Run on flat on an idle wheel
Initially, air was released from the rear wheel on the inside of the curve, so it was idle in most of the corners. In such conditions, the run on flat tyre performed well, and when driving straight, passenger sitting on the side of this wheel could not tell that the run on flat tyre was empty, had it not been for the noise when the mass distribution was shifting in corners, e.g. when driving through chicanes. Driving at 2—40 km/h resembled driving with insufficient pressure, not with a flat tyre. When driving straight on an even surface, the car remained stable. The situation only got a bit worse when the weight was supported by the flat - the noise got louder and the route changed, but the car was still drivable and controllable, even if it got harder than usual.
On weighted wheel
The situation changed when the driving direction changed and the flat was on the weighted side of the vehicle in curves. The car remained stable and still behaved as if the outer tyre had insufficient pressure. It should, however, be noted, that every corner was accompanied by a pretty loud noise. But the run on flat tyre remained on the rim and was not damaged by the tarmac, despite the tyres low profile. It would probably be different on bumpy or even slightly wavy road. When driving at 40 km/h and with tight curves, temporary instability could be felt, but it could be easily corrected by the driver or by a stabilising system (ESC/ESP, DTC in BMWs), unless it was turned off.
Run on flat on both rear wheels
The test with two flats on one axle seemed the most interesting. Driving was similar to the previous situation, but the change in driving route was noticeable at lower speeds. The impression that the pressure in the run on flat tyres is too low could be felt in every curve. There were no transmission issues, but X-drive can move the car even if only one wheel touches the surface or turn a BMW into a front-wheel drive for a while. Sudden manoeuvres, such as avoiding an obstacles at low speeds (approx. 30 km/h) weren’t a problem, but the difference in the car’s behaviour could be felt. We should, however, note, that after the car 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 on two flats in the front or on one side.
Will run flat tyres supplant the good old spare tire?
You cannot drive too long on a flat tyre. For the originally installed RFT tyres, BMW says that the range is 150 km at the speed below 80 km/h (on even surfaces). Run on flat tyre manufacturers, in turn, give values that are half of that, but they add that the range can be extended by slowing down or stopping, as overheating the run on flat tyres may be harmful. As a general rule, when running on a flat tyre, you should choose the possibly smoothest route, take the curves gently and smoothly, always consider a safety margin and remain ready to correct your driving direction.
Pros and cons
You can really drive the run flat tyres when they are flat. What’s more important, you are not confined to driving straight, you can take curves and make sudden movements, such as avoiding obstacles. 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 on 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 unexpected changes in your car's behaviour when taking a corner at higher speeds.
The main advantage of run flat tyres is their increased safety when flat and the ability to continue driving, which is significant. The main drawbacks when compared to traditional 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.
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