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Tyre temperature sensors

Now that there is a single tyre supplier in Formula One, there is no longer a tyre war. Teams cannot produce their own tyres or request customisation of the compound or construction. Yet tyres are one of the biggest performance factors that the teams are able to influence.

Tyres are of course critical to lap time in Formula One, and with the recent trend for high-degeneration tyres, this importance is growing.  Sauber’s Willem Toet in his Lanchester lecture to the Royal Aeronautical Society provided some insight into the proportionate benefits in improvement tyre use. He said that for a given percentage increase in tyre grip, there is a proportionate percentage decrease in lap time by a factor of three; by contrast, aerodynamics provides merely a 1% lap time benefit for a 1% improvement.

Although the interaction of the tyres between the car and the track is a complicated one, a key indicator of how the tyre is being used is its surface temperature, and measuring this accurately gives the race engineers an insight into how the tyre is behaving. To achieve this, teams and their partners have developed increasingly complex tyre temperature sensors.

Taking temperature readings from the surface of a rotating tyre clearly requires a non-contact sensor.

Black rubber tyres have high emissivity, which is the amount of radiated heat they reflect. A mirror may have an emissivity of 0 and perfectly a black object a value of 1. As the tyres radiate very little heat, the temperature sensors need to read in the high emissivity range typically 0.8-0.9595 to take accurate readings.

Then, complicating the issue, is the wide tread of the modern Formula One tyre (330 mm front and 375 mm rear), so either multiple sensors or those with a wide field of view (FOV) are required. With the front wheels being steered, and aerodynamics critical in the area around the tyres, packaging is a challenge, not least the usual installation issues of weight and ruggedness on a Formula One car.

Teams started routinely using infrared tyre temperature sensors in testing some 20 years ago. That was during the tyre war, when each race was followed by a test, usually with the run sheets emphasising tyre assessment.

The first sensors to come into common use were simple infrared modules inside small alloy cases. These sensors convert the radiated heat into a voltage, which is non-linear but is continuous and repeatable, and this analogue output is fed back through the CAN bus to the data acquisition system.

Installation for the rear tyres take the form of a rear-facing sensor embedded in the floor ahead of the rear tyre or sometimes in the flick-up on the sidepod, while the front sensor was set into the trailing edge of the front wing endplate. Since both the sensor and the installation allowed for only a single temperature reading on each tyre (and when the front tyres were not being steered), this was not a perfect solution, so for some tests teams fitted outriggers to the suspension uprights, which mounted three or more sensors to take readings at several points across the tread, even when the front tyres were being steered. This set-up clearly affected aerodynamics though, and would not be legal to run over a race weekend.

More recently, the sensors have been developed further to provide a wider FOV, such that several points on a tyre’s tread can be measured simultaneously. This can be achieved either with multiple sensors in the same module or with a single module with a wide FOV, which then takes readings from multiple positions within the FOV.

With these sensors a team can measure the temperature across the tread, to understand the effect of tyre deformation and suspension camber on tyre temperatures. With some sensors able to measure up to 64 channels from its FOV, from 16 points across the tyre by four positions radially, the variation in temperature] of the tyre can be also be measured.

Installing these sorts of sensors is somewhat easier than simpler infrared sensors, as the distance between the sensor and the tyre can be far greater, allowing the front tyre sensor to be mounted in the sidepod front or the wing mirror pod. With this set-up, the front tyre temperature can also still be read when some steering lock is applied.

There is also an optical means of infrared temperature sensing, which was first used by McLaren more than ten years ago. Using an infrared camera, the entire surface of a tyre can be seen in real time, and such technology has been used by the FIA for the thermal images seen on TV. As this is a real-time device, the interface is USB and requires a separate data logger to support the volume of data and the USB connection. As with the other types of infrared sensors, installation is relatively easy, as the small unit (61 x 42 x 45 mm) can again be fitted in a mirror or sidepod.

Fig. 1 - The rear tyre temperature sensor is typically recessed into the floor

Written by Craig Scarborough

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