I bought a second hand Brompton bike a few months ago, and it's improved my life manyfold! Living in a very small space (a narrowboat), it means I can easily store my bike indoors keeping it safe both from theft and the elements.

Then carrying stuff became an issue. When I started off my mum was kind enough to lend me her Brompton-bag. These are amazingly sturdy, well designed and durable pieces of kit, and their price tags reflect this. Not wanting to part with more money or sacrifice precious storage space (again, narrowboat), and owning 8 Ortlieb panniers, I knew there must be a way to adapt one to fit a Brompton. And that's what this instructable is about.

Here's a breakdown of the pros and cons of the final result, before you get started

Pros:

With Brompton bags costing upwards of £80, you can save £60 or more if you already own a pannier
No modifications to pannier needed - can still be used on a normal bike rack
Ortieb panniers are 100% waterproof (Brompton bags need the included rain cover to be waterproof)
Ortieb pannier offers slightly more carrying capacity

Cons:

Slightly taller bag means
- you can only turn handlebars approx. 45º either way. This doesn't affect 'normal' cycling - it only becomes an issue when weaving through traffic, or restrictions in shared cycle/pedestrian areas. And doorways.
- you can't have your bike light on the lower part of an 'M' handlebar
No backwards-facing compartments

I've kept this instructable quite vague so that it can be used with any type of pannier. Its main purpose it to show people that it can be done, what materials to get and what to expect.

Step 1: Tools and Materials

Make sure you get all of your tools and materials ready before you start! If you have everything ready and are fairly confident, this can take less than 3 hours.

If you have all the tools and an Ortieb pannier, the cost of making this should be between £20 and £30. The only material you need besides the Brompton bag frame (£20) are 2 aluminium poles that shouldn't cost a lot (or they'll be free if you know the right people!

Materials:

1x Brompton bag frame
1x 1m x 13mm aluminium pole
1x 30cm x 9.5mm aluminium pole
8x 4.0 x 10mm rivets

Tools

Drill with an M4 bit
Measure calliper
Hack saw
Junior hack saw
Rivet gun
Epoxy resin
Wire wool

Step 2: Take the Frame Apart

Drill the heads of the rivets until they come out. This should be very easy.

Pull the aluminium poles out of the plastic brackets. They're quite snug in there so will take a bit of tugging, but make sure the rivets are completely out!

Step 3: Reassemble, Measure and Cut

You'll notice I haven't included any measurements in the this instructable. That's because there are far too many types of pannier for that to be useful.

Put the old poles on either side of the plastic handle (see picture), and clip on the pannier.

Take your new length of pole and place it through one of the vertical holes. Then place the plastic bottom bracket (the part that clips onto your bike) so that it goes under the pannier. Mark the point at which you need to cut the pole. Make sure you triple check the length. There's no definitive way of doing this.

Once you're happy with your measurement, cut the pole and put the bottom bracket in and check. If you've cut to too long, adjust it, if it's too short hopefully you'll have enough spare pole to start again!

If it fits, make a second length of pole for the other side and then reassemble.

NB: at this point, you can simply cut the top poles (so they're just long enough to hold the pannier clips), insert rivets and have a simple version of the adapter that basically looks like the second picture. It should work just as well if you only intend to use the frame to carry your pannier. The original purpose of the outer poles is to give shape to Brompton bags that don't have an internal frame. Ortieb and other panniers have an internal plastic frame so the outer arms are not essential.

Step 4: Bend the Poles

I don't have the know-how or tools to bend aluminium poles into shape from straight. If you do, your frame will be much sturdier and elegant than mine! Here's my workaround:

Using brute force and ignorance, bend the outer poles to the shape of your pannier. They will also need to be extended later on, so decide where to place the bends accordingly.

Step 5: Extend the Outer Poles

Cut the bent outer poles and place them in the holes of the frame
Using your measuring calliper, measure the distance between the two ends. NB if you've done a poor a job as me in bending the poles, the measurement will differ slightly on either side
Cut a length of 13mm pole to each of the above measurements
Cut the 9.5mm pole into 4 ± equal lengths
Using 2-part epoxy resin, stick the small 9.5mm pole parts into your 2 13mm lengths as pictured.
Once these have set, place them into the outer poles and apply more epoxy and allow to set
Remove any excess epoxy using wire wool and your favourite paint thinner