Measuring brake disc and hub temperature on a turning wheel, on the road

Why brake temperature is read on the vehicle

Brake testing has to quantify the heat each stop generates. That heat appears in the parts that turn: the disc, the hub and the wheel.

Measuring on the vehicle gives the temperature of these parts during real stops, with the vehicle's own wheels and brakes. The Wheel Instrumentation System fits passenger cars, utility vehicles, heavy trucks and agricultural vehicles.

The thermocouple turns with the wheel

A thermocouple fixed to a disc turns with it, and its wires cannot go straight to a recorder in the cabin. Something has to carry the signal from the rotating side to the stationary side.

A slip ring does this with an electrical connection between a rotating component and a stationary one. A telemetry package does it by radio, with no contact. Both mount outboard on the wheel.

From the disc to the recorder, in four steps

The package bolts onto the original wheel through a plate and collet lug nuts machined for the vehicle. Nothing on the vehicle is modified.

1. 01

   Fix the thermocouples to the disc and the hub

   The thermocouples are attached to the spinning parts at the points to monitor, and their wires are routed to the outside of the wheel.

2. 02

   Bolt the package to the vehicle's own wheel

   The Wheel Instrumentation System mounts on the original wheel with its plate and collet lug nuts. It is waterproof and works in mud and snow.

   Wheel Instrumentation System
3. 03

   Connect directly, or through an amplifier

   The slip ring has a low resistance and is designed for low-voltage signals such as thermocouples, so the thermocouple wires can go straight to it. An amplifier on the rotating side adds the cold junction compensation and sends a high-level voltage through the rings.

   AMP-TC-M1 thermocouple amplifier
4. 04

   Record temperature with wheel speed

   The stator cable goes to the data acquisition system. An optional encoder, up to 1 024 pulses per revolution, adds wheel speed and distance on the same assembly.

Slip ring or telemetry wheel package

The Wheel Instrumentation System carries the signals in analog form through 4, 6, 10, 20 or 36 circuits. Up to 30 measurements can be collected at the same time, so thermocouples and strain gages can share the same wheel. It runs up to 2 000 rpm.

The telemetry wheel package sends strain gage and thermocouple signals by radio, brake rotor temperature among them. It runs on a single 9 V battery, fits most wheel sizes and has a removable cover.

Michigan Scientific telemetry systems use a digital 2.4 GHz link with a range of up to four metres line of sight.

Amplifying the thermocouples before the contacts

The rings and brushes of a slip ring are not made of thermocouple alloys. A temperature difference between its rotor and stator connections therefore adds an error at those dissimilar-metal contacts. An amplifier on the rotating side holds the cold junction compensation and sends a high-level voltage through the slip ring, which the gradient cannot affect.

The AMP-TC-M1 takes one to three thermocouples of type E, J, K or T. It is adhered, potted or strapped to the part and gives a 0 to 10 V DC output. The MicroTC takes one type K thermocouple, weighs 3.9 g and gives a linear 5 mV per °C from −25 to 400 °C.

On an SR Series end-of-shaft slip ring, the AMP-TCN-X2 mounts on the rotor and amplifies up to nine thermocouples per module, with an output of 10 mV per °C. The modules stack for more channels.

Frequent questions