You pull in for fuel, walk round the trailer out of habit, and one hub is too hot to leave your hand on. The tire above it looks exactly like the other three. Its pressure is fine. Nothing has alarmed, and you were about ninety miles from finding out the expensive way.
Heat arrives before pressure does. An underinflated tire flexes and cooks itself; a dragging brake or a dry bearing heats the hub and everything bolted to it. In both cases the first thing that changes is one wheel behaving differently from the wheel opposite it, and on a trailer that is the only thing you can possibly know from the driver’s seat.
The TPMS guide is the argument about whether to buy caps at all. This is what to do with the temperature number once you have them, and — more usefully — what that number is not.
What a cap can actually see
A cap is not a bearing thermometer. It screws onto the valve stem, so it reads the air at the rim’s edge — coupled to the wheel, and lagging the hub and drum where a brake or bearing fault actually lives. It sits in the airstream too, which means it reads cooler than the tread above it. Every threshold below is set against that reality rather than against the temperature you would measure with an infrared gun on the hub face.
What it is genuinely good at is being there. Cap sensors report every 60 seconds from all four corners while you drive, which is four readings nobody has ever had from a moving trailer without stopping to get them. That is the whole value, and it is considerable.
The usual caveat applies before you buy: our rule matches caps whose advertised Bluetooth name begins with TPMS, and a set sold under a different name is never recognized even when the internals are identical. Silence from a cap proves nothing about the wheel.
The ceiling, and the comparison the app cannot make
Set a per-tire temperature ceiling: warn above about 150°F, critical above 170. Coach-installed systems commonly alarm nearer 158, and a cap reads cooler than the tread does, so sitting a little under a coach system’s figure is the right instinct rather than a timid one.
Pressure carries the same information more slowly, because the air heats after the wheel does: roughly two psi for every ten degrees on an eighty psi tire. A pressure ceiling at cold spec plus twenty per cent will eventually say what the temperature already said.
But the earliest real signal is a comparison, and it is not a rule you can set. One tire twenty degrees or more warmer than the tire at the other end of the same axle, with everything else normal, is the thing worth acting on — and alerts here are per sensor, so nothing in the app compares two tires for each other. It is something a person reads off the tiles, at a stop or from the passenger seat, on data that is never more than a minute old.
Compare against the axle mate and nothing else. The sunny side of a rig runs five to ten degrees warmer than the shady side all by itself, a motorhome runs warmer on the exhaust side, and a long descent on the trailer brakes warms every wheel for a while. All three of those are normal and all three will convince you of a fault if you compare the wrong pair.
The combinations
One number is ambiguous here more than almost anywhere else in the cookbook, because heat has so many innocent sources.
- Hot, and that tire’s pressure below its floor — underinflation is making the heat. The tire is cooking itself. If it is well down, do not drive on it to reach somewhere convenient.
- Hot, pressure at or above cold spec, and the axle mate normal — the heat is coming from the hub side rather than the sidewall. A dragging brake or a failing bearing. Stop, and do not resume until you know which.
- Every cap warm and every pressure up by a similar amount — road, speed and weather. Nothing is broken; ease off a little.
- Both caps on one side warm, the other side not — sun, or the exhaust side on a motorhome. Not a fault.
- A temperature alert, and then that cap disappears from the dashboard — the cap has cooked, or the tire has thrown it. Treat it as the worst case and stop.
What none of this tells you
Brake temperature or bearing temperature, which are the two things you actually want and neither of which anything in the catalog measures. A wired probe taped to a hub you already distrust is a one-off diagnostic for a short test tow rather than a method — and the probe we would use has a decode validated at room temperature, with the top of its range unchecked, which is precisely the range this would live in.
Nothing on the trailer sees the tow vehicle’s wheels either, and Lippert’s TireLinc is detected rather than decoded — its advertisement is a presence beacon and the per-wheel data sits behind a connection path that does not exist yet, so read that one on its own display.
And a bearing that has already lost its grease can fail faster than a 60-second poll can report it. That is not a threshold to tune, it is a limit of the method. This is the one topic on the site where the sensor genuinely cannot be the plan: feel the hubs by hand or with an infrared thermometer at every fuel stop, and repack bearings on schedule. The caps are what tell you which wheel to check first.
What to do about it
Set the per-tire ceiling, because it is the one reading that fires on its own and it costs nothing once the caps are installed. Then build the habit the app cannot: at every stop, read the four tiles as two pairs and ask whether either pair disagrees with itself.
That takes about five seconds and it is the earliest warning available anywhere in this topic. The alert catches the wheel that has already got hot. The comparison catches the one that is getting there.
There is no product list at the bottom of this page. The caps that would go on it are sold under a dozen brand names from one internal design, and we cannot confirm which listings advertise under the name our rule needs — so the TPMS guide explains the category and we link nothing rather than send you to a coin flip. The device page has what they broadcast.
