Safety, TPMS and the re-set button

By David Shaw, ERJ editor

A question for motor industry professionals: Is it sensible and acceptable to allow drivers the opportunity to configure a safety-critical system on their vehicle such that the vehicle operates in an intrinsically unsafe mode?

Would any of these professionals offer the driver a simple, easily accessible button which permits them to create such a situation, with minimal checks and warnings?

Would they further permit  the safety system to display the all-clear, even when the system for which it is supposed to provide warnings, is in imminent danger of complete, catastrophic failure?  

No, I didn't think so. Yet this is the frightening world which the car industry wants  to create.

I am talking, of course, about tyre pressure monitoring systems (TPMS). 

The European Parliament in March of this year  passed ground-breaking legislation which will require car makers to fit as original equipment an accurate, effective system to detect a low-inflation pressure in one or more tyres. The law comes into force from November 2012.

There was a two-fold aim. First, because under-inflated tyres consume more fuel, the aim was to ensure tyre pressures are  maintained. This should save some 6 million tonnes of carbon dioxide from being released into the atmosphere each year, as cars run more efficiently. 

Second, when a car gets  a puncture, the system can warn the driver to slow down, stop in a safe place and check tyre pressure, thus preventing a potentially dangerous situation. Also under-inflated tyres can trigger accidents, so a side benefit  would be a reduction in accidents  due  to under-inflated tyres. 

That law law gives no details about the capabilities or performance of the required system. Parliament put responsibility for the detailed technical requirement onto the UNECE, based in Geneva. 

The relevant UNECE committee has been discussing these points for a couple of years and is now approaching crunch time for some tough decisions on cost, performance and implementaton. That committee is made up of car makers, tyre makers and component suppliers as well as some EU and UNECE officials.

There are two broad technologies on the market. The tyre industry favours direct systems, which put  a pressure sensor in each wheel and transmit the information back to the driver, who can then see exactly how much air is in each tyre. This system is initially more expensive than others, but it is more accurate, safer and offers low lifetime costs.

The car industry favours indirect systems, which measure the rotation rate of the tyres. By comparing the rotation speeds of one wheel with those of the other wheels and also with data from previous runs, a computer can work out if a tyre is changing diameter. The software interprets a reduction in diameter as a loss of pressure. 

The indirect system is cheaper and more convenient for car makers to implement, but has a far higher lifetime cost. It has one other absolutely fundamental weakness. The re-set button.

Whenever the  tyres are changed, or the  pressure in any tyre is adjusted, the  driver must - to make the system effective - re-calibrate the system. This is usually done by means of a re-set button on the dashboard.

The idea is that drivers are supposed to find an accurate pressure gauge, measure the pressure in each tyre, and adjust the pressure until it meets the manufacturer's recommendations, and then hit the  re-set button before driving off. This would  be awkward and  time-consuming at the best of times, but pressure gauges in commercial garages are known to be wildly inaccurate and most motorists do not have their own accurate pressure gauge. Thus, it is almost impossible for the average driver to accurately re-set the TPMS system.

The re-calibration process described above is not optional. If any change is made to the tyres or the air inside them, then the indirect TPMS system becomes ineffective until it is properly re-calibrated. Yet there is no safety check to alert the driver to the need to re-calibrate; nor any check to ensure it has been carried out properly. Without that re-calibration, the system is completely unable to give the warnings that it is supposed to give. It becomes redundant, doing nothing except giving the driver a false sense of security.

The first  difficulty is whether drivers really are aware that this recalibration is so essential whenever a tyre is topped up with air. 

The evidence shows that the vast majority of drivers are not aware of this necessity. 

A much more serious weakness, however, is that the system has no checks on the state of the tyres, when that recalibration button is pressed. Once it is re-set, that becomes the base state for the tyre deflation warning. The system will only alert the driver, if a tyre deviates from that calibrated condition by 20 percent.

A driver could, through ignorance, annoyance or any other motivation, hit that button when all four tyres were under-inflated by 50 percent or more. The computer system would  then be calibrated to believe that this is the normal and safe operating condition. It would then alert the driver, only when a tyre becomes 20 percent deflated from that calibration pressure. In the  example, that  would be 60 percent below the recommended  pressure.

Worse, one tyre might be over-inflated, a second at the right pressure, a third at 50 percent under-inflation and a fourth at 80 percent under-inflation. If the driver hits the re-calibration button, the system is fooled into thinking all is well, and will not issue any warnings, even though the vehicle is unsafe and unroadworthy, and is putting the driver and passengers at extreme risk.

As if this frightening prospect were not enough, the car industry wants to impose thousands of euro in extra costs on each consumer in order to  achieve a miserable euro 25 saving for itself.

This system has  built-in threshholds, which are calibrated according to the stiffness of the tyres fitted at the factory. In order to deliver a warning at the required 20 percent under-inflation, the tyres fitted to the vehicle must have the same construction characteristics as those fitted by the vehicle manufacturer. 

Indeed, car makers recommend specifically  in the owner's manual that owners should replace the original tyres with tyres of exactly the same model and make at each replacement. 

Second-generation indirect TPMS systems which rely on vibration characteristics of the tyre are even more dependent on the pre-loaded operating characteristics of the tyres on the vehicle

In the real world, these original equipment tyres can cost euro 500 each, in comparison with cheaper tyres which might cost only euro 100 each. Thus, at each replacement, the consumer has a substantial extra cost. More typical figures might show a difference of euro 50/tyre. Multiplied by four tyres and four or five replacements over a vehicle lifetime, this extra cost placed on the consumer amounts to well over a thousand euro. This is entirely disproportionate to the 25-euro savings achieved by the car makers when they opt for the 'low-price' solution.

Yet a third weakness of the  indirect system is that it  take some  time to report  a problem after the car has started. 

There has been a spate of tyre slashings by vandals in recent months. If a driver fails to notice that his tyre has been slashed, the indirect system will not detect the problem for some minutes.

A direct system, by contrast, will alert the driver within seconds of the engine ignition being turned on.

To summarise, a direct system offers fast response, accurate pressure monitoring, reliable warnings and minimal costs to the vehicle owner. It costs the car makers euro 20 to euro 30 to fit to each vehicle and can monitor the spare tyre. 

An indirect system has a re-set button which can be abused; it is  slow-acting, cannot easily detect low- pressure in multiple tyres and  and requires the vehicle owner to buy expensive premium tyres for the life of the vehicle. The evidence from well-respected research shows that indirect systems have no effect on tyre pressure in the real world and there is no way they can monitor the spare tyre. It costs around euro 8 to fit to each vehicle.

The working group in Geneva is not allowed to decide between indirect and direct, but equally, it must write the specification in a way that ensures the TPMS legislation delivers the safety and CO₂ improvements envisioned by Parliament. The way to do that is to insist that the standards include short time delays for detection; a requirement that the driver cannot accidentally -- or deliberately -- set the system into an unsafe condition and a requirement that the system can quickly and accurately detect multiple low-pressure conditions. 

The key argument here is that the car industry is ignoring major safety weaknesses in order  to save a few tens of euro per vehicle, while it is quite happy to put  an extra cost of thousands of euro onto its customers. 

It is hard to argue, in the face of the re-set button, that an indirect TPMS is effective and accurate. But that is what the car makers will attempt in Geneva next week.