It is not a truck with a tall body
A straddle carrier drives over a container, picks it up inside its own portal frame, and
carries it with the load hanging between the legs. That geometry changes what tire pressure
means. On a truck, pressure is mostly about tire life and fuel. On a straddle carrier the
pressure changes:
- Portal clearance. The frame height above the ground is set by the tyres. A soft tyre
lowers one corner, and the clearance the driver relies on when straddling a stack is no
longer what the drawing says.
- Wheel load distribution. Four or eight wheels per side carry a load that includes the
container and the machine. A pressure split moves load onto neighbours that were not
designed for it.
- Tracking in a narrow lane. These machines work in aisles with very little margin. A soft
tyre makes the machine pull, and the driver corrects all shift.
How many positions, and where they are
Typical straddle carriers run eight wheels (four per side); larger terminal machines can
run sixteen. Each side is a separate load path, so the wheel map has to be written by
side and by position along that side, not by "front left" — the machine moves in both
directions all day and has no meaningful front.
That is the same rule we apply to RTGs: name the mapping geometrically, keep the same
names on the display, in the register map and in the maintenance record. A wheel map that
changes its naming between shifts is worse than no map.
Where the receiver goes
Most straddle carriers have a 24 V electrical cabinet and a controller network. The receiver
mounts in the cabinet, takes DC 10–30 V, and puts the data on **CAN 2.0B (11-bit or
29-bit extended frames)**, RS485, RS232, Modbus RTU, PROFINET or UART. One receiver covers
up to 256 wheel positions, so even a 16-wheel machine uses a fraction of that capacity —
the design question is radio coverage around a steel portal, not capacity.
Two practical points from port work:
1. Antenna and repeater placement is the whole job. The portal is a shield. On larger
machines a repeater inside the frame is often needed; on smaller ones it is not.
2. Alarm ownership has to be decided in writing. The driver needs a fast leak now, on the
machine; the terminal planner needs trend data. Usually that means a local alarm plus
telemetry to the terminal system, not one screen trying to do both.
What to ask before buying
- Which wheels, and what does the supplier call each one? (Ask for a filled wheel map, not a
blank template.)
- Does the receiver support the protocol the terminal controller already speaks, and is the
point table documented?
- What happens when a sensor is lost — is there a distinct status bit, or does the value just
freeze?
- What is the IP rating of sensor, receiver and display separately, and what is the wash-down
procedure?
Honest limits
A TPMS tells you pressure and temperature per wheel. It does not tell you wheel load, it does
not do stability calculation, and it does not replace the inspection the driver does before a
shift. What it changes is that the pressure part of that inspection becomes recorded data
instead of a memory.