From the GermanCarFans posted E90 In depth article, this may help :
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With tyre damage being relatively uncommon in statistical terms, but with punctures being one of the most unpleasant experiences for the motorist, BMW is the first manufacturer in the world to offer the Runflat System Component (RSC), a tyre safety package meeting both the statistical risk as well as any feeling of apprehension on the part of the driver.
RSC is a combination of RSC tyres on EH2 wheel rims and the TPI electronic warning system. Under loss of pressure, RSC immediately warns the driver but allows him to continue for a certain distance even when suffering a complete loss of air, the tyres remaining safely on the rim. Accordingly, there is no further need for a spare or an emergency tyre, a repair kit and car jack. And there is no need any more to change tyres by the roadside. But at the same time RSC still offers 100 per cent safety, as required.
Informing the driver of a loss of pressure – and allowing him to go on.
With their specially designed rim hump, EH2 (extended hump) rims prevent the tyres from “jumping off” the rims under a sudden loss of pressure. An RSC tyre differs from a conventional tyre through its runflat features, reinforced side walls, additional insert strips and a rubber compound with a higher level of thermal resistance enabling such self-supporting tyres to run another 50–250 kilometres at a maximum speed of 80 km/h or 50 mph without even the slightest pressure, depending on the load the car is carrying.
The RSC tyre is its own spare tyre.
Only 20 per cent of all tyre damage involves tyres losing air suddenly or in a quick process. In other words, loss of pressure is a slow-gradual process in 80 per cent of all cases, enabling the driver with RSC tyres to continue for up to 2,000 kilometres or 1,200 miles. So it is fair to say that an RS tyre is its own spare tyre. And this significantly improves the level of safety, also because ABS, ASC and DSC remain fully functional at all times.
The Tyre Pressure Indicator (TPI) included in the system monitors air pressure in all the tyres by permanently comparing tyre rotation speed. The driver is informed of any irregularities by a warning light and a sound signal, the system giving the driver a warning as of a speed of 15 km/h (9 mph) and with a drop in tyre pressure by more than 30 per cent.
It is nevertheless important to note that TPI does not replace the regular supervision of tyre pressure by the driver. And in a simple, convenient process, the driver is required each time after changing tyre pressure and after fitting a new tyre to re-initialise the system in order to save the tyre’s data.
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