Wednesday 11 February 2015

Brompton chain sizing guide

Which chain do I need for my Brompton?

Over time chains and sprockets wear out. Due to the workhorse nature of the Brompton and the fact that the drivetrain is closer to the ground than on larger bikes (it picks up more dirt / grit) they can wear even quicker.

Choosing the correct chain for replacement can be confusing so we made this guide to help.

But first - if you are replacing the chain or rear sprocket(s) then we recommend replacing both parts the chain and the sprocket. We describe it as an old married couple - they get comfortable with each other and wear at about the same rate.

So remember - if you replace the chain alone then the chain may well slip - and the same if you replace just the chain. Check back for our next article on choosing the correct sprocket.

How many teeth does your chain ring have?

The first place to start is to count the number of teeth on the front chainring. Brompton supply them in 3 sizes: 44 teeth, 50 teeth and 54 teeth.

If your Brompton is a 3 speed or 6 speed then the chances are that this is a 50 tooth front ring (as that's the standard) - though it could have been changed to lower gearing (44T) or higher gearing (54T)

If your Brompton is a single or 2 speed version, then chances are it has 54 teeth (as that's the standard) - but again the gearing may have been lowered.

When you have made a note of this - count the number of sprockets (there should be one or two) and the number of teeth on the largest sprocket.

So which chain width?

Now you know the number of teeth on the front and rear - choosing the chain is quite easy!

Historically there used to be two widths of chain used by Brompton:

  • 1/8 inch used on single sprocket bikes (1, 3 and 5 speed) - the part code ended in a W (for wide) such as QCHAIN100W
  • 3/32 inch used on derailleur equipped bikes (2 and 6 speed) - the part code ended in DR (for derailleur) such as QCHAIN100DR
That gave us a total of seven chains to choose from

Since 2013 (we think that was the correct year), all new Bromptons have been equipped with the narrower 3/32 inch chain (which incidentally is easier to remove and install as it has a SRAM power link so no special chain tool is (normally) required.

So, we will assume you are going to replace the sprockets - you are going to aren't you? 

That reduces the number of chains to choose from to four - and which of the four depends upon the sprocket and front chain ring combination as shown in the table below.

The below list shows for each front chain ring size, based on the largest rear sprocket which Brompton chain is required:

44 Tooth Chainring
12t/13t/14t   - QCHAIN096DR (96 links)
16t/15t - QCHAIN098DR (98 links)

50 Tooth Chainring
12t/13t (3 speed default) / 14t - QCHAIN098DR
15t/16t (6 speed BWR default) - QCHAIN100DR

54 Tooth Chainring
12t (single speed default) / 13t / 14t - QCHAIN100DR
15t/16t (two speed default) - QCHAIN102DR

And there you have it - all you need to choose your new shiny chain.  Check back for part two - choosing the correct replacement sprocket.

Brompton chain size document
Brilliant Bikes - Brompton chains


Simon Wood said...

Is the guide to choosing sprockets available? I couldn't find it.

Also, if changing to a 3/32 chain do other things need replacing? Chainring? Tensioner?

Brilliant Bikes said...

Good question - sorry we seemed to miss that - working on it now!

Simon Wood said...

I rang you and got confirmation of my choice of sprockets - and I was advised I didn't need a new chainring or tensioner. New chain now fitted and working nicely :-)