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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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Comments (17)
  1. I'd prefer a tire design that pays attention to performance and safety, not rolling resistance.
     
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  2. Safety first, especially for areas that get lots of wild weather.
    Lots of other ways to be efficient, including SLOWING DOWN
     
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  3. My new Accent has 195/50/R16 I was looking at tires and its quite a rare combo. These are pretty big tires for a subcompact but they are also quite narrow. Tirerack only had a few options in this size.
     
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  4. I noticed that NHTSA is tasked with writing new regulations for specifying the rolling resistance of tires as well as performance metrics (so that people don't sacrifice too much safety for efficiency sake.) They also seem deeply concerned with replacement tires (non-OEM) being significantly less fuel efficient than OEM tires. This comes from OEM tires being a factor in CAFE and replacement tires are not. That should change soon.
     
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  5. Urban or not the i3's said to have 170 hp, and no one will be able to resist wailing around corners and putting it through its paces in the back-country. I'm sure BMW engineers will spend many many hours testing the handling capabilities of their tire choice in all forms of weather. After all, they've been at this for quite a while and have set a pretty high bar for the quality of cars they produce.
     
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  6. I was under the impression that larger diameter tyres had better rolling resistance than a normal tyres. I also thought that the larger the diameter the greater the contact patch, so that by having a tall but thin tyre you could actually have the same size contact patch as a standard tyre.

    Am I right in thinking this? Anyone know?
     
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  7. hi dilbert,

    if you look at the geometry, the larger the radius of a circle, the slower the change in angle.

    so i think you are theoretically correct. but i dont know what the percentage is.

    but i suspect that the percentage is not that high, such that you could only afford a small reduction in width of the tire, before you became worse off with the amount of tire touching the road.
     
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  8. Your impression is correct. Larger narrower tires have lower rolling resistance. They have nothing to do with aerodynamics and the gains are at all speeds, not just at high speeds. The writer was a little confused on this one.

    Go look at the wheels on a horse drawn carriage from 100 years ago. This is not precisely new information.
     
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  9. I agree with the safety factor. I'd rather have wider, safer tires
     
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  10. Wide tires to not necessarily equate to safer tires. Contact patch area is the criteria and this is a function of tire diameter, width, and air pressure. If you go to higher pressure and narrower tires, then yes you are right, but larger diameter brings the contact area back up.
     
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  11. I can see obvious advantages in rolling resistance. As tire pressures get higher, the larger diameter is required to keep the same contact patch length. Also a narrow tire should be better in adverse road conditions such as rain where hydro-planeing is a serious issue. But wind resistance? Wouldn't a tall tire have higher wind resistance? I would say that the penalty of higher wind resistance is an acceptable trade-off for increased contact area and therefore grip and safety especially in the wet.
     
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  12. I am very surprised to see the water channels still being circumferential on narrow tires. When the tire patch is longer than it is wide, then it makes more sense to channel the water out to the side. However this also makes the tire noisier. Now we have a safety vs. noise issue, which is more important to you?
     
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  13. The first car that followed through on the idea that a large wheel provides a good-size contact patch with low wind resistance was probably the Mindset.

    Some information on this topic from two years ago, when BMW first approached it, here:
    http://evworld.com/news.cfm?newsid=21830
     
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  14. Obviously the tire patches can be enhanced merely by automating the tire pressure for circumstances encountered. Sensors/gps/maps could detect curves, obstacles, traffic, and adjust tire pressure for expanding the contact patch for breaking and or hard cornering. A simple brake release of air pressure all around (as appropriate) and re-compression for cruising speed has been available since Ducks crossed beaches in Sicily back in 1943.
     
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  15. And their is always resorting to road anchors, too.
     
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  16. Ah! Funny you mention that. I have recently had that idea. I'm sure also that many other people have had the idea before us too. I assume the reason it doesn't exist on the car is cost and for reasons of safety. If somehow the system breaks and you need to stop in a hurry then your in real trouble.

    I'm sure your right that you could design it in such a way that the act of braking or heavy cornering caused the tyre to uninflate. I believe they are doing this kind of thing on the active front grilles. If they fail they fail open.

    Another idea I have had, which I learned the other day they are actually doing to some degree on the Tesla model S is active suspension. When cruising the car lowers itself for better aero.
     
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  17. I have this other whacky idea which I'm sure will never exist on a car, and probably for good reason. The idea is to have a car with a back end that telescopically extends to create a boat tail shape when the car is cruising. This could be achieved by having the back of the car made of a stretchy material or by having panels that slide to change the cars shape that work a bit like how modern convertible car roofs work.
     
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