Fitting a new Brompton chainring

Ant Cooper
Jun 10, 2016 · 4 min read

Although the 6 speed Brompton has been great fun to ride and coped well with the routes I’ve been on so far, I have decided to fit the smaller 44 tooth chainring as I have some plans for longer distances and hilly touring and would like to be able to spin more on the climbs.

The chart on the Brompton support website gives a good overview of the possible gearing options. A 6 speed with reduced chainring (-12%) definitely gives the easiest option for twiddling up the climbs.

Comparison with a ‘normal’ bike

According to the calculator on Sheldon Brown’s website, my cyclocross bike with 2x10 gears has an easiest gear of 28.9" and hardest of 123.5". The science behind the numbers doesn’t matter too much, only to illustrate that with the smaller chainring my Brompton will be comparable when riding uphill (28.9" vs 29.1"). At the top end I can say that I don’t often find myself pushing the hardest gear as I tend to enjoy freewheeling and admiring the views!

Fitting

Below is a guide to swapping out the chainrings on a newer model Brompton with a ‘spider’ crankset. The process is similar for the previous version with fixed cranks, except you just remove and replace the whole crank arm, rather than unbolting the ring.

Usual disclaimer: any maintenance is done at your own risk. If you’re not comfortable with these steps, please consult a qualified bike mechanic.

1. Brompton stand

I’ve seen this trick on a couple of other websites, to work on the rear wheels and gears you can rest the folded frame on the back of the saddle, using a bit of cardboard to protect it from scuffs.

2. Tools

From left to right:

  • New 44T chainring
  • Multi-tool with 5mm allen key
  • Chain tool
  • Torque wrench*
  • Chainring nut wrench*
  • Masterlink pliers*

* You could get away without these, but it’s always nice to use the right tools and it does makes things a little easier.

3. Disconnect the chain

Find the Masterlink by looking for the plate with elongated hole in and wiggle the chain side to side to loosen the link. This can sometimes be a little stiff — Masterlink Pliers make removal and reconnecting these a lot easier. Hold the chain tensioner to slacken the chain as you separate it and stop it springing apart. An extra pair of hands might be useful!

4. Remove the old chainring

Remove the old chainring using the 5mm allen key and chainring nut wrench. I find it best to work around the crank spider and loosen all the bolts first and then go round again to remove them, leaving the bolt on the back of the crank arm until last. With a little waggling you should be able to remove the chainring.

5. Chainring comparison

You can see the standard 50T and 44T rings side by side. The Brompton cranks use a custom fifth bolt that goes into the back of the crank arm.

6. Installation

Reverse the removal process, alining the chainring holes with the spider and loosely putting each of the bolts in to check all the holes are squared up. Then keep working around each bolt in turn tightening up. If you have a torque wrench, tighten to 10Nm.

7. Shorten the chain

Lastly, you’ll need to shorten the chain. Use the chain tool to remove the last 2 links as shown above. Then put the masterlink in the new chain end and reconnect the chain. Use the masterlink pliers if this is a little stiff.

Make sure you check the chain is connected properly and is running freely through the drivetrain and tensioner.

The whole process only took about 20 minutes and should be straightforward for anyone who has done a bit of bike fettling.

Northern Cyclist

A cyclist in the North of England

Ant Cooper

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antcooper.com

Northern Cyclist

A cyclist in the North of England