Tyre design is a scientific balancing act. The designer’s objective is to create a product that has amazing grip, lasts forever and is wonderfully quiet, but they face a challenging compromise because the required rubber characteristics work against each other. A tyre that handles beautifully wears out in no time, a durable tyre is loud and a flexible silent tyre is a nightmare to drive.
Now, with the arrival of the electric car, the designer’s job just got even more complicated – and Pirelli’s engineers are busy working with a number of manufacturers to develop products that can meet the task. The remarkable torque of electric cars puts immense demands on tyres because they have to be sturdy yet – and this is the difficult part – very grippy, although not so grippy that they use up vital battery power. Meanwhile the silent engine makes tyre rumble more obvious, so reducing noise is also important.
“It is about achieving the right balance – reducing rolling resistance and noise while increasing handling and safety of the vehicle,” says Filippo Bettini, Pirelli’s chief officer of sustainability and risk governance. “It’s possible to make a tyre with low rolling resistance, but it will have a lower grip on the ground and that means it may be unsafe.”
It is about achieving the right balance – reducing rolling resistance and noise while increasing handling and safety
It’s a complex business; there is far more to tyre design than meets the eye. Scientists experiment with an amazing array of variables, including tweaking the recipe of rubber compounds, construction techniques, belt arrangement, sidewall structure, tread pattern, groove width and depth... with the ultimate goal of finding the best mix of characteristics for each car manufacturer and model.
Torque: instant power
When it comes to the specific demands that electric vehicles (EVs) put on their tyres, the most important is the ability to withstand immense torque, the rotational force that drives the wheels. This is down to fundamental differences in the way electric and traditional internal combustion engines generate and use power.
An internal combustion engine produces heat by combining fuel and air in a chamber, burning the mixture to produce a high-pressure gas that expands and drives the pistons, and then transferring this up-and-down movement into rotational motion with a crankshaft. When the engine is idle, it has to suck in fuel and air before it can move, so an instantaneous start is not possible. Mixing the fuel and air also takes time, which limits acceleration.
In contrast, an electric motor, which is essentially an advanced electromagnet, simply takes stored electrical energy and converts it into torque. At the flick of a switch. No complicated chemistry; no delay. The result is that surprising – and eerily silent – sudden acceleration that demands grippy yet robust tyres to deliver the power to the road and keep the car under control, remaining in a safe condition.
A comfortable ride
Then there’s the need to maximize the distance travelled between charges, so more miles per volt is a winner, especially on longer journeys when recharging is necessary. This comes down to the tyre’s rolling resistance, which refers to the tendency of a tyre to deform. The more a tyre changes shape, the more energy it takes to keep the car moving.
The obvious solution is to create a very stiff wheel with extremely low rolling resistance, but there’s a trade-off – the car may go for longer between charges, but the ride will be much less comfortable. A metal tyre has almost no rolling resistance, for example, but would turn even the most luxurious electric car into a bone-rattling disappointment.
A metal tyre has almost no rolling resistance, for example, but would turn even the most luxurious electric car into a bone-rattling disappointment
The objective, then, is to reach a balance that suits the car and its usage. The driver of a compact EV who drives sensibly around town is probably happy to compromise handling and grip to achieve greater range and efficiency. The Tesla owner, in contrast, may prefer an emphasis on beautiful handling and the ability to floor the accelerator safely in pursuit of an adrenalin rush.
The strong and silent type
As electric engines are silent – and tyre noise more evident – acoustics are also a vital factor in tyre design, especially in terms of tread pattern, block size and groove width and depth. Once again, car type affects the manufacturer’s mindset. If they are producing a tyre for a small urban EV, which travels slowly and generates little noise, then acoustics are not an issue; if they are providing a premium product for a heavy and high-powered performance model, then noise reduction is high on the wish-list.
Then there is weight to consider. EVs have heavy batteries and an unusual vertical load distribution. This produces stress during acceleration and cornering, which means the tyre structure needs to be extra resilient. To survive and deliver adequate durability – these tyres do not come cheap – the entire carcass has to be strengthened and improved, which means rethinking the size, number and position of the inner steel belts.
Producing the perfect tyre for the new fleet of electric cars is a truly complex – and fascinating – challenge that will demand rigorous science, innovative thinking and many thousands of miles on the test track. The race is already well and truly underway and the rewards for success will be immense. Like alchemists of old, however, today’s materials scientists may have to overcome the laws of chemistry before they create their dream... the tyre that has it all.
Producing the perfect tyre for the new fleet of electric cars is a truly complex challenge that will demand rigorous science, innovative thinking and many thousands of miles on the test track