What role do tyres play in protecting the environment? Christian Kötz, Head of R&D at the Passenger and Light Truck Tires division at Continental, discusses the major potential in the area of tyre development.

Mr. Kötz, the current economic crisis has to some extent pushed environmental issues out of the headlines. In 2009, people seem to be more interested in discussing car scrapping incentives than CO₂ emissions. However, within the automotive industry, and particularly amongst suppliers to the industry such as Continental, considerable efforts are still underway to ensure the sustainable conservation of resources. What contribution can tyres make towards this?

A huge contribution. As a tyre manufacturer, cutting CO₂ emissions continues to be an incredibly important issue for us, as you might expect.

Tires account for approximately 20 per cent of a vehicle's fuel consumption, with rolling resistance a key factor in this. Lower rolling resistance results in lower fuel consumption and, therefore, reduced emissions. For instance, if we are able to achieve a ten per cent improvement in the rolling resistance of our tyres, average CO₂ emissions would drop by two grams per kilometre per vehicle.

Which doesn’t sound like much to begin with…

It is though! In Germany, there are approximately 48.8 million cars on the road, which together consume approximately 47 billion litres of fuel every year, based on the statistics for 2007. If you do the maths, therefore, solely by using tyres with a ten per cent improvement in rolling resistance, you could save 750 million litres of fuel per year, which is equivalent to over ten million tanks’ worth! This would represent savings of approximately 1.8 million tonnes of CO₂ per year in Germany alone, which is pretty impressive.

All this begs the question: why don’t you make the necessary improvements right now?

We are. But step by step. Reducing rolling resistance is pretty complicated. Tires have various different performance-related properties, and rolling resistance is just one of them. Many of these properties present a direct conflict in goals; rolling resistance, for instance, is in conflict with wet braking/wet grip. Good (in other words, low) rolling resistance primarily requires a rubber compound that exhibits as low material hysteresis as possible. However, for good grip in wet conditions (one of the key safety features of a tyre), you need a compound with high material hysteresis, i.e. which is capable of converting considerable energy into heat. This is a conflict which can never be completely resolved. Thanks to state-of-the-art technologies, however, we are able to keep returning to the problem and approaching it with a greater level of understanding, constantly improving both rolling resistance and handling in wet conditions.

To which new technologies are you referring?

Over the past twenty years, we have successfully developed technologies that have already reduced rolling resistance by a third whilst maintaining the same level of wet grip. This was essentially attributable to the development of silica for the tread compound at the beginning of the nineties. Today, by using new silanes and special polymers, we are developing rubber compounds which exhibit both good rolling resistance and good wet braking performance. In addition to this, we are continuing to reduce the weight of our tyres – a lightweight structure is key to success in terms of rolling resistance. As a rule of thumb, a 100 gram weight difference in the same compounds makes a one per cent difference in rolling resistance. In other words, the more lightweight the tyre, the better its rolling resistance.

But this does not resolve the underlying conflict?

No. It's like this: at every technological level, you have to keep asking yourself which direction you want your product to take. Do you want an optimal low-rolling-resistance tire with very acceptable safety properties or would you prefer a tyre with excellent braking and acceptable rolling resistance properties? Vehicle manufacturers have different philosophies regarding this and end users have different preferences too. I must however stress that where the design of good tyres is concerned; compromising on safety is not an option! Designing a tyre purely on the basis of low rolling resistance is not technically difficult, but neglecting to implement safety properties, such as wet grip, in the process can be downright dangerous – as recently evidenced in tyre tests carried out by ADAC and ACE. It is essential that every tire offers a good basic level of grip at the very least.

Could you tell us a bit more about the different philosophies of the different vehicle manufacturers?

It is important for every vehicle to be fitted with the optimal tyres for its purpose. Let us take the Golf as an example: some models are optimised specifically to provide low fuel consumption through their chassis configuration, engine calibration, or as mentioned, the tyres used. In this case, low-rolling-resistance tyres with good wet grip properties are a valuable part of the overall package. Sports models, on the other hand, are designed for entirely different purposes – to provide ultimate driving enjoyment and impressive road performance. For these models, excellent grip is essential to convert this enormous power; a very low-rolling-resistance tyre would not be the optimal product in this case.

There are no legal regulations at present governing which tyres can be labelled as particularly eco-friendly low-rolling-resistance tyres. Shouldn’t the legislators do something about this?

That is correct. To date, there is no legal limit. However, legislators are taking action and from 2010, a limit will be in place. Once that takes effect, the following will apply for summer tyres: all tyres exhibiting a rolling resistance coefficient below 1.2 will be considered low-rolling-resistance tyres. In my opinion, genuine low-rolling-resistance tyres today have a coefficient of no more than 1.0. From 2016, this limit will be reduced to 1.05. This will lead to very ambitious thresholds, ensuring that the rolling resistance of all new tyres is at a very acceptable, good level.

By way of comparison: what is the coefficient of a good standard summer tyre today?

The best all-round tyres on the market, which have not been manufactured solely for the purposes of good rolling resistance, tend to have a coefficient of less than 0.9. A good example of such tyres is the EcoContact range from Continental. This range was launched some twelve years ago with a coefficient of less than 1.0, which was sensational at the time. The latest version is our EcoContact 3, and we are already in the process of developing its successor.

Could you explain the significance of this rolling resistance coefficient with regard to the environment?

The coefficient is a major criterion in terms of the ecological properties of a tire. By way of a comparison: a tyre with a coefficient of 0.9, a low-rolling-resistance tyre in other words, consumes 3.5 per cent less fuel than a tyre with a coefficient of 1.1. Between 50 and 60 per cent of all cars manufactured today are fitted with tyres that have a coefficient of less than 1.0. This applies to all car segments, not only compact and mid-range cars. Fuel efficiency will certainly continue to be an important issue in the future. And tyres will play a key role in this. However, the important thing is to ensure that safety is not compromised as a result of this environmental focus. Road safety will always remain our top priority when developing tyres.