EV Weight and Torque Mean More Tire Particulates

Battery EVs (BEVs) don’t have a tailpipe and don’t produce any exhaust emissions, but they aren’t entirely emissions-free. Over time, tires wear down, and the rubber that’s no longer part of the tread doesn’t just magically disappear. In fact, much of the rubber worn away by interaction with the pavement ends up as atmospheric particulate matter emissions, similar to diesel soot.

Tire Wear Is a Function of Speed, Weight, and Torque

Though often underappreciated by drivers, tires are one of the most important components of vehicle performance. The point of contact between the vehicle and the road is four patches of rubber that are generally no larger than a human palm. All the forces of vehicle motion are transmitted through these patches. The faster a vehicle accelerates, brakes, and corners, the more load is on the tire and the faster it wears.

Wear is further exacerbated by the weight of the vehicle. A 5,000 lb full-size pickup truck puts much more stress on its tires than a 2,200 lb Mazda Miata. As the world transitions to BEVs, this problem is only getting worse. A typical BEV weighs anywhere from 25% to 50% more than a comparably sized internal combustion engine (ICE) vehicle. For example, a Chevrolet Sonic weighs about 2,896 lbs, while a Bolt EV is 3,624 lbs. A Ford F-150 Lightning weighs as much as 6,893 lbs, compared with 4,948 lbs for the closest gas equivalent.

In addition to the extra mass, electric propulsion provides instant torque, as opposed to the more gradual buildup from an ICE. This makes it more likely that the wheels will spin, and even if slip is constrained, it’s still putting more stress on the rubber. According to a study by Clean Fleet Report, tires on EVs wear out about 20% faster than those on ICE vehicles. Thus new tires must be fitted more frequently, with the corresponding emissions and material consumption associated with manufacturing those tires.

Tires Are a Key Contributor to Particulate Pollution

Since 1980, aggregate tailpipe emissions from vehicles have declined 71%, despite a more than doubling of miles travelled. Black, sooty exhaust from diesels was a prime source of urban particulate emissions. Modern emission control features, including diesel particulate filters, have greatly reduced those emissions, with recent studies now pointing at tires as by far the biggest source of airborne particulates today.

A 2020 study in the UK attributed 60% of PM2.5 and 73% of PM10 to non-exhaust emissions, mainly from tires. Exhaust particulates are limited to 4.5 mg/km by European regulations, but tire particles, which are unregulated, account for 1,000 times as much particulate matter in the atmosphere. Particulates damage air, water, and soil and contribute to a range of respiratory issues.

Regulations Should Account for Tire Particulates

The problem gets worse as vehicles continue to get heavier. Tire manufacturers are actively working to develop new rubber compounds and constructions that both reduce rolling resistance for improved efficiency and increase tire durability. However, this is not a trivial problem to solve. Tires with less wear and resistance also tend to have lower levels of adhesion, which can lead to longer stopping distances and degraded handling.

Finding the optimal balance between these properties has always been a challenge for designers. Improving the emissions characteristics of all tires needs to be a top priority for the industry if society is to gain all the benefits of EVs, and Europe is leading the way. The new Euro 7 emissions standards will be the first to impose limits on emissions from tires and brake dust when they become effective in 2025, and the rest of the world should follow.