... the easiest way to lower the gearing on a 3 speed Brompton?

General cycling advice ( NOT technical ! )
Boogaloo
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Joined: 25 Nov 2016, 12:54pm
Location: Sunny Suffolk

... the easiest way to lower the gearing on a 3 speed Brompton?

Postby Boogaloo » 20 Aug 2019, 8:26pm

I'm looking into this for a friend who recently bought a new 3 speed Brompton and feels the gearing to be too high for him.
From searching the forum I think I have established that it has a 50 t chainring on a 130mm bcd crank. If so this could be pretty easily reduced. I could lend him a 42 for example but mine are track rings for 1/8 chains. I'm guessing the Brompton has a 3/32 chain?
Alternatively we could try a larger sprocket but I assume there is a limit to what will fit. Did I read 15t is the biggest?
Can I make gear inch calculations just like I would with any other bike or Is there anything I should know about Bromptons that would complicate things?

Many Thanks

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NUKe
Posts: 3706
Joined: 23 Apr 2007, 11:07pm
Location: Suffolk

Re: ... the easiest way to lower the gearing on a 3 speed Brompton?

Postby NUKe » 20 Aug 2019, 8:39pm

Smaller chain ring on the front / larger sprocket on the back.
Have a play with this
https://www.sheldonbrown.com/gear-calc.html
NUKe
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geomannie
Posts: 537
Joined: 13 May 2009, 6:07pm

Re: ... the easiest way to lower the gearing on a 3 speed Brompton?

Postby geomannie » 20 Aug 2019, 8:50pm

44 tooth Brompton chainring here. https://brilliantbikes.co.uk/48-brompton-chainsets
I fitted one years ago and lowered the gearing nicely.
geomannie

Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: ... the easiest way to lower the gearing on a 3 speed Brompton?

Postby Brucey » 20 Aug 2019, 8:54pm

6s bromptons have 3/32" chains but 3s models can be fitted with 1/8" chains, even if the sprocket is a 3/32" one. So you can fit a 1/8" chainring and chain, and 'have a go'.

cheers
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drossall
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Joined: 5 Jan 2007, 10:01pm
Location: North Hertfordshire

Re: ... the easiest way to lower the gearing on a 3 speed Brompton?

Postby drossall » 21 Aug 2019, 9:38am

You can buy a 16T Brompton sprocket. I'm not too clear on whether third-party sprockets are compatible, but I doubt that the chain tensioner is designed to work with substantially larger ones.

Changing the chainring is the obvious way to go. Don't forget to shorten the chain to compensate, as the chain tensioner isn't designed to cope with an over-long chain.

simonhill
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Joined: 13 Jan 2007, 11:28am
Location: Essex

Re: ... the easiest way to lower the gearing on a 3 speed Brompton?

Postby simonhill » 21 Aug 2019, 5:02pm

You asked for the easiest.

Take it into a Brompton dealer and ask them to change the chain wheel. It is a standard mod that you can specify when buying.