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How can I determine the correct chain length on a Brompton bicycle?

I have a Brompton bicycle with a non-standard (aftermarket upgraded) drivetrain. My chain broke and I lost some links, so I don't know how many links my chain should be. I do think it's longer than the standard Brompton chain, but I'm not sure.

Brompton derailleur
The Brompton's proprietary dreailer design

Note that the Brompton has a very proprietary dreailer design, so I'm not sure if the normal advice for calculating chain length applies to the Brompton bicycle.

How can I determine the correct number of chain links on a Brompton bicycle?

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2 Answers 2

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There is no one standard Brompton chain length; there are 4 different ones for the various sanctioned drivetrains.

For nonstandard Brompton drivetrains, I've had good luck using the longest length that still allows the tensioner to maintain tension during the fold with the chain on the smaller cog (if applicable). I would recommend starting with this and only go any shorter if the tensioner is clearly not in normal position.

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I start by counting the number of teeth on the front chainring. For instance a commonplace standard Brompton gearing has a 50 tooth front chainring matched with a 100 link chain (that is assuming that you have a 3 or six gear Sturmey Archer setup). See https://brilliantbikes.co.uk/brompton-chains/184-brompton-chain-100.html

If you have a 54T front chainwheel then it's most likely you require a 102 link chain (see https://brilliantbikes.co.uk/brompton-chains/184-brompton-chain-100.html)

** UPDATE ** Since you have a 56 teeth front chainring my suspicion is that you require either a 104 or 106 link chain. But it also depends on the number of teeth on the sprockets on your rear wheel (which I know you have because you mentioned a derailleur). I think that Swiss firm Wiperman make chains whereby you can shorten the length by removing a link without requiring a specialist tool. Maybe buy a Wiperman chain and also a chain tension measuring tool and see what works best. Finally a piece of advice for general chain replacement: It always pays to replace the rear sprockets at the same time as a new chain because over the months/years of usage, the chain and the sprockets develop the same wear-and-tear profile and so work in sync as the age. Adding a new chain with old sockets disrupts that balance and tends to cause problems like skipping, especially when in the higher gears).

There are 44T chainwheels in use on Brompton bikes as well but I don't know about them.

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    fwiw I have 56 teeth on my Brompton's front chainring. Apr 18, 2023 at 18:00

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