How To Fix A Bike Tire That Keeps Going Flat (And Why It Happens)

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Ben Jones

Bike Maintenance, Other

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It can be frustrating if your bike tires keep going flat, can’t it? Thankfully there’s only a short list of reasons why this keeps happening and each of them is generally easy to fix. Let’s take a look at the problems and the solutions.

Why does my bicycle tire keep going flat?

There are three main reasons why your bike tire might be losing pressure:

  • Punctures
  • Time
  • Over-loading the bike

If your tire is going flat quickly when you’re not even riding on your bike – say, within an hour or two – then it is likely to be down to an existing puncture in your inner tube, or a leaky valve, or a previous patch repair that isn’t holding.

It the tire goes flat after a short ride on the bike, then it could be an existing puncture (a slow one), but could also be due to damage to the tire or wheel rim (that is causing damage to your tube), or a tube that is being pinched between tire and rim.

Punctures are by far the most common reason why tires go flat, but they’re not the only culprit. We’re going to look at punctures in detail because there can be a number of causes, some of which can be a little tricky to identify.

If your tire goes a little softer after a week or so, and there’s no other obvious damage to tire, tube or rim, then it may be just that you need to keep a pump handy to top up the pressure before you go out for a ride. Something that I always do before a ride is to check my tire pressures and give them a little extra air with my floor pump – it’s a useful habit that will save you from an uncomfortable bike ride with soft tires.

If your tire pressure looks good before you get on the bike, but then pile on a heap of cargo and they look flat, then it may be that you need to increase the pressure in your tires to compensate. The tires aren’t losing pressure per se, but they need to have the inflation dialled up a little to make up for the higher weight pressing down on them.

We’ll go into each of these reasons in more detail now and look at the solutions for each.

Punctures

Let’s start with a quick definition. A puncture is where something penetrates the rubber of the inner tube, or the tube has a leaky valve, causing air to escape.

There are a few reasons why this can happen.

Valve damaged/leaky

There are three ways that you can check to see if you have a damaged valve: look, listen, or grab a bucket of water. This is also a useful way of finding air leaks wherever they are on the tube.

Valves will either be leaking air through the tip (where you would fit the nozzle of a bike pump) or where the valve is attached to the rubber of the inner tube.

With the tube inflated, take a look at the valve up close to see if there’s any obvious damage – is it bent? Cracked? Is there a hole or tear where it meets the tube rubber?

Check for air escaping from leaky tube valves

If you can’t see any damage, put your ear next to the valve to see if you can hear air escaping.

If you can’t see or hear any damage, then you can try using water. Here’s how: inflate the tube fully, then submerge the tube into a bucket of water. Look to see if there are bubbles of gas coming out of the valve or tube. If you see a steady stream of bubbles it means that there is a hole or leaky valve.

If the valve is damaged then you need to get a new inner tube. Check our inner tube size chart to find the right one for your bike.

Tube worn/cracked/previous repairs not holding

If the valve looks to be in good condition and is holding air, then the next stage is to look at the tube itself.

Check for cracks or holes in the rubber – using the bucket of water trick can be helpful here too. Also look for previous patch repairs that are not holding air.

If there are small holes, say made by a thorn, then it’s worth attempting to do a patch repair. Check out my post on the time it takes for bike tire patches to dry.

If there is cracking in the rubber (rather than small holes), or previous patches that have failed, then it’s best to replace the tube with a new one.

Rim damaged/sharp edges

When you have the tire and tube off the wheel rim it’s useful to check the inside of the metal surface of the rim.

Why? Well, whilst punctures can be caused by things pushing into the tube from the road or trail surface, they can also be down to something sharp inside the rim that pushes into the tube. This might be a thorn stuck inside or a spiky section of metal rim.

Carefully run your finger all around the inside surface of the rim. Make sure you do this VERY CAREFULLY, because you might actually find something sharp!

If it’s something stuck inside the rim (e.g. a thorn) you can take it out. Other times it might be a sharp section of the rim itself. In this case you have two options. Firstly, you can install a strip of wheel rim tape. This cushions the tube from the metal surface and stops anything from pressing into the rubber and damaging it. Secondly, if the damage to the wheel rim is too severe, then it may be time to replace the rim with a new one.

Tire cracked/has holes

Tires are, literally, where the rubber meets the road and they can be the reason for tube punctures in a couple of ways.

On the one hand, they can pick up sharp objects which can become lodged in the tire rubber and eventually poke through and damage the tube. Shards of glass, nails, screws and thorns are all likely candidates for this.

Tires also don’t last for ever. After many miles, they can have cracks develop especially in the rubber side walls. When this happens there will be less protection for the tube and it can get punctured more easily from rocks, gravel and other spiky things.

If you have a damaged tire, then it is best to replace this with a new one. To find the right size, look at the numbers embossed on the tire sidewall. You’re looking for something like “26 x 2.1” or “700 x 38c”. This is the size of your tire and it’s generally best to replace your existing damaged tire with one that’s the exact same size.

Over-enthusiastic use of tire levers

I’ll start this one off by saying that it’s something that I’m regularly guilty of.

When you’re using tire levers to prise the tire edges over the metal wheel rims, you need to be careful not to accidentally damage your inner tube. This can happen in two ways.

Firstly, you can trap the tube between tire and wheel rim. This can slice straight through the tube.

Be careful not to pinch the tube with a tire lever

Secondly, tire levers are fairly pointy and you can easily poke a hole into the tube when you slide the lever between the tire and rim.

Unfortunately, both of these will likely require a new tube as this damage is often not easy to patch repair.

Time

If your bike tire keeps going down but there’s no sign of a puncture, then the most likely cause is time.

Unfortunately, bike tires do go flat over time and there’s nothing we can do to avoid that because the gas molecules in air are small enough that some can escape through the microscopic gaps in tire rubber.

Until we get fancy bulletproof bike tires like Judge Dredd’s Lawmaster, the next best option is to keep a pump handy and top up your bike tires before you head out for a ride. I use a floor pump with a tire pressure gauge for this.

Over-loading the bike

The final thing to consider is that your tube pressure may not be sufficient when you’re carrying extra weight on the bike – either around your midsection (guilty!) or in your pannier bags.

Tires that can feel adequately pressurized with an unladen bike can suddenly feel way too soft once you’re on the bike with your luggage stowed. When this happens the inner tubes become flattened out and sandwiched between the tire and wheel rims. Next time you hit a bump or kerb, the force compresses this sandwich and can cut the tube in a puncture known as a ‘snake bit’ because of the way it looks like a pair of fang marks on the rubber.

If you find this happening then it’s worth pumping your tires up to higher pressures before you load up the bike (but check that the pressure doesn’t exceed the maximum stated on the tire sidewall).

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