Why Measure Tire Temperature?
Tire temperature is one of the most important parameters to measure when it comes to optimizing tire performance and developing accurate tire models. The traditional approach of measuring tire temperatures with pyrometers is useful for evaluating the average temperature behavior but it does not represent the actual surface temperature or contain information about the dynamic behavior. Tire surface temperature changes rapidly with vertical, lateral (cornering), and longitudinal (braking/accelerating) forces and the only way to properly acquire surface temperature data is with multichannel infrared temperature sensors. Remember, everything begins and ends at the interface between the tire and the road.
Given below are some general trends in tire temperature behavior and how multichannel tire temperature data is used to optimize tire performance:
1. Coefficient of Friction vs Tire Surface Temperature
A tire’s coefficient of friction increases considerably with surface temperature, reaches a localized peak, and then falls off beyond the peak, as shown below. In order to exploit a tire’s full potential, it must operate within this optimal “temperature window” (65–75˚C for the profile shown below) where the coefficient of friction is maximized. This profile can be provided by the tire manufacture.
2. Pressure Distribution and Shape of Contact Patch
The instantaneous pressure distribution and size of a tire’s contact patch can be estimated by analyzing the tire’s lateral surface temperature distribution. Surface temperature is mostly generated by the lateral slippage of the rubber across the road (heat generation per unit area = shear stress • sliding velocity) at the back of a tire’s contact patch, as shown below.
The size of this “slippage” region and, therefore the surface temperature, increases with the longitudinal length of the contact patch, load, and slip angle. Some typical examples of the contact patch pressure distribution and resultant lateral temperature distribution are shown below. It’s obviously advantageous to increase the size and pressure uniformity of the contact patch because of the inherent load sensitivity of a tire (i.e., the friction coefficient decreases with increasing pressure/load).
3. Camber, Tire Pressure, Toe, and Vehicle Balance
The most well known use for tire temperature data is the tuning of camber, tire pressure, toe, and vehicle balance. The goal is to adjust the camber, tire pressure, and toe so that the temperature distribution across the loaded tires is uniform. A uniform temperature distribution means that the pressure is evenly distributed across the width of the tire, which will allow the tire to produce the maximum amount of grip. Examples of excessive, insufficient, and optimal camber, tire pressure, and toe are shown below. Tire temperature is also used to evaluate the front-to-rear or side-to-side balance of the car (e.g., a car with excessive understeer will have hotter front tires).
4. Transient Temperature Behavior
Tire temperature typically increases at a high rate for the first few laps before reaching a quasi steady-state value. This profile will change with the tire, conditions, setup, and driving style. Conductive heat transfer from the brake rotors, into the wheels, and into the tire’s carcass is one of the key influences on the average tire bulk & surface temperature. As shown below, the initial rise in rotor temperature coincides with the initial rise in tire temperature.