Solid Rubber Tires
After the discovery of vulcanization tires were made out of solid rubber. These tires were strong, absorbed shocks and resisted cuts and abrasions. Although they were a vast improvement, these tires were very heavy and did not provide a smooth ride.
Today there are still types of tires made of solid rubber
The pneumatic rubber tire uses rubber and enclosed air to reduce vibration and improve traction. Robert W. Thomson, a Scottish engineer, first patented the air filled tire. Unfortunately the idea was too early for its time and was not a commercial success.
In 1888 John Boyd Dunlop of Belfast, Ireland became the second inventor of the pneumatic tire. Dunlop claimed to have no knowledge of Thomsons earlier invention.
The second time around the pneumatic tire caught the publics attention. The timing was perfect because bicycles were becoming extremely popular and the lighter tire provided a much better ride.
Bias Ply Tires
For the next fifty years vehicle tires were made up of an inner tube that contained compressed air and an outer casing. This casing protected the inner tube and provided the tire with traction.
Layers called plys reinforced the casing. The plys were made of rubberized fabric cords that were embedded in the rubber. These tires were known as bias-ply tires. They were named bias ply because the cords in a single ply run diagonally from the beads on one inner rim to the beads on the other. However, the orientation of the cords is reversed from ply to ply so that the cords crisscross each other.
Today you can still find bias-ply tires as authentic equipment for antique and collector cars, as well as for certain type of off-the-road tractor tires.
The first introduced steel-belted radial tires appeared in Europe in 1948. Radial tires are so named because the ply cords radiate at a 90 degree angle from the wheel rim, and the casing is strengthened by a belt of steel fabric that runs around the circumference of the tire.
Radial tire ply cords are made of nylon, rayon, or polyester. The advantages of radial tires include longer tread life, better steering and less rolling resistance, which increases gas mileage. On the other hand, radials have a harder riding quality, and are about twice as expensive to make.
The beauty of the radial design is that it separates the functions of the sidewall and crown of the tyre, allowing greater vertical flexibility whilst ensuring that there is still as much surface in contact with the road as possible. In classic radial tyres the sidewall has a one or two layers of textile cord giving good flexibility, and the tread is made rigid by combining the casing layer with two (or more) layers of steel cord bracing plies. Both these factors give the classic radial tyre excellent road holding capabilities and a longer life span when compared to vintage cross-ply tyres.
Types of automobile tires
Performance tires tend to be designed for use at higher speeds. They often have a softer rubber compound for improved traction, especially on high speed cornering. The trade off of this softer rubber is a lower treadwear rating.
Performance tires are often called summer tires, because they sacrifice wet weather handling, by having shallower water channels, and tire life from softer rubber compounds, for dry weather performance. The ultimate variant of performance tires has no tread pattern at all and is called slick tire. Slick tires are not legal for use on public roads in most countries due to their extremely poor wet weather characteristics.
Winter tires are designed to provide improved performance under winter conditions compared to tires made for use in summer. The rubber compound used in the tread of the tire is usually softer than that used in tires for summer conditions, so providing better grip on ice and snow. Winter tires often have fine grooves and siping in the tread patterns that are designed to grip any unevenness on ice. Winter tires are usually removed for storage in the spring, because the rubber compound becomes too soft in warm weather resulting in a reduced tire life.
Dedicated winter tires will bear the "Mountain/Snowflake Pictograph" if designated as a winter/snow tire by the American Society for Testing & Materials. Winter tires will typically also carry the "M&S" (Mud and Snow) designation.
Many winter tires are designed to be studded for additional traction on icy roads. The studs also roughen the ice, so providing better friction between the ice and the soft rubber in winter tires. Use of studs is regulated in most countries, and even prohibited in some countries due to the increased road wear caused by studs.
These are an attempt to make a tire that will be a compromise between a tire developed for use on dry and wet roads during summer, and a tire developed for use under winter conditions, when there is snow and ice on the road. However, the type of rubber and the tread pattern best suited for use under summer conditions cannot, for technical reasons, give good performance on snow and ice. The all-season tire is therefore a poor compromise, and is neither a good summer tire, nor a good winter tire.
All-Season tires are marked M+S, i.e. the same as winter tires. However, due to the compromise with performance during summer, winter performance is usually not comparable with a winter tire.
All-terrain tires are typically used on SUVs and light trucks. These tires often have stiffer sidewalls for greater resistance against puncture when traveling off-road, the tread pattern offers wider spacing than all-season tires to evacuate mud from the tread.
Within the all-terrain category, many of the tires available are designed primarily for on-road use, particularly all-terrain tires that are originally sold with the vehicle.
Mud terrain tires are characterized by large, chunky tread patterns designed to bite into muddy surfaces and provide grip. The large open design also allows mud to clear more quickly from between the lugs.
Mud terrain tires also tend to be wider than other tires, to spread the weight of the vehicle over a greater contact patch to prevent the vehicle from sinking too deep into the mud.
Depending on the composition and tread pattern, many mud terrain tires are not well suited to on-road use. They can be noisy at highway speeds, and due to the open tread design, they have less of a contact area with the road, limiting traction. The large lugs on mud tires tend to tear and chip on roads, because they are made from hard rubber compounds that do not bend easily.
Tyre and Wheel Measurements
There are a number of relevant dimensions of a tyre or a wheel:
- Diameter of the tyre
- Width of the tyre
- Height of the tyre section
- Aspect ratio (the relation of the width of the tyre to the height of the tyre section)
- Diameter of the wheel (rim size)
All measurement systems refer to two or more of these dimensions, but there are several different traditions of expressing tyre and wheel size.
Most tire sizes begin with a letter or letters that identify the type of vehicle and/or type of service for which they were designed. The common indicators are as follows:
P = When a tire size begins with a "P," it signifies the tire is a "P-metric" size that was designed to be fitted on vehicles that are primarily used as passenger vehicles. This includes cars, minivans, sport utility vehicles and light duty pickup trucks (typically 1/4- and 1/2-ton load capacity). The use of P-metric sizes began in the late 1970s and they are the most frequently used type of tire size today.
If there isn’t a letter preceding the three-digit numeric portion of a tire size, it signifies the tire is a "Metric" size (also called "Euro-metric" because these sizes originated in Europe). While Metric tire sizes are primarily used on European cars, they are also used on vans and sport utility vehicles. Euro-metric sizes are dimensionally equivalent to P-metric sizes, but typically differ subtly in load carrying capabilities.
T = If a tire size begins with a "T," it signifies the tire is a "Temporary Spare" ("space saver" or "mini spare") that was designed to be used temporarily only until a flat tire can be repaired or replaced.
LT = If a tire size begins with "LT," it signifies the tire is a "Light Truck-metric" size that was designed to be used on vehicles that are capable of carrying heavy cargo or towing large trailers. This includes medium and heavy-duty (typically 3/4- and 1-ton load capacity) pickup trucks, sport utility vehicles and full-size vans. Tires branded with the "LT" designation are the "little brothers" of 18-wheel tractor-trailer tires and are designed to provide substantial reserve capacity to accept the additional stresses of carrying heavy cargo.
LT = If a tire ends with "LT," it signifies the tire is either an earlier "Numeric", "Wide Base" or "Flotation" Light Truck size designed to be used on vehicles that are capable of carrying heavy cargo and towing trailers (Numeric sizes), use 16.5-inch diameter rims (Wide Base sizes) or are wider, oversized tires designed to help the vehicle drive on top of loose dirt or sandy surfaces (Flotation sizes). This includes light, medium and heavy-duty (typically 1/2-, 3/4 and 1-ton load capacity) pickup trucks and sport utility vehicles. Tires branded with the "LT" at the end of their size designation are also the "little brothers" of 18-wheel tractor-trailer tires and are designed to provide substantial reserve capacity to accept the additional stresses of carrying heavy cargo.
C = If a Euro-metric sized tire ends with a "C," it signifies the tire is a "Commercial" tire intended to be used on vans or delivery trucks that are capable of carrying heavy loads. In addition to being branded with the "C" in their size, these tires are also branded with their appropriate Service Description and "Load Range" (Load Range B, Load Range C or Load Range D).
ST = If a tire size begins with "ST," it signifies the tire is a "Special Trailer Service" size that was designed to only be used on boat, car or utility trailers. ST-sized tires should never be used on cars, vans or light trucks.
Following the letter(s) that identify the type of vehicle and/or type of service for which the tire was designed, the three-digit numeric portion identifies the tire’s "Section Width" (cross section) in millimeters.
The 225 indicates this tire is 225 millimeters across from the widest point of its outer sidewall to the widest point of its inner sidewall when mounted and measured on a specified width wheel. This measurement is also referred to as the tire’s section width. Because many people think of measurements in inches, the 225mm can be converted to inches by dividing the section width in millimeters by 25.4 (the number of millimeters per inch).
Sidewall Aspect Ratio
Typically following the three digits identifying the tire’s Section Width in millimeters is a two-digit number that identifies the tire’s profile or aspect ratio.
The 50 indicates that this tire size’s sidewall height (from rim to tread) is 50% of its section width. The measurement is the tire’s section height, and also referred to as the tire’s series, profile or aspect ratio. The higher the number, the taller the sidewall; the lower the number, the lower the sidewall. We know that this tire size’s section width is 225mm and that its section height is 50% of 225mm. By converting the 225mm to inches (225 ¿ 25.4 = 8.86") and multiplying it by 50% (.50) we confirm that this tire size results in a tire section height of 4.43". If this tire were a P225/70R16 size, our calculation would confirm that the size would result in a section height of 6.20", approximately a 1.8-inch taller sidewall.
A letter (R in this case) that identifies the tire’s internal construction follows the two digits used to identify the aspect ratio.
The R in the P225/50R16 91S size identifies that the tire has a Radial construction in which the tire’s body plies "radiate" out from the imaginary center of the wheel. Radial tires are by far the most popular type of tire today representing over 98% of all tires sold.
If the R in the size was replaced with a D (225/50D16), it would identify that the internal tire body plies crisscross on a Diagonal and that the tire has a "bias ply" construction. Tires using this construction are for light truck and spare tire applications.
If the R in the size was replaced with a B (225/50B16), it would identify that the tire body plies not only crisscross the tire on a diagonal as before, but that they are reinforced with belts under the tread area. This type of tire construction is called "Belted." Tires using this construction are practically extinct.
Today, the only tires that continue to include the speed rating "in" the tire size (P225/50ZR16) are Z-speed rated tires. In this case, following the two digits used to identify the aspect ratio are the letters ZR to identify the tire’s speed rating (Z) and its internal construction (R). Since 1991, all other speed ratings are identified in the tire’s Service Description (which will be covered shortly).
Tire and Wheel Diameter
The 16 indicates the tire and wheel diameter designed to be matched together.
Tires that have a rim diameter expressed in inches (P225/50R16, as well as 8, 10, 12, 13, 14, 15, 17, 18, 19, 20, 22, 23, 24, 26 and 28) are called "inch rim" sizes, are the most common type of tire size and are used on most cars, minivans, vans, sport utility vehicles and light duty light trucks.
While not as common, two additional "unique" types of tire/wheel diameters are still in use today.
Tires and wheels that have a rim diameter expressed in "half" inches (8.00R16.5LT, as well as, 14.5, 15.5, 17.5 and 19.5) are used on some heavy-duty trailers, heavy-duty light trucks and box vans.
Tires and wheels that have a rim diameter expressed in millimeters (190/65R390, as well as, 365 and 415) are called millimetric sizes. Michelin initiated millimetric sizes for their TRX tires that saw limited use on many different car models in the late 1970s and 1980s.
Michelin PAX System run flat tires have been introduced as an integrated wheel/tire system on a very limited basis as Original Equipment (O.E.) in North America. An example PAX System size of 235/710R460A 104T expresses tire and wheel dimensions in millimeters (235 mm Section Width, tire Overall Diameter of 710 mm and a 460A mm rim diameter, with the "A" in 460A signifying these tires feature "asymmetric" beads in which the outside bead (450 mm) and inside bead (470 mm) are actually different diameters.
All of these "unique" tire/wheel diameters were developed specifically because the tire and wheel design or intended vehicle use required them to be different than conventional tires and wheels. All of these tires and wheels feature bead profiles that have a different shape than traditional "inch rim" sizes.