Is there a long-term future for indirect TPMS?

Ask a leading tyre pressure monitoring system manufacturer what the future is for indirect TPMS and they will probably say there isn’t one. Tyres & Accessories asked this precise question of one of the biggest manufacturers in the business recently and the response was swift and somewhat in line with expectations. Of course direct TMPS makers have economic reasons not to like the solution offered by the competition, but is there more to it than that? As the European legislation mandating TPMS installation in every new vehicle comes into force, Tyres & Accessories engages with both sides of this debate.

The basic thinking is that while all direct TPMS (irrespective of specific manufacturer) physically measures the actual pressure of individual tyres – and often temperatures as well – by their very nature indirect systems are unable to do this. They in contrast calculate pressures using algorithms based on wheel revolution data supplied by ABS and stability control systems. This, say the direct TPMS makers, means all manner of circumstances routinely undermine whatever mathematical wizardry these systems offer.

Examples given as inherent weaknesses of this approach include, false readings given by unevenly worn wheels. In cases like this tyres with different tread depths or pressures can supposedly give false positives, while an axle with one correctly inflated worn tyre and one underinflated new tyre could fail to trigger a warning light due to similar revolution counts and despite faulty pressure on one side. Also because the pressure warnings given by indirect systems are calculations, they require a significant length of time in order to calculate data that may indicate a pressure loss. Because the technology is physically inside each wheel, direct systems on the other hand can provide individual tyre pressures virtually on demand.

Then there is necessity of a reset button. First of all, despite what motorists up and down the country tend to think, it is not a reset button at all. It is rather a re-calibration button. The difference is enormous and the lack of awareness is widespread meaning many people seeking to silence the in-car TPMS chime have actually rendered their indirect TPMS virtually useless by telling the system to receive new baseline data when the car’s tyres are in fact in an underinflated state. In addition, the rise of electronic vehicles, which will in the future use in-wheel direct propulsion systems and techniques such as torque vectoring, are only likely to complicate things further.

Perhaps the most damning criticism T&A has heard from the direct TPMS manufacturers on the subject of indirect TPMS is that their technology only barely passes the European requirements. Quite apart from the fact that few in the direct camp publically claim to understand how this technology can be accurate enough to be compliant, the suggestion is that should the European laws be tightened any further the indirect systems that are struggling to keep up at the moment could suddenly become non-compliant. But is this true, or is it a question of the increasing direct majority of the market sharing their own coloured perspective?

The iTPMS fightback

According to one indirect TPMS or iTPMS manufacturer, currently about 15 per cent of all new cars are equipped with TPMS of one kind or another. While this figure is on the rise – and arguably will rise quickly following the introduction of November’s TPMS rules – current low levels of awareness provide a good opportunity to in their words “dispel some myths” about the technology involved.

NIRA Dynamics AB, an iTPMS supplier from Linköping, Sweden, which supplies a software based system called TPI (Tyre Pressure Indicator) which can be found mainly in Volkswagen group models and is particularly vocal about this issued.

First off, NIRA’s view is that “direct” and “indirect” systems have different strengths and weaknesses. While they agree that direct systems or (dTPMS) are equipped with pressure sensors in the wheels, usually attached to the rim and that they measure the pressure and often temperature data which is sent to the car, NIRA also points out that the sensors require batteries that need replacing “every few years”. Furthermore if this doesn’t coincide with a tyre change, this means a separate visit to the tyre dealership. Most sensors are designed as sealed battery units and therefore need to be replaced as whole unit rather than just the battery. The cost argument continues with the suggestion that sensors cost 50 euros plus each and that they quickly becomes a costly (and perhaps messy) inconvenience to consumer and dealer alike.

For its part iTPMS technology links ABS and stability control data with other signals such as the steering angle or engine torque in order to monitor tyre pressures. According to NIRA, “iTPMS is proven to be very reliable and easy to handle. After each check of the tyre pressure or a change of wheels a system reset is required – often a simple push of a button. The system now knows what the nominal pressure is. This allows detecting both pressure losses on individual wheels caused by punctures as well as the gradual loss of air over long periods of time on all four wheels.” However, as we have seen it is not always as simple as this.

As two of the leading suppliers of iTPMS Dunlop Tech in Hanau, Germany, and NIRA Dynamics from Linköping, Sweden, recently sent their products to be tested by TÜV Süd in order, they say, to add a factual basis to the discussion. Both companies supplied one vehicle each with summer and winter tyres for the tests. The results, say the two companies, answer their critics. But do they answer the right questions?

Firstly they claim that iTPMS not only detects pressure losses on individual wheels, the reliable monitoring of all four wheels of slow pressure losses has also become a standard feature. Furthermore modern second generation iTPMS also use spectrum analysis monitoring certain tyre-pressure-dependent oscillations. This is reportedly done individually for each wheel, enabling monitoring for all four wheels. Therefore indirect systems can detect a pressure drop of 20 per cent on all four wheels in about 15 minutes driving time “four times quicker than legally required.”

The iTPMS makers answer to questions raised over this approach’s sensitivity are answered in pragmatic fashion: their systems simply don’t need to be more sensitive. Their argument is that temperature difference between a cool morning and the hot midday sun alone can cause differences of more than 0.3 bar in tyre pressure. Warming and cooling effects through driving reportedly add another 0.2-0.3 bar. Therefore there is no reason for a system to alarm and force the driver to a garage with a tyre pressure warning just because it is cool morning. And that’s why a finer resolution than 0.2 bar or 10 per cent “makes no sense in practice and would only have to be paid [for] dearly by the drivers.”

The OE only myth

Apparently answering rumours alleging iTPMS systems don’t work properly with non-OE tyres, NIRA and Dunlop Tech said their respective technologies function best with all original tyres and rims and are also compatible with “nearly all aftermarket tyres and rims.” In contrast, they added, direct TPMS with rim mounted sensors do not always fit to aftermarket rims. The fact that the company did not say “all tyres of the correct size” or another blanket approval however leaves continued room for this criticism. The problem appears to be that the current indirect systems are tuned to work optimally with all the original tyres and resonant frequencies associated with these. The implication is that the varying makeup of OE tyres, manufacturer designated tyres (such as Jaguar’s J-marked products and Porsche’s N-marked tyres), and replacement versions differs enough for the TPMS to behave differently – despite them all being of the same model and size. The logic is that outside this narrow stable there is a multitude of further variables and that this could affect iTPMS further.

As far as load is concerned, contrary to other criticisms, NIRA and Dunlop Tech point out that modern indirect systems have sophisticated mechanisms for load compensation and that this was something TÜV Süd looked at in the independent tests the two companies commissioned.

The two companies go on to criticise the European testing methodology’s bias towards sharp changes in pressures. Real pressure losses, they say, are not generally sudden events. “Most punctures lead to gradual, steady losses over many minutes, often hours. Natural pressure loss due to diffusion or microscopic valve leaks take several months before significant losses are achieved.” Their argument is that a real-world TPMS must therefore be particularly sensitive to continuous pressure losses and that the so-called “step response” required by law has “little practical significance, despite the fact that the legal requirements are described as the “pressure step” test.” Pressure step tests they argue often make dTPMS look faster and more accurate in comparison with iTPMS.

What is interesting to note when assessing the claims of the two competing technologies is how little they directly contradict one another’s assertions. Instead they focus on questioning the premise for one another’s approaches and the two respective philosophies. Direct manufacturers say their technology is accurate, fast and specific. The indirect camp says say it is expensive, requires servicing and that theirs is faster than you think. Indirect proponents say their technology works on most tyres and rims, can adapt to weight changes and complies with all legal requirements, while those that make the direct technology point out that it fails to give specific readings, takes longer and is fundamentally undermined by the fact that it doesn’t actually measure either pressure or temperature, but rather seeks to estimate and warn on the basis of other data.

What neither side of this debate seems to have focused on is what this means for the retailers that will be responsible for resetting and perhaps servicing these systems. By arguing that their systems are less expensive than direct technology and that they don’t require servicing the indirect suppliers have basically argued themselves out of the support of the tyre retail business (and quite probably a host of garages and autocentres too). Anyone can see that a car part that either requires checking and servicing (something that is now part of the annual MOT test in the UK) on a regular basis or that brings consumers back to the depot for a safety check provides them with more service opportunities. And with direct technology actually able to flash up real time pressures, there is a strong and demonstrable safety argument to back this up. What indirect system makers win on is the flip side of exactly the same reasoning – their technology is less expensive, which is likely to win brownie points with the OEMs at the other end of the supply chain.

So is there a future for indirect TPMS? As long as the technology remains compliant and OEMs continue to purchase it, yes. However, this doesn’t mean it offers either the best technological solution, best safety argument or the best opportunities to the retail sector.