A machine with four wheels is a radio problem you can ignore. A machine with 160 wheels is a radio problem you have to design, and it does not announce itself as one — it announces itself as "the display sometimes shows dashes".
Why a single receiver is not enough
Our receivers accept up to 250 sensors, which sounds like capacity solves the problem. It does not. Capacity is about how many sensors can be registered; range is about whether the radio link closes. On a long vehicle, the chassis itself is the obstacle: steel structure, axles, hydraulic runs and containers of cargo sit directly between a sensor and the receiver.
At 433.92 MHz, that geometry matters more than the nominal range figure. A datasheet may say 100 metres in open air. Between the front and rear of a 20-axle carrier there is very little open air.
Repeater placement is a geometry exercise
The approach that works is unglamorous: treat the machine as a series of radio zones, and place a repeater wherever one zone's sensors cannot reach the next zone's receiver or repeater.
- Walk the chassis and mark where steel mass sits between the front and the rear.
- Group wheel positions into zones of roughly equal radio distance.
- Place one repeater per zone boundary, in a position with the least metal in the direction data must travel.
- Verify with a link-quality readout, position by position, before the machine leaves the workshop.
On a 160-wheel beam carrier this produced twelve repeaters feeding two receivers — not one repeater per axle, and not a guess.
The failure mode that costs the most
A badly planned installation does not usually fail immediately. It fails as intermittent data: a wheel that reports most days, then does not, usually on a machine parked in a different orientation or loaded differently. By then the vehicle is in service and the fix is a return visit.
That is why we commission with a per-position link check and record it. A written record turns "the system is unreliable" into "position 47 needs a repeater moved 600 mm", which is a repair rather than an argument.
Sensor addressing at scale
At 250 positions, sensor identity becomes a process problem as much as a technical one. Three rules keep it manageable:
- Physical positions are numbered once, in a scheme agreed with the customer, and never renumbered.
- Sensor IDs are bound to positions during commissioning and the binding record is handed over with the machine.
- Replacement procedure is written down, including how to rebind after a sensor swap, so a tire fitter does not have to invent it.
We have seen fleets lose more time to unrecorded sensor swaps than to radio problems.
What this means when specifying a system
The specification that works lists wheel positions and their physical grouping, not just a total. It says who will own the sensor-to-position mapping and in what format it will be delivered. And it asks the supplier to justify repeater count from the chassis layout rather than quoting a rule of thumb.
Those three items are the difference between a system that reports every wheel and a system that mostly reports every wheel.