Joe Walter assesses an EV’s influence on vehicle and tire dynamics

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The market share of battery-powered, plug-in, all-electric vehicles continues to increase worldwide – but with huge regional variations in 2018: from 30% in Norway to 2% in the USA. The uneven global uptick in EV sales is being driven by regional rent-seeking incentives and government regulations in an effort to reduce air pollution and greenhouse gas emissions. It’s interesting to note the evolutionary changes now occurring in tire trends and vehicle dynamics as EV market share continues to climb.

Consider tire design: a given tire has the same rolling resistance coefficient on an electric car as on an automobile powered by an ICE – but the contribution of the same tire to vehicle energy loss is significantly higher on the EV. This is because gasoline engines effectively deliver only 15% of their onboard fuel energy to the drive axle, while 65% is lost as heat. This is not so with battery-powered cars, which suffer negligible thermodynamic losses and have onboard efficiencies (by not accounting for upstream losses) of around 80% – with most of that going to the drive axle to overcome tire and aerodynamic drag, as well as braking losses.

Additionally, EVs are, on average, 20-30% heavier (about 450kg/1,000 lb) than a comparably sized, conventionally powered car due to battery weight. As a consequence, the tires carry heavier loads. It follows that tire rolling resistance forces that consume energy are proportionally higher on an EV even though tire rolling resistance coefficients are the same. Thus, there will be incentives (other than market forces) to further reduce the already low values of rolling friction of EV tires without compromising traction or tread wear.

Also consider the inverse relationship between overall tire diameter and rolling friction – i.e. with other conditions remaining the same, larger tires are more fuel efficient than their smaller diameter counterparts. Specifically, a 1% outer diameter increase results in approximately a 1% rolling resistance decrease. While the general concept was known to Coulomb in 1785, it was first exploited in a public way in the tire industry with Bridgestone’s somewhat oversized 155/55R19 tires fitted to the BMW i3 in 2013.

But now consider another tire variable affecting vehicle fuel efficiency in the opposite way: rotational inertia. Some automotive energy is consumed in overcoming the inertia of rotating components during vehicle velocity variations – principally by the tire-wheel assemblies. The physical property governing this behavior is known as mass moment of inertia, which depends not only on tire-wheel dimensions and mass, but also on the distribution of mass about the spin axis. All other variables being equal, tall, narrow tires will have higher rotational inertia and less fuel efficiency than lower, oval, wider tires. These negative aspects could be consequential during city driving with frequent stop/start for which EVs seem ideally suited – and possibly negate the positive effects of lower friction from increased rolling radius.

Another consequence of tall, skinny tires is the resulting reduction of rubber in road contact compared with conventional counterparts. Less rubber in the footprint, coupled with heavier tire loads imposed by EV batteries, results in faster tread wear and reduced tire life. This fast wear tendency is exacerbated by the instant torque available on vehicles powered by electric motors during stop/start driving. Conversely, a positive characteristic of these taller tires is a more compliant sidewall, which will tend to reduce the number of bent wheels and damaged tires due to pothole impact.

Because of the heavy battery packs and electric motor(s) replacing an ICE, the added weights (mainly as sprung mass) and locations will affect EV ride and handling, consume additional onboard energy, and lower vehicle center of gravity – with positive and negative consequences.

For example, altered EV weight distributions will modify transient responses in roll, pitch and yaw. These tire and/or vehicle changes can be aptly handled by consumers and manufacturers; none are earth-shaking. The broader disruptive influence of electric vehicles, driven by pending government mandates, has yet to be experienced at the economic, environmental and societal levels. Time will tell.

Illustration: Phil Hackett

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