Why the Skinny Tires?

One of the most-asked questions I get about my i-MiEV is what’s with the skinny tires on this car?

With EVs, we focus mostly on efficiency, since we don’t have capacity to burn. My wife’s new car, a Jeep Compass, has large, 235 section, 16″ tires. Those were the second-largest available on the car. Each one weighs around 40 pounds, mounted on the wheel.

2014-Jeep-Compass-Latitude

Jeep Compass with 16″ wheels

Maybe we should be asking this another way: Why do gas-powered cars have such large tires? The answer? Because they look cool. There is no reason a 3,097 pound vehicle should need 235-55R16 tires. Tires that wide were the size of the rear tires on the ’82 Corvette.

The Vette needed wider tires to get the power to the road. Why does a Jeep need ’em? Because they look cool.

What are the penalties of having these heavier tires? The first is “unsprung weight”. This means a part of the car that doesn’t have a suspension component between it and the road. Since the tires touch the road, they are considered unsprung weight.

Colin Chapman, one of the founders of Lotus Engineering, Ltd., was meticulous about unnecessary weight, specifically unsprung weight. (Here is a great reference.) He knew that not only does a heavy car burn more gas, it’s slower. Lotus vehicles are well known for Colin’s penchant for “adding lightness”.

Gas-powered cars can get away with these wastes of energy because they have energy to burn — literally. EVs don’t have this luxury. Hence, the skinny tires. Tires used on EVs are lighter (for less unsprung weight), skinnier (for less aerodynamic drag), and have lower rolling resistance (for better range, but lower traction).

The BMW i3 has an interesting work-around: BMW uses skinny tires, but large diameter. This allows for a 16″ tire-sized contact patch (the part of the tire that actually touches the road at any one time) in a 19″ tire, but with the lower aerodynamic losses. I just don’t want to see how much they cost to replace.  🙂

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