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Flat Tire Repair

4 min read

Parent
🔧
Common Repairs
Fixing the most frequent breakdowns
Current
🛞
Flat Tire Repair
Patching tubes, finding leaks, improvised patches

To fix a flat, take the wheel off, pry one tire bead off the rim with levers, pull the tube out and find the leak by feel or by submerging the inflated tube in water, then roughen an area about 3 cm across, apply rubber cement and let it dry until tacky, about 2-3 minutes, and press the patch down firmly for 30 seconds. At a pressure of 400 kPa, air that gets under a 3 cm patch pushes on it with about 283 N, which is why the surface preparation and the drying time matter.

Part of Bicycle Repair & Fabrication

Flat tire repair is the finding and patching of a puncture in a bicycle inner tube, or the improvisation of a repair when no kit is available.

The Patch Is Under Load

A patch is not just covering a hole: if the bond fails at the hole, air gets under the patch and pushes it off over its whole area. Take an assumed tire pressure of 400 kPa (about 4 bar; actual pressure is marked on the tire’s sidewall) and a patch with a 3 cm diameter, which has an area of π × 1.5² = 7.07 cm² = 7.07 × 10⁻⁴ m².

  • Force = pressure × area = 400,000 Pa × 7.07 × 10⁻⁴ m² = 283 N

That is the weight of a 29 kg mass hanging on the patch, held only by the bond over the cemented area, and a leak that starts at one edge peels the rest. A clean, roughened surface and cement that has dried tacky give a bond that holds this; a patch pressed on wet cement or over dirt peels off in a ride.

What You Need

  • Tire levers, or screwdrivers and spoons, any flat prying tool
  • A pump
  • Patches and rubber cement (vulcanizing glue)
  • Sandpaper or a rough stone
  • A bowl or pool of water for finding leaks
  • A rag

Method: Patching A Tube

Step 1: Remove the wheel from the bicycle.

Step 2: Deflate completely. Push one side of the tire bead off the rim with the levers.

Step 3: Pull the tube out through the gap.

Step 4: Find the leak. Inflate the tube and listen and feel for the leak. If you cannot find it, submerge it in water and watch for bubbles.

Step 5: Mark the hole and deflate.

Step 6: Roughen a 3 cm area round the hole with sandpaper or a rough stone.

Step 7: Cement. Apply rubber cement and let it dry until tacky, about 2-3 minutes.

Step 8: Press the patch firmly over the hole and hold pressure for 30 seconds.

Step 9: Check the tire before the tube goes back: run a finger carefully round the inside and find what caused the flat. A thorn left in the tire punctures the new tube.

Step 10: Reinstall and inflate.

Improvised Repairs When The Kit Is Gone

  • Patch from another damaged tube: cut a patch from an old tube and stick it with contact cement, super glue, or rubber cement made from dissolved latex (Rubber Coating for dissolving rubber in solvent).
  • Temporary wrap: electrical tape in several layers round the punctured area.
  • Stuffing the tire: pack the tire with grass, rags or strips of rubber for a solid but rough ride. This ruins the tube but gets you home (Tire Alternatives).

Common Mistakes

  • Pressing the patch on wet cement — it has to be tacky, not wet.
  • Skipping the roughening — smooth rubber does not bond.
  • Not finding the cause — the object is still in the tire.
  • Pinching the tube under the bead on refitting — it makes a new hole.

Safety

Solvents and high pressure

Rubber cement and solvents give off flammable vapour: use them outdoors, away from flames and sparks. Do not pump a tire past the pressure marked on its sidewall, since a tire that comes off the rim bursts with a bang; keep your face away from the valve and bead when inflating.

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