| Flat tire | - Punctured inner tube
- Failed valve
- Failed rim tape or protruding spoke end
- Cut or damaged tire casing
- Tires perished with age
- Dented or cracked rim
| - Look for cuts, bulges, cracked sidewalls and anything stuck in the tread.
- Spin the wheel and watch the rim for dents, cracks and wobble.
- Read the size off the tire wall and note the valve type.
- Try pumping it up. A tire that holds air for the viewing had a slow leak, not a blowout.
| - Photo of the tire wall marking and of the rim.
- How it went flat: slowly over weeks in storage, or suddenly while riding.
- How long the bike has been standing.
| Basic to Intermediate, depending on the cause Diagnosis time: Minutes Safety-relevant | A tube is cheap. Two perished tires, rim tape and tubes together are not. Rear wheels with a hub gear, coaster brake or hub motor take noticeably longer to remove and refit correctly. | - Dented or cracked rim.
- Any frame or fork damage.
- Both tires perished on a bike whose comparable price does not cover new ones.
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| Brakes weak or not working | - Worn brake pads
- Stretched or corroded brake cable
- Contaminated disc pads or rotor
- Hydraulic leak or air in the system
- Rim braking surface worn out
| - Squeeze each lever: it should firm up well before the bar.
- Look at pad thickness and, on rim brakes, the wear line or concave wear on the rim.
- On hydraulic brakes, look for oil at the lever, hose and caliper.
| - Which brake type and model.
- When pads or cables were last changed.
- Whether a hydraulic brake has ever been bled.
| Basic to Advanced, depending on the cause Diagnosis time: Under an hour Safety-critical | Pads and cables are inexpensive. A hydraulic bleed needs the maker’s kit and fluid. A worn-through rim means a new wheel. | - Rim worn past its wear indicator.
- Leaking hydraulic system if you have no bleed experience and no shop quote.
- You cannot get both brakes working — then it is not a rideable bike.
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| Gears skip or will not shift | - Cable stretch or indexing out
- Worn chain and cassette
- Bent derailleur hanger
- Internal hub-gear fault
| - Shift through all gears on a stand or during the ride.
- From behind, check that the derailleur cage hangs parallel to the sprockets.
- Look for hooked, shark-fin teeth on the sprockets.
| - Drivetrain make and speeds.
- Whether it skips under load or only mis-shifts.
- For hub gears: whether it has ever been serviced.
| Basic to Advanced, depending on the cause Diagnosis time: Under an hour Low safety relevance | Cable and adjustment cost little. Chain, cassette and chainrings wear together, so replacing one often means all three. Hub-gear internals are a shop job. | - Hub gear that slips under load, unless priced as a wheel to replace.
- Drivetrain fully worn on a low-value bike.
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| Saddle, grips, pedals or accessories missing | - Saddle, grips or pedals simply missing
- Seized seatpost
| - If the saddle or post is missing, check that a seatpost can actually move in the frame.
- Read the seatpost diameter stamped on the post, or measure the seat tube.
- Check handlebar diameter before buying grips.
| - Why the part is missing.
- Whether the original parts exist somewhere.
- Seatpost diameter.
| Basic to Advanced, depending on the cause Diagnosis time: Minutes Safety-relevant | Saddles, grips and pedals are ordinary parts. A seized seatpost is not: freeing one can take hours and can damage the frame. | - Seatpost seized in the frame.
- Parts missing because the bike was stripped — ask about ownership.
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| Crack, dent or bend in frame or fork | - Cracked frame
- Bent fork
- Damage to carbon parts
| - Look along the top and down tubes for ripples, and at every weld and joint for hairline cracks.
- Check the fork legs are symmetrical and the wheel sits centrally.
- On carbon, look for cracks, soft spots and paint damage at impact points.
| - Any crash history.
- Close-up photos of head tube, bottom bracket area and dropouts.
| Professional Diagnosis time: Under an hour Safety-critical | Frame and fork repairs are specialist work and usually exceed the value of an ordinary used bike. | - Any crack.
- Bent fork or misaligned frame.
- Impact damage on carbon without a specialist report.
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| Play or doubt at the folding hinge or latch | - Worn folding hinge
- Latch that does not lock securely
| - Unfold and lock every hinge. Lift the bike by the saddle and bars and feel for movement at the joints.
- Look for cracks around hinge plates and welds.
- Check that every latch closes fully and has its safety catch.
| - Make and model of the folding bike.
- Whether hinge parts have ever been replaced and by whom.
| Professional Diagnosis time: Under an hour Safety-critical | Hinge pins, clamp plates and latches are maker-specific. Assessment and adjustment belong with a dealer for that brand. | - Any play you cannot explain.
- A latch that does not lock every time.
- Cracks near a hinge.
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