Electric vehicles are conquering the world's roads, and the first autonomous trucks are already in operation. What do these new mobility trends mean for tires? Frank Walloch, who is responsible for the development of commercial vehicle tires at Continental, talks about the new challenges facing the industry – and why tires sound different in Asia than they do in Europe.
Mr. Walloch, do electric vehicles need different tires compared with cars that have a combustion engine?
It depends whether we're talking about truck or car tires. There are significant differences between the two. The one thing that tires for all electric vehicles have in common is the objective of reducing energy consumption. This is essentially achieved by reducing the rolling resistance. With electric vehicles, a lower rolling resistance also has a positive effect on the range, which is a key issue for us in tire development. The characteristics of an electric drive are also very different to those of combustion engines. In electric vehicles, you get the maximum torque when accelerating right from the very start. Vehicles with combustion engines take longer to get up to speed. Tires on electric vehicles can therefore be subject to greater levels of stress during acceleration, which in turn affects their endurance and wear values.
Tires on electric cars are relatively narrow. Why is that?
We tire developers refer to this trend as 'tall and narrow'. There is a direct correlation between tire diameter and rolling resistance. The larger the tire diameter, the lower the rolling resistance for the same tire load. Tire deformation, which can occur while driving due to the vehicle weight, for example, is lower overall compared with smaller tires. Rolling resistance is also an important aspect here. It must be reduced in order to increase the range, for example.
Is a reduction in the rolling resistance not achieved at the expense of other vehicle handling characteristics, such as the braking distance?
That's correct. It's a case of conflicting goals. It's not possible to achieve both to the same degree. Obviously, we can optimize new product generations with respect to all key properties using innovative technologies, thus lessening the effect of these conflicting goals. Unfortunately, however, we cannot completely resolve this conflict. This is due to the physical nature of things. A longer range with lower rolling resistance can only be achieved in compliance with all safety aspects. Braking distance, handling and directional stability must not be overlooked. Our challenge is to find the best compromise between a safe braking distance and low rolling resistance.
Will tires for trucks and buses also become narrower and taller? They have to support heavy loads…
No, in the case of trucks and buses there is another important aspect to consider: the hold. Commercial vehicles often have one job, namely to transport goods or people. If we were to increase the diameter of commercial vehicle tires, this would stand in direct conflict with efforts to increase the size of the hold. For truck tires, the trend has actually been moving toward smaller tires in recent years in order to allow an increased loading height in the vehicle. I don't believe that we will see narrower tires for commercial vehicles, for example. The stress on tires in this sector is too high.
What changes can we expect to see in the commercial vehicle sector?
It's often the small details that you don't notice at first. If truck tires become smaller, the load capacity must be adapted. We express this using the load index, which is information about the load capacity that is noted on the tire sidewall. This load index can be controlled by means of components inside the tire. In the tire we have reinforcement in the form of the carcass. It consists of a steel wire construction that absorbs all the pressure exerted on a tire. To increase the load capacity, you must therefore reinforce the carcass, for example. However, this has no effect on the shape of the tire. All our product lines in the truck and bus segment have already been extensively optimized with respect to rolling resistance and safety. Good rolling resistance hasn't just become a priority on foot of the trend toward electrification. In long haul operations, fuel and energy consumption is an important cost factor for our customers. The better a tire performs in terms of rolling resistance, wear behavior and above all safety, the more satisfied our customers are.
Continental presented the Conti e-MotionPro, a concept tire for electric vehicles, at the most recent IAA Commercial Vehicles. What are the main features of this prototype?
This tire is a concept, with which we are demonstrating future trends in tire development for electric vehicles. Our picture of the future is a world in which there are individually adapted tires for a wide variety of possible uses. In particular, we envisage specially designed treads that meet the new requirements for future vehicle generations. Our vision is of a tread that is highly resistant to wear, for example.
Rolling noise is also going to be a key issue. Because electric vehicles are comparatively quiet on the road, other noise can be heard more clearly.
That's right. A positive side effect of the electric drive is reduced noise emission from traffic. For us, however, this means that tire noise may become an issue. This is an extremely important field of research and development. One possible option is to modify the tread pattern geometry, as is the case with our Conti e-MotionPro concept tire. The aim is to make tires quieter.
So how do tires for electric vehicles sound compared with conventional tires? Do they hum, or does it sound more like a roar?
Tire noise contains higher-frequency components that make a buzzing sound. There are also low-frequency components that make a humming sound. This low-frequency humming tends to be regarded as annoying. A higher-frequency, slight buzzing sound is subjectively perceived as more pleasant. In this respect, our aim with new tire generations for electric vehicles is to reduce low-frequency humming.
Will this noise be the same all over the world? Or do you plan to develop country-specific tire noises?
Yes, tire noise can actually vary from region to region. As a first step, statutory requirements with respect to pass-by noise and other limits must be met. In tire development, however, there are also soft factors that influence subjective perception of certain factors by end users. We therefore have test drivers who are specially trained to assess tires on the basis of these parameters. These can vary from market to market. Noise is a major issue in Asia in particular. Here, higher-frequency tire noise is perceived as more disruptive than it is in Europe, for example. Tires can therefore sound different in Asia.
Let's talk about autonomous driving, which is an important issue in the commercial vehicle sector. Will special tires have to be developed for driverless trucks?
We are certainly thinking in that direction given the likelihood that tires on autonomous trucks will be subject to less stress. We need to investigate this in greater detail. Braking and starting off that is controlled electronically by an on-board computer is without doubt easier on tires than a human stepping on the brake or the accelerator. Drivers brake less calmly. They may also enjoy accelerating. The autonomous vehicle doesn't understand the concept of fun.
Autonomous vehicles can brake in a more anticipatory and gentle manner than a vehicle driven by a human. Does the importance of braking distance have an influence on tire development? Coupling trucks to create long, driverless transport caravans, the likes of which are already being trialed in Shanghai, reduces the risk of rear-end collisions.
It makes no difference whether we're talking about car or truck tires – safety is always our top priority. We will not bring tires with reduced braking performance onto the market, and that includes autonomous vehicles. An autonomous truck has, of course, a shorter reaction time, which has a positive effect on the braking distance. This provides a measure of additional safety that we will never give up. It also represents a further step in the direction of Continental's Vision Zero, to a world without accidents.