Why TPMS for semi-trailer and tractor-trailer fleets is its own problem
TPMS for semi-trailer and tractor-trailer operations fails for reasons a rigid truck never hits.
On a rigid vehicle the tyre positions are fixed for life. On a combination they are not: a tractor works
with different trailers, a trailer works with different tractors, and every swap changes which sensors
are within radio range of which receiver.
That is the whole engineering problem in one sentence, and it is not about sensors. It is about **what
stays with the unit when the unit is unhooked** — and about the steel coupling that sits between the
tractor and the trailer.
The three configurations fleets actually run
| Configuration | Where the receiver lives | What breaks first |
|---|---|---|
| Receiver on the tractor only | One unit in the cab area | The trailer's rear axle is far away, and the coupling blocks the path |
| Receiver on the trailer only | One unit on the trailer chassis | The driver cannot see trailer pressures unless the data leaves the trailer |
| One receiver per unit + a gateway | Both, with data combined on a platform | The mapping: which sensor belongs to which trailer this week |
Most fleets that start with a pilot pick the first or second and then move to the third once they have
more than a handful of trailers. Multi-axle and long combinations are covered in more depth on
The coupling is the radio weak point nobody plans for
A 433.92 MHz link does not care about distance as much as it cares about what is in the way. Between
the tractor's receiver and the trailer's rear wheels there is: the fifth wheel, the frame, a steel deck,
and often a loaded container. Two practical rules:
- Plan from the drawing. Work out which wheels have a realistic path and which do not; put a
repeater where the geometry needs it, not where the harness was easiest.
- Give the trailer its own receiver if it keeps its own wheels. A trailer that is swapped between
tractors needs its identity to stay with the trailer, not with the cab.
On a combination with more than roughly ten metres between the furthest wheel and the receiver, budget
for at least one repeater.
Dual wheels: the sensor question that decides installation
Most trailers run dual tyres. That changes what you can fit:
| Position | Typical solution | Note |
|---|---|---|
| Outer dual | External valve-mounted sensor | Fastest to fit and to replace; accessible at every walk-around |
| Inner dual | Extended valve / flow-through valve stem | The sensor stays reachable from outside, so it is not a wheel-off job |
| Band-mounted internal | Steel band around the rim well | Protected, but only reachable when the tyre is off; length is cut to the rim circumference — 1650 mm covers what most truck and bus wheels need, and longer rims are a cutting step, not a development |
Ask for the valve arrangement before specifying sensors. Getting the inner dual wrong is the classic
reason a "pilot" stays a pilot.
The data contract
Whatever the configuration, the numbers are fixed, and they should be written into the RFQ:
| Item | Value |
|---|---|
| Pressure range | 0–1400 kPa gauge (100–1500 kPa absolute), ±7 kPa |
| Data period | 500 ms on the output side |
| Wheel positions | up to 256 per receiver — a combination never runs out of capacity |
| Sensor | 433.92 MHz, IP68, −40 to +125 °C (internal; external −20 to +85 °C) |
| Output | one interface per unit: CAN 2.0B (11-bit or 29-bit, 250 kbps default), RS232, RS485 or Modbus RTU |
| Modbus role | slave only — your PLC polls the receiver; we do not act as a master |
For J1939 networks: we work in 29-bit extended frames and have deployed J1939-format message sets in
construction machinery integrations with major OEMs. We hold no J1939 conformance certificate and we say
so rather than imply one.
What a fleet platform needs from us
If the data is going to a platform rather than a cab display, the useful integration is a documented
payload and a stable identity per trailer. Three things to agree:
1. Vehicle identity — plate, VIN, or your own unit number; and what happens when a trailer is swapped
(does the data follow the trailer, or the combination?).
2. Position label convention — "left rear inner" means different things on different fleets; fix it once.
3. Uplink path — a gateway publishing over MQTT to your broker (change-triggered, buffered when
coverage drops) versus a receiver that the tractor's controller reads on CAN.
Our truck and fleet page covers the fleet-side view, and the interface details sit on
CAN bus TPMS integration. If you are running a trailer fleet and want the configuration
in writing — wheel count, rim type, valve arrangement, and where the data must land — [tell us those four
things](/contact) and we will answer with a bill of materials rather than a brochure.
The one mistake that wastes a pilot
Specifying sensors before deciding where the receiver is. On a combination this single choice decides
the repeater count, the trailer identity design and whether the driver ever sees a trailer reading. Decide
that first; the sensor choice is the easy part.