Run Flat Tyres
Over the last few years a number of car manufacturers offer new car models with run-flat tyres - Audi, BMW, Ferrari, Lexus, Mercedes-Benz.
Run Flat tyres improve vehicle safety and remove the need to carry a spare tyre and the associated wheel jack etc giving significant increase in boot space and reduced vehicle weight.
Tyre Punctures always occur at night or in heavy rain and most times are simply inconvenient. The greatest danger occurs with a rapid loss of tyre pressure at high speed. As soon as the tyre sidewall disconnects from the wheel rim steering control will be lost.
Run-flat tyres work by keeping the tyre bead fixed to the rim flange even when running at zero tyre pressure.
Run-flat tyres have a reinforced sidewall or internal support ring construction to cope with the enormous stresses when used at zero pressure.
Zero pressure running stress can cause hidden damage to the tyre that is not easily identified so run flat tyres are not normally repaired but should be replaced after a puncture.
The following Run-Flat Tyre Systems are currently available:
Support Ring Tyre Construction
- Continental CSR
- Michelin PAX
Sidewall Reinforced Tyre Construction
- Bridgestone RFT
- Continental SSR
- Dunlop DSST
- Goodyear EMT
- Michelin ZP (Zero Pressure)
- Pirelli Euforia
Cars fitted with runflat tyres do not normally carry a spare wheel but will allow you to complete a short journey at reduced speed until a replacement run flat tyre can be fitted.
A run-flat tyre running at zero pressure will allow the vehicle to steer and handle as normal. This makes it difficult for a driver to notice when a puncture has occurred.
Vehicles designed to use run flat tyres should have a Tyre Pressure Monitoring System (TPMS).
Direct Sensor Based TPMS
Sensors inside the wheel directly monitor local pressure and relays pressure information to the vehicle ECU and warn of a tyre pressure loss.
Interpretive monitoring - ABS Based TPMS
A punctured tyre has a reduced wheel circumference compared to a normal tyre so the ABS system can use relative rotational speed (A smaller diameter wheel needs to rotate faster to maintain a given speed) to sense a puncture or loss of tyre pressure.
If a puncture or tyre pressure loss occurs then the resulting increase in wheel speed triggers the TPMS of the ECU and warns the driver.
Retro Fitting Run Flat Tyres
Many cars are already fitted with a functional ABS based interpretive TPMS and retro-fitting run-flat tyres is possible. Cars designed to use Run-flat tyres, as standard will have the suspension system optimised for handling quality.
Run Flat Tyre Fitting
Fitting Run-flat tyres can be more difficult due to the stiffer sidewall construction. Rim mounted pressure sensors can also make wheel fitting more difficult. 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.