Author Topic: Brompton derailleurs differ?  (Read 1698 times)

LittleWheelsandBig

  • Whimsy Rider
Brompton derailleurs differ?
« on: July 26, 2020, 05:09:38 pm »
Apparently Brompton changed the derailleur dogleg connector when they swapped to the underbar shifter in 2017. Does anybody know if the derailleur springs are interchangeable between the two?

I have purchased an aftermarket Ti spring to replace the somewhat corroded decade-old original but it is a slightly larger overall diameter and does not look like it would fit into the plastic cable casing stop.
Wheel meet again, don't know where, don't know when...

LittleWheelsandBig

  • Whimsy Rider
Re: Brompton derailleurs differ?
« Reply #1 on: July 27, 2020, 09:26:07 am »
A reply on the Brompton e-list suggests that there is no functional difference for the derailleur spring on pre-2017 and current derailleurs. In either case, a Ti Parts Workshop derailleur spring is a larger diameter than the original and needs some bodging to fit within the plastic cable stop. I am inclined to drill a larger recess to allow the spring to fit comfortably.

The major difference seems to be the cable fixing location. Old derailleurs/ cables use a tiny grub screw within the lever. New ones are secured at the derailleur.
Wheel meet again, don't know where, don't know when...

LittleWheelsandBig

  • Whimsy Rider
Re: Brompton derailleurs differ?
« Reply #2 on: August 03, 2020, 08:41:29 pm »
I have fitted a stiffer and shorter aftermarket external spring and a Brompton gear cable and casing in accordance with the instructions. The derailleur does not shift into the bigger cog and my guess is that the internal spring is merely compressing when the lever tightens the cable.

The original external spring is about the same stiffness as the internal spring, so the derailleur moves when the lever pulls the cable. The only way the stiffer spring would compress would be if the internal spring is fully compressed at all times.
Wheel meet again, don't know where, don't know when...

LittleWheelsandBig

  • Whimsy Rider
Re: Brompton derailleurs differ?
« Reply #3 on: August 29, 2020, 11:11:04 am »
Went back to the original spring and everything works properly.
Wheel meet again, don't know where, don't know when...

Geriatricdolan

Re: Brompton derailleurs differ?
« Reply #4 on: December 17, 2020, 09:53:01 am »
Got fed up with the mini derailleur getting jammed every time the bike is dirty, it shifts from the 12T to the 16T but doesn't want to go back until I clean everything... so I converted my S2L to a single speed. Removed shifter, derailleur and even the second sprocket, which I replaced with a spacer... All looks neat and tidy and my 73 inch gear is plenty to go everywhere...  ::-)

AND it's lighter...  ;D

LittleWheelsandBig

  • Whimsy Rider
Re: Brompton derailleurs differ?
« Reply #5 on: December 17, 2020, 09:56:51 am »
So you reused the B chain tensioner with no modifications to the pulleys?
Wheel meet again, don't know where, don't know when...

Geriatricdolan

Re: Brompton derailleurs differ?
« Reply #6 on: December 17, 2020, 12:15:13 pm »
So you reused the B chain tensioner with no modifications to the pulleys?

Yes, why would I need to change the pulleys?

LittleWheelsandBig

  • Whimsy Rider
Re: Brompton derailleurs differ?
« Reply #7 on: December 17, 2020, 12:18:28 pm »
Uncontrolled lateral movement of the pulleys might have been an issue but the proof of the pudding is in the eating.
Wheel meet again, don't know where, don't know when...

Re: Brompton derailleurs differ?
« Reply #8 on: December 17, 2020, 12:18:55 pm »
The 1 and 2 speed tensioners are very different - the 1 speed is much simpler and doesn't allow the pulleys to move sideways.

It probably weighs less too.

Geriatricdolan

Re: Brompton derailleurs differ?
« Reply #9 on: December 17, 2020, 12:22:23 pm »
My thinking is that the derailleur doesn't help keep the chain in place, it's just there to guide the shift... so if you remove it, then you can't shift, end of the story.
As far as I can see, the chances of the chain dropping are the same with or without the deraileur and since it has never happened, I don't think it's a major issue...  ::-)

EDIT: as we are on the topic, any idea if the brake pads of a 2018 model are the same as Shimano road?

LittleWheelsandBig

  • Whimsy Rider
Re: Brompton derailleurs differ?
« Reply #10 on: December 17, 2020, 02:01:00 pm »
Yes
Wheel meet again, don't know where, don't know when...

Geriatricdolan

Re: Brompton derailleurs differ?
« Reply #11 on: December 17, 2020, 02:38:59 pm »

Geriatricdolan

Re: Brompton derailleurs differ?
« Reply #12 on: December 17, 2020, 03:51:07 pm »
Pads changed, as I always keep a stack of Shimano pads...

Now I just have to winterise it for the January weekly commute... I've ordered a Schwalbe Winter for the front...
I used to bus into work on the icy days, but with Covid I'll rather take my chances on the bike... I could drive, but the fuckers have put up the parking fee... £ 7 a day if you don't have an annual permit... £ 7 a day for the privilege to work... unbelievable!  >:(

Re: Brompton derailleurs differ?
« Reply #13 on: December 17, 2020, 06:38:36 pm »
in the derailleur arrangement the tension pulley is free to move but doesn't because the position of the guide pulley is constrained. The chain doesn't fall off but it does get noisy when riding out of the saddle etc.   If the guide is not properly constrained the chain can of course unship from the sprockets.

It isn't at all difficult to pack out the pulley bushings with a stack of washers and therefore define the position of the guide pulley at least, so why not do that?

cheers

Geriatricdolan

Re: Brompton derailleurs differ?
« Reply #14 on: December 17, 2020, 08:17:00 pm »
in the derailleur arrangement the tension pulley is free to move but doesn't because the position of the guide pulley is constrained. The chain doesn't fall off but it does get noisy when riding out of the saddle etc.   If the guide is not properly constrained the chain can of course unship from the sprockets.

It isn't at all difficult to pack out the pulley bushings with a stack of washers and therefore define the position of the guide pulley at least, so why not do that?

cheers

I could, I just don't see how removing the mini derailleur would make any difference in how likely the chain is to unship. The derailleur does not touch the chain at any point, unless you are shifting, the shift whacks the chain, that moves to the adjacent sprocket and the tensioner adjusts laterally a tad. If you don't whack the chain, it should not unship. I don't think the derailleur acts as a guide when you don't operate it

Anyway, I'll give it a try... the side to side movement of the jockey wheels is barely noticeable

Re: Brompton derailleurs differ?
« Reply #15 on: December 18, 2020, 10:41:07 am »
The derailleur does not touch the chain at any point, unless you are shifting...…
When you are pedalling normally this is usually the case. But 'usually' isn't 'always' and I think the pusher occasionally contacts the guide pulley at other times too.

Quote
Anyway, I'll give it a try... the side to side movement of the jockey wheels is barely noticeable

Most derailleur systems fail by allowing unintended lateral movements between the guide pulley and the sprockets.  To engineer this in from the start (especially when it can be engineered out eg for the cost of a few washers) seems a bit mad to me.   But who knows, it might work for you.

cheers

Geriatricdolan

Re: Brompton derailleurs differ?
« Reply #16 on: December 18, 2020, 11:11:20 am »
Thanks,
I'll see if I can find a couple of washers

I guess another thing I could do is to shorten the chain by a couple of links (it's due a replacement soonish)... if I no longer use the 16T sprocket, I should be able to live with a shorter chain and therefore more tension/fewer chances of things going wrong. Fold worked fine when in the large sprocket, so I can't see any drawback...

On a related note, do I need a Brompton specific chain, or any 8-9 speed Shimano chain would work equally well? The 16 T sprocket did look like a Cassette one, but the 12 T seems quite a bit thicker...

Geriatricdolan

Re: Brompton derailleurs differ?
« Reply #17 on: December 18, 2020, 01:43:28 pm »
I've disassembled the tensioner, the shims need to have an ID of 17 mm, so I have ordered a pack of 10 on Ebay.
The adjustment can only be done so that the jockey wheel sits on the left side, so it's probably best I place the 12 T sprocket as far as possible on the left and move the spacers to the right

LittleWheelsandBig

  • Whimsy Rider
Re: Brompton derailleurs differ?
« Reply #18 on: December 18, 2020, 01:52:48 pm »
I think that may result in a slightly better chainline.
Wheel meet again, don't know where, don't know when...

Geriatricdolan

Re: Brompton derailleurs differ?
« Reply #19 on: December 18, 2020, 02:02:36 pm »
I think that may result in a slightly better chainline.

Good to know... I would have thought it would be worse... it's a narrow hub and I would have thought the chainring would be better aligned with the RH sprocket...

LittleWheelsandBig

  • Whimsy Rider
Re: Brompton derailleurs differ?
« Reply #20 on: December 18, 2020, 02:07:01 pm »
Most of a Brompton rear hub’s ‘narrowness’ comes out of the LH side. The BB is slightly offset to the left also. Easy enough to sight along the chainring to check the chainline anyway.
Wheel meet again, don't know where, don't know when...

Re: Brompton derailleurs differ?
« Reply #21 on: December 18, 2020, 03:57:27 pm »
I've disassembled the tensioner, the shims need to have an ID of 17 mm,.
The adjustment can only be done so that the jockey wheel sits on the left side,..

The pulleys float by virtue of there being a long centre bushing. I spaced one of these out for singlespeed use a few months ago and

a) I used washers, I forget which exact size but it (sure as eggs is eggs) wasn't 17mm
b) it was of course possible to put the pulley anywhere within the normal float range, by putting washers one side or the other of the pulley.

This is with the current model of tensioner.



see the bushings in the photo above (centre)?  The washers need to fit over those.

cheers

Geriatricdolan

Re: Brompton derailleurs differ?
« Reply #22 on: December 18, 2020, 04:56:33 pm »
Ah OK, I thought I would place the washers on the plastic arm that holds the wheel, which is 17 mm, that would prevent the wheel from going right and would keep it left only