Most drivers do not know that the most important parts of any car are its tires. The tires have an enormous influence on the safety and performance of an automobile. While tires may appear to be simple devices, developing them is an extremely complex process. It is based on subtle design changes that can have significant effects on performance. It is essential that drivers understand automobile tires because they only work properly when they are driven under specific operating parameters. For instance, operating a tire at an improper inflation pressure can accelerate wear and cause dangerous damage to the tire. While engineers are developing entirely new tire design concepts for the future that solve some of these problems, it is important that drivers understand how their tires work today. With such knowledge, they can prolong the life of their tires and avoid dangerous driving situations.
Tires are the single most important safety and performance feature of any automobile. Since they are the medium through which the car and road cooperate, the performance of a car’s tires greatly affects the engineering that goes into the overall vehicle design. Tires are in many ways very simple because they contain a minimal number of components and are relatively easy to manufacture. However, they undergo extensive, complex development with subtle design changes that have major performance implications.
How We Got Here
The theory behind the modern automobile tire is almost as old as the automobile itself. In 1895, Frenchman Andre Michelin placed air-filled, or pneumatic, tires on an automobile for the first time. The theory was to use air to support the weight of the car and cushion the ride for the passengers. The air-filled tires had many benefits over the solid tires of the day, which transmitted all the imperfections of the roads directly to the passenger compartment .
The next revolution was made by the B. F. Goodrich Company, which used the first synthetic rubber in its tires. The use of synthetic rubber enabled engineers to make more improvements to their tires because they could change the compound, or “recipe”, used in the tire to create the desired result. This also reduced the dependency of the tire industry on natural resources. Engineers, however, have found that a mixture of natural and synthetic rubber is the ideal solution. As a result, most modern tires are composed of a mixture of the two types of rubber .
In terms of construction, modern tires are made almost exclusively with a radial construction. This has been the case for decades ever since Michelin applied the radial construction it was using for its racing tires to passenger tires.
Radial tires are built around two circular cables, which can be found on either side of the tire where the tire is mounted to the wheel. These steel cables are referred to as the bead bundles. Various materials are stretched between these two cables perpendicular to the tread. These materials, often polyester, form the tire casing, or tire body. They vastly improve the strength of the tire. Two steel belts are bonded to the outside of the tire casing around the circumference of the tire. These belts support the tire tread and give the overall structure of the tire more strength. Finally, the tread and sidewalls are built around these components .
Design and Development
The main challenge for tire engineers is to develop tires that will perform well for an extended period of time. Engineers are slowly beginning to use computer software in their design process. However, it is proving difficult to use software modeling systems on tire design. It is very challenging to analytically predict how the various materials in a tire are going to work together and behave during use. Furthermore, engineers constantly tweak the chemical formulas of the rubber compounds. It is difficult to account for these changes and accurately predict their performance. As a result, tire engineering is to an extent still a trial and error process. Large test rigs are used to evaluate the tires at many load and speed combinations .
How Do They Work?
Gripping the Road
Tires work by creating friction between the rubber of the treads and the road surface. Friction is a force that opposes motion, so the friction prevents the tires from sliding across the road surface, which they naturally want to do. When one hears engineers or automotive enthusiasts talk of “grip”, they are referring to this friction between the tire and the road. Grip is what makes an automobile a useful machine. The grip between the tire and the road enables the driver optimal control over the vehicle.
There are two types of grip. The first type translates a car engine’s propulsive force into forward motion. This grip is in the same direction of the motion of the car. It makes the tire rotate and prevents the tire from slipping or spinning over the road surface. The second type of grip is in the lateral or side direction. Lateral grip acts perpendicular to the motion of the vehicle. It keeps the car moving in the same direction that the wheels are pointed, meaning that the car will not spin or skid while changing direction. Engineers want grip to be as high as possible.
Another type of friction created by a tire is rolling friction, or rolling resistance. It is an internal characteristic of the tire that tends to oppose rotation. Tire engineers want to minimize rolling resistance for this reason. Forward acceleration, braking, and fuel economy can be improved by decreasing rolling resistance. However, while the goals of tire engineering are relatively simple—maximize grip, minimize rolling resistance—the design process used to achieve them is very complex. While modern tires do not contain many components, they do undergo surprisingly complex development that seeks subtle design differences that often have major performance impacts.
The Contact Patch
Tires are intended to slightly deform and not remain perfectly round. This deformation creates a flat spot called the contact patch at the road surface. If tires did not deform, an almost infinitesimally small amount of rubber would actually touch the road, and little to no grip would be created. It would be analogous to trying to drive a train on a highway. The contact patch is the area of the tire that is actually in contact with the road at any given time. Since the tire is rotating, the actual section of the tire that is creating the contact patch is constantly changing. However, since the same contact patch is always created, we will refer to it as if it were a part of the tire. The contact patch is so important because it is where the precious grip is created.
However, tires are not designed to operate with just any contact patch. Tires are designed to operate effectively and safely with a very specific contact patch that is determined by the inflation pressure and suspension settings. If these settings are changed from the ones under which the tire is designed to operate, the size and shape of the contact patch changes, and the tire does not operate properly, and wears abnormally.
The importance of the air pressure in your tires cannot be stressed enough. A tire’s pressure has enormous safety and performance implications. All automobile manufacturers publish tire inflation values for every one of their vehicles. These values can usually be found on a manufacturer-installed sticker in the door sill next to the driver’s seat or in the glove compartment. Ideally, tires will be kept at this pressure; however it is widely accepted that under-inflation is much more dangerous than over-inflation. Under-inflated tires build up more heat and wear faster. This is because they deform more and thus create a larger contact patch. This means that more energy goes into the tire compound as it is deformed. This additional energy is converted into heat in the tire components. Furthermore, more friction is created at the contact patch, which also creates more heat.
While one might think that a larger contact patch is better, it actually is not the case. While it does create more grip, it also increases rolling resistance. Furthermore, the added heat and altered contact patch accelerate tire wear. Under-inflated tires wear faster at the tire shoulders—the curved transition between sidewall and tread. This type of tire wear can lead to catastrophic tire failures such as tread separation, where the tread detaches from the tire body, or blowouts.
In the year 2000, tire manufacturer Firestone recalled 14.4 million of its Wilderness AT, Radial ATX, and Radial ATX II tires. These tires had experienced an extremely high failure rate, particularly when mounted to Ford Explorer SUVs. The recall was prompted by numerous complaints concerning tread separation, where the tread and outer steel belt had separated from the rest of the tire. The complaints prompted an NHTSA investigation, which led to Firestone’s voluntary recall of its tires. The failure of the Firestone tires resulted in many fatalities worldwide .
Berkeley professor Dr. Sanjay Govindjee studied the causes of the Firestone tire failures on Ford Explorers. He concluded that cracks which formed around the steel belts caused these layers to separate from the tire body. He believes that the cracks formed not because of the distance traveled by the tires, but by the number of fatigue cycles, or heat cycles, the tires had gone though. A heat cycle is the complete process of starting the car with the tires cold (not at operating temperature), warming the tires to operating temperature, and then allowing the tires to cool once the trip is complete. The heat fatigue was exacerbated when the tires were under inflated (Govindjee).
On the other hand, additional safety implications for operating over-inflated tires have not been observed. Over inflation results in the center of the tread wearing faster than the shoulders. However, wear due to added fatigue and additional heat build-up is not apparently a concern. The complete effects of inflation pressure on tire performance and wear are not fully understood, and it is therefore essential to operate your tires only at the manufacturer-suggested pressures.
Caring for Tires
Since tires are designed to operate at very specific conditions, it is the driver’s responsibility to ensure that the tires are at the proper inflation pressure and to monitor tire wear. Operating your vehicle at the improper pressure causes uneven wear and reduces the performance of your tires. In passenger cars, grip is often reduced, meaning that the car will lose control at slower speeds if the driver needs to swerve to avoid a collision. Braking distances are also increased from their minimum if the tires are under inflated.
It is essential that one check a vehicle’s tire pressure often: once a month is a good idea. Air slowly leaks from the tires through the valve and through a natural, and very slow, process of diffusion through the rubber. Pressure is also sensitive to temperature change. Therefore, tire pressure should be measured and adjusted when the tires are cold. Cold does not refer to the ambient temperature but to the temperature due to operation. A cold tire is one that has not been driven within three hours . However, pressure will also change due to seasonal changes in air temperature. Pressure drops with temperature, so a properly-inflated tire during the summer months will most likely be under inflated during the winter.
When measuring and adjusting tire pressure, one must use an accurate tire gauge. People often fill their tires until the bulge in the sidewalls that forms above the contact patch disappears. This is a dangerous practice as it results in over-inflation. The bulge is normal in radial tires.
More complex adjustments may need to be made in order to correct uneven tire wear. These adjustments need to be made to the suspension settings and require precise equipment and training to perform.
You Say You Want a Revolution
The sensitivity of modern tires to inflation pressure has prompted tire engineers to look at safer ways to design tires. The Michelin Company is currently developing revolutionary new constructions that reduce the dependency on drivers to maintain proper operating parameters. They recently unveiled airless concepts that take advantage of new material and manufacturing techniques to eliminate the need for air to support the weight of a vehicle. Michelin displayed its Tweel concept at the 2005 North American International Auto Show in Detroit. The Tweel uses rubber components within the tire. They allow the tire to deform and behave much like a pneumatic tire, yet the car is ultimately supported with solid structures. Michelin does not expect an airless tire to be available to consumers for at least another decade.
The engineering behind automobile tires is full of nuance. Although the tires themselves are relatively simple, the design of the individual components and the performance of the overall package is a result of exhaustive research and development. However, the success of the engineering that goes into the tires is dependent upon the responsibility of the car owners. The industry is currently working to reduce the variable effectiveness of its tires that results from improper use.
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