very expensive month in worn out bits.
Quote from: tiermat
that's not science, it's semantics.

Re: Brompton derailleur
« Reply #4 on: April 23, 2014, 03:03:25 pm »
Sorry to revive an old thread, but I'm having a similar problem with my S2L.

Fine in first, won't change up to second when I accelerate.  So I am spinning like mad.

Have lubricated with GT85, cleaned dirt out etc.  Have adjusted the little screw.  Everything moves freely, but just won't change up.

Having another go tonight, and if still playing up, will use hubby's S2L tomorrow.

Mine is 3 years old and does 10 miles a day.  The chain does not appear to be stretched, but would it be a good idea to replace it and the two jockey wheels that are inside the derailleur (the cogs)?  They don't look that worn to me but I guess some wear could be hidden by the chain.


Re: Brompton derailleur
« Reply #5 on: April 23, 2014, 03:17:33 pm »
At 8,000 miles I'd expect the chain to need replacing, but that shouldn't stop you changing gear.

Have you tried adjusting the cable? Take the back off the shifter and move the plastic doobrey into another notch.
Quote from: tiermat
that's not science, it's semantics.

Re: Brompton derailleur
« Reply #6 on: April 23, 2014, 03:22:18 pm »
Hubby thought about that last night, but when we looked on the Brompton website, it said you shouldn't need to adjust the cable, so I have visions of him trying to take it apart and vital springs pinging across the living room followed by much cursing.  Might try that tonight though.

Zipperhead

  • The cyclist formerly known as Big Helga
Re: Brompton derailleur
« Reply #7 on: April 23, 2014, 04:42:13 pm »
From memory, because I haven't had to look at mine for a while. If you swing the chain tensioner out of the way (you don't need to remove it from the axle) and look at how the gear change mechanism works, it's a fork on the chain (one of the tensioner jockey wheels keeps the chain in place in the fork) but the cable pulls more than it needs to, it pulls a spring which moves the fork.

That's why Brompton say that you shouldn't need to adjust the cable.

However, the things that I would check would be - do the jockey wheels move from side to side freely?

I might also suspect that the cable isn't moving freely inside the outer housing. Again, from memory it goes in the reverse way to a normal gear cable - you insert it at the change mechanism end and push it towards the change lever and the nipple is at the change mechanism. It might be worth taking the inner cable out and seeing what state it's in, is there any rust visible on it for instance? If there is then you might get away with cleaning it and putting a drop or two of oil on before it goes back in.

[Modified to add]
I've just remembered that when my S6 started doing this on the two speed changer I went through all of the above with no joy, changed the chain, no joy. Went through my entire book of swear words, no joy. Got a friend to teach me to swear in Hindi, no joy. Put new sprockets on, sweet changing restored.
Our son does know who Boz Scaggs is, we've done ok as parents.

Re: Brompton derailleur
« Reply #8 on: April 23, 2014, 04:53:37 pm »
Zipper

When you say the sprockets, do you mean the two metal ones on the back wheel?  I'm not very good with bike terminology I'm afraid.  Does the chain move from one sprocket to the other to change gear (in which case wear on those would make sense not working).


Zipperhead

  • The cyclist formerly known as Big Helga
Re: Brompton derailleur
« Reply #9 on: April 23, 2014, 05:21:05 pm »
Yes, sorry the two metal sprockets on the back wheel. They probably look scarily worn with teeth like a junkie, but that's how they're meant to be (to allow the chain to move from one to the other easily).

Again, yes, the chain moves from one to the other to change gear. The plastic assembly with the plastic jockey wheels keeps the chain tensioned by the arm with the bottom wheel moving against a spring, and the other wheel holds the chain into the fork mechanism which moves the chain from side to side to persuade it to jump from one metal sprocket to another.

Both of my Bromptons have got hub gears as well, and on those the sprockets are held in place by a big circlip which can be removed with a couple of screwdrivers and some swearing - I don't know about the two speed, but it's probably the same.

Looking at the SJS website it looks like it is - this is a set of sprockets for two speed (not necessarily the ratios that you have) and if you look at the picture you'll see that the sprockets have little teeth sticking inwards, which is how they transmit your power to the hub (so they are not screwed on requiring much force and swearing to remove). Once the circlip is removed from the outside end then the sprockets and spacers will just slide off.

It's always a good idea to take note of what order they are in. The sprockets are easy, the bigger one goes nearer the spokes, but the spacers may be different thicknesses. I normally line the old ones up across a newspaper while I clean things up. It helps keep the swear word count down.
Our son does know who Boz Scaggs is, we've done ok as parents.

Re: Brompton derailleur
« Reply #10 on: April 23, 2014, 08:10:55 pm »
Thanks Zipper. Hubby is on derailleur disassembly and repair duty tomorrow as he has to stay at home all day while we get the boiler replaced. I am using his S2LX until mine is fixed. His is a few years older but lightly used, will probably do it good to get some miles on it. Just went up the road and it changes gear as sweet as anything  >:(

Zipperhead

  • The cyclist formerly known as Big Helga
Re: Brompton derailleur
« Reply #11 on: April 24, 2014, 12:33:18 am »
If he has problems getting any of it apart due to it being bastard tight, let me know. I have a yoof who is well trained in standing just out of arm (and spanner) reach and gently flinging in insults.

He will of course apologize once the offending items have been freed off.
Our son does know who Boz Scaggs is, we've done ok as parents.

Re: Brompton derailleur
« Reply #12 on: April 24, 2014, 08:43:30 am »
Watched the Brompton video and tried their 'if all this adjusting screws stuff doesn't work' option of adjusting the cable at the shifter end.  Take off the shifter housing and there is a set of slots that the cable hooks in to, to tighten and loosen just move up and down a slot.  Moved up a slot to tighten, and yay  :thumbsup: :thumbsup: :thumbsup: it all works now.  No cursing involved.  May have seen lightbulb above hubby's head when he saw the video though.

Re: Brompton derailleur
« Reply #13 on: February 08, 2019, 07:17:29 pm »
After a bout of head scratching/swearing I replaced the chain and the cogs* and that fixed it and it changes beautifully again.

Can anyone give a good mechanical reason why that would work?

Having had the same problem, I found this (very old) thread, followed up the links and suggestions and so on, and found it very helpful. I'd already stripped off and cleaned the sprockets (and cleaned the chain, obviously), and stripped and cleaned the chain tensioner, derailleur etc. Following this thread, I had a fiddle with the cable in the shifter. That seemed to help, and for a week or so it's been OK. Now it's sticking again.

Logic says change the cable, so I popped into a shop, but they stuck a gauge on my chain and said change the chain and sprockets. The chain's probably 6-8 months old; the sprockets came with the (lightly used) bike nearly a year ago.

The Brompton replaced the Dahon I broke. That was getting new chains every 18 months maybe, doing the exact same commuting trips. It even had a new cassette once in three and a half years! The Brompton uses basically a similar chain - none of this 11-speed stuff that wears out as soon as you walk out of the shop. On my old 5-speeds, 40 years ago, I'd have expected 3-5000 miles at least before changing a chain. I'll allow that stop-start commuting probably causes more wear than sport riding, mind you.

But the killer is that worn chains and sprockets, if anything, cause a derailleur to change too often. They don't stop it changing. So what's going on? Just to confirm, the issue is the chain not dropping onto the small sprocket.

I'll measure it myself tomorrow, and it may well need a change. But I'm not sure I won't still need a cable too...