Tall, Skinny Tires: Newest Green-Car Efficiency Trend

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BMW i Launch Event, Frankfurt, July 2011

BMW i Launch Event, Frankfurt, July 2011

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There are many unusual things about the BMW i3 Concept electric minicar revealed last month, including its glass doors and roof, not to mention BMW's first production battery-electric powertrain.

But the BMW i3 is also at the leading edge of another efficiency trend: tall, narrow tires that reduce the vehicle's wind resistance compared to the usual short, wide tires fitted to cars today.

Fitting low-rolling-resistance tires to a line's most efficient models is now de rigeur. Over the years, these harder, lower-friction tires have gone through several generations, so that's today's versions produce a more comfortable ride and less road noise than the earliest attempts.

One approach to raising fuel efficiency is to make the wheels and tires smaller, largely to cut their weight. Michelin has developed a new 10-inch tire for that very purpose, and you have to go back to the days of the original 1959 Morris Mini to find tires that small fitted as standard to a production car.

You can think of the i3's tall, narrow tires as a variation on that theme. They're heavier than a tiny tire, but the gains in reduced aerodynamic drag presumably outweigh the gains. That will be especially true at speeds above 40 mph, where the energy used to overcome air resistance starts to multiply.

Audi Urban Concept launch, 2011 Frankfurt Auto Show

Audi Urban Concept launch, 2011 Frankfurt Auto Show

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But making tires taller and narrower changes the shape of the contact patch with the road, which can reduce grip. In the case of the BMW i3, which is envisioned largely as an urban car, we doubt its owners will be flinging it through mountain curves or attempting to travel at 150 mph on the autobahn.

And the effect of a differently shaped contact patch is significantly lessened in a small, light minicar like the i3, compared to its effect in a full-size luxury sedan like BMW's 7-Series.

Other cars in which we've seen this trend lately include several urban electric concepts from the recent Frankfurt Motor Show--the Opel Rak E, the Volkswagen Nils, and the Audi Urban Concept--as well as the Renault Twizy electric two-seater.

2011 Audi A2 Concept

2011 Audi A2 Concept

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But perhaps more significant, we also see something of the same trend in the BMW i3's future arch-competitor, the Audi A2. Both cars are five-door hatchback battery electric vehicles with optional range-extending engines, and they will compete directly in many markets.

Once is interesting, twice can be a coincidence, but three or more times is clearly a trend.

What do you think about the styling and performance implications of carmakers fitting taller, skinnier tires to future vehicles?

Leave us your thoughts in the Comments below.

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