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TYRE PRESSURE MONITORING
Pressure monitoring is a bush travel essential - May 2016
Outback Travel Australia has been evaluating different types of tyre pressure monitoring systems over the past five years and we wouldn’t go bush without one.
The classic ‘bush flat’ is a blown-out tyre, with a ring of tread rubber atop a smoking, shredded sidewall. This outcome tempts most people to believe that a sidewall puncture caused the problem, but it’s usually not so. When a tyre blows out spectacularly it’s often the result of an undetected, slow leak, caused almost always by a small tread puncture. This slow leak creates a pressure drop and that lets the tyre flex more, building up heat in the process. As the pressure continues to drop the flexing increases and so does heat build-up. Eventually the ‘cooked’ casing gives up.
A tyre pressure and temperature monitoring system can alert a driver to such a leak in its very early stages. This allows the driver to pull up and repair the tyre temporarily with a plug, or fit a spare, long before there’s permanent damage to the tyre.
Tyre pressure monitoring saves tyres and can also prevent an accident.
Tyre pressure monitoring history
The USA has had mandatory tyre pressure monitoring since 2007 and Europe mandated it in 2012. As with electronic stability control it’s only a matter of time before tyre pressure monitoring is mandated in Australia as well.
On the face of it tyre pressure monitoring systems are excellent safety devices and ideal for recreational drivers, who are more likely to suffer punctures than the metropolitan brigade, but there are different designs and some are definitely better than others.
The after-market devices being sold here use valve-integrated pressure senders, wirelessly connected to an in-vehicle display, but there is an alternative that some makers adopted for USA compliance.
Indirect pressure monitoring
Indirect pressure monitoring is cheaper and doesn’t need wheel-mounted senders. Instead, ABS software is rewritten. As a tyre deflates, its rolling radius decreases and its speed relative to the other three tyres increases. Indirect pressure monitoring detects this rotational speed change and the tyre warning system is activated.
Indirect pressure monitoring sounds like a cheaper, more reliable solution, but that’s not how it’s worked out in the USA. The problems centre on the fact that all the tyres’ rolling radii need to be calibrated, so that the computer knows when one gets out of whack.
Whenever tyre pressures are changed – for a high-speed, loaded-vehicle trip, as an example – the dash-mounted reset button must be pushed and a re-calibration procedure followed. Procedures vary, but most involve driving for some distance until the computer ‘learns’ the new setting. If this isn’t done the monitoring system won’t operate. Worse, if the pressure gauge is inaccurate the tyres can be set to an incorrect pressure, which the system recognises as its new benchmark.
In an effort to overcome these deficiencies, second-generation units have inbuilt ‘vibration signature’ detection. The OE tyre’s vibration characteristics at different pressures are stored in the computer and when low-pressure characteristics are detected the system warns the driver. The big problem with this design is that a change of tyre make and size makes the system useless.
And yet another problem with indirect pressure monitoring is a time delay of some minutes in detecting a low-pressure tyre after start up. Since many punctures don’t show up as a flattish tyre until morning and not everyone does a visual tyre check before each day’s drive, that’s a major defect with the indirect pressure monitoring design.
Have a guess why vehicle makers continue to use indirect pressure monitoring? It’s dirt cheap.
Direct pressure monitoring
OK, so indirect pressure monitoring isn’t ideal: what’s the alternative? Direct pressure monitoring uses pressure and temperature sensors mounted either inside the tyre or on top of the valve stem. Both types have their advantages and disadvantages.
Internal sensors can be fitted only when the tyres are demounted from the wheels, so unless you love changing tyres by hand you’ll get this type of pressure monitor fitted by a tyre specialist. The obvious time is when you’re buying new rubber.
Internal direct pressure monitors come in two designs: a unit that’s attached inside the wheel well, or one that’s integrated with a metal valve stem and fits snugly inside the wheel, behind the valve hole. The wheel-well type attaches by way of a large ‘hose clamp’ that fits around the wheel interior before the tyre goes on.
External direct pressure monitors replace the existing valve cap, by simply screwing into place. They come with a locking system that’s intended to prevent theft or loosening through vibration.
All direct pressure monitors have integrated batteries that power the sensors’ mini-transmitters. In-wheel units have batteries with claimed lives of up to seven years, while valve-cap types typically have two-to-six-year battery lives.
Digitised pressure and temperature signals are received by an in-car display monitor that plugs into a cigarette lighter socket, or can be wired in. In most cases the display’s aerial is sufficient to receive signals from the road wheels and the spare tyre(s), but trailer tyres and spares may be out of range, in which case a second aerial can be installed under the rear of the towing vehicle. Most car-type pressure monitoring systems can handle 11-14 tyres and truck types can cope with many more.
Obviously, the protected nature of an in-wheel system prevents theft and in-service damage, but we’ve had transmitters damaged by tyre fitters when demounting punctured tyres. The valve-stem type is easy smashed by the tongue of a rotating tyre demounting machine, so the wheel-well type is better in this respect.
Tyre valve cap units are of two types: sealed and unsealed. The sealed types have a captive battery and when that runs out of puff you have to buy a new sensor. The unsealed types can be fitted with replacement batteries, but this design makes them more vulnerable to damage from water, mud, dust, rocks and track-side branches.
We're currently doing long-term testing of sealed and unsealed types and will draw conclusions at the end of 2016.
At Outback Travel Australia we have first hand experience with different direct tyre pressure monitoring systems and we wouldn’t go bush without these systems. The systems are so good at detecting a slow leak well before damage is done that we now carry only one spare wheel on most trips.
Here are our real-world, Outback test findings.
We used successive generations of SensaTyre for four years and it saved us at least four tyres.
Living with SensaTyre pressure monitoring proved to be slightly complicated. Once the transmitters were fitted inside the wheels the display module was clipped into the fresh air vent louvres on the dashboard and plugged in.
Calibration wasn’t the easiest operation, but having a computer-literate 11-year-old on hand sure helped!
Thereafter, tyre pressure maintenance could be virtually forgotten, because the display gave a constant readout of either pressure or temperature, at the push of a button. Any sudden deviation from the pre-set safe pressure range triggered an alarm and a flashing light.
Getting professional repair done on a plugged tyre proved a nemesis of the SensaTyre units, because every time we had to demount a tyre to replace a temporary plug with a mushroom patch the tyre fitter smashed the sensor! We warned the fitters every time, but they still broke the sensors.
Another issue was the need to fit a booster aerial at the rear of the towing vehicle so the display unit could receive signals from camper trailer tyres.
These experiences put us off the in-wheel types, so we next checked out some valve-cap-replacement types. Valve-cap replacement units are theoretically more vulnerable, although we haven’t lost or damaged one in thousands of kilometres of bush travel.
We've replaced standard rubber valve stems with metal ones and we've also run them with rubber stems. We chose not to use the locking collars, to check whether they’d unscrew through vibration, but we haven’t lost one yet.
The TPMS Australia valve-cap transmitters were 10-gram units that replaced the tyre valve caps, simply screwing into place. The kit included sticky-back 10g balance weights, to mount opposite the valve if required, but we didn’t bother. There were also Allen-key locking rings for the transmitters.
We mounted the monitor on the console using double sided tape, plugged in the socket power lead and installed the transmitters. In about 10 minutes we fired it up and found calibration was easy. Also, the screen showed pressure and temperature of each tyre in turn, in a rolling display that scrolled through one tyre at a time.
The TPMS system gave ample audible and visual warning of low pressure or high temperature and also picked up trailer tyre data without the need for a booster aerial.
Unfortunately, the distributors of this system seem to have gone out of business and we've been unable to find an alternative supplier for either this
TPMS sytem or replacement sensors. We'll keep searching.
This valve-cap system was also easy to program and had a large dashboard monitor that’s clearly visible in strong sunlight and has large pull-out aerials to detect transmitter signals from the valve-cap units.
An advantage of the PressurePro for owner/users is that it doesn't need to be recalibrated every time you change pressures. Once the transmitter caps have been off the valve stems for a minute they'll happily monitor whatever pressure is currently in the tyre.It easily picked up trailer tyre data without an auxiliary aerial.
However, one day it just stopped working, so we sent it back to the distributors, who detected a power cord issue and gave us a new one FOC. That replacement
unit also gave up the ghost, so we abandoned the test.
The monitor is designed to detect punctures and/or excess temperature, and to deliver a visual and an audible warning to alert the driver. The monitor also includes a 5V/1A USB output which can charge devices such as mobile phones and GPS units.
Monitoring pressure range is 10-75 psi (0.7 - 4.8 bar) and temperature range, -40°C to +70°C
The DIY kit comes with screw-on valve cap sensors that are factory paired and labelled to the corresponding tyre, allowing the system to operate in a matter of mintues.
The internal sensors must be fitted by a tyre-fitting professional and the wheel must be balanced after installation.
A TPMS Sensor Partner is an optional accessory designed to pair internal sensors to the display when the position of a tyre is changed.
We requested a loan test unit from ARB and said we'd buy it if it tested well. ARB said we'd have to buy it first and that's not the confident approach
we expected! However, common sense eventually prevailed and we have a test unit that we'll evaluate during 2016. We'll report in December.
This is the best unit we’ve tested so far: it’s easy to calibrate and picks up trailer tyre signals even as we’re coupling up!
The Doran valve cap units are larger than the TPMS and Pressure-Pro units, but battery size is larger and life is at least twice that of the smaller-cap units.
We replaced the caps after four years, when two of them became erratci, suggestiong dying batteries. We have five years’ testing and a collective 350,000km so far completed.
We thought the larger caps might be vulnerable in the bush, but so far we haven’t damaged a cap. On rubber stems the caps move around a lot, but a fat 'washer' of foam rubber at the base of each valve stem damps that action.
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