A car hangs alongside you a moment too long, the driver keeps glancing across at the trailer, and then they point. Anyone who tows knows what that means, and knows they are already late.
You will not have felt a thing, and that is the specific problem. There is too much rig between you and a trailer tire, the tow vehicle soaks up everything, and people routinely cover miles on a flat one without the faintest idea. Which is how a slow leak becomes a blowout, and a blowout is the most expensive thing that regularly happens to RVs. Not the tire, which is nothing. It is what a disintegrating tire does to the wheel well, the underbelly, the plumbing run behind it and occasionally the whole side of the rig on the way past.
Heat gets there before the pressure does
Everyone buys these for pressure. The reading that catches a failure early is usually temperature.
An underinflated tire flexes more through the sidewall, and flexing turns into heat. The heat is what destroys the tire; the low pressure is only how it got there. So a tire heading for failure normally announces itself as one tire running hotter than its neighbours, well before the pressure has dropped far enough to look alarming.
Which is why the alert worth setting is not a fixed number. It is one tire behaving differently from the others on the same axle. Four of them running warm on a hot afternoon at highway speed is physics and a black road. One running warm while its neighbour does not is a problem you can deal with at the next exit, rather than on the shoulder in the dark.
The same physics explains why the pressure figures wander about, and why that is normal. Cold inflation pressure is the spec on the sidewall, and pressure climbs as a tire heats up in use, so a tire reading well above its cold spec after two hours on the interstate is behaving correctly. Set cold pressures cold, in the morning, before you have driven anywhere. Set them at a truck stop after a hundred miles and you will bleed a warm tire down to a number that leaves you underinflated once it cools.
Three ways people keep an eye on this
1. A gauge and five minutes
Free, assuming you own a decent gauge, and genuinely better than a monitoring system you have stopped opening. Five minutes in the morning catches slow leaks, and slow leaks are most of what goes wrong.
What it cannot do is follow you down the road. The screw you pick up at hour two stays a secret until it becomes a story you tell at the next rally. A gauge tells you the state of the tires at the moment you crouched down and looked, which is worth knowing, and is not monitoring.
2. Screw-on Bluetooth cap sensors
The valve caps sold in sets of four under a dozen brand names. They replace the caps you already have, take about a minute a wheel, and broadcast pressure, temperature, battery level and an alarm flag. They share one internal design and one broadcast frame, which is why a single rule reads the lot of them regardless of what is printed on the box.
That cuts both ways, and it is worth knowing before you order.
The buying trap. Our rule matches on the advertised Bluetooth name, which starts with TPMS. Caps that advertise under a different name will not be recognised even when the internals are identical. There is no way to tell from the product photo, and sellers rename these constantly. If Bluetooth monitoring is why you are buying them, buy from somewhere you can return them, and check within the return window.
There are two honest limits on the caps themselves, and both sit in the compatibility database rather than the small print. They read pressure at the valve rather than inside the tire, which is close enough to be useful without being the same measurement. And they hang weight off the end of a valve stem, spinning, every mile you drive. On a light trailer at moderate speed nobody notices; on a heavy fifth wheel it is worth a thought, and rubber stems in particular were never designed to swing a lump around, which is why metal stems are the standing advice.
Treat them as early warning, which they are good at. If you tow heavy, do not treat them as a replacement for a coach-fitted system, because they are not one.
3. Sensors that live inside the wheel
Mounted inside the wheel, on the valve stem or banded to the rim, with a dedicated repeater and a display of their own. This is what people towing heavy fit, and what a lot of Class A owners have from the factory. Nothing hangs off the stem, no spinning weight where you do not want it, nothing on the outside of the wheel for anyone to unscrew. Reception is better too: a purpose-built repeater does on purpose what a hub inside the rig does at a disadvantage.
You pay for that in money, in tire shop time when a sensor needs servicing, and, for now, in us. The Lippert TireLinc sits in our database as a candidate rather than supported: what it advertises is a presence beacon and nothing more, with the per-tire data behind a connection path we have not built. It is a high-value target for us and it is likely to move. It has not moved yet, and we would rather say so than have you buy on the strength of a plan.
So which one?
- Light trailer, a few trips a year. A gauge and a habit. Add caps if you want the reassurance.
- Travel trailer, regular long drives. Cap sensors. This is where they earn their money, and the metal-stem advice applies.
- Heavy fifth wheel or a Class A. A coach system, accepting that you will read it on its own display for now.
- Dual rear wheels. Coach system. The inner tire on a dually is the one that fails unnoticed and the one you cannot easily reach with a gauge, which is the whole argument in a sentence.
Go and read the date code
RV tires usually age out long before they wear out. A trailer covering four thousand miles a year can sit there with plenty of tread and sidewalls that are seven years old and quietly finished. The four-digit code on the sidewall gives you the week and the year it was built, and it is worth ten minutes with a torch to find out what yours say.
No monitoring system catches age. It will report correct pressure and ordinary temperature on a tire whose rubber has given up, right until there is nothing left to report from. Which is roughly how the category divides: the caps tell you about the trip you are on, and the date code tells you whether you should have set off on it.
