Lowering the gearing on a Brompton

[fivebikes]

I use the 44t chainring and the standard twin sprocket set up with the SA sourced wide range hub that Brompton have used on their six speeds.
To ease the legs on 35-40 mile hilly rides I’ve added a Swytch kit. Adds a bit of weight but definitely helps on hills!
The go to sources for Brompton mods though are Steve Parry (Google is your friend) who fitted different derailleur set ups including front doubles and 7 speed rears.
Also Ben Cooper at Kinetics https://www.kinetics-online.co.uk/foldi ... hloff-kit/ who can create all sorts of gearing/braking/luggage carrying options. Spoken to him a couple of times and he’ll have a solution…..including bigger wheels on a Brompton if that’s your thing! Lots of options but some are expensive as the components aren’t cheap…Rohloff hubs for example.
The extra chainring at the front, shifted using a stick or greasy finger at the start of a climb is the cheapest way to go. You pays your money and takes your choice!

[iandusud]

Thanks Philip, that's very helpful. I like to climb with very low gears, and I'm used to a bottom gear of about 18" (and even with that I'd be walking up anything that said 11.3%!). It was mentioned upthread (and I can see for myself) that a large sprocket may foul the chain tensioner; when you fitted your 18t sprocket, was that the maximum possible, or was there clearance for a bit more?

Now I am in front of my laptop I give exact figures.

My gear ratio are as follows
CR 44
13= 34.7, 54.4,85.3
18 = 25.1,39.3,61.6

I think you may be able to fit a 19 tooth just? see photo

This looks like a sensible range of gears (no need for anything bigger than 85" for touring IMO) and adding a smaller inner chainring for particularly hilly sections would be a real plus. A 34t chainring would give a 19" bottom gear. What I don't know, which is relevant, is the capacity of the chain tensioner.

[djnotts]

" 34t chainring would give a 19" bottom gear. What I don't know, which is relevant, is the capacity of the chain tensioner."

I did no tech calcs, but no problems in practice with a 38/34.

[2_i]

I can see the benefits of lowering the gears but is there a ratio where the torque at the hub can become excessive ?

I ride on 20T chainring pretty regularly in winter and in the mountains. In heavy snow you hardly ever go off the 20T.

[Philip Benstead]

Some on here said they have they fitted a double or even a triple chainset on a brompton, do they have front derailurer, I been told they do not work can we have some photos please?

[djnotts]

I don't think anyone who has mentioned a double chainset has used a front mech. I haven't - "My 6 speed runs a 38/34 up front on a standard hub. Manual/greasy finger change."
However if you Google "front derailleur on a Brompton" then many examples will be revealed (including I think from past discussions on this Forum).
With the massive increase in interest in Bromptons in other countries, often with huge spend on modifications, I am sure that the full conversion has been done!

[jimlews]

The early pioneers of 'double clangers' used to use a short stick for up-changes and their heel for down-changes.

https://en.wikipedia.org/wiki/Paul_de_Vivie

[fivebikes]

Some on here said they have they fitted a double or even a triple chainset on a brompton, do they have front derailurer, I been told they do not work can we have some photos please?

This earlier thread shows photos although the links to where the brackets come from are out of date!
viewtopic.php?t=117916
I’m sure Google will locate something for you…Steve Parry front derailleur at a guess!

[nirakaro]

When Brompton were prepared to deal with Spa we regularly used to fit an additional smaller chainring on the crank side of the existing chainring, to be changed by hand.

The Brompton chain tensioner looks to have a very limited capacity. Does this mean that the chain runs slack if it's on the smaller chainring? Is that a problem?

[rualexander]

Some on here said they have they fitted a double or even a triple chainset on a brompton, do they have front derailurer, I been told they do not work can we have some photos please?

https://youtu.be/FUVr0y_KDec

[2_i]

The Brompton chain tensioner looks to have a very limited capacity. Does this mean that the chain runs slack if it's on the smaller chainring? Is that a problem?

Quite opposite is true: the Brompton's tensioner may well have the highest capacity of any commercially available tensioner.

[2_i]

Some on here said they have they fitted a double or even a triple chainset on a brompton, do they have front derailurer, I been told they do not work can we have some photos please?

And one more:

https://www.youtube.com/watch?v=KgxxOImRpHI

[nirakaro]

The Brompton chain tensioner looks to have a very limited capacity. Does this mean that the chain runs slack if it's on the smaller chainring? Is that a problem?

Quite opposite is true: the Brompton's tensioner may well have the highest capacity of any commercially available tensioner.

Sorry, I wasn't clear - I was referring to the chain tensioner on the 6-speed, not the full derailleur.

[rualexander]

The Brompton chain tensioner looks to have a very limited capacity. Does this mean that the chain runs slack if it's on the smaller chainring? Is that a problem?

Quite opposite is true: the Brompton's tensioner may well have the highest capacity of any commercially available tensioner.

Sorry, I wasn't clear - I was referring to the chain tensioner on the 6-speed, not the full derailleur.

You're still not clear

[nirakaro]

Quite opposite is true: the Brompton's tensioner may well have the highest capacity of any commercially available tensioner.

Sorry, I wasn't clear - I was referring to the chain tensioner on the 6-speed, not the full derailleur.

You're still not clear

Maybe I'm being stupid here, or maybe I don't know enough about Bromptons. AIUI, some of them have a full multi-speed derailleur, others have a 3-speed hub, and a 2-sprocket derailleur (making six ratios in all), with only a few teeth difference. This version has a derailleur that looks like it will take up only a little slack, or am I mistaken?
Am I being clear now?