What is a run-flat tyre and how do they work?
Run-flat tyres have been around since the 1980s but are only now becoming common as original equipment and even as retro-fit options but, just what is a run flat tyre?
A RUN-FLAT TYRE is, and the hint is in the name, a tyre that doesn’t become ‘flat’ in the same way a non-run-flat tyre does. Meaning, even if the tyre is punctured and loses air pressure it can still be driven on. But there’s still a lot of mystery surrounding these things…
What is a run-flat tyre?
This is a type of tyre that allows you to continue driving after suffering a puncture. The idea is that you’ll limp your vehicle to the nearest tyre repair centre and have it changed. See, you can’t drive on a run-flat tyre indefinitely after it’s been punctured; like a temporary spare they have a speed limit (generally 80km/h although this may vary depending on the maker) and a set distance of around 80km although, again, this may vary depending on the maker – some car makers suggest a distance of double this but tyre makers don’t. And, if you’re towing and your run-flat cops a puncture, it’s not recommended you keep driving and, you should never touch a run-flat tyre that’s been punctured because it will heat up significantly. Wait for the tyre to cool down before handling it.
How do run-flat tyres work?
There are two types of run-flat tyres available on the market but the idea behind them is the same and that is that there is something in the tyre that allows it to keep its shape even after a loss of some or all the air pressure.
The most common type of run-flat is one with a reinforced sidewall and this works by helping the tyre to keep its shape, allowing it to be driven on after losing air pressure. Then there’s with an internal ring of either rubber or some other sort of supporting material. This ring sits inside of the wheel and helps to take the weight of the vehicle if the tyre is punctured, essentially propping it up.
Run -lat tyres are fitted to a special type of wheel rim called EH2 (Extended Hump) which acts like a bead lock to provide improved retention of the bead when the tyre is at zero inflation pressure. Standard wheels are known as H2. It is possible to fit a run-flat tyre to a standard H2 rim but it’s not recommended due to the reduced bead-lock functionality of a standard wheel rim.
Aren’t run flats hard and hopeless for those living in the bush?
Run-flat tyres have copped a fair amount of bad press from Aussie motoring writers who claim those driving in the countryside and copping a flat will be stuck, because most bush-based tyre retailers won’t carry stock of run flats and so you’ll need to wait around for a replacement to be delivered. And, on the surface that makes sense. But, and there’s always a but, while car makers and tyre companies don’t like the idea it is possible to fit a standard tyre with identical dimensions, load index and speed code to get you out of trouble – it’s recommended once you get back to the big smoke that you swap the non-run flat tyre for a run flat.
In the early days, driving on run-flat tyres was akin to driving across cobble stones; the ride felt very hard and knobbly. These days, however, you wouldn’t be able to tell the difference between run-flat tyres and regular tyres.
Never try and repair a run flat tyre
A run flat is a sacrificial bit of kit. It’s designed to keep you moving across a certain distance and not a single tyre maker recommends repairing them. More than this, you can’t inflate a run-flat tyre once it’s suffered a puncture to try and get more life out of the thing.
Can I put run flats on my car?
If your vehicle doesn’t have run-flat tyres as standard, yes, but only if your vehicle has a tyre pressure monitoring system – this is the theoretical minimum requirement. See, without a TPMS it’s unlikely you’ll notice when your tyres have suffered a loss of pressure. Car makers that don’t fit run flats as standard to their vehicle don’t recommend fitting them, although it is possible, but you’d need to switch to EH2 wheels. More than this, fitting run flat tyres might require adjustments to the suspension and steering to compensate for the slightly stiffer characteristics of a run-flat tyre. It’s not worth it.
That said, Bridgestone released its DriveGuard series of tyres back in 2016 which are a run-flat tyre designed for vehicles that weren’t fitted with run-flats originally – meaning you can retro-fit them. But that’s only as long as you’ve got a TPMS fitted to your car (this has been mandatory on all new cars sold in Europe since 2014 but not here in Australia) and the tyre is available to fit your vehicle then you can fit a Bridgestone DriveGuard tyre; the company claims its new-generation run-flat performs well in wet and dry weather and offers superior bump absorption – a complaint of some run-flat tyres.