Tire Tech: How does the BMW i3 handle so well on such narrow tires?

We take a closer look at Bridgestone's Ecopia EP500 tires for the 2015 BMW i3 electric car.

Antuan Goodwin/CNET

When I had the pleasure of spending time testing the 2014 BMW i3, one of my first impressions was "Wow, what was BMW thinking with those narrow wheels?" By the end of my evaluation, that question had changed to "How does it handle so well with such narrow tires?" Recently, I got my answer to both of those questions when I met with Bridgestone to discuss the rubber in question: the Bridgestone Ecopia EP500 tires.

According to Bridgestone, the tire manufacturer worked with BMW from the i program's inception, so the i3 was designed and built almost literally from the ground up with the tires on mind. Everything from the suspension geometry to the ride quality to the energy efficiency depends on the EP500's oddball proportions.

The i3's tires are almost ridiculously narrow. Measuring just 155mm wide, the rubber is significantly narrower than the Fiat 500e (185mm), the Nissan Leaf (205mm), and the Ford Focus Electric (225mm). The only other tire on the market that I could find near the EP500's dimensions is a temporary spare tire replacement. On paper, conventional gearhead knowledge points to the i3 having less grip and poor performance, but there's more to this tire than its width.

The i3's Bridgestone Ecopia EP500 tires are some of the narrowest we've ever seen. Antuan Goodwin/CNET

Let's take a look at EP500's entire designation: 155/70R19 for the stock, base setup. The 155 represents the width in millimeters, which we've already discussed, but things get interesting from there. The number 19 at the end indicates that the tire is designed for a 19-inch wheel -- larger than anything offered by the i3's competition -- and the 70 in the middle designates the tire's aspect ratio. I won't get into too much detail about how aspect ratio is measured, suffice it to say that the larger this number is the more sidewall the tire has to work with. An aspect ratio of 70 on a 19-inch, narrow tread works out to an almost obscenely tall wheel for a vehicle of this size. We're talking motorcycle tire narrow.

All tires are perfectly round when they're on the rack, but once you get them mounted under the weight of a car, the rubber deforms and flattens against the road ever so slightly, forming a grippy contact patch. The EP500's larger diameter and circumference mean that this tire has a slightly larger contact patch when measured front to rear. This makes up for the compromised width and means that when it's time to turn, there's still enough contact patch to provide adequate grip. On the road, the electric Bimmer's seat-of-the-pants performance felt as good as any of its wider-wheeled competition.

It's difficult to tell from curbside, but the EP500s have a long, narrow contact patch. Antuan Goodwin/CNET

What Bridgestone and BMW have done, essentially, is rotated the i3's contact patches by 90 degrees -- going long and narrow rather than broad -- but why?

For the i3, a tall and narrow tire carries a few advantages, according to Bridgestone's tire geeks, the narrow EP500 creates less aerodynamic drag as the electric hatchback rolls along and works with the Ecopia low rolling resistance compound to reduce efficiency losses to friction when cruising. Bridgestone also claims that the narrow width is more resistant to hydroplaning, which boosts the i3's wet driving performance.

Conventionally, tall aspect tires like the EP500 run into issues balancing sidewall stiffness, tire weight, and responsiveness. Being designed for an electric car where every watt of energy matters, Bridgestone needed to dig deep into its bag of tire design tricks to keep the EP500s particularly light and stiff.

Check out CNET's full review of the 2015 BMW i3 for more details. Antuan Goodwin/CNET

Tires wear, so replacing them is inevitable. I should note that designing the i3 and the Ecopia EP500s together means that though they're a perfect pair, such specialization means that the BMW i3 driver only has one choice in when the tires need swapping. For Ford Focus Electric, for example, there are dozens of brands, compounds, and seasonal/specialized choices. On the i3, it's Bridgestone or bust. I like having a wide range of options at a wide range of budgets, but drivers who are already willing to pay the i3's higher-than-average price may prefer the simpler choice. Considering how well the Ecopia EP500 performed during our on-road and closed course testing, it's not a terrible choice to be locked into.

Be sure to check out the rest of our full review of the 2014 BMW i3 to get the whole scoop, more pictures, and videos of this little electric.

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