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Tread
The tread is that portion of the tire that comes in contact with the road. The tread is a thick rubber compound formulated to provide a high level of traction that does not wear away too quickly. The tread pattern is characterized by the geometrical shape of the grooves, lugs, and voids. Grooves run circumferentially around the tire, and are needed to channel away water. Lugs are that portion of the tread design that contacts the road surface. Voids are spaces between lugs that allow the lugs to flex. Tread patterns feature non-symmetrical lug sizes circumferentially in order to minimize noise.
Treads are often designed to meet specific product marketing positions. High performance tires have small void ratios to provide more rubber in contact with the road for higher traction, but may be compounded with softer rubber that provides better traction, but wears quickly. Mud and snow tires are designed with higher void ratios to channel away rain and mud, while providing better gripping performance. When installing new tires, you should try and replace all four at once. Installing just two new tires on the front or rear will encourage under steer or over steer, depending on the vehicle and position of the new tires.
Snow tires are a very[citation needed] special kind of tire. They actually gain traction when it is cold, whereas, other tires lose traction on the ice and snowy roads.
Tread lug
Tread lugs provide the contact surface necessary to provide traction. As the tread lug enters the road contact area, or footprint, it is compressed. As it rotates through the footprint it is deformed circumferentially. As it exits the footprint, it recovers to its original shape. During the deformation and recovery cycle the tire exerts variable forces into the vehicle. These forces are described as Force Variation.
Tread void
Tread voids provide space for the lug to flex and deform as it enters and exits the footprint. Voids also provide channels for rainwater to flow out of the footprint. Voids also provide space for mud and snow to be channeled away from the footprint. The void ratio is the void area of the tire divided by the entire tread area. Low void areas have high contact area and therefore higher traction.
Rain groove
The rain groove is a design element of the tread pattern specifically arranged to channel water away from the footprint. Rain grooves are circumferential in most truck tires. Many high performance passenger tires feature rain grooves that are angled from the center toward the sides of the tire. Some tire manufacturers claim that their tread pattern is designed to actively pump water out from under the tire by the action of the tread flexing. This results in a smoother ride in different types of weather.
Sipe
Tread lugs often feature small narrow voids, or sipes, that improve the flexibility of the lug to deform as it traverses the footprint area. This reduces shear stress in the lug and reduces heat build up[citation needed]. Sipes also provide greater traction in icy conditions.
Wear bar
Wear bars are raised features located at the bottom of the tread grooves that indicate excessive tire wear. When the tread lugs are worn to the point that the wear bars connect across the lugs, the tires are fully worn and should be taken out of service.
Contact patch
The contact patch, or footprint, of the tire is the area of the tread in contact with the road. This is the area that transmits forces between the tire and the road via friction. The length-to-width ratio of the contact patch will affect steering and cornering behavior.
Bead
The bead is that part of the tire that contacts the wheel. The bead is reinforced with steel wire, and compounded from high strength, low flexibility rubber. The bead seats against the wheel tightly to ensure that the tire holds air without leakage. The bead fit is tight also to ensure the tire does not shift circumferentially as the tire rotates.
Sidewall
The sidewall is that part of the tire that bridges between the tread and bead. The sidewall is reinforced with rubber and fabric plies that provide for strength and flexibility. The sidewall transmits the torque applied by the drive axle to the tread in order to create traction. Sidewalls are molded with decorative ornamentation, government mandated warning labels, and other consumer information.
Over time, rubber degrades. Ford has recommended that tires be replaced when they are 6 years old to prevent sudden failure, even if the tire looks undamaged. In tropical climates, such as in Singapore, tires degrade sooner than in temperate climates.
Shoulder
The shoulder is that part of the tire at the edge of the tread as it makes transition to the sidewall
Inner tubeBicycle tires and some passenger and truck tires are designed for use with inner tubes. Inner tubes are torus shaped balloons made from a material impervious to air leakage. The inner tubes are inserted into the tire and inflated to give the tire its final shape.
Wheel
Tires are mounted to wheels, or rims, that bolt to the drive axle. Automotive wheels are either made from cast metal alloys, or stamped/welded metal. According to The Merriam's Webster Dictionary the rim is the outer part of the wheel, the tire does mount to the rim but a rim is not a wheel
Valve stem
The valve stem is a tubular rubber shape with a metal valve used to inflate the tire with air. Valve stems usually protrude through the wheel for easy access for inflation. Tires are inflated through a valve, typically a Schrader valve on automobiles and most bicycle tires, or a Presta valve on high performance bicycles. The rubber in valve stems eventually degrades. Replacement of the valve stem at regular intervals reduces the chance of failure.
Tire pressure monitoring system
Tire pressure monitoring systems are electronic systems that continuously monitor tire pressure on a vehicle, and alarm when the pressure goes below a warning limit. There are several types of designs to monitor tire pressure. Some actually measure the air pressure and some make indirect measurements, such as gauging when the relative size of the tire changes due to lower air pressure. These systems are becoming mandatory in countries such as the United States.
Load rating
Tires are specified by the manufacturer with a maximum load rating. Loads exceeding the rating can result in unsafe conditions that can lead to steering instability and even rupture. For a table of load ratings, see tire codes.
Inflation pressure
Tires are specified by the manufacturer with a recommended inflation pressure that permits safe operation within the specified load rating. Most tires are stamped with a maximum pressure rating. For passenger vehicles and light trucks, the tires should be inflated to what the vehicle manufacturer specifies, which is usually located on a decal just inside the driver's door.
Speed rating
The speed rating denotes the maximum speed at which a tire is designed to be driven for extended periods of time. The ratings range from 99 mph (160 km/h) to 186 mph (300 km/h). For a table of speed ratings, see tire code.
Rotation
Tires do often exhibit irregular wear patterns once installed on a vehicle and partially worn. Tire rotation is the procedure of moving tires to different car positions, such as front-to-rear, in order to even out the wear, thereby extending the life of the tire.
Wheel alignment
Once mounted on the vehicle, the tire may not be perfectly aligned to the direction of travel, and therefore exhibit irregular wear, often quite substantial. A wheel alignment is the procedure for checking and correcting this condition through adjustment of camber, caster and toe angles. These settings also affect the handling characteristics of the vehicle.
Retread
Tires that are fully worn can be re-manufactured to replace the worn tread. Retreading is the process of buffing away the worn tread and applying a new tread. Retreading is economical for truck tires because the cost of the replacement is small compared to the cost of the tire carcass. Retreading is less economical for passenger tires because the cost is high compared to the cost of a new tire.
Flat
A flat tire occurs when the tire deflates to the point that the metal of the wheel comes to ground level. This can occur as a result of a wear-and-tear, a leak, or more serious damage. A flat tire seriously impairs the ability of the vehicle to be driven, requiring the vehicle to be safely removed from the road, and the tire to be changed. Continuing to drive a vehicle with a flat tire may result in damage to the tire or the vehicle, and may put the occupants and other vehicles in danger.
Hydroplaning (or aquaplaning)
Hydroplaning, also known as aquaplaning, is the condition where a layer of water is present between the tire and road. Hydroplaning occurs when the tread pattern cannot channel away water at an adequate speed to ensure a complete dry footprint area. When hydroplaning occurs the tire loses traction and steering, and creates a very unsafe driving condition. When hydroplaning occurs, there is less responsiveness of the steering wheel. The correction of this unsafe condition is to gradually reduce speed.

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