A little while ago I bought this case on eBay. Since I bought my Brompton, I've been on the lookout for a hard case that was big enough to fit it. The B&W hard case designed especially for the Brompton is very expensive, and I could not justify it for something I may use once every couple of years if that. A mate of mine bought himself a Vincita bag, but wasn't really impressed by it. This old United Colors of Benetton case looked ideal. And it was! The Brompton fitted perfectly, although I have to remove the seat. But I intentionally ordered my Brompton with the extension seat post, so it would be super easy to remove to fit into a bag. I also have a right-hand folding pedal I can swap with the current, standard pedal, to make it fit extra easy if need be. I've often thought how cool it would be to be able to turn the Brompton carry bag into a trailer, or have a set-up whereby the bag fits to the Brompton, to allow riding to and from the airport. This would completely eliminate the need for taxis, lifts from friends or family, or expensive bus or train transfers. The Brompton's unbelievably brilliant design did allow me to strap the hard case to the bike's front luggage frame, but it was not the most comfortable (or controllable) set up for navigating unknown streets in a strange city. So the only real solution was a trailer of some sort. Radical design also makes such a bag, but like the Vincita it's a soft bag, and I really want a hard case lined with foam to protect my beloved Brompton as much as possible during baggage handling. I've seen some great solutions on the internet (for example this clever trailer), which inspired me to try to modify my recently acquired hard case.

The hard case on its own weighs almost 8kg even with all the pockets and linings taken out, which is about the same as the B&W case. Add to that the weight of a Brompton H6R, and the weight limits for airline luggage suddenly looms worryingly near. So my conversion had to be adding as little weight as possible. Designing a small trailer which was compact enough to fit in the case with the Brompton, and robust enough to carry the case, was going to add significant weight. After much thought I decided to try to utilise the hard case's natural integrity as a substitute for a trailer frame. After all, this solution was not meant to be a touring trailer. It won't allow me to use the case to lug heavy gear on a cycling holiday. It is designed simply to pull its own weight on wheels from home to the the airport to a hotel (or somewhere else where the case can be left secure until the return home). Actual luggage could be carried on the Brompton itself.

The design I settled on was very, very simple. A 45 degree angle bar direct to the rear axle as a tow arm, which should give a fair bit of turning clearance given the small 16" wheel of the Brompton, and doesn't require any tricky pipe bending. The axle would be a second steel tube through the centre of the case, with quick release everything including the wheels, to make it easy to pack and unpack. The simple and most useful hitch solution I've seen online was the use of air hose connections. So on my ride home after work one night, I stopped by a hardware store and picked up the following bits:

Not pictured is a small piece of plastic tubing I used to attach the hitch to the bike, and a couple of thoer bits and pieces I bought halfway through the build.

I was concerned about the wheels, which looked very heavy duty (and just too damned heavy) for my intended use, but their advantages were that they would ensure a decent ground clearance and I wasn't quite sure how this was all going to go together and what sort of angle towards the bitumen the back of the trailer would have, and they happened to fit perfectly onto the chromed steel tubing the store had. And they only cost $10 each, which made them the most expensive item purchased for the project.

The first job after working out where everything could go, was drilling holes in the hard case. This was the point of no return, so I measured up everything as much as I could to make sure it would work, before putting holes all over the case. While it cost bugger all on eBay, it was still a good case. My plan was to keep a roll of black gaffer tape handy when travelling, to tape over the holes when checking in. I could easily remove the tape at the other end.

and the hole for the axle, with the rivets showing for the clamp

The plan was to attach the tubing to the case by conduit clamps, riveted directly to the hard case. The holes were necessary, obviously, to let the steel tube pass through the case and through the clamps. I figured clamps on the outside of the case would get busted off pretty quickly by baggage handling, leaving me very much stranded at the other end of the trip. Riveting to the hard case shell without any additional support wouldn't be very strong, but the intention is to tow the hard case with minimal weight in it. I won't be using it as a cargo trailer.

I drilled a small hole in the 'trailer' end of the tow arm, to allow an R clip to pass through and secure it in place. It should be secure enough, and also very quick and easy to install and remove while sitting in an airport lounge:

The end of the tow arm.  The R clip will hold the tow arm in place once it is passed through the clamps riveted to the hard case. — The end of the tow arm. The R clip will hold the tow arm in place once it is passed through the clamps riveted to the hard case.

My original plan was to use about 6 R clips at various places through the axle to stop the wheels and axle sliding laterally during travel and allowing the wheels' rotating surfaces to rub against the case, but a more elegant (?) solution seemed to be just a couple of spacers made out of plastic washing machine hose, slid over the axles between the wheels and the case. So back to the hardware store I went for a couple more bits:

Hose pipe for spacers, and a hose clamp for the hitch arrangement.

The hose clamp pictured was for attaching the clear plastic piping to the air hose connection. At first I'd planned to have the axle running close to but just behind centre of the hard case, to allow for a more balanced towing weight and to lessen the stress on the tow arm. Unfortunately, given the dimensions of the case, I was limited to how long I could make the axle and still allow it to fit within the case. This meant that the wheels had to be placed further back, where they wouldn't interfere with the handle of the case and other protrusions. Here's the finished trailer:

The hitch was made by the air hose connection being screwed into the end of the tow arm, with a securing bolt through the middle. I used a longer than necessary bolt, so that I can attach a safety strap later on. I'll make the safety strap from an old bicycle helmet strap and a carabiner. The advantage of this type of connection is that it rotates, and is spring loaded so it is very easy to snap on and off, leaving very little attachment to the Brompton.

The hitch attachment for the tow arm.  The connecter is a spring-loaded air hose clamp — The hitch attachment for the tow arm. The connecter is a spring-loaded air hose clamp

The connector mounted onto the Brompton is the male air hose connector, attached with a hose clamp to a small length of plastic water pipe. The pipe in turn bolted onto the Brompton's rear axle. I've used this type of water pipe for other trailer designs (albeit larger pipe), and it works very well giving plenty of movement through all axes. While not as robust as a mechanically hinged setup, it should be good enough for getting to the airport and back without a heavy load on the trailer. I also used a spare washer to prevent the Brompton's axle nut turning on the plastic:

The hitch attachment for the Brompton.  This will stay on the bike while riding about at the destination, with the hard case left at the hotel.  While it looks a bit iffy, I've used this type of home-made hitch on a previous trailer projec… — The hitch attachment for the Brompton. This will stay on the bike while riding about at the destination, with the hard case left at the hotel. While it looks a bit iffy, I've used this type of home-made hitch on a previous trailer project and it works well enoughfor light loads, and there's no real possibility of it bending far enough to foul the rear wheel.

One of the things I really like about the Radical Design hitch is it's ability to still allow the Brompton to be 'parked' by folding the rear triangle under. While my set-up requires a bit of finesse to fold the Brompton's rear wheel, it can still be done. Here's the finished set-up:

It tows fine, but as you can see from this pic it is a long way off centre.  I may need to bend the tow arm. — It tows fine, but as you can see from this pic it is a long way off centre. I may need to bend the tow arm.

The Brompton can still be 'parked' without unhitching the hard case.

The turning circle is surprisingly good considering the straight tow arm

All the final pieces.  Nice and simple and relatively uncomplicated. — All the final pieces. Nice and simple and relatively uncomplicated.

And it all fits!  The lime green foam is a $4 workout mat cut in half, for extra padding when packed. — And it all fits! The lime green foam is a $4 workout mat cut in half, for extra padding when packed.

All up, the 'trailer conversion kit' adds about about 4kg to the hard case. Most of that is the heavy duty wheels, which weigh in at 1.5kg each!

The trailer tows far to the left side of the bike, which is okay if I remember but I will ask around to find someone with a pipe bender, to put a shallow 'S' bend in the tow arm to improve the tracking. I'll also keep an eye out for some lighter wheels. As it is, weights are as follows:

Brompton approx. 14 kilos
Carradice saddlebag with wet weather gear & tools & 'D' lock approx. 1.5 kilos
Hard case approx. 8 kilos
Trailer accessories approx. 4 kilos.

That all adds up to a little under 28 kilos. That fits within the limits for most international flights, but I'd probably have to pay extra for a second piece of luggage if I wanted to take anything else or take the wheels, 'D' lock and saddlebag as carry-on. Domestic flights will definitely involve paying extra, but that would be the same with any hard case no doubt.

Travel trailer case for the Brompton