Run flat tyres

In recent years there has been a marked increase in the number of vehicle manufacturers offering run-flat tyres as a safety system on new vehicles - BMW, Mini, Lexus, Audi to name a few.

Run-flat systems have been around for a number of years in many different forms - Dunlop Denovo, Michelin/Dunlop TD system, Bridgestone/Dunlop Denloc system. To name but a few. Over the years manufacturers have adopted several different systems but all with one goal in mind - vehicle safety.

Most motorists will, at one time or another, suffer the inconvenience of a puncture. This will involve jacking up the vehicle, emptying the boot, removing the spare wheel and changing the damaged wheel over. It is at this point that the motorist realises that punctures usually occur when the boot is full of shopping or when it is raining!

There are inherent dangers associated with changing a wheel at the roadside, particularly if the puncture occurs on a motorway where the work has to be carried out on the hard shoulder. Disabled or particularly vulnerable motorists need to avoid the risk of being stranded in areas of greater risk such as when travelling alone through city centres late at night.

Whilst a puncture is usually more of an inconvenience and unwanted expense, the dangers involved with a tyre failure at high speed are far more serious. The greatest danger being loss of control of the vehicle. Loss of control occurs when the sidewall of the tyre, which is usually kept secure against the rim by the internal air pressure, becomes separated from the rim flange and drops into the well of the wheel.

As soon as the beads are disconnected from the rim flange, loss of steering control will occur.

Run-flat tyres are designed to keep the tyre beads securely anchored to the rim flange area of the wheel even when operating at zero pressure. In this section of our website we will concentrate only on those Run-flat tyres that are currently available from tyre manufacturers. We will not go into details of systems that are designed to prevent the tyre beads from dropping into the wheel well by physically blocking of the well area of the wheel. An example of this system is Tyron which is covered separately.

One area of particular interest is the question of repairs to Run-flat Tyres. At the moment the majority of tyre manufacturers DO NOT recommend a Run-flat tyre be repaired following operation at reduced pressure. When a standard tyre is punctured and runs at reduced or zero pressure it will suffer damage to the sidewall that will render it unfit for repair and re-use. This damage is easily spotted once the tyre is removed and is usually seen as a creasing to the inner liner of the tyre in the sidewall area. This creasing indicates that damage has occured to the internal construction of the tyre and further use could result in casing failure.

Due to its heavily reinforced sidewall construction a Run-flat tyre will not exhibit these signs and there may be internal damage that cannot be seen. A Run-flat tyre will undergo a great deal of stress while operating in a zero pressure situation and will therefore be subject to greater stresses than a standard tyre. The actual tyre does not carry the weight of the vehicle - it is the pneumatic effect of the air inside the tyre that carries the weight.

It is believed that the British Rubber Manufacturers Association (BRMA) are shortly due to make an official recommendation regarding repairs to Run-flat tyres.

Run-flat systems can also generally be split into two separate categories. Those incorporating a significantly strengthened sidewall, and those that use an internal support ring that is mounted on the inside of the wheel rim.

The following systems are available from tyre manufacturers as Run-flat Systems:

Sidewall Reinforced	EMT	Goodyear
	RFT	Bridgestone
	DSST	Dunlop
	EUFORIA	Pirelli
	ZP (ZERO PRESSURE)	Michelin
	SSR	Continental
Support Ring	CSR	Continental
Support Ring	PAX	Michelin

Both systems will, subject to distance limitations, allow the motorist to continue their journey without having to stop and replace the punctured tyre. Accordingly most vehicles that are equipped with Run-flat tyres do not have a spare wheel.

The dramatic increase in the sale of Run-flat tyres since 2002 is due to the increase in fitment of these tyres to vehicles as Original Equipment (O/E). BMW being the biggest user. Many models of BMW are now available with Run-flat tyres - the 1 series, new 3 series, 6 series and Z4 and Z8 models are all equipped 100% with Run-flat tyres, 5 series and 7 series are available with Run-flat as an option. Other manufacturers offering Run-flat tyres as O/E fitment include Audi, Daimler-Chrysler and Ferrari to name a few.

The other essential item of safety equipment for a vehicle with Run-flat tyres is a Tyre Pressure Monitoring System (TPMS). Run-flat tyres are designed to minimise the difference in ride quality when a tyre is punctured. This is especially the case with motorway driving where there is relatively little cornering. For this reason it is difficult to determine that a tyre has lost pressure or is operating in zero pressure conditions. Should the driver not be aware of a loss of pressure they may well exceed the safety limits imposed by the manufacturer. This would eventually cause the tyre to fail. Run-flat tyres are not indestructible!

There are two main systems for detecting a drop in tyre pressure. One involves a sensor that is fixed to the wheel, inside the tyre. The other uses the vehicle ABS to detect the differing speed of revolution of a wheel with a reduced circumference., caused by a reduction in tyre pressure.. These are detailed further as follows:

Sensor Based TPMS

A sensor based TPMS has a pressure monitoring sensor fixed inside the wheel and tyre assembly. This is usually clamped to the wheel and constantly monitors the internal pressure of the tyre. This information is relayed to a receiving unit on the vehicle body which is connected to a processing unit in the electronics system of the vehicle. This alerts the driver to a loss in tyre pressure.

Below is a selection of sensors available on the market today:

A typical combination type TPMS sensor fitted to a rim

3 typical combination type TMPS sensors

Advantages of the combined TPMS sensor include accurate monitoring of the actual pressure inside the tyre and an immediate warning of a tyre deflation. Disadvantages include the fact that it a more complex system (sensor, receiver plus processing unit) and it can be more difficult to fit the tyre due to the possibility of damage to the sensor if extra care is not taken.

ABS Based TPMS

This system uses the ABS of the vehicle to monitor the rotation speed of the individual wheels. If a deflation of a tyre occurs the resulting increase in wheel speed triggers the TPMS and advises the driver accordingly.

Advantages of the ABS based system include the fact that the system uses technology and equipment that is already fitted to the vehicle. Also there are no sensors fitted inside the wheel/tyre assembly which makes the tyre fitting process easier than the sensor based systems.

There are many vehicles that operate a TPMS but do not offer run-flat tyres as an option. Some tyre manufacturers state that it is acceptable to mount Run-flat tyres to these cars providing the TPMS is in working order. Other manufacturers recommend that only vehicles with Run-flat tyres fitted as OE should use them. When a Run-flat tyre is supplied and homologated with a vehicle manufacturer it is tested under extreme conditions, including operating at zero pressure. During these tests the handling characteristics of the vehicle is studied both in normal and zero pressure situations. These observations assist in determining the vehicle suspension settings because the tyre sidewall flexing forms part of the vehicle suspension. A recent example of this was recently highlighted when journalists slated the BMW 5 series for an "uncompromising and uncomfortable ride" when fitted with Run-flat tyres. The new BMW 3 series however, fitted 100% OE with Runfla,t is now praised for its ride quality.

From a vehicle manufacturer's point of view Run-flat tyres are preferable for numerous reasons.

A reduction of the weight of the vehicle. Although heavier than standard tyres, four Run-flat tyres weigh less than five standard wheels and tyres, including the spare wheel, plus jack and wheel brace. Weight reduction is important because it increases fuel efficiency.
The absence of a spare wheel increases the amount of boot.space.
The reduction in the amount of materials required in the manufacturing process reduces environmental waste and energy consumption.

Difficulties can arise during the fitting of Run-flat tyres due to the nature of the stiffer sidewall construction. and possible presence of a pressure monitoring sensor mounted on the rim. As tyre sidewall aspect ratio reduces the amount of flex in the sidewall also diminishes and it is this makes the tyre more difficult to mount onto a rim. As the Run-flat tyres will have both a low profile and a reinforced sidewall fitting can be a difficult process. All etyres mobile tyre fitting vans are equipped with the latest machinery to facilitate accurate fitting and balancing of Run-flat tyres. etyres tyre fitters are trained to fit Run-flat tyres.

Below is a typical example of one of our Run-flat tyre fitting machines.

Run-flat tyres offer distinct advantages but from the first run-flat systems from Dunlop 30 years ago to the first sidewall reinforced tyre from Bridgestone in the late 1980's there has been a lack of popularity in the marketplace. This was caused by a lack of enthusiasm by vehicle manufacturers in fitting them as OE fitment.

The latest generation of Run-flat tyres and the introduction of TPMS has removed this lack of enthusiasm. As stated above, major vehicle manufacturers such as BMW and Mercedes now regularly fit them as standard equipment. Ride quality, which was previously a cause for concern, has been vastly improved.. Now when a vehicle is designed to be fitted with Run-flat tyres as standard the suspension system is tuned to incorporate them into the design.

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