Can run flats really run flat?

Run flat tyres allow drivers to carry on driving when they have a puncture, preventing the need for the driver to change the tyre at the side of the road in potentially hazardous traffic. When a run flat tyre has deflated it should allow the driver to travel around 50 miles at 50mph, meaning they can get home or to a garage, where an appropriate replacement can be fitted.

Run flats have a reinforced sidewall which can take the weight of the car should all of the air escape. Because the sidewall on a run flat tyre won’t deform very much when there’s a puncture, it’s difficult to see a flat with the naked eye. As such all cars fitted with run flats have a tyre pressure monitoring system installed, which alerts the driver of a loss of pressure with a light in the dashboard.

A downside to the strengthened sidewall is that it impairs ride comfort. Lumps and bumps in the road are easily absorbed by the flexible sidewall in most tyres, but because the sidewall on a run flat is more rigid it transmits the shock through to the cabin more easily. This was very noticeable on early run flats but more modern tyres do a better job of absorbing shocks than their predecessors.

The theory behind run flats is sound, but how do they perform in practice? We tested the performance of Goodyear RunOnFlats at the MIRA testing facility in the midlands. First of all we took a BMW 5 Series on fully inflated RunOnFlats on a slalom course and around a figure of eight, in order to gauge the level of grip and traction offered.

The surface was too smooth to detect any issues with ride comfort but, as you’d expect from a set of brand new premium tyres, there was sufficient traction to drive much faster than the average road user would ever need to, and handling was predictable and precise.

Goodyear _Run On Flat _ Comparison 1

Next, the air was let out of the front passenger side tyre by pushing in the valve. Once the air had completely escaped we had a quick look at the tyre. Pushing on the sidewall made it move very slightly and it did appear to bulge a little in comparison to the inflated tyres, but it wouldn’t cause alarm to most drivers.

We took the car back onto the slalom and figure of eight course and tried to replicate the same speeds as before. The performance of the flat tyre was very impressive – it was possible to drive at normal speed without any difficulty. Because the one tyre had less pressure it created a bit of drag, and the car would veer very slightly to the left as a result.

Additionally the tyre would rumble under heavy load. When turning right weight shifts over to the left of the car, which made the flat tyre work harder and groan audibly, but it still afforded enough grip to allow easy progress.

It was a genuinely impressive experience. If a driver climbed into a car with a punctured run flat tyre they would almost certainly notice a difference in handling, but it would be so subtle that pinning it down to a flat tyre would be difficult if not for the fitment of a tyre pressure monitoring system.

Obviously in the event of a blow out a run flat tyre won’t afford any extra benefit over a standard tyre. This can cause a problem if there’s a can of foam instead of a spare, but blow outs are less common than simple punctures.

Now that they offer a better level of ride comfort, there’s a good case for run flat tyres. Stopping by the side of a motorway or B-road to change a wheel is hazardous and can be extremely difficult without a breakers bar to help undo wheel nuts – a tool few drivers carry.

Run flat tyres provide drivers with the opportunity to reach somewhere safe to change a wheel. Additionally, the tyre pressure monitoring system that is precluded by their fitment makes it much easier to keep tyres at recommended pressures, which can improve fuel consumption and, in the long run, save you money.

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