You almost never build a bike from scratch. You make a good one out of three bad ones. The whole skill is knowing which parts go together.
UNDERSTAND
A bike is a frame and fork, two wheels, a drivetrain, brakes, a saddle and handlebars. The frame is the only part that's hard to make; everything else can be salvaged and replaced.
Abandoned bikes are everywhere: basements, tips, courtyards, garages. A rusty bike with a buckled wheel often has a perfect frame. Collect several and build the best of each.
The real trap is standards: parts that look alike and won't fit. Those are what you need to know before taking anything apart.
How a bike works and how to maintain it is in Le vélo.
ACT
1. Tools
The bare minimum: open-ended spanners from 8 to 17 mm, 4, 5 and 6 mm Allen keys, a screwdriver, two tyre levers (or two spoon handles), a pump, a chain tool. Useful later: a crank puller, a spoke key, grease.
2. Choose the frame
This is the part that decides. Check:
- Welds and head tube: no cracks, especially around the head tube and the bottom bracket shell. A thin line under flaking paint that follows a straight path is a crack in the metal.
- Alignment: look at the bike from behind, both wheels must be in the same plane. A bent frame can't be straightened properly.
- Rust: on the surface, it brushes off. If it has eaten through or swollen the tube, walk away.
- Size: standing over the top tube, you need a few centimetres of clearance.
A steel frame can be repaired, welded, and it forgives. Cracked aluminium and carbon go in the bin.
3. Compatibility to check
| Part | What must match | Where to read it |
|---|---|---|
| Wheel and tyre | Rim diameter in mm: 622 (700C or 29 in), 584 (650B or 27.5), 559 (26 in), 406 (20 in) | On the tyre sidewall, for example 37-622 |
| Wheel and frame | Dropout spacing: 100 mm front, 130 or 135 mm rear | Measure between the dropouts |
| Seatpost | The diameter, to a tenth of a millimetre (27.2 is the most common) | Stamped on the post |
| Headset | 1 inch (old bikes) or 1 1/8 inch | Measure the fork steerer |
| Bottom bracket | The thread, English (BSA) on the vast majority | Marked on the cups |
| Chain | Width according to the number of sprockets | A wide chain won't fit a narrow cassette |
| Brakes | Type and reach: calipers, cantilevers, V-brakes, discs | Look at the bosses or tabs on the frame |
Simple rule: take the wheels, fork and brakes from the same donor bike if you can. They came together from the factory.
4. Assemble in order
- Fork and headset. Grease the bearings, tighten until there's no play but the fork still turns freely.
- Bottom bracket. Grease on the threads, and watch the direction: the right side often tightens anticlockwise.
- Wheels. Spin them: a slight wobble is fixed by tightening the spokes on the side opposite the wobble, a quarter turn at a time. Replace any cracked tyre.
- Brakes first, before the drivetrain. A bike without gears rolls; a bike without brakes kills. Pads must hit the rim flat, never the tyre.
- Drivetrain. Chain at the right length, derailleurs adjusted, or a single-speed setup.
- Saddle and bars. Saddle height: leg almost straight, heel on the pedal at the bottom.
5. The simplest: single speed
If the derailleurs are shot, remove them. Keep one chainring at the front, one sprocket at the back, lined up, and a tight chain. On a frame with vertical dropouts, a tensioner or an old derailleur locked in place holds the chain. A single-speed bike never goes out of adjustment. Choose a low gear: you'll climb hills, and accept going slower on the flat.
6. Test it
Brake hard standing still, then at low speed. Shake the bars, the saddle, the wheels: nothing should move. Ride a hundred metres, then tighten everything again. Check it all again after the first day.
ADAPT
No inner tube left. Stuff the tyre with rope, foam, or a coiled garden hose. It's heavy and harsh, but it rolls to get you by.
Repeated punctures. There's still a thorn or a shard in the tyre: run your fingers gently around the inside, all the way.
No frame at all. A bamboo frame can be built: dry bamboo tubes 3 to 5 cm across, joints of fibre (hemp, flax) soaked in resin, reusing the head tube, bottom bracket and dropouts cut from an old frame. It's a project of several weeks, not a quick fix.
Carrying loads. A two-wheeled trailer made with two front wheels and a wood or tube frame carries 30 to 50 kg (65 to 110 lb). Attach it to the seatpost or the rear axle, never to the rack alone. See Démonter, déplacer, rapporter.
Building a bike for someone else. Take their size first, not afterwards: a bike too big is dangerous, one too small wears out the knees.
Nothing will come apart. Penetrating oil, time, then heat. See Récupérer sur un véhicule to get the most out of a whole vehicle.