Tyres are a matter of life and death and there really is ‘pressure’ to ensure they’re safe…
By Richard Robertson
Buying tyres is often viewed as a ‘grudge purchase’ but they are truly a matter of life and death – especially in an RV. That’s because our vehicles regularly operate close to or at their maximum gross vehicle mass (GVM) and on roads of dubious quality. In these situations any failure is dangerous, but vehicles with single rear wheels are especially susceptible as more weight is carried by the back axle.
This issue is particularly close to home because we had a rear tyre side-wall blowout on our Ford Transit-based van conversion motorhome. Fortunately, it happened on a straight stretch of freeway in light traffic and there were no dramas, but on a twisty or steep backroad it could've had a very different outcome.
Checking and maintaining RV tyre pressures is vital, yet can be difficult. Manufacturer-stipulated pressures of 50-90 psi are common, but service station air pumps often don’t go above 50 psi. That means finding a truck stop or tyre dealer, which isn’t always easy.
At the very minimum, tyre pressures should be checked before every journey. On average, tyres lose one to two pounds pressure per month, so experts reckon, and on longer trips a weekly check is recommended. Most importantly, you need to check pressures before starting out, because pressure increases with heat build-up from the road and rising daytime air temperatures. This means the minimum amount of ‘kit’ you need to carry is a tyre pressure gauge.
A pencil-type gauge is a good starting point, but be sure to get the one that measures up to 100-psi. I have one and also a Digital Tyre gauge (0-100 psi) and both concur with my ancient home air compressor’s gauge. They also agree with my tyre pressure monitoring system – TPMS – which adds confidence.
Speaking of tyre pressure monitoring systems, one of these should be your next line of defence and is really a must-have. Budget $300-$600 for a name-brand system, and the more wheels and tyres you have, the bigger/more expensive the system needs to be. In this article I’m referring to a basic, four-tyre system to suit a campervan or small motorhome. Many of you will need to monitor six tyres, plus if you tow a trailer or car you should add sensors to cover them. While bigger systems are more complex to install and monitor, the basics described here hold true. However, before continuing let me tell you a little story…
I set out to buy an ARB-brand TPMS, having done a fair bit of research balancing reputation with budget and the degree of installation difficulty. Firstly, however, I decided to go the absolutely cheap-and-cheerful way and see just how good or bad a bargain-basement eBay special could be.
There are innumerable cheap Chinese-made TPMSs on eBay and in no time I’d pressed Buy Now and for $33.67 – including free postage – the ‘experiment’ was on its way. Sooner than later the parcel arrived and I got straight down to the task of installation. Well, as soon as I could make sense of the detailed-but-not-too-clear ‘instwuctions’…
What attracted me to this unit in particular was its similarity to the desired ARB system, but at literally a fraction of the price. How bad could it be I wondered, as I commenced the installation process?
This style of TPMS comes with a sensor that screws on to each tyre’s valve stem and is secured by a locknut. Each has a battery and connects via Bluetooth to a small display unit, which sits in a 12-volt socket (for power) and includes a pair of USB charging outlets on the side.
When first installed the display unit flashed as it established contact with the sensor, after which it displayed the pressure and also internal air temperature. Our motorhome runs 52 psi in the front tyres and 72 psi in the rear, but for some reason every wheel was flashing 50 psi and the system was beeping. Further investigation of the instructions revealed the beeping to be an over-pressure alarm. Then a light went on (in my mind, not the display unit).
Rechecking the fine print of the eBay ad showed the system to have a pressure readout range of 20-50 psi. That meant it thought the tyres were already over pressure and was letting me know. Bugger. Well, at least the alarm function worked…
Following that setback I installed the system on our old Falcon ute. Before installation I set each tyre to its recommended pressure – 30 psi front/35 psi rear – and was amused but not surprised to see the display showing some quite different pressures: 31/34 front and 37/32 rear. While disappointing, I’ve realised the value of this system now is as a guide to each tyre’s pressure, and that should a leak develop that beeping alarm will again sound if pressure drops more than 15 percent. For less than $37 it’s basic insurance and to this day the colourful little display unit lights-up on starting the car and I can see all is well. Unfortunately, at night the display is too bright and I either have to cover it up or pull it out because it's so distracting. Like they say, you get what you pay for…
And so it was back to square one and searching online for the ARB TPMS. The system I was after came in two parts – wheel sensors and head unit – and total cost was close to $400.
The ARB system certainly seems better than its cheap competitor, which is as you’d expect. Again, it has a sensor for each wheel that’s secured by a locknut and a display unit that sits in a 12-volt socket and includes a pair of USB charging ports. Importantly, it covers a pressure range of 5-99 psi. ARB also sells in-wheel sensors rather than the screw-ons, but they are a lot more work to fit and replace if there are issues (plus they are prone to damage by tyre fitters). Also, you need a separate one for the spare, which is included, rather than just swapping across the screw-on unit, should you get a flat tyre.
The ARB’s display unit is bigger, more substantial and easily read, with what appears to be an old fashioned liquid crystal display that only shows air pressure. Importantly, it includes alarms for under and over-pressure. There’s also an upgrade that lets you monitor up to six vehicle tyres and six trailer tyres, with four customisable pressure presets for different driving conditions. It’s a heavy-duty system designed for 4x4s and caravans/trailers, but equally suited to RVs.
Set-up was similar to the cheap eBay system, but with (thankfully) clearer instructions. I adjusted the tyre pressures and installed each unit, a task difficult on the front wheels because of the hubcaps, but easy on the rear wheels because they have longer valve stems. Once installed and with the display unit inserted into the 12-V socket, it was time to set the target pressure for each tyre. Ford recommends 50 psi front and 71 psi rear for a fully laden Transit. I have generally used around 52 psi and 72 psi as our little motorhome is close to its 3550 kg GVM when we travel.
Once set-up and driving, the (monochrome) pressure readout flashes until the unit establishes contact with each tyre sensor. One by one the readouts stop flashing and you get the actual pressure reading. It's the same every time you start driving and it can take anything from a few seconds to a few minutes for all four tyres to establish contact and update their readouts. The factory default setting for the under/over pressure alarm is 25 percent and I've kept it at that.
On The Road
So far the ARB system has worked well, with the displayed pressures matching my tyre pressure gauge readout to +/- 1 psi when cold. I set 55/72 psi front/rear as the reference pressures and initial runs in spring weather showed readings of 60/61 psi front and 82 psi for both rears. However, I’ve noted that some of the numbers in the display unit are ‘fading’ and difficult to read. Meanwhile, the cheap and brilliantly colourful Chinese unit continues to work like a charm. Hmm…
Experts reckon the best rule of thumb for tyre pressure increases while driving is to allow 10 percent for the ambient air temperature plus 10 percent for rolling friction. That puts Polly’s cold pressures of 52/72 to around 60/84 on the highway, which is about right.
On days in the high 20s to low 30s Celsius, I keep 52 psi up front but go to 75 psi rear, which gives highway readings of 60-62 front and 88-92 rear, depending on the time of day. Our Continental light truck tyres have a maximum cold inflation pressure of 79 psi, so we’re getting close to maximum and I think I’ll try that on the next run just to see what the highway temperatures are.
I’m told the old 4-5 psi rule-of-thumb for pressure increase when driving is a hangover from cross-ply tyre days and that the sorts of tyres most of us now run work quite safely with the 10%+10% formula. I’m also told that if you’re not getting those sorts of increases you’re starting with pressures set too high, but notes you do need to play around depending on the vehicle’s load and prevailing weather conditions to ensure correct results.
If you’ve got a tyre pressure gauge and TMPS you’re well on the way to tyre Nirvana. The final piece of the puzzle is a 12-volt air compressor so you can adjust pressures as you go. It also means if you get a slow leak you can keep topping it up while keeping an eye on the pressure as you head to get it fixed. Just be sure the 12-volt lead and air hose are long enough to comfortably get from power in your vehicle (a 12-V socket or battery clips) to the most distant tyres.
Having said that, I’ve just purchased a portable air compressor that is part of the Ryobi One+ 18V system sold at Bunnings. It’s one on many tools/accessories (Ryobi calls them ‘skins’) that work using a rechargeable and interchangeable 18V lithium battery, which you buy separately. We use the Ryobi One+ system to power a range of units in the garden and around the home and so far have no complaints. Watch for a review of this handy little compressor soon as it does away with a power lead and a single battery lasts for yonks…