The challenges of developing EV tires

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Martin Winter, head development engineer at the Hankook European Technical CenterMartin Winter, head development engineer at the Hankook European Technical Center, discusses how the growing market of electric vehicles has affected the tire development landscape.

The requirements for an e-car tire aggravate the performance conflicts that are always present (such as grip versus mileage, rolling resistance versus handling etc).

Low rolling resistance to save battery power and improve the vehicle’s range is important, as well as a reduced noise level to match the noiseless engine.

Another issue is grip, as an electric engine provides full power immediately. This is regulated by acceleration limitation, but the tire still has to cope with the force of the e-engine. A high grip level is thus required and the potential for irregular wear becomes an issue. Since batteries are heavy, a bigger tire size could be a possible solution.

The vehicle dimensions dictate the space available for the tires – as long as car manufacturers use the same platform for all engines (one car – different engines) the common dimensions of regular tires can be kept the same. To handle the weight of the vehicle [including the heavy battery], tire pressure could be changed; however this is already regulated and limited by EU regulation.

For EVs, we can decipher between four vehicle segments where the emphasis in development may slightly differ. The most important requirements summarized for the popular segments are below:

Compact and small (for example VW E-UP) through to high duty vehicles (for example the Tesla Model X):

• Low rolling resistance and low tire weight

• Low tire noise

• Robustness at high loading capacity

• Low tire wear (regulated through shape and evenness)

The tire needs to be lightweight – however this can increase the cavity noise, which is easily heard in a car where the engine is almost soundless. Hankook’s Sound Absorber technology offers a solution here. Additionally the tire noise caused by the profile can be reduced through an adapted profile design and tread cap mixture.

Low rolling resistance can be achieved through a special mixture of tread cap compounds, however these are often detrimental for wet and wear performance, even though new compound combinations have helped to reduce this conflict.

Cars that are already heavy such as SUVs, which become heavier due to the battery robustness at high loading capacity, present an issue for tire manufacturers. In some cases the driver license critical weight of 3.5 tons might be reached. The tire must be lightweight as well as robust.

Adoption of a higher inflation pressure to address the load increase is under discussion in order to improve the durability of the tire. The trade-off between comfort and tire performance is also under review by car makers as they strive to develop EVs with the same driving performance as gasoline-powered cars. For some EVs the solution might be a change in the tire’s measurements. Cars have to be adapted to cater for a high rim and a narrow tread width, which alter the driving performance through side stiffness and other performance characteristics. For most e-cars under development, the chassis space in height is limited (same or similar chassis module as for gasoline engines).

Tire aerodynamics will become important to enable the car to achieve a good CW-value (air resistance) and to ensure optimum battery life.

A ‘cold’ tire has a higher rolling resistance compared with a warm tire in the ISO 28580 cycle test – this means that the battery needs more ‘power’ within the first 30 minutes of driving, for example 10km compared with a warm tire. This could lead to research into new tread compounds, which as a result, may lead to changes in the manufacturing process, particularly in terms of mixing and extruding, so infrastructure gains in importance.

Tire noise can be reduced by fine-tuning of the construction, the contour design and the tread pattern. With regard to wear, car makers may decide to limit the torque to prevent intensive acceleration – braking is less sensitive due to the recuperation process.

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