What’s it like to use run flat tyres when the air has gone? We’ve conducted such a run flat tyres test to see what happens - we’ve also changed the position of the flat tyres to see how this changes the experience.
Running On A Flat Idle Wheel
Initially, air was released from the rear wheel on the inside of the curve, so it was idle for most of the corners. In such conditions, the run flat performed well and, when driving straight, the passenger sitting on that side of the vehicle could not tell that the tyre in question was empty. However, there was an increase in noise and mass distribution when shifting in corners that strongly indicated the tyre was flat.
Driving at speeds of up to 25 miles per hour (40 km/h) resembled driving with insufficient tyre pressure, not a completely flat one. When driving straight on an even surface, the car was able to remain stable.
The situation only got a little worse when the weight was supported by the flat wheel, as this lead to louder noise and a change in direction but, overall, the car still drivable and controlled.
On a weighted wheel
When the flat was changed to the weighted side of the vehicle (in accordance with the direction of the corners) the situation changed. The car remained stable for the most and still behaved as if the outer tyre had insufficient pressure.
It should, however, be noted that every corner was accompanied by particularly loud tyre noise. Yet the run flat remained on the rim and was not damaged by the tarmac, despite the low profile of the tyre.
It would probably be different on bumpy or even slightly wavy roads. When driving at 25 miles per hour, with tight curves, temporary instability was experienced, although this could be easily corrected by either the driver or a stabilising system - such as ESC, ESP or the DTC in BMWs).
Run On Flat On Both Wheels
For us, the test involving two flats on one axle seemed the most interesting. Driving was similar to the previous situation, but a change in direction was noticeable at lower speeds. The impression that the pressure was too low in these tyres could be felt in every curve or corner.
There was no transmission issues, but the X-drive in the test can move the car even if only one wheel touches the surface, or even turn the BMW into a front-wheel drive car for a period of time. Sudden manoeuvres, such as avoiding an obstacle at low speeds (such as 18 miles per hour) were not a problem, but the difference in the car’s behaviour could still be felt.
Can run flat tyres supplant the traditional spare tyre?
We should, however, note, that after the vehicle seemed to lose stability, it returned (after a correction), but the situation may be different in the event of a car with a single axle drive, especially when the axle is weighted. This also applies to running two flat tyres on the same side, or both in the front.
Of course, you cannot drive too long on a flat tyre. For the originally installed RFT tyres, BMW states the maximum range is 93 miles (150 km) at speeds below 50 miles per hour (80 km/h) on even surfaces.
Many run flat tyre manufacturers, however, give values that are half of this, yet they add that this range can be extended by slowing down or stopping, as overheating can be harmful to these products.
As a general rule, when running on a flat tyre, you should choose the smoothest possible route, take the curves gently and smoothly. You should also always consider a safety margin and remain ready to correct your driving direction.
Pros and cons
As their name suggests, you really can drive in run flat tyres when they are flat. However, more importantly, you are not confined to driving straight, as you can still take curves and other sudden movements, such as avoiding obstacles.
During the test, the stiffness appears sufficient, and the rims were not damaged, which is also important. Every time the “damaged” tyre becomes idle, it emits a noise reminding the driver to drive more carefully. This is good, as you can easily forget that your run flat tyre is damaged if you drive calmly.
That’s why run flat tyres should not be used in cars without a pressure control system, as you may not feel that the tyre is punctured when driving straight, while a successful repair becomes less and less possible with every kilometre you drive - not to mention the unexpected changes in your car's behaviour when taking a corner at higher speeds.
The main advantage of run flat tyres is, of course, their increased safety when flat and the ability to continue driving. The main drawbacks, when compared to traditional car tyres, are the decreased comfort, especially on bumpy roads, higher purchase price and the need to replace the tyre after driving longer distance using run on flat, which often means replacing two tyres on the same axle, as the tyres should not only be identical in type, but also in tread wear to avoid aquaplaning.
Also, the repair can be harder - not all repair shops have suitable equipment and properly trained staff - and the damage is usually not visible on the outside. Tyre failures caused by incorrect repairs were widely publicised in Germany, and we should not expect that the situation is any different in the United Kingdom.