To fix a flat on the road, swap in a spare tube and patch the punctured one at home: it’s the fastest fix and, according to Park Tool, the safest. If you run tubeless and the sealant won’t close the hole, insert a plug (a rubber strip that fills the hole); if the cut is big, remove the valve and put in a tube. Here are all three repairs step by step, how to tell what caused the flat, and what to always carry in your bag.
What to carry in your saddle bag
With this kit you can handle almost any flat, with or without tubes:
| Item | What it’s for |
|---|---|
| Spare tube in your size, with the right valve | The quick fix; make sure the valve is long enough for your rim |
| Two plastic tire levers | Getting the tire off and on without damaging the rim |
| Mini pump or CO2 with an inflator head | Reinflating; CO2 is fast but each cartridge works only once |
| Vulcanizing patches and glue | Repairing the punctured tube if you flat twice |
| Tire boot | Covering a cut in the tire from the inside so you can get home |
| Plugs and plug tool (tubeless) | Filling holes the sealant can’t close |
| Multi-tool with a valve core remover | Quick adjustments and removing the core from a tubeless valve |
Valves don’t fit every rim: a Presta valve (the skinny road valve) is about 6 mm in diameter and a Schrader (the car-tire valve) about 8 mm (sizes and length for your rim are in the bike valve chart). If you’re not sure what size tube you need, the tire size calculator decodes the ETRTO code on the sidewall, and the tire size conversion chart lists them all.
How to replace a tube step by step
- Remove the wheel. On the rear, shift to the smallest cog first: the wheel comes out and goes back in much more easily.
- Deflate completely. Press the valve while pushing the wheel against the ground. On a Presta valve, unscrew the tip first.
- Unseat the beads. Push the edge of the tire (the bead) toward the center of the rim all the way around: the rim is deepest there, so the tire gets more slack.
- Insert the tire levers. Hook one under the bead opposite the valve, and the second 1-2 in (3-5 cm) away. Slide the second lever around the rim until one side comes off completely. Never use a screwdriver or a knife.
- Pull out the tube, starting on the side opposite the valve.
- Find the cause before you install anything (explained below). If the thorn is still stuck in the tire, you’ll flat the new tube in the first mile.
- Put a little air in the new tube, just enough to give it shape: that way it won’t twist or get pinched.
- Fit it inside the tire, with the valve pointing straight at the hub. If the tire has rotation arrows, follow them.
- Push the bead on by hand, starting at the valve and finishing on the opposite side. Use a lever only if there’s no other way, and be careful not to trap the tube.
- Check both sides for any tube peeking out between the tire and the rim.
- Inflate a little and check the bead line. The molded line on the tire next to the rim should sit at the same height all the way around. If it rides high in one spot, deflate and reseat; if it dips, sometimes a bit more air does it. If it won’t seat, use soapy water, never grease or oil.
- Inflate to your pressure and tighten the valve nut finger-tight only.
To avoid going too low or too high when you reinflate, the tire pressure calculator gives you the number for your weight and tire width, and the guide to bike tire pressure explains why it changes.
Find what caused the flat
Inflate the punctured tube until it’s twice its normal width and find the leak by sound, with your cheek, or by dunking it in water and watching for bubbles. The shape of the hole tells you what happened:
| Tube damage | Likely cause | What to do |
|---|---|---|
| One small hole | Thorn, wire, glass or a nail | Find the object in the tire and remove it; it can be patched |
| Two parallel slits | Pinch flat against the rim (snake bite) | More pressure or a wider tire |
| Hole on the inner side | Rim tape shifted or damaged, or a spoke end poking through | Check and replace the rim tape |
| Tear next to the valve | Tube installed crooked or very low pressure | Keep the valve straight and run the right pressure |
| Long cut or tear | Blowout or cut tire | Can’t be fixed: new tube, and inspect the tire |
Using the position of the hole in the tube, find the same spot on the tire (that’s why it pays to always line the tire label up with the valve) and carefully run your fingers around the inside, because whatever caused the flat may still be there.
How to apply a vulcanizing patch
This is the classic patch, the one you glue on. Applied properly, it’s as solid as the tube itself.
- Mark the hole with a marker, around it and not on top of it.
- Sand lightly with the sandpaper from the kit. Sanding too much or too hard leaves grooves in the rubber and the patch won’t grip.
- Clean with rubbing alcohol or a solvent that leaves no film, and let it dry.
- Spread a thin layer of glue, slightly larger than the patch. You don’t need a blob: an even coat is enough.
- Wait a few minutes until the glue loses its shine. This is the step people skip most, and the most common reason a patch peels off.
- Apply the patch centered and press it hard for more than a minute, especially at the edges. Check that no edge lifts.
- Leave the clear plastic on top: it helps the patch slide inside the tire.
Glueless (self-adhesive) patches are faster, but Park Tool treats them as a temporary fix: replace the tube as soon as you can. Watch out for TPU tubes (the light plastic ones): Park Tool’s vulcanizing kit doesn’t work on them, so use the patches their brand recommends.
If the tire is cut
If the tire has a cut you can see the tube through, air pressure will push the tube out and it will blow again. Put a tire boot on the inside, covering the cut with some margin and not reaching the bead. In a pinch, a folded energy bar wrapper or a dollar bill works. It’s only to get home: Park Tool stresses that a booted tire should be replaced as soon as possible.
Tubeless flat: sealant, plug or tube
Sealant closes small punctures on its own while you ride; sometimes it’s enough to spin the wheel with the hole at the bottom and reinflate. If it won’t seal:
- Find the hole and remove whatever caused it.
- Prep the hole with the tip of the plug tool, pushing it in and out a couple of times.
- Thread the plug through the fork of the tool to the middle.
- Push the tool into the hole: the plug folds in half as it goes in. Stop when about 1/4 in (6 mm) of each end is sticking out.
- Pull the tool out while holding the plug so it doesn’t come out with it (Park Tool’s has a retainer for this).
- Inflate and check. If the hole is big, add a second plug.
Park Tool considers a plug a temporary repair. If the cut is longer than two plugs can fill, or the bead has come off the rim, it’s time to put in a tube: remove the tubeless valve, wipe out what sealant you can with a rag and install the tube as you would in a regular tire. Schwalbe confirms the sealant left inside won’t damage the tube. To inflate, a pump beats CO2: Schwalbe warns that CO2 degrades its sealant.
If you’ve never set up tubeless, the tubeless guide explains how much sealant to run and how often to top it up, which prevents more flats than anything else.
Common mistakes
- Not finding the thorn. The second flat of the day usually comes from the same piece of glass.
- Pinching the new tube with a tire lever while installing the last section of bead. Finish by hand.
- Pumping to full pressure without checking the bead line. A badly seated bead makes the wheel hop or the tire blow off.
- Running low pressure “for comfort” and getting pinch flats. If it keeps happening, you need more pressure or a wider tire.
- Carrying an old spare tube that’s been in your bag for years. Check it every few months: the rubber cracks at the folds.
- Forgetting the valve core or valve nut when switching from tubeless to a tube.
If flats keep happening with no visible cause, check the rim tape and tire wear as part of your maintenance schedule by mileage. The names of every part in six languages are in the wheels and tires glossary.
Frequently asked questions
Is it better to replace a bike tube or patch it?
On the road, replace it: it's faster and Park Tool considers it the safest option. Put a vulcanizing patch on the punctured tube later at home, and the repaired tube goes back in your bag as the spare.
How long should patch glue dry before applying the patch?
A few minutes, until it loses its shine. Spread a thin layer slightly larger than the patch, let it dry, press the patch hard for more than a minute, especially at the edges, and check that they're stuck down.
What do two holes side by side in a tube mean?
That's a pinch flat (snake bite): the tube got trapped between the rim and an impact. It means your pressure was too low for your weight or the tire was too narrow. Run more pressure or go to a wider tire.
Is a tubeless plug a permanent repair?
No. Park Tool considers it a temporary repair and recommends replacing the tire as soon as you can. It often holds for weeks, but check it before a long ride.
Can you inflate a tubeless tire with CO2?
To get home, yes, but Schwalbe says not to use CO2 cartridges with its sealant because it reduces sealing ability. Once home, deflate, check the sealant and reinflate with a pump.
