With the continuing increases in fuel prices, there is an increasing focus on engine performance and rolling resistance in tyres. The latter is one of the components used to measure tyres’ fuel efficiency on the European labels put into place in 2012. But what is rolling resistance exactly?
A force to be reckoned with
If we say someone has had an "easy ride", we mean everything has gone smoothly for them. They have met with neither resistance nor friction along the way. However, we all know this is only true in figurative speech because any object in motion is subjected to resistance. A truck has to overcome, or at least compensate for, different forms of resistance in order to move forward: its own mass (inertia), air resistance, engine friction and the rolling resistance of its tyres. Truck manufacturers are constantly working to make their vehicles lighter and facilitate the mobility of their moving parts. Indeed, the greater the reduction in resistance, the less energy is required to move the truck, and of course less energy means less fuel or more autonomy for electric vehicles. However, with the sharp increase in the price of petrol, and even diesel, fuel efficiency has become a key priority in the transport sector. Furthermore, with the combustion of fossil fuels directly linked to the emission of harmful particles and gases (CO, CO2, NOx, etc.), there are also sound environmental reasons for reducing fuel consumption.
Rolling resistance is estimated to represent around 30% of a truck’s fuel consumption. Tyre manufacturers are therefore also involved in arace for technological innovation. Furthermore, each axle contributes to rolling resistance to a greater or lesser extent. In a semi-trailer made up of a tractor and triple axle trailer, the axles represent 50 – 60% of the rolling resistance (estimations vary according to the manufacturer). The tyres on the steering axle account for just 15 – 17% whilst the powered axle tyres proportionally offer more resistance as they account for between a quarter and a third of the total.
Like engine friction, rolling resistance is the result of energy losses. In an ideal world, the energy consumed (whether electric, solar, wind or power fuelled by combustion etc.) would be used in its entirety to move the vehicle or power its systems. In reality, a certain amount of energy is always lost, usually in the form of heat. Tyre rolling resistance is the result of energy dissipation within the tyre. If wheels were still made of wood, without a rubber exterior, the loss would be reduced to sliding on the road surface, due to a lack of grip. However, the tyre's primary purpose is to ensure that the vehicle’s driver and passengers can travel comfortably and safely. The flexibility of the rubber allows it to act as a shock absorber by hugging the road surface. The tyre is therefore constantly subject to flexion, extension and shearing stresses. Unlike a spring, the rubber does not immediately return to its initial form when the stress it is subject to is removed. It does not expend the same amount of energy when it returns to its resting state as some of this energy has dissipated as heat. This is what is known as hysteresis.
A question of compromise
Reducing these energy losses in turn reduces rolling resistance. In the 1990s, the introduction of silica into rubber tyres significantly reduced tyre heating. Manufacturers could make them harder to limit flexion but his would compromise comfort and road holding. They therefore need to reach a compromise in order to simultaneously preserve all these qualities at acceptable levels.
The driver themselves can limit the rolling resistance of their tyres by ensuring they are always inflated to the optimal pressure according to the manufacturer’s recommendations. When they are underinflated, tyres tend to flatten out more easily. They therefore undergo more flexion and extension in motion leading to abnormal heating.
Furthermore, thecomponents which make up the tyre rubber which contribute to lowering rolling resistance may have negative consequences on wet grip . This is why European labelling emphasises these two criteria. They do not allow consumers to fully judge a tyre's performance but they do give an indication of the efforts manufacturers have made to offer a combination of safety and fuel efficiency.
A question of labelling
Photo credits ©: rezulteo
The rolling resistance score displayed on European tyre labels is calculated in a laboratory test, carried out by each manufacturer according to the corresponding ISO standard. The tyre is placed on a test bench to measure the Rolling Resistance Coefficient in kg/t. At 4.0 kg/t or below it is awarded the highest score (A); from 4.1 to 5.0 kg/t gets a B rating; from 5.1 to 6.0 kg/t, a C rating; and so on down to F for coefficients of over 8.1 kg/t. Unlike the labelling for passenger cars and light-duty vehicles, the scale for trucks includes a D ranking but does not go down to G.
By Renaud Lacroix