The tire/vehicle system
Tires play a vital role in vehicle operational safety. However, they have not always been viewed as products ripe for safety enhancements.
In the public’s mind, tires have reached a status of infallibility, as proven every day by the one billion of them statistically operating safely in North America. But try to tell this to those that have suffered from the consequences of tire failures, from flats to loss of traction and structural integrity!
Still, one might think that there is no need for major tire innovations today, when in reality there is. Some of us who witnessed the original introduction of radial-ply tires by Michelin in France in 1948 and are still around have a different view of the product: its design, its methods of manufacture and its validation procedures for a particular vehicle application.
From June 2008 to March 2009, Modern Tire Dealer published a series of articles I wrote, which covered the following vital tire performance criteria:
• tire structural integrity,
• tire tread wear resistance,
• tire traction and
• tire power wastage.
The logical conclusion to my tire series focuses on the vehicle and its effects on tire performance and vice-versa.
Many years ago, while working at Ford Motor Co. (1955-1970), I observed that tires were not that well understood by my colleagues, nor by Ford’s management, to the level that they should have been, considering the critical role they play in vehicle operations.
At the time, tire engineering product acceptance specifications were limited and applied only to bias-ply tires, then still universally used in the United States. During an informal conversation I had with my chief engineer, I stressed the point that “in my view, nothing affects the operational economics and behavior of a vehicle more, one way or the other, than the type and quality of the tires Ford selects for its vehicles as original equipment, and that, among many other factors, insufficient tire manufacturing precision and uniformity can have detrimental effects on vehicle operation, from the operating smoothness and other standpoints.”
Therefore, I strongly recommended that Ford take tires very seriously, and start to develop technologically-based tire/wheel/valve system specifications and validation testing procedures.
This was conveyed to Ford’s top management, including Robert McNamara, one of the 10 whiz-kids, as they became known, hired by Henry Ford II in 1946 to map out Ford’s peacetime business strategy following World War II.
McNamara was to become the president of Ford, but left Ford soon after, in January 1961, to become the U.S. Secretary of Defense. McNamara, who passed away last June at age 93, was a much different and very inspiring leader, who left his mark on Ford. He, already back then, wanted sensible, user-friendly, affordable, smooth-running cars, featuring fuel economy, safety and low emissions, as well as long-life tires, something, I mentioned at the time, that could not be achieved with the bias-ply tire architecture.
McNamara also wanted simplicity, reliability and vehicles that would better protect their occupants in the event of a crash. This resulted in the 1956 Fords being equipped with seat belts and other safety features. The car retail price was $2,550, the monthly payments $56, and the sales slogan was “56 for 56.” However, the safety features, named “the lifeguard design,” did not sell, as the public failed to understand the safety message and, psychologically speaking, did not want to be told that cars could be life-threatening. The market failure of the lifeguard design was disappointing to McNamara. Years later, however, safety sold because it was mandated by the U.S. government through the creation of NHTSA (National Highway and Traffic Safety Administration).
Another McNamara project was the Ford Falcon, the sensible car mentioned earlier. The compact Falcon was introduced in 1959, and soon after reached the 600,000 units per year sales mark. At that production level, and in the tradition of Henry Ford, tremendous economies of scale were derived.
The Falcon was relatively light and featured unitized body construction. It was spacious, accommodating five passengers in comfort, and was adequately powered, durable and reliable, with good fuel economy and, on bias-ply tires, had a good level of overall operating smoothness. It was also affordable to anyone with a steady job, retailing for $1,912.
The Falcon was the first unitized-body Ford car that I radial-tuned in 1965, in collaboration with Pirelli in Milan, Italy. From the overall operating smoothness standpoint (NVH, or noise, vibration and harshness), the Falcon was unmatched, even by the Lincoln. Simultaneously, we also radial-tuned a 1965 Ford Galaxie, a body-on-frame vehicle. The entire Falcon/Galaxie radial-tuning operation was completed in two months and for me was, at that point, the most technologically rewarding project I'd undertaken.
Upon my return to Dearborn, Mich., I was asked to duplicate the Falcon/Galaxie operating smoothness, and with a small Ford team and using the Ford facilities, we did, based on a Ford Fairlane, a car fitting between the compact Falcon and the full-sized Galaxie. As with the Falcon, the radial-tuned Fairlane resulted in a “super NVH car,” as we called it.
These early tire/vehicle system tuning projects clearly demonstrated what can be achieved through dedication, ingenuity, perseverance and enthusiasm, and this at minimal cost, and within a short period of time. “Radial-tuning” was, indeed, “in.” Today, and particularly in view of our current socio-economic situation, I wish I could repeat such performance.
What prevented the radial-tuned Falcon, Galaxie and Fairlane from reaching production at that time? The three European radial tire producers (Michelin, Dunlop and Pirelli) were not ready to provide radial tires for American cars. The European radial tires were small as compared to American tires, and tire loads were higher in America as compared to France, England and Italy.
The Europeans also did not know that the American tire market conditions, which then were, and still are, tough. Furthermore, although Ford by the early 1960s had already developed comprehensive bias-ply tire product acceptance engineering specifications, including for uniformity, based on radial and lateral force variation acceptance criteria, we were still learning about the real world capabilities of radial tires operating under American conditions. We were particularly concerned about the tire structural integrity, a critical criterion, because of the much longer radial tire tread life potential (50,000 miles), as compared to bias-ply tires (25,000 miles).
As for the American tire producers, they, after an initial resistance to radialization, finally began to realize that they better get involved. However, the problems confronting them were: 1) the massive capital investment required; and 2) even with money available, how to use it efficiently, and how to acquire the technological know-how and implement the manufacturing process needed to produce consistently high-quality radial tires at minimum scrap levels.
These were tough concerns, indeed. At the time, we finally concluded that Michelin was up to the task. Michelin eventually provided Ford with large size radial tires, 225x15, in accordance with Ford tire product engineering acceptance specifications for standard original equipment for the 1970 Lincoln Continental MK3, a U.S. automotive industry first. The rest, as they say, is history.
Tire/vehicle system development
There are three basic vehicle sub-systems interacting with each other within a given vehicle’s overall architecture, all acting and/or reacting through the tires: 1) the vehicle propulsion, 2) the vehicle chassis, and 3) the vehicle body. There is a fourth critical input involved: the road.
The whole system is, of course, under the control of the vehicle driver. Therefore, the role of the tire/vehicle system engineer is to make sure that all vehicle sub-systems, including the tires, work in harmony. This is what in the early 1960s I called “tuning” (as in radial-tuning), not an easy job then, I can assure you, and not an easy job today.
Vehicle propulsion. The full power of a given vehicle propulsion system can only be used effectively if the tires are up to the task. This also applies to the power of the vehicle braking system, which is part of the vehicle chassis. These days, many, including politicians, are concerned with vehicle economy/fuel efficiency.
The development of a fuel-efficient car starts with an efficient propulsion system, which consists of a light, powerful, relatively small, low friction engine, coupled with an equally efficient, low friction transmission and axle. As for the tires, they also must exhibit low friction, particularly within their structures.
The tread must, of course, provide high grip with the variety of road surfaces encountered, in order to maintain vehicle control, even when the road surfaces become slippery when covered with rain, ice or snow, all real-world conditions, and therein lies the dilemma. Finally, in order to derive the full advantage of an efficient propulsion system, the overall vehicle mass must be low (3,000 to 3,300 lbs.) yet provide comfortable accommodations for six occupants. Such a vehicle must also be of dimensions and shape to derive a low aerodynamic drag, and be equipped with reasonably sized lightweight tire/wheel assemblies. The wheels must be light and strong, a difficult balance to achieve with cast aluminum. My preference is high strength steel, including stainless steel.
Vehicle chassis. The vehicle chassis consists of the suspension, steering and braking mechanisms and the four tire/wheel assemblies. Vehicle chassis come in a variety of architectures and must also be light, efficient and work in harmony with the tire properties.
The chassis is directly related to vehicle control, assuming that a sufficient amount of friction is available between the tire tread and the road surfaces. This relationship continues from the time the tires are new to the time they have reached their end of life, preferably after the tires, for economic reasons, have accumulated 80,000 miles of service.
Vehicle body. Two types of vehicle bodies are used. One is called “unitized,” whereby the vehicle chassis structure or frame is incorporated into the body.
The other is called “body-on-frame,” whereby the body is separated from the frame through rubber insulators called “body mounts.” This isolated frame covers the full lower perimeter of the vehicle body and uses up to 12 body mounts to isolate the body from the rest of the vehicle system.
The body-on-frame design is very effective, and this was clearly demonstrated when the light truck architecture was used for passenger cars called “SUVs,” resulting in good NVH performance.
The original promise of the unitized body design was that one could save weight. However, this has not always been achieved, because most, if not all unitized bodies use rubber isolated, non-connected sub-frames at the front and rear of the vehicle, thereby adding weight.
From my experience, unitized body passenger cars are more difficult to radial-tune than body-on-frame passenger cars, and in the end, little, if any, weight is saved.
However, and again according to my personal experience, it is possible to radial-tune a unitized-body car without the use of sub-frames, as I demonstrated years ago with the 1965 Falcon, whereby the body was de-sensitized from the negative NVH effects of the radial tire architecture.
As the critical part of the assemblies fastened to the vehicle hubs, tires — working in synchronism with the vehicle suspension systems — are supposed to absorb road surface irregularities. However, by themselves, tires are complex resonant systems, which often act as transmitters rather than attenuators of impact harshness.
The result is a particularly specific noise called “boom,” as well as other annoying radial tire specific frequencies, such as certain road noise and shake, that can be highly noticeable within the vehicle cabin (body).
Even more complex are the transfer functions of the radial tire’s road-induced vibration and noise inputs via the wheels and through the vehicle hubs, which propagates unabated through the vehicle body if the vehicle is not tuned properly.
Very early on, the radial-ply tire clearly indicated a high transmission of road-induced, detrimental NVH characteristics as compared to bias-ply tires. This required new chassis suspension and body design, as well as new development approaches.
Radial tuning is more than a matter of working on the suspension; it also requires, as mentioned earlier, body de-tuning, so as not to amplify the detrimental resonant effects of the radial-ply tire architecture. Therefore, impact harshness has to be treated differently than its resulting boom noise.
Shake also presented a new set of problems at certain vehicle resonant speeds. The wheel fastening to the vehicle hubs had to be modified in order to maximize precision and uniformity.
These problems, to variable extents, still exist today.
It is unfortunate for the new/young generation of engineers that there are no really good or practical treatises commercially available regarding the tuning of a particular vehicle architecture for a particular tire application. Today, many still think that radial tires are attenuators of NVH. In reality, this is not always the case.
One must realize that all tires are different, that all tire/vehicle combinations are different, and that some brands of replacement tires are significantly different from the original equipment tires. In view of the current high proliferation of tire sizes and types, and the equally high proliferation of vehicle types and architecture, the tire/vehicle system engineers certainly have their work cut out for them.
All in all, one must keep in mind that the use of a particular type and size of tire, including the wheel, should always ameliorate a vehicle’s NVH performance, not make them worse. The bling-oriented vehicle stylists should remember that.
The manufacture of precision tires was an important consideration, even in bias-ply tire days. This was recognized early on by the great tire mathematician, John F. Purdy, in the 1920s. In fact, it was in bias-ply tire days that precision tires and tire uniformity became part of the tire engineering product acceptance specification first implemented at Ford in the early 1960s.
However, it was with the advent of radialization that precision tire manufacturing became an even more critical tire performance criterion, for radial tires are much more sensitive to manufacturing variations, anomalies and non-uniformities than bias-ply tires were. Many tire producers found this out the hard way.
The precision radial tire manufacturing process requires tremendous capital investments, from compound mixing on, and including special tire manufacturing know-how. Insufficient tire manufacturing precision — particularly with stiff tires, stiff suspensions and other factors — measured as local changes in radial tire longitudinal, radial and lateral forces, can become quite evident within the vehicle depending on the vehicle’s level of sensitivity. The problems associated with insufficient tire manufacturing precision persist to this day, but could be dramatically improved.
The main function of the wheel, besides providing ease of vehicle motion, is to transfer loads and torques between the tire tread in contact with the road and the vehicle chassis. All wheels used today on passenger cars and light and medium/heavy duty trucks are of the drop-center type, invented and patented by C.K. Welch of London, England, in 1890.
An important aspect of the Welch invention was that the pneumatic tire section did not need to be a fully enclosed toroid (ring-shaped structure), hence saving materials, and could be manufactured with an open base, which, in turn, led to significant advances in tire manufacturing.
The Welch idea also resulted in easier tire mounting and dismounting, as compared to the clincher type tire/wheel assembly mainly used in the U.S. Today, the Welch tire/rim system is used throughout the world on most vehicles. Full torus, beadless tires, dating to the time of Robert Thomson, the original inventor of the pneumatic tire in 1845, are used today on racing bicycles.
To provide its functions, the wheel must be designed, developed and produced in such a way as to meet high standards of strength and precision, as well as impact and fatigue resistance, particularly in view of today’s use of limited vertical stroke tires, also called “ultra-low section height tires.” With the use of such tires, the frequency of wheel failures due to structural integrity degradation has created a new and lucrative business: wheel insurance, with premiums of up to $500.
A more effective solution to the basic problem would have been: 1) a return to more normal-size section height tires and more normal-size, stronger but lighter wheels; 2) the construction of better roads, and improvements in their maintenance.
Tire/wheel assembly fastening to vehicle hubs
Once tire/wheel assemblies are torqued in place on the vehicle hubs, they must not compromise the painstakingly specified and applied tire and wheel manufacturing precision and uniformity.
The tire/wheel assembly centering and fastening systems used on today’s vehicles, including hubs, remain a problem to be addressed, as the current method often results in creating several pounds of assembly radial force variations, due to the eccentricities generated by the wheel/hub combination. This must be remedied.
Tire mounting and dismounting
Through the process of mounting and dismounting tires on drop-center rims, torque is generated. Torque must be minimal, in order to prevent tires and wheels from being damaged during these procedures.
Years ago, the rim well profile and clearance dimensions required to mount and dismount tires without damaging either was determined by a cut-and-try method, an expensive and time-consuming operation.
The more scientific approach consisted of measuring tire mounting and dismounting torque by using strain gages applied to the tire mounting and dismounting machine arm. For a given range of tire bead zone and carcass stiffness and a range of rim well dimensions and profiles, the proper tire rim well clearance was determined. This original work was conducted in those days at Ford when passenger car wheel diameters never exceeded 15 inches, wheels were made out ofsteel, and tires were only of bias-ply construction.
When radialization emerged, new tire mounting and dismounting problems appeared, and in time, were handled in an efficient manner. With today’s larger bead diameter, wider and stiffer ultra-high performance radial-ply tires mounted on larger diameter wheels (20 inches and above) and wider rims (eight inches and above), other conditions developed. Under certain circumstances, tire and wheel damages can occur during mounting and dismounting.
In addition, today’s extensive use of softer aluminum alloy cast wheels can result in wheels being more easily nicked and scratched.
Finally, direct type TPMS (tire pressure monitoring system) sensors can, at times, interfere with tire mounting and dismounting, resulting in sensor damage (an expensive proposition), and loss of tire pressure.
Tire to wheel rim bead seating
A tubeless tire develops a seal on its rim when the tire/wheel assembly is pressurized. It is actually quite simple: The rim ledge has a 5-degree taper, and the tire bead zones directly adjacent to the rim ledges also have a 5-degree taper, but at a slightly less diameter. Therefore, when the tire is mounted/pressurized on its rim, the tire lower bead zones are compressed by about 1mm when in place, hence creating the seal.
To provide a reliable seal and to minimize the effects of variations that could create leaks, both tires and wheels must be produced at consistently high precision and uniformity levels. It is also important to assure a uniform tire bead to rim ledge pressure distribution, and that the wheel rim bead seat area is free of scratches.
Of equal importance is the quality of the air used to pressurize the tire/wheel assembly. The air must be dry and clean (free of contaminants and oil). This requires the use of high quality, well-maintained air compressors, air storage tanks and lines, careful tire and wheel handling, and, of most importance, well-trained tire technicians. Clean and dry air consists of about 80% nitrogen and 20% oxygen.
A well-designed, developed and produced new tire mounted on an equally well-designed, developed and produced wheel fitted with a high quality valve should not lose more than 4 to 5 psi of pressure per year. A good portion of this pressure loss is attributable to the absorption of the oxygen portion of the air by the tire structure.
Although a nearly 100% nitrogen-inflated tire also loses some of its nitrogen by being absorbed into the tire structure, it does so at a much slower rate than a tire inflated with regular air (nitrogen being an inert gas).
The development of the bromobutyl tire innerliner years ago has made a significant contribution to tire inflation pressure retention. A very recent innerliner development claims to further improve on the already high performing bromobutyl innerliner, and features a reduced thickness (hence reduced weight and cost), while still maintaining its higher level of pressure retention.
What happened to American ingenuity?
The future has never been riper for the introduction of practical, simple, affordable and exciting vehicle and tire innovations that would benefit the consumer in tangible ways, and this particularly in view of our current socio-economic circumstances. This future does not necessarily lie in conspicuous, frivolous consumption of products that do not offer intrinsic value, which includes vehicles fitted with extravagant tire/wheel assemblies and other superfluous accessories.
Even though one cannot schedule an invention, I refuse to believe that good old-fashioned American ingenuity has vanished. I prefer to believe that it is merely dormant.
This is the time to waken it. Many people are out of work, having lost their jobs or been forced to retire early, including all types of engineers with valuable experience. This is the time to tap their talent and creativity. ■
For more than 50 years, Jacques Bajer 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.
The French-born engineer also owns a number of patents, including one for a “casing preparation method” used in producing quality retreaded tires.
He is perhaps best known, however, 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.