The skinny on tall, thin tires
They were on the Ford Model T a century ago. They were on the Citroën 2CV in the 1950s. Today they’re on hybrid and electric concept cars. They’re also used in racing.
Are tall, thin tires going to be the next big trend in the tire industry?
That’s what we asked the five largest tire manufacturers in the world: Bridgestone Corp., Continental AG, Goodyear Tire & Rubber Co., Groupe Michelin and Pirelli & Cie SpA.
Some were forthcoming with details and information; others, not so much.
“We are constantly working on many technical projects, but we don’t always want to talk publicly about them,” a Goodyear spokesperson told Modern Tire Dealer.
Two other tire companies also declined to participate in this story at first, then later agreed to. Two of the tire makers would not comment on the details of manufacturing tall, thin tires. Two would not discuss the tall, thin tires their companies offer.
There appears to be a great deal of secrecy surrounding the design of tall, thin tires. One thing is certain: There is a trend for better fuel economy in vehicles, and tires play a significant role in the equation. Less rubber on the road means better rolling resistance, less noise and less wasted energy.
Several OEMs have designed concept cars that feature tall, thin tires. The BMW i3 Concept Coupe, unveiled at the 2012 LA Auto Show in November, has tires size 155/60R20 up front and 175/55R20 in the rear. The Audi Urban Concept car has 125/60R21 tires in front and 145/50R21 in the rear. The Volkswagen XL1, a production vehicle that is available in Europe, has Michelin Energy tires size 115/80R15 on the front and size 145/55R16 on the rear.
As OEMs develop more electric vehicles and hybrids, tire makers continue to develop taller, thinner tires to go on them.
Continental AG says that the Conti.eContact, introduced in 2012, is pioneering tire dimensions for the growing electric vehicle market. The company estimates that 2.8 million electric vehicles will be registered worldwide by 2020.
The Conti.eContact’s dimensions include a larger outer diameter, and the size is 195/55R20 instead of a traditional 205/55R16, resulting in reduced rolling resistance. A more flexible sidewall helps conserve energy loss when deflecting or rebounding.
“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. The narrow size improves the aerodynamics of the tire.”
Strübel says Continental’s tall, narrow size concept focuses on reducing the section width by 10 mm to improve the aerodynamics of the tire while keeping the aspect ratio constant to conserve handling. Next, the rim size is increased by four inches. The result is that the new size for a 205/55R16 is 195/55R20.
“This was tested in the eContact project with great success. Other sizes are, of course, also possible and in development for some OEMs,” says Strübel. “Our concept is to keep the tire pressure constant due to the same load index of the new tire size.”
“Very simplistically, the skinnier the tire, the less material that’s going to be in that tire to generate heat and rolling resistance,” says Steve Carpino, vice president, R&D, North America for Pirelli Tire North America.
What is a tall, thin tire?
“A tall and narrow tire is very different in terms of global shape from current tires,” says Jim Knowles, senior product category manager, Michelin North America Inc. “In general, they have a reduced tire width and an increased overall rim diameter compared to current tires.”
“It’s a higher pressure, skinnier tire,” says Pirelli’s Carpino. “Even if it has the same diameter or is taller, there are just certain match-related effects that will generate less heat and rolling resistance.”
Carpino says that one of the challenges in designing tall, thin tires is maintaining load-carrying capacity.
“Say you have a Ford Fusion that has a 225/55R17 tire today. If you try to replace that tire with a tall, skinny tire, it still has to be able to carry the same load as that conventional tire. You can get that load-carrying capacity by keeping the same air volume. Air is what carries the load in a pneumatic tire. You need to have a certain volume of air to inflate, to carry that same load. You can get that by going to a taller sidewall and a narrower tire.”
Carpino says that one of the biggest benefits of tall and thin is better fuel economy if they are designed to offer comparatively lower rolling resistance. Depending on the tread pattern and compound, a skinny tire also will perform better in aquaplaning conditions.
Unlike in the days of the Ford Model T, tall, thin tires today are generally designed to replace a specific shorter, wider tire.
“The size range for tall, thin tires depends on what your starting point is,” says Carpino.
“If 255/40R19 is where you’re starting, then you go up to something like 235/55R17. It’s going skinnier, but it’s not going any taller.”
As far as speed ratings go, Carpino says the kinds of vehicles that would be riding on these tires generally are not the unlimited speed range vehicles.
“I would expect they would still have to be in the H and V category at the higher end, and probably you could get away with S and T on some of the lower-level vehicles. On most vehicles today, if you’re talking car tires, H and V are not uncommon at all. If you’re doing this on conventional vehicles, you’d have to maintain that kind of performance.”
The tall and thin driving force
“Everyone’s looking for (energy) efficiency right now, and taller and skinnier is a way to be more efficient,” says Rafael Navarro, vice president of communications, media relations and motorsports, Pirelli Tire North America.
“You can infer that that is what spiked the curiosity. I don’t know that it’s a trend. I don’t know if it’s anything that’s going to take the tire industry by storm.”
While it still remains uncertain whether or not tall and thin tires will become an industry trend, it is clear that vehicles are getting lighter and more fuel efficient.
“I think the trend toward lighter vehicle weight is growing,” says Hank Hara, chief technology officer, Bridgestone Americas Inc. “The vehicles equipped with large and narrow tires are the next generation in this process, following the lighter-weight vehicles.”
Hara says he sees many more lighter-weight vehicles on U.S. roads today than several years ago. Those vehicles include models such as Versa, Soul, Sonic, Accent, Fit, Fiesta, Mazda2, Yaris, Rio, Juke, Rouge, CR-V, CX-5, RAV4, Forester and Escape.
“There are two key drivers behind the trend in tall and narrow tires — style and efficiency,” says Michelin’s Knowles. “In terms of style, automakers are looking for new ideas to help differentiate their vehicles. Larger rims and outside tire diameters have an appealing look while allowing manufacturers to revisit how they distribute the space inside the vehicle.”
Fuel efficiency and function are even more important than a vehicle’s appearance. Automakers want to reduce the fuel consumption of their vehicles and they also want to reduce the external noise of their vehicles on the road, especially in Europe where the European Union’s regulations on exterior noise are in effect.
“Energy efficiency is enhanced thanks to the tire’s lower rolling resistance and better aerodynamics,” says Knowles.
“Continental has been promoting the tall and narrow concept for some time now,” explains Strübel. “This was part of the new tire line project, the Conti.eContact. The tall and narrow concept was also shown in the company eCar and it was presented to several OEMs in the last several years. In the meantime, we see a movement to this new tire size segment in the market.”
Where is the tall, thin market heading?
“The market will start first with niche vehicles that will help consumers begin to accept these new types of car and tire designs,” says Michelin’s Knowles. “That is why we will likely see them on hybrid or electric vehicles first.”
He says we are then going to see tall and narrow tires start to appear on European city cars and then European city utility vehicles. In this category of European city utility vehicles, tall and narrow tires are interesting because of increased vehicle ground clearance.
“Tall and narrow tires are gaining interest among several OEMs as they explore technologies to improve safety and to meet the new 2016 and beyond CAFE and CO2 emission regulations.”
“If I had to guess — and this is based on what we hear from our OE customers — I don’t see it in the next five or six years for sure,” says Pirelli’s Carpino. “It’s both technical and cosmetic at this point, more for the appearance of the vehicle.”
Carpino says Pirelli has heard from at least one of its OEM customers that for style, a popular look is to see the wheel well fully filled by its tire. That, at least today, requires a tire that is similar dimensionally to tires that are currently common.
“For a while, it will just be more in the concept and electric and hybrid vehicles.”
Even if they did become more popular, manufacturing them would not require a major change in the existing infrastructure. Tire engineers told us that in addition to new molds, manufacturing plants would only require some equipment adjustments to produce the tires en masse.
What are the drawbacks?
There are disadvantages to using taller, thinner tires on cars.
“The main disadvantage for the tall and narrow concept is the building space in the car, where often the wheel house must be changed,” says Strübel. “That is more likely to happen in cases of the new architecture of electric and hybrid vehicles, where the car planning is starting from the beginning.”
There are no issues with traction when they are used on electric cars and hybrids, says Bridgestone’s Hara. “High-performance cars like today’s Corvette are not in the scope of applications.”
Hara says he does not anticipate any wear issues with tall, thin tires, either.
“One of the challenges, but not necessarily disadvantages, of tall and narrow tires is that the overall diameter of the tire is larger,” says Michelin’s Knowles.
“Vehicle weight and packaging are key factors that determine the tire dimension needed on a vehicle.”
Knowles says that in order to maintain the tire load capacity required for current vehicles, the overall diameter of a narrow tire must be slightly larger, thus the term “tall and narrow.”
“To take full advantage of the performance benefit of tall and narrow tires, vehicle manufacturers must adapt their designs to accommodate the larger diameter tires,” he explains.
Continental’s Strübel says the outer diameter of the tire is an important consideration in the beginning of the concept phase of a new car and must, therefore, be discussed early in the vehicle’s development. A longer lead time is necessary for this new size concept. “For electric vehicles, the rolling resistance of a tire is even more important than for combustion engines due to the limitation of the battery capacity,” says Strübel.
He adds, however, that a European OEM “is developing a vehicle with a combustion engine with such a tire size for market entrance soon.”
“We anticipate that these tires will be equipped on a wide-variety of environmentally friendly vehicles,” says Bridgestone’s Hara. “These tires are ideal for EVs (electric vehicles) and HEVs (hybrid electric vehicles), as the lower rolling resistance is a significant benefit. However, we do not feel that their use should be limited to these EVs or HEVs.”
“That’s going to be more for the OE to drive the market,” says Pirelli’s Navarro. “If a premium OEM partner comes to us and says we’d like your support in doing this, we believe in it and as a company that prides itself on innovation and research, we’re going to go through with it.
“Until you get the OE to agree to bolt that up as part of their strategy for their cars, I don’t think it’s anything we can impose on them.” ■
Europe, noise reduction and tall, thin tires
Part of the movement toward tall, thin tires is being driven by the European Union, where regulations on exterior noise are in effect. The EU says that a tire’s exterior noise grading is expressed in decibels, with one, two or three sound waves for the label.
One black wave indicates the best noise level performance. It means that the noise output level of the tire is at least 3dB below the future legal limit.
Two black waves indicate sound levels that are in the moderate range.
Three black waves indicate the weakest performance in terms of tire noise output. It represents a noise output level between the current maximum and the new lower limit that will be introduced in Regulation 661, coming into force by 2016.
Two very big advantages: Bridgestone’s tall, thin technology saves fuel and lives
1. Environmental performance: By giving tires a larger vertical diameter and increasing their internal air pressure, it is possible to limit changes in the shape of the contact patch.
By optimizing the structure of tires and using the most appropriate materials, Bridgestone achieved a rolling resistance coefficient 30% lower than conventional tire sizes such as 175/65R15.
Making the tread area narrower reduces air resistance, a major factor that needs to be considered when working to improve fuel efficiency. Due to these features, tires that have this new technology have levels of fuel efficiency exceeding those of the fuel-efficient tires Bridgestone has already put on the market.
2. Safety: The narrower size of tall, thin tires helps reduce wet road water pressure placed on the tires. In addition, the tires have been specially designed to have a longer contact patch and higher contact pressure, which helps these tires prevent water from accumulating between the tire and the road. These features combined with the benefits of the tires’ newly developed, specialized patterns and compounds enable the tires to achieve an improvement in wet grip performance of 8% in comparison to conventional tires, thereby enhancing safety.