Tire Terminology

All Terrains - All Terrain tires or AT are a compromise.  All Terrains are an attempt to offer good performance both on road as well as offroad.  The ALL TERRAIN TREAD is intended to perform well under a variety of conditions found offroad while still offering acceptable on-highway performance.  This is accomplished by using a tread pattern design where the lugs are tighter together than a more aggressive mud tire's tread. The result is usually a quieter ride on the street than a mud tire due to its lesser aggressive tread pattern.  When compared to a street tire, All Terrain tires usually  produce more noise.  The payoff of an All Terrain Tires is that they perform well on a variety of terrains: rocks, sand, somewhat in the mud while still offering decent traction on the paved road. One drawback of an all terrain is that the tread design tends to pack with mud; however, some of the AT designs perform surprisingly well in muddy conditions.  The AT All Terrain is typically the tire for the 4-wheeler who drives their 4x4 as a daily driver and will see minimal trail use and more on highway driving.

Mud Terrain Tires - MUD TIRES or MT (Mud Terrain) are as you might have gathered from the name, designed to perform most specifically in the mud.  But when you look at the tread design of many mud tires, they generally perform well in other conditions such as on the rocks, in deeper snow, as well as in loose gravel and in the softer, constantly changing terrain of wooded trails.  This is because mud tires are usually designed from a softer compound with wider gaps (voids) between the lugs, which grab onto anything it can hook one of its lug edges around, especially when aired down.  Tread designs typically are what make or break a mud tire and vary widely from manufacturer to manufacturer. Drawbacks of  MT Mud Terrain tires are they perform poorly on the highway especially in the rain where the wide lug pattern results in less of a tire footprint on the road.  Even worse, the MT can be downright dangerous in icy conditions. Mud tires also tend to wear quicker than an all terrain or a street tire and depending on your perspective, the on-highway noise level can be considerably higher especially after they wear down with highway use.   

Bias-Ply Tires and Radial Tires
There are two basic types of tire construction that mud, all terrain and street tires use as their foundation.  They are bias-ply and radial designs.  Each type of tire construction has its own unique set of characteristics that are the key to its performance, whether on road or off road and these characteristics can help to define the purpose of the tire.  The following information will explain what identifies the difference between a bias ply tire and a radial type tire.

The bias ply tire construction utilizes rubber-coated layers known as plies composed of textile cords, usually nylon and sometimes Kevlar.  The plies are layered diagonal from one bead to the other bead at about a 30 degree angle. One ply is set on a bias in one direction as succeeding plies are set alternately in opposing directions as they cross each other and the ends are wrapped around the bead wires, anchoring them to the rim of the wheel.  The layers of plies are them covered with more rubber to form the tread of the tire.  Bias ply tires are sometimes called cross-ply tires.

Performance and Purpose of a Bias Ply

Bias ply tires have a limited purpose in life and are only used for specific purposes or jobs.  The reason for this is because of its performance characteristics.  However for some jobs the bias ply tire is an idea tire for the purpose such as for the tires of a towed trailer, farm equipment tires, some purpose built tires like extreme terrain tires and some forms of racing still use bias ply tires.  The reasons for this limited use are:

  • The bias-ply tire casing is constructed to form one working unit. When the sidewalls deflect or bend under load, the tread squeezes in and distorts. The distortion affects the tires footprint and can decrease traction and increases wear depending on the terrain.  The tread distortion also causes abrasion from the ground surface, which reduces the life of the tire. These factors are why bias ply tires are not idea for passenger car tires or as tires that my see highway use unless used as tires for a towed trailer.
  • Bias Ply Strength - The way to increase the strength of bias-ply tires is by increasing the number of plies and bead wires. More plies means more mass which increases heat retention and reduces tire life.
  • Because of the bias ply inherent construction, sidewall strength is less than that of a radial tire's construction and cornering is significantly less effective.  This is probably one of the main reasons bias ply tires are not used for passenger cars and trucks.
  • However because of the bias ply construction and inherent strength of a properly inflated tire, the bias ply is ideal for straight line towing.

The radial is a type of tire that is constructed with rubber coated, reinforcing steel cable belts that are assembled parallel and run from side to side, bead to bead at an angle of 90 degrees to the circumferential centerline of the tire.   (As opposed to the 30 degree alternating application lengthwise as in bias ply tires).  This makes the tire more flexible which reduces rolling resistance to improve fuel economy.  Then numerous rubber coated steel belts are constructed into the "crown" of the tire under the tread to form a strong stable two-stage unit.

Performance and purpose of Radial tires

Radial tires are the preferred tire of choice in most applications for several key reasons.  

  • The combination of steel stabilizing belts in the single-layer radial casing allows the tread and sidewall to act independently. The sidewall flexes more easily under the weight of the vehicle and its cargo, while the tank-track type tread provides even contact with the ground. Greater vertical deflection is achieved with radial tires. This is desirable because extreme flexing greatly increases resistance to punctures.
  • To increase a radial tire's strength, larger diameter steel cables are used. Larger steel cables can help reduce punctures, tears and flats. Larger steel cables also help distribute heat, resulting in a cooler running tire and improving fuel economy.  Unlike bias ply tires larger steel cables have little negative affect on performance.
  • The parallel stabilizing steel belts of the radial minimize tread distortion. As the sidewalls flexes under load, the belts hold the tread firmly and evenly on the ground or object and thus minimizing tread scrub and greatly increasing tread life.
  • When cornering the independent action of the tread and sidewalls keeps the tread flat on the road. This allows the tire to hold to its path.
  • When offroad, the radial tire's stabilizing steel belt design aids in greater traction by holding the tread evenly over obstacles allowing the tread of the tire to have a better chance of finding traction. 

Sipes are the small slots that are cut or molded into a tire tread surface.  These slots are meant to aid in increasing traction in snow, ice, mud, and wet road surfaces.  The name of the concept of siping a tire comes from a man named John Sipe, who received a patent in the 1920's, after realizing that an array of small transverse cuts in the heels of his shoes gave him better traction. Later Goodyear received a US patent claiming that the "sipes" improved traction characteristics in tires.

Tire tread is a series of block shapes, groove configurations, and sipes, all of which have an affect on the tires traction and noise level.  Typically, wide, straight grooves running in the direction that the tire travels will have a lower noise level and good water removal. More lateral grooves running from side to side will usually increase traction while increasing noise levels. Sipes are the small grooves or slits that are cut across larger tread elements. Up to a point, more sipes give more traction in snow and mud as well as over various terrains found offroad.

All tires are required to have certain information molded into the side of the tire in a location known as the sidewall.  Some of the information is self explanatory while other information requires a little knowledge to decipher.  The following will help you understand what this information means.

 WIDTH - This is the width of the tire measured in millimeters from sidewall to sidewall.  An example might be 215 representing 215 millimeters.

 ASPECT RATIO - This is the the ratio of the height of the tire's cross-section to its width. An example of this might be 65, which means that the height is equal to 65% of the tire's width.  To calculate the aspect ratio, multiple the first number (e.g. 215) by the second number with a decimal before the number (e.g. .65).  Using the example numbers the tires aspect ratio would calculate as 215x.65=139.75 where 139.75 is the tires height in millimeters.  This is the height of the rubber from rim to tread on one side of the tire. 

To convert the aspect ratio to a full tire height in inches, do this:
Convert the above calculated tire height (aspect ratio) in millimeters to inches by multiplying the millimeters by .03937 (139.75 x .03937 = 5.5 inches).  Then take the inches and multiply by two and add the rim size. Example: 5.5 x 2 + 15 (rim size in inches) = a 26 inch tall tire.

 CONSTRUCTION - This indicates how the how the tire was put together and will say much about the tires handling characteristics. R indicates the tire is a radial type tire.  For more information about what a radial is, click here.  B indicates the tire is a bias ply type tire.  For more information about bias ply type tires, click here.

 DIAMETER - This is the width of the opening in the tire where it would be mounted to a wheel.  This is measured from one bead across the opening to the other side of the same bead.  This measurement is in inches and an example would be 15 and indicates that this tire is for a 15 inch rim, or wheel.

 LOAD RANGE (NOT HI-LIGHTED) - This is a number corresponds to the maximum load in pounds that a tire can support when properly inflated. You will also find the maximum load in pounds and in kilograms molded elsewhere on the tire sidewall.

Tire Load RangesInflation Pressure Assigned
For "Maximum Load" Ratings
P-metric
Standard Load (SL) 35 psi
Extra Load (XL) 41 psi
Light Truck
Load Range C (LRC) 50 psi
Load Range D (LRD) 65 psi
Load Range E (LRE) 80 psi

- This is a number that corresponds to the maximum service speed for a tire.  See Chart for ratings.  Note: Speed ratings are specific for passenger car tires and not light truck tires.  Light truck tires (LT) are not speed rated. 

Rating

Maximum Speed
Q 99 mph
S 112 mph
T 118 mph
U 124 mph
H 130 mph
V 149 mph
W 168 mph
Y 186 mph
Z Above 149 mph

Disclaimer for the common sense challenged: Please note that this rating relates only to tire speed capability based on tire tires limits, and is NOT a recommendation to exceed legally posted speed limits; always drive within the legal speed limits.

 PSI - Pounds per square inch - used to measure air pressure in a tire.  The PSI rating on tires is typically the maximum recommended tire pressure for that tire.  Tire pressure should always be checked periodically and when the tires are cold. Under normal operation, tires can lose approximately 1 PSI of pressure every month. For every 10 degree change in ambient temperature, tire pressure can change by approximately 1 PSI.