What's up with the tall wheels but skinny tires?

[tiburonh]

Bridgestone's explanation:

http://www.thegreencarwebsite.co.uk/blog/index.php/2014/01/10/bridgestones-ologic-tyre-delivers-better-efficiency-for-bmw-i3/

 

[Plug]

Some interesting info in this article too (especially the last paragraph on page 4 about the wet weather grip).

http://www.moderntiredealer.com/channel/retailing/article/story/2013/08/the-skinny-on-tall-thin-tires.aspx

 

[ecoangel]

Nothing new,

Bridgestone made ECO tyres for the Honda Insight Mk1, Toy-mota Prius and VW Lupo 3L.

My A2 1.2 runs magnesium wheels with even skinnier 145/80 R14 Bridgstone B381s.

They have finned tyre walls to reduce drag and are very quiet on the road when crusing. The winter tyres are Blizzaks. Sadly Continental stopped making 145/80R14s - they had better Snow and Mud ratings than the Blizzaks.

Part of the reason the steering feels so good on the i3 and A2 1.2 is the narrow tyres - same was true of 1970s Fiat 127s !

The other good news is skinny tyres tend to cost less.

It's only since the 1990s we have gone for fashion and ultimate cornering grip with ultra low profiles. 1980s Rally cars still used tall tyres eg: 205/70R15s and even todays F1 cars have relatively high profiles because that forms most of the suspension !

 

[Plug]

Nothing new

Sorry but it is. The width is only half the story, the main development here is the increase in diameter to reduce the rolling resistance. Your old examples are all 14" or 15" wheels but these new tyres are designed for much larger diameter wheels (19" and 20" for the i3 and 21" for the Audi) and these ratios haven't previously been seen on modern road cars.

From the first article regarding the diameter

A larger diameter tyre has several advantages over smaller tyres. The tread on a smaller diameter tyres can be inclined to excessive movement or ‘deformation’ during driving, while a larger diameter and higher belt tension significantly reduce tyre deformation and therefore conserve energy that is otherwise lost through internal friction, helping to reduce rolling resistance.

and from the second article

“The height is because the increased outer diameter of the tire improves rolling resistance significantly due to reduction of tire deformation,” explains Dr. Christian Strübel, head of Continental AG’s Expert Field Rolling Resistance PLT Tires Business Unit in Hannover, Germany.

“A 1% outer diameter increase results in approximately a 1% rolling resistance reduction. Therefore, the size strategy should be to get as tall as possible with respect to the overall tire/vehicle performance.”

 

[Yiiikes]

Thanks for this TiburonH, very informative. Yet another example of the lengths that BMW went to in making the i3 the most efficient EV on the planet - they rethought everything. Nice!

 

[tiburonh]

Some interesting info in this article too (especially the last paragraph on page 4 about the wet weather grip).

http://www.moderntiredealer.com/channel/retailing/article/story/2013/08/the-skinny-on-tall-thin-tires.aspx

Cool. Thanks for sharing. I love that the design of these tires isn't only about improving mileage, that it also seems to convey some additional safety benefits and that it also makes the tires quieter.

 

[ecoangel]

Tall side of the argument sounds rather suspect from an engineering standpoint:

http://www.caranddriver.com/features/effects-of-upsized-wheels-and-tires-tested

Weight and, at when anything over 40 mph, aerodynamics, are far more of an efficiency issue than tyre tread rolling resistance. Larger wheels weigh more and have greater gyroscopic effect when turning the steering wheel.

Indeed wheel bearing design is of greater importance and the fact that the i3 has a coast mode (although it's not easy to accomplish) has more benefit on efficiency than pure wheel diameter.

Regarding smoothing out road imperfections, perhaps bmw have a point but I think it's more about fashion.

More on i3 aero here:

http://www.mybmwi3.com/forum/viewtopic.php?f=11&t=534

 

[ecoangel]

Skinny tyres used in International rallying:

http://cdn.epicski.com/a/a9/a9b58845_wrc-subbiesnow.jpeg

Note contact patch of narrow tyres is very different to modern low profile rubber. Narrow tyres have narrow width but proportionally longer contact patch e.g: 135/80R16 where as something like a 245/50R16 has a very wide but relatively short contact patch which is much better for high g cornering. Contact pressure is also very different between the two styles and cross tyre temperature variations affect grip and drag more on the low profiles.
On my race car we measure 3 temps across each tyre after qualifying to set pressures for the race ahead.

The larger the diameter the wheel the longer the patch and more contact time per revolution - another reason to be suspect of tall is better eco theory!

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=10

However, Bridgestone claim their new concept 155/55R19 with much higher air pressure achieves a rolling resistance coefficient that is approximately 30% lower than conventional (and lower pressure) sized tires 175/65R15.

Larger wheels do offer more space for bigger braking systems and perhaps given that the i3 weighs 340kg more than the old A2 means bigger discs (rotors) to meet regulatory requirements.