Managing tire temperature during force and moment testing is always a headache. As soon as you do anything with the tire, such as increase slip angle or slip ratio, the sliding friction causes the tire to heat up rapidly. If this is not managed and all the sweeps are tightly packed together, the tire temperature will creep up during testing, which causes the tire’s performance to change, leading to inconsistent results. The test engineer must therefore allow time between each test for the heat to dissipate, letting the tire cool back down to a steady baseline temperature.
The amount of time required for cooling varies according to a number of factors, such as the severity of the preceding test sweep, the tire’s bulk temperature, and its mass and conductive properties, among others. Empirical testing must therefore be conducted. Based on the results, engineers then estimate – as best they can – the cooling times required. This is a challenge for the engineers, and a potential source of error, as different judgment calls could lead to the tire being tested at different temperatures. In addition, if a change is made to the test procedure all the following pauses change and need to be re-estimated, then the test procedure reiterated, which costs time and reduces efficiency.
To address this, TIRF (Tire Research Facility, part of the Calspan Corporation in Buffalo, New York) has developed a Thermal Pause control system for its tire test rig. When triggered, this automatically holds the tire in a cool-down load case until a target baseline temperature is met. Once reached, the control system automatically triggers the next test sweep. This is a simple and novel approach that I found effective and easy to use.
When writing the command file for a test procedure, the engineer adds an extra channel for the thermal pause logic, indicating where the thermal pauses are to take place. At these locations the cooling load case is also given: this is typically an average velocity with zero slip and at an appropriate load to give the most efficient cooling for the tire type. During the test, when the rig reaches a thermal pause command, it loads the tire to the given cool-down load case and waits… The control system then monitors tire temperature measurements in real time and compares them to the given target baseline temperature, which reflects the requirements of the tire. Road car tires typically operate at around 40-50°C, while race car tires can be much hotter, but any reasonable temperature could be specified. Once the control system observes that the tire temperature is consistently below the baseline temperature, it automatically wakes up the rig and triggers the next dynamic test sweep.
The result of testing with this system is a perfectly flat plot of baseline temperature versus time, where all sweeps start from the same temperature, regardless of any outside influences. Six feet of snow outside or an Egyptian-style heatwave may cause the pause lengths to change and the total test duration could change as a result; nonetheless, the baseline tire temperature will remain constant.
The only drawback to such a system is that longer pauses may be needed when the tire’s bulk temperature has increased, as the bulk takes longer to cool. The control system addresses this by including internal tire temperatures, measuring the temperature of the innerliner. This additional measurement allows for a more advanced control system that holds the thermal pause until both the tire’s surface and inner-belt temperature reach their respective targets. Care should be taken with such functionality, however, as identifying appropriate target temperatures for a range of locations around the tire may prove challenging. As a failsafe, a timeout can be set for the thermal pause; if the target temperatures prove unachievable, the next sweep is triggered, irrespective of temperature after a given amount of time.
Finally, the control system can be used to target a high (in-test) temperature. Instead of cooling the tire, it can be used to hold it at a high slip condition, burning rubber until it reaches the high target temperature before triggering the next sweep. This could be useful when gathering data for thermally sensitive tire models or for racing applications, where the tire needs to be more substantially heated to reach its operating temperature.
The thermal pause system is a handy new addition to the tire testing toolbox, however engineers might choose to use it. tire
Gregory Smith is the director of Tyre CAE and Modelling Consultants, providing tire testing and modeling services to the OEM and motorsport industries.
More information at www.tyremodelling.com