There are several types of tyre damage which can affect the safety and performance of your vehicle, but the type of damage which results in a flat tyre is invariably a puncture. There are two types of punctures, rapid punctures, and slow punctures. Rapid punctures can easily be detected, as they will seriously affect the handling of your vehicle and the tyre will rapidly lose pressure. A slow puncture, on the other hand, can go unnoticed for a long time. Slow punctures are caused when damage results in a loss of air slowly over time, and finally results in a drop in pressure. When a slow puncture occurs, tyres fail to perform properly, and this can compromise driver safety. Uneven road surfaces, and potholes, will impact your tyres more significantly if you’re suffering from a slow puncture.

What causes a slow puncture and how to spot one?

an upright nail that a tyre almost runs over

Common causes of a slow puncture are the tyre being pierced by a sharp object, such as a shard of glass, or a severe impact on a kerb or pothole. If you hit your tyre pretty hard on anything, or fear you may have driven over some broken glass, it’s a good idea to check your tyres over.

There are tell-tale signs of a slow puncture that you can watch out for. When you’re driving on a flat road, relax your grip on the steering wheel and feel for the vehicle pulling to the left or the right. If this is happening, this indicates low pressure in at least one of your tyres. If after repressurising your tyres, your tyre pressure drops again, it is likely that your tyre has a slow puncture. A good rule of thumb is if you notice that a tyre is losing more than two pounds of pressure in a month, you may have a slow puncture. Other indicators are a vibration through the steering wheel, though this can also be a sign of other faults, or any noticeable changes to the handling of the car.

a car with a flat tyre

It can be tempting to carry on as normal when you have a slow puncture, repressurising the tyre regularly as it gets flat, but this compromises the safety of your vehicle. A partially deflated tyre is unstable, offering less grip and potentially making your steering unsafe. If the slow puncture leads to a blowout, you could face serious injury or even a fatality if it occurs at speed.

Many slow punctures are due to a leaking valve, and you can easily check whether this is the case. Simply rub some soapy water around the valve after inflating the tyre and watch to see whether any bubbles form as air escapes. If this is the case, you simply need to arrange for a replacement valve to be fitted. If the valve is not the issue, you need to find the puncture spot. You could do this yourself by removing the tyre and placing it in water to track where bubbles indicate air escaping.

Can I repair my Flat Tyre?

a nail is in a car tire and it is pulled out with pliers

If you get a puncture while you’re driving, you may be able to carry out a repair on the roadside. You can use tyre repair kits that employ sealant to plug the hole caused by the puncture and then re-inflate your tyre back to its optimal inflation to make sure it’s safe to drive on for the time being. One important note here is that you should never attempt to change or repair a tyre on the hard shoulder of a motorway. You should put your hazard warning lights and handbrake on, and stand well clear while waiting for roadside assistance. Flat tyre repair kits are available, and it's good practice to keep one in your car, but these will only be a temporary solution.

Not all punctures can be safely repaired in this way, and the location of the puncture is hugely important. For tyre repair purposes, the surface of your tyre is divided into the Minor Repair Area (the central third of the tyre) and the Major Repair Area (the remaining two thirds). If the damage is to the Minor Repair Area, a temporary repair can be attempted. If the damage is to the Major Repair Area, then the tyre will need to be replaced, as the damage is too close to the sidewall of the tyre. Damage in this area of a tyre can lead to a blow out, which can cause a collision resulting in serious injury or fatality.

Just because the puncture has occurred in the minor repair area, doesn’t necessarily mean it can be mended. British standard BSAU159 also defines the maximum size of the damaged area in order to carry out a safe repair. So, if the diameter of the damaged area is larger than 6mm, a repair should not be attempted. This means it may be possible to repair a tyre that has been pierced by a nail or a screw, but if the damage is caused by a larger item such as a bolt or other metallic debris, the tyre may need to be replaced. This also means that tyre damage such as splits, cuts, and gouges cannot be repaired.

How do I repair a Flat Tyre?

Most repair kits will require you to manually squeeze all of the sealant into the tyre, via its adapter. You may have to remove the tyre valve to do so, but instructions in either your repair kit or vehicle handbook will tell you how to do this. Once all of the sealant is in the tyre, attach the compressor to the tyre valve. Then plug the compressor into the cigarette lighter or 12v socket inside your car. You may have to roll the car forward slightly to ensure the sealant spreads through the entire tyre.

Some sealant kits, however, will have an additional adapter, allowing you to fit the bottle of sealant into the compressor, so you can pump the sealant into the tyre with the help of the compressor.