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Tyre repairs and sealants

Dealing with a puncture

If you get a puncture you might be able to get it repaired for a fraction of the cost of a new tyre if the puncture is in the main tread area and you stopped quickly without overheating the deflated tyre.

British standard BS AU159 sets out the rules for repairs to car tyres. It defines the type of damage that can be repaired and the way that repairs should be carried out.

One of the most important requirements is that the car tyre has to be removed from the wheel to check for internal damage. If not spotted this could later result in sudden failure of the tyre. Because of this, externally applied plugs and liquid sealants injected through the tyre valve can not be thought of as permanent repairs.

Buying a new car

If you are buying a new car don't assume that there will be a full-size spare wheel and tyre in the boot.  It is increasingly common for car manufacturers to provide a non-standard or 'skinny' spare or even simply an emergency tyre sealant and compressor/inflator pack.

If carrying a full-size spare is important to you then raise it with the dealer; some offer a standard spare wheel as a cost option if the design of the boot floor can accommodate one.

Our car reviews include information on the type of spare supplied as standard.

Tyre sealants

Many car manufacturers now supply a tyre sealant and tyre-inflator pack instead of a spare wheel. The sealant and compressed air are injected through the tyre valve but the result achieved will depend on the cause of the puncture and how far and fast the flat tyre has been driven on.

There are two basic types of sealant, those that are put into the tyre in case of a puncture and those that are put in following a puncture.

Pre-puncture sealants

These are injected through the tyre valve as a preventative measure. The aim is to prevent air loss if a puncture does occur so you can drive on without interruption.

If the sealant works then the seal will be pretty much instantaneous after the tyre has been punctured. This raises the concern that you will have no idea whether there's been a puncture or not. A large screw or nail in the tread of the tyre will cause further damage over time and could lead to more catastrophic failure.

Post-puncture sealants

These are used following a puncture. You inject the sealant through the tyre valve and then use the compressor or cylinder of compressed gas to re-inflate the tyre.

If you regularly drive alone then carrying a can of tyre sealant can bring peace of mind but it is important that you spot the puncture early and stop quickly - running in a partially or fully deflated condition will cause weakening of the tyre structure and irreparable damage.

Permanent or temporary?

Because a tyre has to be removed from the wheel and checked for internal damage before it can be repaired permanently, sealants can't be considered permanent repairs.

The standard for tyre repair also defines the materials and methods that can be used to repair tyres permanently. It states that the "use of liquid tyre sealants does not meet these criteria and hence cannot be considered suitable as a permanent repair."

Tyre experts recommend that you check your tyres every time you use the vehicle if they contain a pre-puncture sealant. If the sealant has been activated you should see signs of the sealant (usually white latex based) on the outside of the tyre. If there are any signs of this or of a penetrating object in the tyre then it will have to be taken off the wheel rim and checked to see if it is suitable for a permanent repair.

(25 April 2012)

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