A bus moving from conventional dual rear tyres to wide-base, or super single, tyres is usually a
weight and fuel decision. It also quietly changes the tyre-pressure monitoring system: fewer
sensors are needed, but the radio link they depend on becomes harder to make reliable. That
second half is the part that gets underestimated in specifications.
This article explains what actually changes, why the radio side gets harder, and which numbers
you should ask a supplier to commit to before you approve a design.
Two rear tyres or one: the sensor count changes
On a conventional six-wheel bus, the rear axle carries dual tyres on each side. Each tyre has
its own valve stem and its own pressure, so each tyre needs its own sensor. A rear axle with
duals therefore needs four sensors — two per side.
A wide-base, or super single, fitment replaces that pair with a single, much wider tyre on each
side. The axle still carries the load, but there are now two tyre cavities on the rear axle
instead of four. For TPMS that means the rear axle drops from four sensors to two. On a
two-axle bus the whole vehicle goes from six sensor positions to four.
The width class is what tells you which family you are dealing with. Conventional bus tyres sit
in the 275 and 295 width class. Wide-base sizes sit in the 435, 445 and 455 width class. Those
numbers describe the section width; they do not by themselves define the complete tyre
specification, and you should always work from the tyre manufacturer's own data sheet for the
exact fitment.
Fewer sensors sounds like a simpler project. In practice the simplification is on the mechanical
side, and the difficulty moves to the radio side.
Why the radio path gets harder
A tyre-pressure sensor is a small radio transmitter with a very small antenna, sitting on the
valve stem. It has to get its reading to a receiver mounted somewhere on the vehicle, through
whatever sits between them.
When you fit a wide-base tyre, the amount of tyre between the sensor and the rest of the vehicle
increases substantially. More tyre mass and more reinforced structure in the path means more
shielding of the radio signal. The 433.92 MHz link that was comfortable on a 275 or 295 tyre now
has a harder job on a 435, 445 or 455 tyre, and the margin that used to be spare becomes margin
you are relying on.
That is the whole mechanism, and it is worth stating plainly because it is often described in
the wrong terms. The issue is not where the sensor physically sits on the vehicle, and it is not
about a longer or shorter distance to the receiver in any simple sense. It is that the wider
tyre, as a physical object, attenuates the radio path more. If a supplier explains the problem to
you in terms of sensor position alone, you are not getting the full picture.
The practical consequence is that a system which performs well on a conventional bus will not
automatically perform well on the same bus after a wide-base conversion. It has to be verified on
the actual fitment.
What to put in the specification
Because the difficulty is on the radio side, the specification should pin down radio behaviour
rather than just list sensor features. These are the numbers we publish for our own industrial
line, and they are the ones worth asking any supplier to commit to:
- Operating band: 433.92 MHz, licence-free.
- Wheel positions per receiver: up to 256. A single receiver can cover the whole vehicle, which
matters because fewer receivers means fewer mounting points and fewer places for a connector to
work loose.
- Sensor-to-receiver update interval: 3 seconds or less, configurable. On a wide-base fitment
where the link is already working harder, do not accept a vague "fast enough" — get a number.
- Receiver-to-vehicle-bus period: 10 ms on CAN 2.0B, default 250 kbps and configurable. This is
the number the vehicle integrator actually needs, because it determines how quickly a pressure
alarm can reach the dashboard or the fleet platform.
- Pressure accuracy: plus or minus 7 kPa; tyre temperature accuracy plus or minus 3 degrees C.
- Ingress protection: IP68 for the sensor.
- External sensor operating temperature: minus 20 to plus 85 degrees C.
Ask for the update interval and the CAN period in writing. Those two numbers decide whether the
system is usable for a fleet, and they are also the two that are easiest to leave vague.
Retrofit and factory fit are different projects
A retrofit onto buses already in service has one set of constraints: the receiver has to go
somewhere that already exists, the wiring has to follow a route that is acceptable to the
operator, and the whole thing has to be installable in a workshop without pulling the vehicle
apart.
A factory fit, where you are the body builder or the tier-one supplier, has different
constraints: you can decide the receiver location, you can specify the harness, and you can
define the CAN message layout that your own vehicle controller expects.
We work on both. On the factory side the deliverable is usually a sensor and receiver pair plus
the message map and DBC file that your integration team needs, and support while you bring it up
on the bench. On the retrofit side it is usually a kit plus an installation procedure.
If you want to see how the two are usually structured, our OEM/ODM page describes the
component supply model, and the receiver product page lists the output interfaces
we support — CAN, J1939, RS485, RS232, Modbus RTU, PROFINET and UART. For the bus case specifically,
see the city bus installation notes.
What we would need from you
To give you a real answer rather than a catalogue answer, we need four things:
1. The tyre fitment you are moving to, including the actual tyre data sheet.
2. The axle layout, so the wheel-position count is right.
3. Where the receiver can be mounted, and what it will connect to.
4. Whether this is a retrofit or a factory fit.
With those, the conversation becomes specific: how many wheel positions, which output, what
update interval, and what has to be verified on the vehicle before you sign it off.
Wide-base tyres are a reasonable decision. The mistake is assuming the TPMS carries over
unchanged. If you are planning a conversion and want the radio side checked before you commit,
send us the fitment details and we will tell you what we expect to see.