Are green tires truly green?
Second in a series
Learn a trade. Become an expert. Make a difference in your field of expertise. That path has taken Jacques Bajer from France to the United States, specifically Grosse Pointe, Mich. And industry-wide respect has followed.
For more than 55 years, Bajer, a mechanical engineer by trade, has championed the cause of tire structural integrity in one way or another. His work with Ford Motor Co., beginning in 1955, led to the creation of a tire uniformity grading machine.
In 1964, he almost single-handedly developed low-profile tires for the 1964 Lincoln and Thunderbird. They performed so well that Ford made them standard equipment on all its vehicles the following year.
In our industry, he is best known for radializing the domestic automotive industry. Bajer didn’t invent the radial tire, but when he convinced Ford to offer radial tires as options on its vehicles, a new era began.
In the second of a three-part series on tire construction and how it affects vehicle performance, Bajer presents his thought-provoking views on the environmental, also known as green, tire movement.
Question 1: Are green tires truly green? Answer: Not as much as the tire manufacturers would like you to believe. In reality, there is little tire manufacturers and their suppliers can do at this point in time to significantly reduce tire rolling resistance without detrimentally affecting other equally or more important tire performance characteristics. This includes skid and tread abrasion resistance, traction and other aspects of tire performance that are vital to the operating smoothness, quietness and safety of a vehicle. And, of most importance, there’s the satisfaction of consumers to consider. They want long reliable tire life, all at an affordable price.
Vehicle fuel consumption has an immediate effect on vehicle CO2 emissions. Lowering CO2 emissions to 100 grams per kilometer (3.215 oz./.6214 miles) would be ideal today, but would require fundamental modifications of tire/vehicle system design, which the industry might not be able or willing to readily implement. From the environmental pollution standpoint, one aspect directly related to tires and seldom mentioned is the amount of tread rubber dust created by the wear of the tire treads. This rubber dust, which includes chemical molecules, is deposited on the roads, gets blown around by winds, is washed away by rains and ends up where? In the air we breathe, the soil we grow crops in and the groundwater we drink. (Please also refer to my article in the March 2009 issue of MTD, “Get a grip on winter tires.”)
The choice of tires
The choice of tires vehicle producers make for OE, and the choice of tires consumers make at worn tire replacement time, has a significant effect on practically all aspects of the vehicle operational characteristics, including economics.
Since the advent of the U.S. radialization at Ford with Michelin tires for the 1970 Lincoln Continental MK3 as standard original equipment, in which I played a key technical role, tire aspect ratios have been dramatically reduced from .80 to .50 and even lower. Back in 1970, I would have never dreamed that this would ever take place, for in my view it would have been contrary to improving the match between tire and vehicle, something tire and vehicle designers should always be striving for.
Also, since 1970, tire inflation pressures have increased from 24 psi in bias-ply tire days to 35 psi in today’s radial-ply tire days, an increase of nearly 50%. This, in combination with the use of ultra-low section height tires, mounted on larger diameter and wider wheels, for example 20-inch x 10-inch wheels, so-called “plus-sizing,” sacrifices tire performance and good tire economics for appearance, and violates the basic purpose of pneumatic tires: to swallow road surface irregularities or obstacles.
These plus-sized tire/wheel assemblies, inflated at 35 psi, are very stiff radially, laterally and tangentially, due to their high tri-dimensional spring rates. They are also heavier, requiring more power to accelerate, decelerate and maintain road load. This goes against the efforts to reduce tire rolling resistance and power wastage.
These tire/wheel assemblies are also significantly wider, and this increases windage (the resistance of the tire/wheel assembly going through the air).
All in all, the plus-sizing movement did not ameliorate the efficiency of tire/wheel assemblies; quite the opposite. The question is: when will the tire, wheel and vehicle manufacturers get back to basics, particularly in consideration of today’s difficult economic times, for consumers today, particularly mainstream consumers, deserve better.
Finally, increasing tire inflation pressures significantly results in tires operating at higher than desirable stresses, hence significantly reducing the level of vehicle operating smoothness, and increasing ride harshness, and not swallowing road surface irregularities effectively.
To conclude this vital aspect of tire dimensional proportions and tire/vehicle system dynamics, and assuming that the proper tire/wheel selections and tire inflation pressures have been made, the bulk of significantly improving a given tire/vehicle system’s fuel efficiency, hence its environmental friendliness, essentially rests in working on the vehicle itself, particularly its exterior to interior dimensional ratio, its aerodynamic drag, as well as reducing its weight. Such an undertaking requires extensive tire/vehicle system engineering and development effort, and a deviation from current thinking, including new manufacturing techniques and the use of high-level personnel talent, expertise, enthusiasm, dedication and a serious long-term financial commitment from the top management of tire, wheel and vehicle producers.
As for the tire (tire/wheel system) contribution to a reduction of vehicle rolling resistance or power wastage and fuel consumption, at best, and under real-world everyday tire/vehicle/driver system operating conditions, a net 1% to 1½% reduction in vehicle fuel consumption per year can be expected, if the tires operate on a vehicle of a mass not exceeding 2,900/3,000 pounds, a low aerodynamic drag body shape, and a propulsion system consisting of a small four-cylinder turbocharged 1.8-liter diesel engine, and an efficient transmission and axle, as well as a precision, well-aligned chassis. Such a vehicle would have to be well (but not frivolously) equipped with the basic essential necessities mainstream consumers are looking for today, and this at an affordable price.
Other aspects of green
Other aspects of green are related to the energies needed and the pollution generated in the process of producing tires, wheels and vehicles. Keep in mind that the larger and heavier the tires, wheels and vehicles, the more energies and materials they require for production, the more pollution they generate, and the higher their cost.
Don’t forget the cost, in reality the cost/price (the greenback). This aspect of “green” particularly applies to tires which require frequent replacement, such as every two years, at a cost of $1,000 a set and up, and after having provided only 30,000 miles of service. A truly green tire, in my view, would provide 90,000 miles of service, last six years and could be retreaded 80% of the time to provide another 90,000 miles of service, with the 20% casing mortality attributable to normal tire damage from the effect of road hazards. Such a 90,000-mile tire proposition, however, may not be welcomed by OE tire producers and their dealers.
Question 2: What goes into the manufacturing of low rolling resistance/low power wastage tires?
Answer: Assuming such tires are designed and developed to be light, precise and uniform throughout production, they should be made from rubber compounds consisting of high grade, low hysteresis polymers, such as natural rubber, except for the tread and the innerliner. The tread is made of synthetic rubber, such as SBR, reinforced by carbon blacks of specific types, and silica. More about the tread later. As for the innerliner, it is also made of a synthetic rubber, called butyl, because butyl has excellent air retention (it is not prone to air diffusion throughout the tire cross-sectional structure).
Low rolling resistance tires are also made from low hysteresis cords, such as rayon, Kevlar and steel, and from nylon cords for the tire cap ply or plies. Nylon cords, in contrast with the other cords previously mentioned, are made of high hysteresis materials. But at the end of it all, it is the tire architecture, such as radial or bias, which is the dominant factor as far as low tire rolling resistance is concerned. With the advent of the radial tire architecture, the rolling resistance of tires was reduced by 25% as compared with bias tires, quite a difference indeed, which translated into a reduction in vehicle fuel consumption, particularly with small and light vehicles. By small I mean a vehicle featuring an efficient outside dimension to inside dimension ratio, such as is exemplified by a few recently introduced vehicles, with a maximum weight of 3,000 pounds.
Selecting the proper tire size
A tire size should be selected conservatively, and with an adequate load reserve. As for the tire inflation pressures, they should be as low as possible, in order for the tires to operate at their lowest possible stress level, and within the physical capabilities of the materials used for the tire construction, and at their best balance of performance. Operating tires at higher than ideal inflation pressures, and with insufficient load reserve, is detrimental to these objectives and the green tire movement.
Question 3: Is the oil saved using low rolling resistance tires lost because such tires provide less tread life?
Answer: Yes, a large percentage of it is lost, particularly if the tires only last two years and provide only 25,000 to 30,000 miles of service instead of 60,000 miles over five years (ideally 90,000 miles). The functions of the tire tread, when well-matched to the tire architecture, is to provide economical tire life in terms of kilometers or miles, skid resistance and traction, as well as stability. A tire tread life of five years or 60,000 miles under everyday U.S. road/tire/vehicle/driver system operating conditions, and wearing uniformly, is highly desirable (ideally 90,000 miles).
Again, correctly matching treads to the dynamic operational characteristics of the tire architecture is essential, but not an easy task. The treads must be produced from high grade synthetic rubber, as well as the proper type of carbon blacks and silicas. This, however, presents a dilemma, for such treads, in spite of recent advances, still contribute about 55% of the total tire rolling resistance when they are new. This percentage gradually diminishes as the treads wear off.
As for the rest of the tire rolling resistance, it comes from the tire structure, and represents about 40% of the total tire rolling resistance and, at this point in the life of the tire, is essentially related to tire flexing when rolling under load. The last 5% of the tire rolling resistance is related to windage. Also contributing to the overall vehicle rolling resistance are the inertias of all the vehicle rotating masses, from tires to wheels, brake rotors and drums, etc., as well as brake lining to rotor drag, and the friction of all the bearings from the vehicle spindles, engine, transmission and axles.
Needless to say, the subject of tire rolling resistance or power wastage is quite complex, highly intertwined and not for the faint of heart. Any simplification of it would only mislead the consumers, which should be better informed about real-world tire performance capabilities, particularly at tire replacement time.
For a reality check on tire rolling resistance, please refer to my article in the August 2008 issue of MTD, “How tire power wastage affects vehicle fuel consumption.” Considering the high cost of replacement tires, some might say the “green” in “green tire” stands for “greenback,” the mighty dollar.
Here is my definition of a green tire: a green tire is a sensibly dimensioned, lighter, more fuel-efficient, lower rolling resistance, long-life, retreadable tire, produced from renewable materials as much as possible, and with a process that does not require an excessive amount of energies, and does not generate a lot of pollution. To achieve all this is difficult, and in fact can be impractical.
Didn’t Kermit, the (Muppet) frog, sing “It’s not easy being green”? Well, I agree whole-heartedly.
By the way, how green are your tires? ■
Jacques Bajer opened his own consulting company, Tire Systems Engineering Inc., in Grosse Point, Mich., in 1970. The firm specializes in the design of advanced manufacturing systems for the economical mass production of tires, power transmission belts, lathe-cut seals and air springs. He can be reached at (313) 886-6860.
To read the first article in this series, click here.