The Folding Society

The Brompton Trailer Bike

Do not attempt any job on your Brompton or any other cycle yourself unless you have the knowledge and experience to do so - inappropriate modifications, or incorrectly executed modifications could endanger your and other people's safety.

{Trailer bike}The following article on a novel Brompton modification was kindly supplied by Andrew Holland. Please note the usual warnings and disclaimers about making modifications to your or anybody elses bicycle(s).

With two Bromptons and child seat life was OK. As a 4-year-old became a 5-year-old then a 6-year old boy it got progressively harder getting up the hills. "Daddy why don't you talk to me ?" says Clement "Gasp Gasp" goes Daddy

One of the many good things from the Folder forums was the try other people's bikes out so Clement & I had a test ride on a conventional Trailer Bike. Just the job Clement can contribute to the pedal power. As Edward and Henry would say "We can do it together!" (Thomas the Tank engine quote no doubt Clement would correct me for getting the wrong engines)

Well there are no ready made folding trailer bikes and I had been considering how to modify the Brompton so we had a folding trailer bike. I didn't want to cannibalise or hack up a Brompton too much, so as Clement grows bigger it can be restored to a normal Brompton.

A series of modifications

Clement having short legs we needed to find short cranks. A quick test showed us there would be no hope of standard cranks. Shortened cranks are not standard items for the bike shop. The folder outing to Uckfield did have a good outcome because Janette Edge had just the job and David put us in contact with High Path Engineering. So we now had the cranks (130mm) and a set of 3 chain rings to chose from.

Folding the handlebars into the horizontal position solved Clement's short arms. (See Photo)

Mk 1 coupling

The next problem would be the coupling. I considered the front luggage block upside down and remove all the forks etc. I then hit on cutting of the forks and using the top half as the coupling swivel. At Brompton prices that was a bit excessive for project I was not sure if it would work. The solution was a cheaper set of Mountain Bike forks (1 1/8 inch). The pivot was to place in front of the axle to prevent the front Brompton folding with the weight on the rear carrier. With the forks cut off and a plate on the rear carrier of the front Brompton as a pivot it should be ready for a trail ride

Clement was not so ready for the trial ride. It wobbled excessively and there were a few tears. Future modifications and the use of elastic made a workable proposition but limited the turning circle. The first outing was to Battersea Park for the London folder meeting. The coupling worked but the set up and clear down time was not ideal. The handlebars were independent so could be held all the time, Clement thought it was great to be able to steer and it not affect the direction of the bike.

The Mark 2 coupling

The engineering facilities at home are limited to cutting and drilling. Some steel was procured, buying wood is easy but small quantities of steel seemed a problem. This time the steering was linked by using the brake bolt hole in the cutdown forks as the pivot coupling. (See Photo) The pivot is a High Tensile Bolt in the slot so if the bolt breaks then the coupling does not fall apart but becomes slack.

Clement's next test ride ended in protests because on the start of the ride the folded down handlebars went out of reach on tight corners. Other considerations were the ability to get over the humps for road calming thus a plus and minus 10-degree movement are necessary to keep all three wheels on the road without stressing the coupling.

It took a week of me thinking if there were an alternative solution to the handlebars and Clement deciding that perhaps roads were not all very sharp bends to compromise and have another try but starting on a straight piece of road. Cutting a longer slot in the socket end of the coupling solved the speed hump problem. Once confidence had been established there was no problem apart from the over confidence from Clement let's go faster.

Folding the trailer is easy and the package is held together with a luggage fastener. The handlebars are left sticking out to save wear on the centre joint. If there was a compactness problem these could be folded also.

Folding the front Brompton, the folding is OK but because the coupling is mounted on the rear carrier the folded bike is not so compact. The coupling can be removed by removing 4 bolts. The biggest disadvantage is with the coupling inplace the Brompton does not stand on its own but fall on its side.

Mk 3 requirements

The coupling arrangement stands proud of the rear carrier so the compactness of folding is compromised and it adds 700g to the weight so the plans are afoot for a Mk3 coupling but this will need some welding or epoxy resin work. More of that later. We will probable do some more riding to gain some experience before moving onto build the Mk3 coupling. The ride for me on the front is quite hard because of the wobbly nature of the rider at the back, the usual tandem problem. With Clement looking back to see where Jane is this doesn't help stability. Clement does quite enjoy this new arrangement. It makes a great difference on the uphills to have a contribution of effort.

Parts needed

50mm of 6mm internal dia thick wall tube. Or 12mm round drilled with 6mm on its axis. 12mm * 3mm steel strip to make a new rear carrier ? or 2 more pieces of tube and a lot of epoxy resin and mat

We are still getting to grips with who uses which gears and to keep Clement from pedalling too fast. The best so far is me in 1st and Clement in 2nd for all but the worst uphills. Moderate hills me in 2nd and Clement in 3rd. On the flat 3/3 is the answer.

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Copyright (C)1999 Andrew Holland
Last updated: 18 June 1999