Illustration courtesy of BMW.
Before Scottish chemist John Dunlop created his pneumatic tyre late in the 19th century vehicles rolled on wheels shod with solid tyres.
There wasn't much you could say about them that was positive, the roadholding was appalling, the braking feeble, and the ride backbreaking, but they were impervious to punctures.
Dunlop's creation transformed motoring in virtually every way, but unfortunately they were subject to punctures that would regularly strand motorists on the side of the road. The answer was to carry a spare wheel that could be fitted to get you on your way again, but carrying a spare also has its issues. Over the years carmakers have mounted spares on the front guards, the rear bumper, under the rear and in the boot, but no matter where they have located it the spare has got in the way.
For just as long they've also been looking for ways to get rid of them. The only time we appreciate them is when we have a flat, for the rest of the time they are just going along for the ride and getting in the way. Getting rid of the spare would save money and weight, reduce fuel consumption, and liberate space that could be used for other purposes.
Carmakers have tried a number of things to rid themselves of these unwanted items. They've used smaller, space-saver wheels, they've used small conventional wheels and tyres, they've even done away with the spare completely by using a foam to fix a puncture, but the most recent way is through the use of run-flat tyres.
Run-flat tyres have a stiff sidewall that will support the weight of the car in the event of a puncture so you can get to a tyre retailer to repair or replace the damaged tyre. The upside of run-flat tyres is that you don't have to carry a spare wheel, and because you can drive on you don't have to change the wheel on the side of the road.
The downside is that the ride is much harder than with a regular pneumatic tyre, the cost of run-flats is about 50 per cent higher than a regular tyre, and not all tyre dealers have the knowledge and equipment to handle run-flats. Many tyre dealers are also unaware that run-flats can be repaired and pressure owners into buying expensive replacements instead of repairing tyres.
Run-flats can be repaired if owners re-inflate tyres when the puncture warning comes on instead of driving on with a flat tyre and damaging the sidewall to the extent it is beyond repairing.