When no replacement bicycle part exists, improvise the parts that fail gently, such as tires, patches, cable housing and chain links, and treat brakes, fork, steering and cranks as safety-critical parts that must be tested with care. Ride slower on any improvised part: halving the speed cuts a 90 kg rider’s energy from 1,620 J at 6 m/s to 405 J at 3 m/s, and cuts the braking distance to a quarter.
Part of Bicycle Repair & Fabrication
Improvised parts and fabrication are the substitutes made, cut or salvaged to keep a bicycle running when manufactured spares are not available.
Speed Is The Safety Margin
Improvised parts are less reliable than made ones, so the one thing in the rider’s control is speed. Kinetic energy is ½ m v². For a 90 kg rider and bicycle (assumed):
| Speed | Kinetic energy |
|---|---|
| 3 m/s (about 11 km/h) | ½ × 90 × 9 = 405 J |
| 6 m/s (about 22 km/h) | ½ × 90 × 36 = 1,620 J |
| 9 m/s (about 32 km/h) | ½ × 90 × 81 = 3,645 J |
Halving the speed leaves a quarter of the energy to dissipate, and since stopping distance at a fixed braking force is proportional to speed squared, it also leaves a quarter of the distance. A bicycle running on an improvised brake or tire should be ridden at walking-to-jogging pace until the part has proved itself.
Which Parts May Be Improvised
| Part | Improvise? | Notes |
|---|---|---|
| Tires and tubes | yes | Tire Alternatives; patches from other tubes |
| Cable housing, shift cables | yes, with care | Cable & Brake Substitutes |
| Chain links | yes, temporarily | Chain Repair; a weaker joint |
| Brake cable and pads | with great care | test before every ride |
| Spokes | yes, if the length is right | Spoke Replacement |
| Frame cracks | only with a sound method | Frame Repair |
| Fork, steering tube, crank arms | do not improvise | a failure here is sudden and total |
The dividing line is how the part fails. A tire that fails goes soft and you stop. A fork, steerer or crank that fails does so suddenly, with the rider’s full weight on it.
Sources Of Parts
A scrapped bicycle is a parts bin: spokes, cables, chain sections, bolts, bearings. The parent article makes a high salvage priority of every bicycle found, and strips a frame for parts only when it is beyond repair. Vehicle & Machinery Cannibalization covers the larger stock of parts in cars, including wheel bearings that suit a bicycle hub when sizes match.
What You Need
- A stock of salvaged parts, sorted by type and size
- Basic fabrication tools: files, drill, hacksaw, pliers, a vice
- Wire, tubing, and rope for substitutes
- A way to test: a safe, flat, empty space
Method: Improvise Safely
Step 1: Classify the part by what happens when it fails.
Step 2: Pick the closest salvaged part and match the size, thread and material.
Step 3: Make it simply, with as few parts as possible.
Step 4: Test at standstill: load it by hand, hang weight on it, or work the lever hard.
Step 5: Test at walking pace, then slowly faster, and re-inspect after the first ride.
Common Mistakes
- Improvising a safety-critical part to save time — the failure is not recoverable at speed.
- Skipping the test — most improvised parts fail on the first real load.
- Riding at normal speed on a patched bicycle — the energy figure above is four times higher at double the speed.
- Mixing thread standards — a bolt that almost fits strips and fails.
Safety
Improvised brakes, steering and wheels can fail without warning
A bicycle with an improvised brake, fork or wheel part can fail suddenly, throwing the rider at speed. Test each part first at rest and then at walking pace, keep speed low, and keep clear of other people and of slopes until the part has proved itself.
What’s Next
- Learn next: Tire Alternatives — when tubes and tires run out
- Combine with: Cable & Brake Substitutes — wire and tubing in place of cable