Fullscreen HTML5Med Res (??MB)HTML4

Keeping track of your TYRE PRESSURE ...without leaving the car! Do you take the tyres on your car for granted? Being the sole connection between your vehicle and the road, correct inflation is vital for good road holding. If you get a nail or screw in your tyre, how soon will you notice the deflation? By the time it’s obvious, it may already be too late. We review two tyre pressure monitors that will alert you before it becomes too serious. by Nicholas Vinen & Leo Simpson I f you have to take evasive action while driving, the last thing you want is for one of your tyres to lose traction due to under-inflation. In the worst case, this could cause you to spin or fail to avoid an obstacle, leading to a costly and perhaps very dangerous crash. So you need to make sure that you have good tyres on your car, that the tread is not overly worn and that they 48  Silicon Chip are all properly inflated at all times. There are a few different scenarios which lead to either gradual or sudden tyre deflation. Picking up a nail or screw is an obvious one but there are other ways that they can fail such as cracks or tears in the sidewall due to age, impacts with the kerb or manufacturing faults. We’ve seen tyres from what you might consider a reputable brand blow out siliconchip.com.au their sidewalls after just a few days, without any obvious impact or other damage! The key point is that regardless of the reason for the tyre deflation, you need to know right away so that you can pull over and either replace it with your spare tyre (we hope you have a proper one and not one of those silly donut emergency spares!) or have your car towed to a workshop where it can be repaired. Besides avoiding a serious accident, another advantage of finding out early that your tyre is going flat is that it maximises the chance that it can be repaired. Some tyres cost $500 or more to replace, which is something you’d obviously like to avoid, especially if it’s still quite new! If you doubt that figure, take a look online at the price of a high-performance road tyre such as the Michelin Pilot Super Sport in larger sizes. Larger off-road tyres also carry an inflated price tag (pardon the pun!). And we haven’t even mentioned the likely damage to alloy wheels – some fancy/performance types can cost up to $1000 each and even more! Of course, even if none of the above scenarios ever occur with your car, it is still most important that your tyres are always correctly inflated. Even a modest degree of under-inflation will seriously affect your car’s fuel economy. So how will you know a tyre is going flat? If you’re perceptive, you may notice that your vehicle’s handling has become worse but that will only happen once the tyre is already pretty flat. And you probably won’t notice sound or vibration from the tyre until it’s quite far gone. You need a Tyre Pressure Monitoring System (TPMS). Not all TPMSs are created equal Many newer vehicles list a TPMS as one of the features of the car in the brochure but our experience with two such vehicles is that these systems are often very limited in their capabilities. Some have a proper TPMS which will display the four (or even five) tyre pressures in PSI or bar on the dashboard. If your vehicle has this, then you can rest easy, knowing that siliconchip.com.au it should alert you pretty quickly if one starts going flat. But many other vehicles have a much more crude TPMS which works based on wheel rotation sensors (which are also used by the traction and stability control systems). Basically, these work on the principle that if one tyre has much lower pressure than the others, it will rotate at a different rate on average since its circumference has effectively changed. And eventually, by keeping track of the average rotation rate of each wheel, the vehicle should be able to warn you that your tyre pressure is low. You can be pretty sure that you have one of these systems if your vehicle’s feature list includes “tyre pressure monitoring” but you have no pressure readout, only the ability to “reset” the TPMS. These systems have multiple drawbacks, including false warnings if they are not reset periodically, an ill-defined low tyre pressure threshold and an ill-defined time delay between the tyre pressure being low and you receiving an alert. If this is the case, you will be much better off with a proper TPMS which actively monitors the pressure of all wheels and alerts you as soon as any tyre drops below a set threshold. Both of the units we’re reviewing here have this capability (and others) and both cost less than $100. How they work There are two basic types of after-market TPM systems. They work similarly and the main difference is how the sensors are installed. The type we are reviewing here are the easiest to install and these consist of four units that replace the dust caps on the valve stems on your tyres. They are a bit larger and heavier than the dust caps (a few grams each) and they just sort of hang off the stems, so they are best used with tyres that do not have especially long stems. When fitted, they press in the centre of the valve so that the pressurised air is applied to the underside of the sensor. The sensor has a “rubber” O-ring on its underside that May 2017  49 The second model is “solar powered” and sits on the dashboard of the vehicle. This one has been set to read PSI but is otherwise essentially the same. Its sensors (one shown fitted to the valve stem below) are slightly different but operate the same way. forms a seal with the top of the valve stem so that the tyre does not deflate. It has pressure and temperature sensors on its underside and an internal battery for power. These sensors are activated by the g-forces they experience when your wheel rotates, thus consuming no power when the vehicle sits idle. Once activated, they periodically measure the tyre pressure and temperature and transmit it wirelessly (at 433MHz) to a base unit inside the vehicle, where it is displayed. The base unit is programmed to sound an alarm (flash display and beep) if any tyre pressure drops too low or if continuous deflation is sensed. There is another type which we are not reviewing but which is available for a similar price. These work basically the same way except for the way the sensors are fitted. Rather than being screwed onto the valve stems, they are fitted inside the wheel and replace the valve stem. This is a neater solution but it has two major drawbacks: the need to remove the tyres from the wheels to fit the sensors; and the fact that the units are often sealed for life and need to be replaced when the internal cell goes flat after a few years. If you’re going to replace your tyres anyway, or you have the tools to remove and replace the tyres from your rims, then you may want to order a TPMS with internal sensors. They are no more difficult to obtain, the cost is similar and you don’t have to worry about your valve stems or sensors being damaged due to g-forces at high speeds or from damage from impact with kerbs/stones/etc. 1) remove the dust cap 2) screw the security lock nut all the way onto the valve thread 3) place the synthetic rubber O-ring provided over the valve stem until it rests on the lock nut 4) find the sensor labelled for that wheel (FL = front left, etc) 5) screw the sensor tightly onto the valve. You will notice air escaping once the sensor has been screwed in sufficiently to operate the valve but it should stop once you screw it on further and it forms a seal. 6) using the provided spanner, “unscrew” the lock nut behind the sensor until it compresses the O-ring between the nut and sensor. This makes it difficult to remove the sensor (or have it vibrate off) without loosening the nut using that tool. Once all four sensors have been fitted, store the spanner in your vehicle where you can easily find it (in case you need to remove a sensor to reinflate a tyre etc) and then plug the receiver unit into the cigarette lighter socket. In the case of the solar-powered version, simply place it on top of the dashboard where you can see the display clearly and where it will be exposed to sunlight while driving during the day. The system will automatically power up the next time you go for a drive and you can check its operation then. Once you’ve confirmed that it’s operating normally, you can set the over/under pressure warnings and so on, using the two buttons on the unit and the instructions provided. Fitting the TPMS Setting the limits Assuming you’ve purchased one of the models we’ve reviewed (with the external sensors), installation should take less than half an hour. Essentially, the process for each wheel is as follows: The units we purchased had a default setting in “Bar”. If you prefer to think in kPa, conversion is easy: 1 Bar = 100kPa. If, though, you’re like the vast majority of people At left are the four valve “caps” (sensors) from one of the two TPMS sets we purchased online. They are clearly labelled as to which wheel they need to go on (otherwise the cabin display, shown at right, will not show the right wheels). The display, which plugs into the car’s cigar lighter, is quite small, as shown in the dimensions alongside the pic. 50  Silicon Chip siliconchip.com.au Got a mobile phone? The cigar-lighter model incorporates a handy USB (5V) socket so you don’t need a separate phone charger. It even supports fast charging. and haven’t yet been “metric converted” when it comes to tyre pressure, conversion to PSI is a tad more difficult: 1 Bar = 14.5038 PSI. A rough conversion, usually good enough, is 1:15. Fortunately, most systems can be set to read out in PSI. By default, the systems we purchased sound an alarm if the pressure of any tyre drops below 2 bar (29 PSI) or rises above 3 bar (43.5 PSI). These are reasonable limits however if you have high-performance tyres, because they’re quite stiff, you may find the pressure rises significantly on a hot day after some driving and could easily exceed 3 bar if set at cold to say 2.5 bar (36 PSI). You will find that the front tyres get hotter and thus rise in pressure more than the rear tyres while driving, partly because the front brakes do most of the work and thus dissipate more heat than the rear brakes and partly because of the hot air flowing out from the radiator and engine and out from under the engine bay (mid-engined and rear-engined vehicles excepted!). This is normal. Both settings are adjustable using the two pushbuttons on each unit and it’s relatively simple to change the limits to suit your tyres. We found that on our test AWD vehicle with Continental ContiSportContact 3 tyres, the cold pressure (from the dealership) was 2.4 bar (35 PSI) per tyre, rising to around 2.7 bar (~38 PSI) after a long trip on a hot (35°C+) day. So the default pressure limits worked fine on that car. On the other hand, the low-profile Michelin Pilot SuperSport tyres on our rear wheel drive test car started out around 2.6 bar (37 PSI) but the front tyres rose over 3 bar (44 PSI) even during relaxed driving. We put this down both to the type of tyre as well as to the prodigious heat output of the rather large engine in this vehicle but once we had adjusted the upper alarm limit, we had no false alarms. In addition to over- and under-pressure alarms, the unit will also sound an alarm if it detects the pressure in a tyre continuously dropping, indicating a likely puncture. There are also alarms for high tyre temperature (>65°C), indicating imminent tyre failure or low sensor battery. because it lets you see how the tyres warm up as you drive. As they warm up, the pressure will normally rise. There may be a situation where your tyre has a slow leak but because it’s warming up as you drive, the pressure won’t initially drop. So if you notice the temperature of a tyre increasing, without an associated increase in pressure, that might be a sign that you need to pull over and check it. While the cell powering each wheel sensor is necessarily small, due to the efficient design, they should last several years of typical use. Each TPMS unit comes with a tool which allows you to open the waterproof housing of the sensors and replace the cells if and when required (see photo). They use fairly standard button cells (CR1225) which should not be difficult to obtain. One side-benefit of fitting a TPMS is that it seems to greatly reduce the rate at which air leaks out of tyres. On some older vehicles, you may find you need to “top up” your tyres every month or even more frequently to keep them inflated to a proper pressure. We have always assumed this was due to pinholes in the tyres or air leaking around the bead but after fitting a TPMS to an older (~10-year-old) vehicle, we noticed that the tyres held their pressure month after month, suggesting that air was actually leaking out past the valves and the dust caps. Because the TPMS sensors seal the valve stem, you are no longer relying on the integral valve to keep the air in and this appears to be a big (and unexpected) advantage. Another bonus feature we should mention is that the cigarette-lighter powered TPMS we tested has a USB charging port, so you don’t need to unplug it to charge your phone (or whatever). That’s quite handy and it also saves you the cost of purchasing a car charger. It even supports fast charging, while many cheap car chargers don’t. The cigarette-lighter powered TPMS also shows battery voltage which we feel is very handy as it may give you advance warning of an ailing battery or alternator and also makes it easier for you to determine whether you’re driving a “weekend” car often enough to keep its battery charged. Using it Disadvantages Each unit generally also includes a temperature sensor so the tyre temperature is also displayed. This is very handy One of the few disappointments of fitting the systems reviewed here is that neither of them monitor the pres- siliconchip.com.au May 2017  51 sure of your fifth wheel, ie, the spare tyre. This may seem pointless but it isn’t. If you’ve ever had a flat tyre and gone to swap it with your spare, only to find out that it too has gone flat, you will understand! Of course, it’s a good idea to check that your spare tyre is inflated on a regular basis but let’s be honest, how many people actually do that? Most people don’t even check their oil level or windscreen washer reservoir level on a regular basis. So monitoring the spare tyre pressure would be a plus. You could do this with a second monitoring set-up but that’s expensive and inconvenient since you would have to use a double adaptor to power it and where would you put it? The only other criticism we have, really, is that the cigarette lighter powered displays are hard to read during the day. They aren’t that bright and there can be a lot of glare, depending on where you install it; with the type which plugs into the cigarette lighter/accessory socket, you don’t get a lot of choice in that department. We fitted ours with an anti-glare coating that we cut down from one designed for a mobile phone screen, however, this provided limited (albeit visible) benefit. Having said that, since you only really need to check the display periodically (after all, it should make a noise if something is really wrong), you can simply wait until you are stopped and then place your hand over the unit as a shade when you want to read it. It’s quite easy to read at night, though. On the other hand, the solar-powered unit is quite bright and only difficult to read if you are wearing polarised sunglasses. And another advantage of the solar-powered unit is that it will “wake up” with the slightest vibration of the car. That means you can check the tyre pressures without even getting into the car! While the solar-powered unit is sound-activated and its battery always seems to have sufficient charge, the 12Vpowered unit is normally switched on with the ignition. If your accessory socket is always powered, the unit goes into “sleep” mode when not in use and wakes up automatically, or with a button press. Changing the battery or adding air As mentioned earlier, the button cells in the sensors are rated to last for a few years. Their life will depend on how much you drive; if you’re a courier, a taxi driver or have a long commute they might not last for one year but if you only drive on weekends, they may last longer than five years. Happily, changing them is quite easy. The TPMS is supplied with a plastic spanner-like tool which clips into the rear of the sensors and a few rotations will have it apart. As we said, CR1226 batteries (12mm diameter, 2.6mm thick) are a common type of button cell so finding replacements should not be difficult. You will want to keep the metal spanner in your car since if you need to add air to a tyre, you need to undo the anti-theft/anti-vibration nut and then you can simply unscrew the sensor with your fingers. You can then add air as usual and reverse the process to re-attach the sensor. Then again, since the sensors form such a good seal, you probably won’t have to do that very often. Rotating tyres If you rotate the tyres, you can simply remove the four 52  Silicon Chip Included in the TPMS kit you should find a pair of “keys” used to pull the sensor apart to replace the battery. This might not be needed for a few years, so remember where you put the keys! sensors and re-attach them to the correct wheels. However, if you wish to avoid this, you can also use the unit’s pushbuttons to swap the location where two of the sensors are shown on the display. So the “FL” sensor may end up on the front right-hand side of the vehicle and you can then set the display to show the output of that sensor in that position, saving you from physically moving the sensors. Conclusion Given the low cost and ease of installation, we strongly recommend either of these units. If you like the sound of the vehicle-powered device, check that your car has the accessory socket in a position where it won’t get in the way of the controls (eg, gear shift lever) and that there is sufficient clearance to plug the unit in. The solar-powered version should suit just about all vehicles. We also suggest that you check how long your tyre valve stems are and how flexible they are. If you can easily push the valve cap over so that it’s nearly in contact with the rim, you may risk rim and/or sensor damage under hard acceleration/braking if you fit a TPMS as described here. In that case you have three main options: (1) purchase an in-wheel system and have it fitted by your tyre retailer; (2) find or make a stiff (but not totally rigid) tube or foam section to fit over each stem to reduce how much it flexes, or cushion the sensor should it contact the rim or (3) have your valve stems replaced with shorter, more rigid versions. Pricing and availability The two TPMS we have reviewed here are not the only ones available but we think they are among the best. You can find plenty of options, including multiple vendors selling both these systems, on ebay and Ali Express. Most of them are sent from China and are available under $75 including delivery (but note that the free delivery option may take more than four weeks). You can purchase them via the following shortlinks (they will automatically expand in your browser): 12V-powered version: siliconchip.com.au/l/aabt Solar-powered version: siliconchip.com.au/l/aabu SC siliconchip.com.au