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09/07/2018
Tires and the electric transformation

 Market developments

 

Heavy and with loads of torque; electric cars might be quite hard to handle for most tires. What is the industry doing about it?

Nicodemo Angì

Will electric cars reach the same widespread distribution of other more traditional vehicles? All signals, albeit inconsistent at times, seem to be pointing in that direction, even though significant changes are expected soon.

At the moment, electric cars look no different from any other car, at least externally: the real revolution is taking place under the body and will affect many traditional components, which will probably be done away with.

Even tires will not be spared, and many technological developments are expected to meet the needs of electric traction.

 

Heavy and continuous use

We are well aware that the perfect tire is the result of a delicate balance between opposing objectives. The ultimate goal is to create a product capable of guaranteeing maximum grip, a long life and low rolling noise, but tires that “stick” to the road normally wear out fast. If a long lasting tire is the goal silence is compromised while a silent and flexible tire is unable to guarantee proper handling.

Designing cars is already a complex and difficult endeavour, and electric cars have, if possible, complicated it even further. Rolling resistance has suddenly become the Public Enemy Number 1 being able to reduce mileage (which is still a weak point in electric vehicles), while the silent motors require keeping rolling noise down to a minimum.

To make things worse, electric motors are able to deliver maximum torque immediately (or almost): great fun for sure, but this means also added stress on the tires, as all the limits mentioned above, in addition to the significant weight of an electric car, remain.

This is why Filippo Bettini, chief executive officer of sustainability and risk governance at Pirelli, wrote: "It’s all about achieving the right balance between reducing rolling resistance and noise and increasing handling and safety. Creating a tire with low rolling resistance is certainly possible, but this will lack adequate grip, thus reducing safety".

 

A delicate balance

Actually, a simple solution to a rather complex problem exists - creating a stiff tire with a very low rolling resistance – but this would be detrimental to driving comfort, not to mention rolling noise and poor grip.

When someone hears the words "electric cars" the name Tesla probably comes to mind, and no wonder as these luxury cars deserve the credit for adding some glamour to a sector traditionally filled with anonymous looking vehicles able to guarantee very little mileage.

Elon Musk's great intuition was to produce high-end luxury cars so that their price would partially offset the cost of those huge battery packs able to guarantee anxiety-free mileage.

In fact in the US, the impressive (almost 5 meters long) Model S is the best-selling luxury sedan, which represents a rather demanding challenge for tire manufacturers.  The same goes for the all-wheel drive P100D and its 762 hp able to deliver a torque of nearly 1,000 Nm, pushing a mass of around 2.3 tonnes.

Here we are looking at a large and heavy sedan and, therefore, the tires are called upon to ensure adequate comfort and durability, not so important if one has the fortune to drive a light and hot-blooded Lamborghini or Ferrari of equal power.

 

Back to the drawing board

Michelin accepted the challenge and responded with a special version of its Sport Pilot 2. In an interview on Wired, Tire engineer Ed Gliss said: "We are very proud of the work done with Tesla engineers. On smaller electric cars rolling resistance is crucial in ensuring greater mileage. Tesla, on the other hand, requires also good handling and low rolling noise. But given the large and heavy battery pack, and the unusual weight distribution, we were forced back to the drawing board, and had to rethink the tires currently under production. We therefore had to reconfigure the tires to cope with greater loads and stress preserving, at the same time, the traditional comfort and durability expected of a Michelin tire. The result was a compound that effectively disperses heat and allows the tread blocks to maintain the right stiffness while driving ".

These words clearly highlight the different needs of electric cars, which can differ considerably even between models. The BMW i3, for example, favours driving smoothness and mounts tires with unusual dimensions.

 

Urban vehicles and sports cars

The normal version, with a 125 kW engine, adopts 155/70 tires mounted on large 19-inch wheels. These tires are rather "thin" and have been designed to optimize rolling resistance. The version with a Range Extender (equipped with a small two-cylinder petrol driven unit used to recharge the batteries in motion) uses different tires on the rear axle to cope with the increased weight while the front tires remain the same 175/60 R19. The "S" version is a little sportier and shares the same tires. However, the rims, are 20 inch thus confirming the same "thin" tire approach.

On the other hand, the Nissan Leaf prefers sportier tires: 16" wheels are equipped with 205/55 tires while the 17" wheels have 215/50 tires.

The entry level of Nissan’s “cousin”, Renault’s Zoe, mounts tires with an aspect ratio similar to those found on the i3, since it mounts 185/65 R15 but can also use a set of 205/45 R17.

Meanwhile Tesla, being high-performance vehicles, are equipped with very generous tires: the Model S can boast a set of 245/35 R 21 while the SUV Model X uses a set of generous 285/35 R22 on the rear axle, and it is interesting to note that many Tesla use Pirelli tires, both as standard equipment and as optional. The "small" Model 3 starts from a set of 235/45 R18 but can also be equipped with larger 235/35 R20 at the front and 275/30 R20 at the rear.

 

Looking at the future

Chevrolet’s Bolt, instead, uses tires we could define unusually “normal” such as a set of 215/50 R17 on both axles.

However, going beyond mere sizes, inches and so forth, we can safely say that the tires chosen to equip electric vehicles belong to the Energy-saving category, albeit with greater resistance to stress and noise.

Michelin’s Energy Saver E-V, for example, can increase Zoe's mileage by 6% and allow the little French city car to do away with a spare tire as these are self-sealing: weight saving is another factor in maximising mileage. Even Chevrolet’s Bolt can use them, and compared to normal tires, in terms of efficiency the difference is quite remarkable. Replacing them with sportier tires, the result is better handling and grip, although this will penalize noise levels and reduce mileage by about 10%.

Goodyear, for its part, presented its prototype EfficientGrip Performance with Electric Drive Technology in Geneva last March. A lighter and more resistant casing, together with a new tread design, allows to manage the weight of the batteries and the sudden torque of the electric motors. The design of the sidewalls reduces aerodynamic drag while rolling resistance is maximised by a special compound.

Hankook too unveiled its Enfren Eco, which reduces resistance, has special anti-noise technologies and an aerodynamic design able to reduce noise levels even further.

In short, these are just a few examples of how the tire industry is keeping pace with the changes are affecting passenger vehicles, and more surprises are expected in the near future!

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