This is my first instructable. Probably nothing new to most of you but i thought id give it a crack anyway. I like the brompton fold up bikes but can no way justify the money for the amount i would use it. After reading sheldon browns article about his fold up raleigh twenty i decided to build my own but on the cheap. After a week or so i got this folder of ebay. If i were to do this again i would make sure i got one with a threaded bottom bracket. I have now got hold of a suitable bottom bracket from germany as these thompson bbs are popular there ( i also have an old bb tube i could turn down and braze in). The crank shown in the picture was one i had laying around.
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Modern folders are multispeed and i was looking for a way to replicate this and also i heard that the best modification was to replace the chromed steel wheels with alloy as these brake much better. I had an idea that a kids 20" wheeled mountain bike would answer both of these points at very little cost and a suitable donor was bought . The wheels and derailleur were quickly unbolted from the kids bike and offered up to the folder frame. Also at this time i removed the mudguards and rack as i wont need them.
I thought that the frame looked a bit weak so i welded a piece of 1" conduit in as a brace. If nothing else i think it is more astheticaly pleasing but should give a firmer ride.
Next i brazed on some 5mm nuts for water bottle cage bosses . I could have used rivnuts but im going to repaint anyway and also as this is the only tube holding the head tube on, brazing will keep the tube stronger.
Next i turned my attention to brakes. At first i toyed with sticking with the origional sidepull calipers that came on the bike but they looked way past their best. I then decided to use the cantilever brakes that came "free" on my donor bike. I hacksawed the bosses for the brakes of the kids bike, shapped them with a round file and welded them to the folder frame. To work out the centre height for the bosses i set the brake blocks at their mid point then held it against the rim of the wheel showing roughly where the boss should go. Even if they are slightly out of position there is plenty of adjustment on the blocks latter.
Then i repeated this on the forks. as the forks looked a bit on the flimsy side i decided to braze these on. I fitted each wheel then marked roughly where the brake bosses should go. I drilled two holes in a piece of scrap to hold them in place whilst brazing which wasnt a problem with the rear bosses as i tack welded before fully welding.
I got one of these twisted wire brushes to go in my angle grinder and it stripped the whole frame in about half an hour.
I wore a pair of safety goggles as these can "spit" bits of the wire out at you but nowhere nearly as bad as the untwisted variety.
I now have a stripped frame hanging in my garage waiting for a free day to be painted. I use smooth finish hammerite spray cans as it dosnt need a primer and gives a good looking and rugged finish,
Next i spray painted the frame and forks. I had to wait for the weather to warm a bit but still used a heater in the garage. During coats i also managed to clean the wheels up.
I couldnt wait a week and got too carried away rebuilding to remember to take photos. The forks were greased using bike grease then refited. Then the wheels were bolted into place. The deraileur, seat post and handlebars were next. It went together really quickly and im well chuffed at the look of the bike. The adjustment levers (for the seat height,handlebar height and to fold the bike) havent come up very well and are a bit rusty but to replace or rechrome would cost too much, i suppose i could always paint them silver.
Ive had to go down the route of getting an old bottom bracket to fit into the bb tube as the thompson one dosnt fit as the rear stays come through the tube. Fortunately i found a bottom bracket tube on an old rear suspension that was the same diameter as the tube on my folder with a bit of tweaking it fitted (using a mallet). I t was only when i tried to screw the bottom bracket cones in that i realised that the first end to be pushed into the frame had colapsed a bit ( it was a very tight fit). I thought about buying a tap and found that they are a fortune. I took an old shell and ground four slots then backed the relevant side away to produce my own tap. It may not be hard like a real tap but it worked. The shells and axle were then screwed into the frame and the crank arms fitted.
I finally got a free afternoon to get the chain, brake cables and deraileur sorted. I bought a set of gear and brake cables for £4 and a chain at asda which seem ok quality (for my needs). The chain was wrapped around the rear cog then through the deraileur then back around the chainwheel where the spring link was fitted to join both ends together. I ran the cable outer casings into place through the brazons then when happy with the lenghts fed the inners through and trimmed them to length. A bit of adjustment to the deraileur and I was ready to ride. Im pleased with the speed and the chunky mountain bike tyres are great for going up kerbs!!!