Differences between Direct and Indirect Tyre Pressure Monitoring Systems (TPMS)
The Tyre Pressure Monitoring System (TPMS) fitted to most Volkswagen vehicles with the exception of those in the US relies on inputs from the ABS wheel speed sensors to detect pressure loss, this is know as Indirect TPMS.
Differences in cost between Indirect TPMS and Direct TPMS at the factory are USD $10 for Indirect and USD $75 for Direct (figures taken from submissions to the NHTSA).
Fitting a direct TPMS is reasonably easy – Fitting a direct Tyre Pressure Management System (TPMS)
Indirect TPMS works be reading data from the existing ABS sensors to calculate pressure loss based on pre stored values and rotation of other wheels. For the system to operate you must “set” the system after you have pumped up the tyres to the correct values, press the TPMS button (has an exclamation mark on it) until a beep is heard. At this point the system is in learn mode and depending on how straight the road is will learn the tyre parameters over the next few hours (generally 5-15 minutes before the system will trigger).
The formula used by both first and second generation Indirect TPMS explains why they will only alert after 25% of pressure has been lost. The system will not give a warning if two tyres are equally underinflated on the same side or same axle or when all four tyre pressures are equally low and will alert only when the pressure drop is greater than 25% (the second generation is slightly more accurate due to the use of another sensor).
?LF = left front wheel angular velocities
?RF = right front wheel angular velocities
?LR = left rear wheel angular velocities
?RR = right rear wheel angular velocities
?a = average angular speed
? = tyre inflation (0 for normal tyre pressure)
First Generation Indirect TPMS takes information from the ABS wheel speed sensors and looks for small deviations from normal. Lower pressure results in a smaller tyre radius, which increases the speed of that wheel relative to the other wheels. The system operates by comparing the relative speed of one tyre to the other tyres, it has to allow for turning of corners, driving over rought roads, over uneven surfaces, with wet tyres on one side and dry on the other and this is why it won’t detect pressure loss until around 25% has been lost.
Second Generation Indirect TPMS system utilises the wheel speed sensors of the ABS system to learn the radii and various sensors to learn frequency characteristics of tyres what correctly inflated are able to meet the requirements of FMVSS 138 and will be able to be used on vehicles sold in the US. The requirements of the FMVSS are that the TPMS must be able to detect 25% pressure drops in one, two, three or four tyres within 20 minutes whilst driving between 50 and 100 km/h. This system is also able to warn which tyre has suffered the pressure loss however will still take up to a few minutes to warn of large pressure loss when driving or when driving off on a flat tyre. The Audi A6 MY09 ownwards is the first car sold in the USA with an Indirect TPMS however the NTHSA has not yet confirmed that it meets the requirements of the regulations. The following formula explains how the system works with frequency characteristics, there is another type that uses vertical force and vertical deflection which requires a height sensor or accelerometer but none are commercially available yet.
k = tyre sidewall stiffness
^k = change in tyre stiffness
m = mass acting on tyre
The calibration process is generally very similar for different variations of second generation Indirect TPMS and only requires the driver to drive normally, the times vary depending on driving style and speed variations. The calibration process will continue to be performed each time the car is started until complete.
- 0 -> 5 minutes – system able to detect tyre deflation greater than 25%
- 5 -> 20 minutes – system able to detect tyre deflation of up to 25%
- 20 -> 120 minutes – system fully calibrated
Whilst that all sounds good there are disadvantages to the indirect TPSM
- no way of telling which tyre is under inflated as there is only a single warning light
- some combinations may not be detected, ie two tyres on the same side or
the same axle under some specific circumstances won’t trigger the
- no checking of the spare tyre
- the calibration time after pressing the TPMS button can be hours (generally 5-15 minutes)
- the detection of pressure loss can take anywhere from a few minutes to hours
- they cannot detect a small pressure loss, generally around 20 percent is required to trigger the warning
- no monitoring of tyre temperature
- systems with frequency measuring require the owner to fit similar tyres as fitted at the factory as tyres with softer or harder compounds will have different frequency characteristics to those programmed in at the factory
- relys on users to set the system, if they can’t be bothered to check tyres are they going to bother to set the system?
The last point is really the major downfall of Indirect TPMS, it requires the driver to recalibrate the system every time the tyres are inflated. If this does not occur the system becomes ineffective and there is no warning to the driver that the TPMS system requires recalibration. Many drivers are unaware of what TPMS is and even if they are they are not aware that they have to recalibrate the system and this can lull them into a false sense of security and less frequent checking of pressures (after all the car will let the driver know when a tyre needs inflating. Right?).
If the TPMS warning does go off many users see the TPMS button as a reset for the warning, the car than calibrates for this lower pressure and won’t warn until pressures drop a further 20%. The driver assumes that everything is ok as the warning hasn’t come back and keeps driving oblivious to a problem with the pressures.
The system used mainly in the US doesn’t have these disadvantages. The US brought in laws requiring tyre monitoring to reduce accidents and decrease fuel usage, they tested a number of systems however the ABS sensor based systems (Indict TPMS) could not reliably detect a loss of under 25% hence the choosing of the in wheel based sensor. Estimates have shown there to be a USD 300-500 million dollar saving by going to the in wheel based system over having no system at all.
The only disadvantage to the Direct TPMS is the 10+ year life of the sensors and having to have additional sensors for winter tyres. The cost of the sensors is around USD $50 from Volkswagen. VW along with most other manufacturers fit antenna’s in the wheel arches which serves two purposes, the transmitters can have lower power output which means longer battery life/lighter weight and moving wheels around doesn’t require reconfiguration of the sytem.
Aftermarket system have a central antenna and you have to update the system if you move the wheels around, often they have a colour or number on the valve stem to identify them. Benefits of the aftermarket system is that they can often monitor 7-8 tyres so you can add your spare wheel and trailer. Disadvantages are no integration into the car, battery life can be lower and/or the sensors bigger to accommodate larger batteries.
Direct TPMS is a far superior system that only offers a minimal additional cost at manufacture, it’s main drawbacks are having to replace the sensors after 10 years and keeping multiple sets of wheels requires additional sensors. There are systems in testing today that have lowered the weight and size of the sensors to 7grams by removing the requirement for a battery.
Indirect TPMS requires the driver to calibrate the system and is slow to alert the drivers to loss of pressure. If the driver maintains the system correctly and realises that they still need to check tyre pressures it can be better than no TPMS. Many of the reasons why the US and soon the EU (2012) have made TPMS mandatory can be circumvented through incorrect use of Indirect TPMS.
Research has shown that Indirect TPMS makes no difference in the real world to tyre pressures which means the safety and environmental arguments for mandatory TPMS are pointless.
Both TPMS systems can lull the driver into a false sense of security, the difference is that with Direct TPMS the system will still operate and alert the driver to pressure loss whereas the Indirect TPMS may fail to alert if not properly maintained.