Tires and Wheels

Tire and wheel assemblies on your car provide the only connection between the road and the vehicle. Modern tires require increased attention to achieve their full potential of extended service and correct ride control. You can expect to replace at least one set of tires on your car. Tire life depends on tire quality, air pressure, vehicle weight, driving conditions, suspension condition and wheel alignment.

 

TIRES

• The primary purpose of tires is to provide traction. Tires also help the suspension absorb road shocks, but this is an extra benefit. They must perform under a variety of conditions. The road might be wet or dry: paved with asphalt, concrete, or gravel: or there might be no road at all. The car might be traveling slowly on a straight road, or moving quickly through curves and over hills. All of these conditions call for special requirements that must be present, at least to some degree, in all tires.
  
  In addition to providing good traction, tires are also designed to carry the weight of the vehicle, to withstand side thrust over varying speeds and conditions, and to transfer braking and driving torque to the road.
  
  
  
  
   

TIRES CONSTRUCTION  

• Tires are constructed of several layers of rubber materials, cords, and two rings of wire, called beads. The bead is the portion of the tire that helps deep it in contact with the rim or the wheel. It also provides the air seal on tubeless tires. 
  
  

• The casing or carcass is the external structure of the tire. A ply is metal or fabric cord that is rubberized (covered with a layer of rubber). The plies provide strength to the tire to support the load of the vehicle.
  The ends of the plies wrap around the steel bead before being bonded to the side of the tire. The beads are coils of with at the side edges of the tire. These give the tire the strength to stay firmly attached to the wheel. Chafing strips are hard strips of rubber that protect the beads from damage that could result from chafing against the rim. 

• TRACTION
  
  A Tire’s traction is defined as how well it grips the road. Traction is affected by the road surface and contaminants such as water, ice, or debris. It is also affected by the tire’s tread material inflation pressure, width of the tread, cord ply design, wheel alignment, and other things. 

• TIRE TREAD
  
  

  The tread is a band made of a rubber compound designed to have various traction and wear characteristics. A federal grading standard that is cast into the sidewall of the tire describes a tire’s traction and wear characteristics. Grooves in the tread allow traction on we surfaces, giving the water a place to go. They also allow the tire to flex without squirming, which would cause wear.
  
  Treads are designed for specific types of weather and conditions. The design selected is always a compromise. The best traction on a dry pave road would be with a racing slick, or a bald tire. The same tire would be dangerous in the rain. Water form a wedge under a tire that can actually float the car. This is called hydroplaning. A deep tread pattern will break through a water film and grip the road at low speeds, but at high speeds high speeds the tire can hydroplane.
  
  Tires with large grooves are designed for use in mud and snow. But the large tread pattern can result in noise on the highway. Treads are often spaced at random intervals to minimize noise.
  
  Sipes are small grooves in the tire tread that look like knife cuts. They allow extra griping as the tire flexes. Sipes also clear water off the road, wiping the contact area to provide a better grip. Ribs in the tire tread are design to pump water from the road through the groove to the back of the tire, where it is thrown out onto the road.
  
  
  Most of today’s tires are radial-ply tires, although some trucks and RVs still use bias- ply tires
  
  Radial-Ply tire: Radial tires have casing plies that run across the tire from beat seat to bead seat in the ”radial” direction of the wheel. The outside circumference of the tire is held together by reinforcing belt rings of slightly angled cord material.
  
  Bias-Ply tire: Bias ply tires have casing plies that cross each other at angles of
  35-45o. They ride softer that radials, but their tread tends to squirm when rolling. This results in tire wear.
  
  A tire acts like a part of the suspension system as it supports the load of the car, isolating the passengers from road shock as its sidewall defects. The sidewall deflection allows more of the tread to actually be in contact with the road surface. The larger area of contact, called the tire’s footprint, allows the load on the tire to be spread across a wider area of the tire. A large footprint also causes the tire to grip better so it can transmit forces of the engine and brakes to the road surface.
  
  A radial tire flexes on its sidewall and is more resistant to wear because its tread surface stays flat on the ground. Because of the bulging sidewall a properly inflated radial will appear to be low on air.
  
  Radial tires have less resistance to rolling, which improves a car’s gas mileage. Because radial tires help the vehicle to achieve better fuel economy, they are included on all new cars as original equipment.

• TIRE SIDEWALL MARKINGS 

 Included on the tire sidewall for a typical passenger car are:

• • The tire size. 
  • Maximum permissible cold air pressure. 
  • Load rating - an indication of the load limit for each of the vehicle tire’s. 
  • The name of the material that the cords of the tire are made of. 
  • The number of plies in the tread and sidewall areas. 
  • If the tire is a radial tire. 
  • Whether the tire is tube-type of tubeless. 
  • The DOT (Department of transportation) manufacturing code. 
  • M+S - this indicates that the tire meets the RMA definition for a mud and snow tire. 
  • Uniform tire quality grade standard (UTQG) - DOT grading for traction, treadwear and temperature.
    
    
• TIRE SIZE
  
  A tire information sticker called a placard is located on the door post, the edge of the door, the gas filler door, or the glove box door. The placard indicates the correct OE (original equipment) tire size, and the cold inflation pressure, and the gross vehicle weight.
  
  The tire’s size is listed on the sidewall of the tire. There are several types of rating, but the most common is the P-Metric. It looks like this:
  
  P205-75 R14 

• • The P stands for passenger car. 
  • 205 is the width of the tire in millimeters. 
  • 75 is the hieght-to-width ratio (profile). 
  • R stands for radial construction. 
  • 14 is the diameter in inches. 
  • The M+S stands for mud and snow. 

 

 

If you are considering making a change in the size of an original equipment tire three things must be considered in the replacement tire:

Its overall diameter must not be changed by more than +2% or -3% from the original tire’s diameter. Changing tire diameter can affect the operation of antilock brakes, calibration of the speedometer, gear ratio, and four-wheel drive.

Its speed rating must be equal to or greater than of the original tire.

It must have an overall load carrying capacity that is equal to or greater than the load index number listed on the original tire.

• SPEED RATING
  
  Sometimes a speed rating is listed on the tire after the size designation. Speed symbols for passenger cars range from the L rating 75 mph to ZR (over 149 mph. Examples of the most common ratings are as follows:
  
  S - 112 mpg (180 km/h)
  T - 118 mpg (190 km/h)
  H - 130 mpg (210 km/h)
  
  These mean that a properly inflated tire with an H rating has been designed to operate at up to 130 MPH for short period of time (such as when passing cars). Sustained high speeds can damage the tire. 

• LOAD RATINGS
  
  It is important that tires be of sufficient size and capacity to support the load it has to carry. This includes the weight of the vehicle and its passenger. This gross weight rating can sometime be found on the tires. Normally the load rating can only be found on light trucks. Load range B is the normal rating for passenger car. Load range C and D are used for light trucks or vans. The actual gross weight can be found on the door jam decal. 

• DOT  

The DOT symbol signifies that the tire meets Department of Transportation standards.

• ALL-SEASON TIRES 

Since the mid-1970s, all-season tires have been available. Before that you had to change to specific tires twice a year. Any combination of the letters M and S (M+S, MS, M&S, M/S) on the tire side wall means that the tire meets snow tire definitions set by the Rubber Manufacturers Association. These tires have treads with specially designed pockets and slots in at least on tread edge.

• SNOW TIRES 

Snow tire has deeper tread grooves designed to provide a better grip when driving on snow-covered roads. Most manufacturers recommend that they be installed on all four wheels to prevent handing problems.

• TIRE QUALITY GRADING 

American manufacturers us the uniform tire quality grade system, which rates tread wear, traction, and temperature resistance. It is printed on the sidewall of the tire.

• TREAD WEAR
  
  Tread wear is a rating that compares tires tested on a government test course. The number 100 represents a standard tire. A 200 would be expected to last twice as long on the government test course, and a 15 would last about one and one-half times as long. The actual tire life can vary due to road conditions, climate, air pressure alignment, driving habits, vehicle loading, and other factors.
  
• TRACTION GRADE
  
  Traction grading uses a letter rating; A is the highest rating, while C is the lowest. This rating covers braking in a straight line, and not cornering.
  

   

   

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