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How To: Brompton Flat Fix

How To: Brompton Flat Fix

Posted on 05 November 2020

The world has changed quite dramatically with COVID-19 and it's had a major impact on the cycling industry. In most cases in the past, getting a flat fix done was an in-and-out kind of thing - at the very worst, it took our mechanics a one-day turn-around. However, with cycling now becoming the top method of getting around Toronto, bike shops are overwhelmed with bicycle service requests and require appointment bookings, sometimes weeks in advance (even for flat fixes).

With this maintenance guide, we hope to help those that are mechanically confident to replace a tube on their Brompton bike that they purchased at Curbside Cycle! Many of the steps are very critical to safety, especially when reinstalling everything. Do not force anything into place - if something isn’t working properly, you must take your bicycle to a bike shop to be inspected by a mechanic. 

This blog we'll focus on the rear wheel, which takes more know-how than the front as we have to deal with the gears!

Recommended and Required Tools:

To replace a flat on a bike, you'll need tools (you may already have some of these at home). If you need wrenches, your local hardware store can likely help you out. You will need to head to a bike shop for tire levers and air pumps: we recommend Tacx T4600 Tire Lever and the Lezyne Sport Floor Drive Pump. All stock Bromptons require the use of a 15mm wrench for both front and rear wheel removal, and you might need the aid of pliers for the Barrel Adjuster Lock Nut as seen below.
brompton folding in action gif animation

Unfold your Brompton

As you do before every ride, start by unfolding your bike if it isn't so already!

Place Brompton Upside Down

Brompton Upside down for maintenance
This will be the easiest way to perform any maintenance or cleaning of your bike.
Brompton Chain Tensioner Assembly Tensioner

Important Terminology

Throughout this flat fix blog, there will be times where you'll need to reference this image. These are important bits to know their name and what they do, as they're the immediate parts we'll work on in the next steps.

Step 1: Disengage The Hub Gear

Brompton bike maintenance rear wheel removal disengaging hub gear
If needed, use your pliers to hold the barrel and loosen the lock nut with your 2nd pair of pliers. We generally tighten down these parts during our tune ups or gear adjustment services.
Brompton hub indicator removal

Step 2a: Unscrew the Indicator Rod From the Hub

Step 2b: Set Aside in a Safe Spot

Brompton Sturmey Archer Gear Indicator Rod
Brompton Rear wheel folded in

Step 3: Fold rear wheel in to lessen the tension on the Chain Tensioner Assembly

This will reduce the tension on the chain tensioner assembly (CTA) so it doesn't fling back, potentially damaging something when you remove the chain.

Step 4a: Chain Tensioner Assembly (CTA) Removal

Brompton Chain Tensioner Removal
Use your 15mm wrench to remove the Chain Tensioner Nut (CTN).

Step 4b: Set aside CTN


Step 4c: Carefully Remove CTA

Brompton with Chain Tensioner Assembly Removed
Brompton rear wheel folded out without chain tensioner assemly

Step 4d: Unfold Rear Wheel Again

This is what your Brompton should look like once you have the indicator chain, Chain Tensioner Assembly and Nut removed. You will now have access to the axle nut on the drive side (the side with the gears and chainring).


Step 5a: Remove Wheel

Brompton Rear wheel removal loosening axle nuts
With the use of your 15mm wrench, loosen the axle nuts on each side.Brompton with chain off the cog ready for removal

Step 5b: Remove Chain From Cog

As shown here, set chain on rack stay. If you don't have a rack, set it on your fender stay. Going fenderless? Just let it hang.

Step 5c: Remove Wheel

Brompton Rear Wheel being removed
Make sure axle nuts are loose enough so the slotted washers are free of the frame. Pull the wheel out of the drop outs. Keep in mind, the tire will have to be completely deflated to remove or reinstall (to get the wheel past the brake pads).
Brompton tire removal

Removing one side of tire and tube 

These are the more involved steps of this blog. With the tire completely deflated and bead loosened, take your tire levers and begin inserting and prying off one side of the tire and removing the tube. For a more detailed video, Park Tool has a step by step of removing a tire off of a wheel. Link: https://youtu.be/eqR6nlZNeU8?t=89


These next images are depictions of the different types of flats. The most common flat is the top side puncture, followed by the pinch flat and rim tape failure.

Top Side Puncture

Tire Puncture from glass
This will generally be caused by glass, nails, thorns and other sharp objects on the road or trail. It's a one-two punch that goes through both the tire and the tube. Be sure to remove the object from your tire before replacing the tube so you don't immediately get another flat.
Tire Puncture Pinch Flat

Pinch Flat

A very common way to get a pinch flat is by having low air pressure in your tube. A great way to avoid this is by topping up your tires every couple of weeks (for bigger tires) or every several days (for skinnier tires like those on a road bike).

Inner Puncture

Tire Puncture Inner puncture rim tape

This kind of flat usually means that there is a protruding spoke, or the rim tape is in need of replacement due to age. We recommend going to your local bike shop for them to install new rim tape before putting in a new tube to avoid a second flat.


These next steps will guide you to reinstalling your tube, tire and putting the wheel back into place!

Brompton tube installation

Reinstalling the Tube and Tire

Once you have found the cause of the puncture (and remedied it if necessary), start by adding a little air to your new tube so it is round, inserting the valve into its hole, and tucking the tube into the tire. Once the tube is all tucked in, work your way around the tire pushing the tire edge back over the inner edge of the wheel rim (use your tire levers for the final tuck if needed). Leave the tube deflated until the final step to make re-installing it on your Brompton a lot easier. You can refer to this Park Tool instructional video again to install your tire: https://youtu.be/eqR6nlZNeU8?t=335

Wheel Installation

In this graphic we have made the hex nut transparent so you can see the TOP engraving on the washer (washer outlined in red). Since your bike is upside down, you NEED to ensure the washers on both sides are placed so TOP is towards the ground as shown in the image. Firmly tighten the axle nuts on both sides.
Brompton Chain Tensioner Assembly Engaged

Reinstalling the Chain Tensioner Assembly

This is how it should look like when you're done. Follow these instructions exactly:

Ensure non-pivoting pulley fits between the Wing tips of the chain pusher

Brompton Chain Tensioner Assembly Terminology
Ensure that the non-pivoting pulley wheel goes on top of the chain and between the Wing Plate Tips of the Chain Pusher
Brompton Chain Tensioner Assembly and Nut Terminology

Install Chain Tensioner Nut

Ensure your pulley runs on top of the chain between the wing tips of the chain pusher. Do not over tighten the Chain Tensioner Nut as you could break the plastic on the assembly.

Tension the Chain

Brompton Chain Tensioner Assembly Terminology
Push chain tensioner arm forward and ensure chain goes above as shown in image.
Brompton Chain Tensioner Assembly Terminology

Once again, this is how an installed Chain Tensioner should look once installed

You can spin the crank to ensure the chain is running smoothly and no abnormal sounds are occurring.

Reinstalling Indicator Rod

Brompton Gear indicator rod Terminology
Insert the indicator rod into the hub. Screw in clockwise until it stops turning. Do not tighten it! Unscrew half a turn and screw other side into the adjuster barrel. Do not tighten the lock nut.
Brompton Indicator window gear adjust

Adjust the 3 speed hub

Once you have the indicator rod attached on both ends, pedal the bike and shift the gear hub to 2nd gear. Look through the "Indicator Window" circled in yellow. Twist the barrel adjuster until the "step" circled in red, just protrudes out of the axle by approximately 1mm.

Completed View with out graphics

Brompton Indicator window gear adjust
This is what you should see through the indicator window when you're done and the bike is in 2nd gear.
Brompton Chain Tensioner Assembly

Tighten Lock Nut against Barrel Adjuster

We recommend finger tightening the nut if you wish not to use these tools in the future, or securely fasten them with your pliers. Do not over-tighten this nut.

Filling Up the tire

Lezyne Sport Floor Drive
If you are using the recommended pump, the Lezyne Sport Floor Drive, ensure you use the correct side! Stock Brompton tubes use a Schrader Valve.
Lezyne Sport Floor Drive

Pump up!

For Schwalbe Marathon tires that come with the standard Bromptons, inflate your tubes to between 65-110 PSI and do so every week to two weeks to avoid flats! Now go and ride your bike!

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