Rolling Resistance

Cars – in fact all vehicles – have many different forces put on them and manufacturers have many teams all researching and developing new ideas to improve the performance and economy of a vehicle. In many cases this means working with third party providers such as oil companies, electronics, software and even tyre manufacturers to find ways to make these improvements.

Tyres are valuable components for any car, whether it is a family sedan, SUV, sports car or a hybrid/electric powered vehicle. It is the tyre that keeps the car on the straight and narrow in all conditions and it helps the car maintain comfort by acting as a mini shock absorber through the wall of the construction. Keeping tyres correctly inflated and looked after can literally be a life saver when conditions turn bad or an incident occurs where the driver has to make sudden inputs to steering, acceleration or deceleration.

One topic that many drivers do not consider is the impact on fuel economy that tyres have – having the pressures correct for their usage can have a significant saving on fuel costs, hence the need for Tyre Management Systems on heavy trucks and equipment. The same applies to a vehicle used for commuting and family runs.

Rolling resistance is the metric by which tyre manufacturers can assess the efficiency of a tyre on a particular vehicle. Physics is involved here with the rolling resistance being the battle between traction and power delivery. The vehicle needing power to actually counter the grip that the weight of the car is applying to the tyre. There is friction between the surface of the road and the tread of the tyre and then there are forces inside the tyre construction that use more energy to counter them.

When you think about it, the power of an engine is mostly lost through dealing with the drivetrain and other components hanging off the engine. There already discussions on how much power is lost between the flywheel and the differential and then more is lost in the actual moving of the vehicle! Some experts say that approximately 25% of a vehicle’s fuel is used to deal with the resistance of the tyres.

I read a report recently that suggested a 5% reduction in rolling resistance could equal a 1% increase in fuel economy. This doesn’t sound much however over the life of the tyre it could add up to a good saving. Importantly, that equates to more power being used for its original intention – torque and pushing/pulling the vehicle along.

There is a fine balance between comfort, tyre life and grip when trying to improve the rolling resistance of a tyre. Adjusting an input to the tyre can have an adverse affect on that balance, for example, a tyre inflated to a high pressure will reduce comfort and wet weather grip whilst providing a longer life (depending on usage!), conversely having a lower inflation will get better grip and more wear as more of the tyre is in contact with the surface which will reduce the tyre life or damage the tyre (again depending on usage). It is not an exact science as so many factors are at play – with usage and driving style making it more complex.

The manufacturers spend a fortune on trying to test different materials and compounds to help with the battle between rolling resistance, tyre life and traction. Silica was first added by Michelin to help provide less resistance yet give better grip in wet/dry conditions. Bridgestone used silica in their Potenza S02s that I had on the Honda although they were a relatively soft compound and when warm gave great traction.

Carbon Black is also used in a similar way to silica however in higher quantities – it is a byproduct of catalytic cracking in the refining process of crude oil. It can also come from tar and vegetable oils. The rubber/silica/carbon black mix is constantly being developed with the intention of keeping the tyres cooler and subsequently reducing the amount of energy lost. The compounds work to move heat caused by friction away from the tread thus keeping the outer layer cooler and therefore harder. If you read any material on these compounds, you will need a science degree as they talk about molecule chains and other technical reasons why the compounds work the way they do!

Today, as with many industries, there are new names for old technologies, a tyre that has a reduced rolling resistance has now been given a make-over by marketing: they are called fuel-efficient or energy-efficient tyres – even eco-friendly!! They still however have to deal with the basic issues surrounding tyres of old namely the battle between life and grip.

Before any purchase it is important to consider the usage of the tyres to be bought – and don’t simply buy the cheapest set because you may find that to be false economy. The life might not last as long as a quality tyre and you might find that wet weather grip is compromised, subsequently you might have to replace them sooner.

As I’ve said in other articles, tyres are a critical component of your car and it is worth the while to understand them such that you know their limitations and how they can help provide a better driving experience.