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Car tires

Cars tires

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After you read this, you will be able to know about.

  1. Car Tire Description and construction
  2. Tire Code
  3. How to select your tire
  4. Take attention to

Car Tire description and construction

A tire (or tyre) is a ring-shaped covering that fits around a wheel‘s rim to protect it and enable better vehicle performance. Most tires, such as those for automobiles and bicycles, provide traction between the vehicle and the road while providing a flexible cushion that absorbs shock.

The materials of modern pneumatic tires are synthetic rubber, natural rubber, fabric and wire, along with carbon black and other chemical compounds. They consist of a tread and a body. The tread provides traction while the body provides containment for a quantity of compressed air.

Tire Construction

Tire Construction

Tire Construction

Tire Construction


The tread is the part of the tire that comes in contact with the road surface. The portion that is in contact with the road at a given instant in time is the contact patch.

The tread pattern is characterized by the geometrical shape of the grooves, lugs, voids and sipes. 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 and evacuate water.

Car Wheel

Tires are mounted onto wheels that most often have integral rims on their outer edges to hold the tire. Automotive wheels are typically made from pressed and welded steel, or a composite of lightweight metal alloys, such as aluminium or magnesium. These alloy wheels may be either cast or forged.

Tread void or grooves

Grooves are the voids between treads, which provide channels for rainwater, mud, and snow to be channelled away from the footprint.


Tread lugs often feature small narrow voids, or sipes (on tire shoulders),  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 due to air circulation in its grooves.

Wear bar

Wear bars (or wear indicators) are raised features located at the bottom of the tread grooves that indicate the tire has reached its wear limit and should be replaced.


The bead is that part of the tire that contacts the rim on the wheel. The bead is typically reinforced with steel wire and compounded of high strength, low flexibility rubber. The bead seats tightly against the two rims on the wheel to ensure that a tubeless tire holds air without leakage.

Ply / body plies

Plies are layers of relatively inextensible cords embedded in the rubber to hold its shape by preventing the rubber from stretching in response to the internal pressure. The orientations of the plies play a large role in the performance of the tire and is one of the main ways that tires are categorized. This is important for road holes, to hold on, when the car fall in, without tire explosion. Plies are rated in categories (4,6,8) and can be reinforced to hold over the road holes. Three types of ply are: Diagonal (Bias-Ply) tires, radial tires, and steel belts. For more…


Cushion is a flexible pillow that absorbs shock.

Tire Codes


  • P: Passenger Car
  • LT: Light Truck
  • ST: Special Trailer
  • T: Temporary (spare wheel)
  • R#: Radial rim diameter in inch (multiply by 25.4 to get R in mm)

Load code is the maximum load should be loaded by single wheel. Make attention that the wheels which hold the motor take higher load than the other two. Usually the front wheel (or where the motor) the weight distribution is 56% for front and 44% for the rear wheels. The tire load capacity has its own safety factor. But if you live in a country or area with a big or un-normal road gore or cavity take 50% as additional weight and if no metallic plies in the tire. Usually you have this metallic plies if you have a problem with your road in your country.

Tire diameter calculation as follow: (Rim diameter [inch])* 25.4 [mm] + 2x 9Ratio of height to width) x (nominal width of the tire [mm]) = tire diameter [mm]


for 35 PSI, 35 degree C (95F)Load code 35PSI


Speed rating is represented by an index. The allowable maximum speed is for limited time which is 10 minutes.

Speed ratings make a difference not only in regards to speed, but in regards to ride comfort, wear and cornering ability. Typically, the higher the speed rating, the better the grip and stopping power, but the lower the tread life. You can always increase the speed rating of the tires on your vehicle for improved performance.

The speed rating code mean that the tire can hold up to its rated speed, but the load should not be 100% of the select tire maximum load.

For example: selecting ‘S’ it mean we can load 100% of its rated maximum load 100% until the speed 180km/h. But for higher rated speed such ‘W’ we can reach 100% of the maximum rated load up to 240 km/h, over that we must decrease 5% each additional 10km/h over 240km/h. Meanwhile the pressure increase by 1.5PSI each 10km/h, so for that do not load a heavy load if you want to go near the maximum speed.

Additional marks

There are numerous other markings on a typical tire, these may include:

  • M+S, or M&S: Mud and Snow; A tire that meets the Rubber Manufacturers Association (RMA) and Rubber Association of Canada (RAC) all-season tire definition.[20] These are all-weather tires, with self-cleaning tread and above-average traction in muddy or very light snowy conditions, and for low ambient temperatures. Spike tires have an additional letter, “E” (M+SE).
  • DOT code: All tires for use in the USA have the DOT code, as required by the Department of Transportation (DOT). It specifies the company, factory, mold, batch, and date of production (two digits for week of the year plus two digits for year; or two digits for week of the year plus one digit for year for tires made prior to 2000).
    • TL: Tubeless
    • TT: Tube-type, tire must be used with an inner-tube
    • Made in …: Country of production
    • C: Commercial; tires for light trucks (Example: 185 R14 C
    • SL: Standard Load; tire for normal usage and loads
  • XL: Extra Load; a tire that allows a higher inflation pressure than a Standard Load tire, which increases the tire’s maximum load
  • RF: Reinforced — for Euro-metric tires, the term ‘reinforced’ means the same thing as ‘Extra Load’[20]
  • Arrows: Some tread designs are “directional”, and designed to perform better when driven in a specific direction (clockwise or counter-clockwise). Such tires will have an arrow showing which way the tire should rotate when the vehicle is moving forwards.
Tire rubber characteristic / traction rate

Traction is rated either A, B or C; with A being the best case traction and C being acceptable in traction but inferior to both A and B. Traction test are often very misleading as it is, by actual testing, a measure of the tire’s ability to stop in a ” straight ahead” condition, on a wet surface of concrete or asphalt. It has no testing for cornering or acceleration capabilities. One may infer things about the latter but one may be incorrect in these inferences.

Temperature is the test of the tire’s ability to withstand heat. Like traction, temperature is rated A, B or C; with A being the best case of a tires’ ability to withstand heat and C being acceptable but inferior to A and B. Heat is a killer of tires and the largest offender is either under inflation of overloading of the tire. Another offender is selecting a tire with an insufficient temperature rating for your specific needs. So make sure if you select the appropriate tire temperature rate suitable for your country.

Traction Grades

The tire traction of grade ‘AA’ on Asphalt is 0.54 g-Force, and from the table the grade ‘AA’ is the best and can stop the car faster than the other grades. In bad asphalt quality we prefer to use the grade ‘AA’ or at least ‘A’ to help the car to stop. And from the table below, the asphalt quality is good one. So bad asphalt quality assumed as concrete (no gravels on surface) ‘AA’ gives 0.41 g-force which is not bad, and better than B or C which is 0.26 on bad asphalt.

Traction Rates
Traction Rates

The test should be performed with a special measuring vehicle where the  tire is braked with locked wheels on wet asphalt and wet concrete road  surfaces.

  •     A – indicates the best classification in accordance with certain     requirements;
  •     B – is a slightly lower classification;
  •     C – indicates the worst (or lowest) value.

A tire with value C, therefore, has the poorest wet traction.

Temperature Grades

Temperature Grades
Temperature Grades

The temperature grades represent the tire’s resistance to the  generation of heat when tested under controlled conditions on a  specified indoor laboratory test wheel.

  •     Sustained high temperatures can cause the materials of the tire to     degenerate and thus reduce tire life
  •     Excessive temperature build-up can lead to tire failure
  •     Federal law requires that all tires meet at least the minimal     requirements of Grade C
  •     The temperature grade is established for a tire that is properly     inflated and not overloaded

for more… and more…


How to select your tire

From your car catalogue select the appropriate tire.

For example 255/60 R17 106H All season and (option) 275/55 R17 109H for winter. Your tire diameter is equal to 17″x25.4+2x(255*0.6)=737.8mm.   Car weight 2093kg (empty) so each tire should be rated as load:  Total weight*0.56 /2, the  total weight is 2093 kg plus 80kg per person and for safety fill the trunk with some load (SUV or this example is for 4 wheel drive) 300kg => (2093kg+5person*80kg) *0.56/2= 698. when we load the extra weight on the trunk (300kg) the load will be distributed (fast estimation) by half (front-rear) and the load symbol will calculated as follow: ((2093kg+5person*80kg+300kg) *0.5/2) x safety factor[1.2 for road conditions] = 837 kg. or for 32 PSI your selection is 106 (830 kg). And at 35 PSI the tire rated load is 870 kg.

So when you load your car with some weight, increase the tire pressure and reduce your speed.

If your car maximum speed in speedometer is 240km/h and your country allowable maximum speed is 120km/h (and you will respect the regulation) you can select tire speed rating between 180km/h and 240 km/h(not less due to some safety in case of you emergency car use) so symbols can be from ‘S’ to ‘V’.

You should know if you need ‘all season tire’ or ‘winter tire’. If you are in a region such mountain we prefer to buy winter tire for winter season, this is best for slopped or skidding roads, bad asphalt (Traction class AA or A). also make sure you have M+S symbol (Mud and Snow).

winter Mud and snow

Winter Mud and Snow

Before selecting your tire, read the specification and manufacture data. you should to know (as we will describe the most of them) what you want to care about:

Dry braking, Wet braking, Handling, Hydroplaning, Snow traction, Ice braking, Ride comfort, Noise, Rolling resistance. Usually you will find a rate for each option or characteristic in the manufacture website. If you are leaving in a hot country, select temperature rate A and if your road way is very slopped (up or down, traction or breaking) you should select traction rate ‘AA’ or ‘A’ in case of wetted or bad asphalt quality.

Make attention

Passenger car Tire inflation

Every vehicle should have a manufacturer recommended tire inflation  value, usually on a sticker on the driver’s side door jamb.

Small car tire pressure at front 30 to 28 PSI and rear 28 PSI

Medium car size, front (motor) 32 to 30 PSI and for rear 30  to 28 PSI

SUV / 4×4 truck need 35 to 32PSI for front and 32 to 30 PSI for rear, in case of heavy load, use for rear 35 PSI.

Those as fast pressure selection, and below there are a scientific decision.

If you want to load some weight on the trunk you should modulate the rear tire pressure.

Reduce top speed

The faster a tire goes, the more heat it retains and the faster it wears.  A 20-mph increase in average speed, from 55 mph to 75 mph, can decrease tread life by up to 30%.  This change in speed will require a 60,000-mile tire to be replaced after only 42,000 miles.

Rotate your tires regularly:

Tires wear differently depending on their position on the vehicle.  To keep all tires wearing evenly and to maximize tread life, they must be rotated at regular intervals.  Industry recommendation is to rotate your tires every 2nd oil change (about 6,000 miles). You should refer to your owner’s manual for manufacturer recommendations specific to your vehicle.

Maintain proper tire inflation pressure: Tires that are underinflated by 20% will reduce tire life by as much as 16%.  Using real numbers, a car’s tires that are underinflated by 6 psi will cause a 60,000-mile tire to last only 50,000 miles.  Temperature changes also have an effect;  a 10-degree Fahrenheit change in temperature will result in a 2-psi difference in tire pressure. (1 mile = 1.61 km)

Speed and load

If you’re going very slowly, you may be able to carry heavier loads than at “normal” speeds.  With a tire like an 11R22.5, load capacity can increase 185 percent when the vehicle is stationary if you boost inflation by 40 psi. Also increasing the speed will increase the road friction with the tire, and this yield to decrease the tread life or increase the wear. Speed increase the pressure and tire-air temperature. If we take tire speed rated for example ‘W’ as reference of car speed 190km/h with 35 PSI, the pressure will increase 1.5 PSI each additional 10 km/h until a certain limit where this become stable at 240km/h at 7.5 PSI as additional pressure. For ‘V’ rating the pressure start increase from 160 km/h up to 210 km/h 1 PSI each additional 10km/h. Speed rating ‘S’ start from 160km/h up to 180km/h increasing by 1PSI each 10km/h. When the pressure For more info and other rating speed…

Load and tire pressure

The rated load as from the table above is rated at 35 PSI (1 PSI = 6.893 KPa). To calculate the maximum permitted load at a specific pressure, use the following approximated formula: Load = (rated load) x SQRT(PSI / 35) within 1% error if your selected table is from below tables at 35 PSI or if the selected table is from above at 42 PSI the formula will be Load = (rated load) x SQRT(PSI / 42). For best estimation refer to the table below

TireLoadLimitsAtVarious PSITireLoadLimitsAtVariousPSI -2(Click to enlarge)

Relation of speed and Pressure to load.

Relation between speed and pressure or speed and load are estimated as the following formulas :

Speed-Pressure (Km/h – PSI) , Rational Model

Logestic SpeedPSI

Speed-Load (Km/h – weight % change), Hyperbolic Decline


Speed - Load - Pressure

Speed – Load – Pressure

Increasing the load on tire, yield to increase air-tire temperature and friction, this increase the wear of the tire. So when driving, the tire pressure may increase by 5 to 7 PSI due to the speed and temperature. (Note that in summer the asphalt temperature may exceed ambient air temperature by 15 degree Celsius).

For further reading about Speed – Pressure


Decreasing the car speed, allow you to increase tire pressure or put more load instead.

(Note: the relation between % load and extra PSI is not linear)

Manufacture Tire tests

There are many tests should be applied to new tire model. Federal has one of these methods and another standard has something similar.

In a rotating test machine, and in each step, the velocity start from lower speed up to the maximum rated speed (for this model), and 10km/h each 10minutes the speed will increase. Then a series of test that combines the speed changes and load changes. The load start from 75% of the rated load then increase by 5% each step plus the modification of the speed. Overall phases are around 24, but some tests will applied to 48 hours at 110% of the rated load at speed 80 km/h and at higher rated speed (100 km/h) apply 90% of the rated load. The tires rated for high speed (‘S’ rated and up) will subjected to higher speed test such 160km/h for one hour.                                                   This is a brief description and for more refer to… http://www.nhtsa.gov/cars/rules/rulings/upgradetire/econ/TireUpgradeII.html  http://federal.eregulations.us/rulemaking/document/NHTSA-2010-0132-0001

 Tire pressure at ‘rated speed’ temperature relation

When checking the pressure of your tires, it is best to develop a routine. That is, try to check them in a shaded area at the same time be it morning of evening and make sure that your check the pressure when the tires are ‘cold’. We try to check our tire pressure at least once a month and always before a highway trip or performance driving.

 As approximate estimation of pressure T1 / T2 = P1 / P2   , Where T is the temperature in Kelvin and P is the pressure in any unit (PSI or Pa). For example if you regulate the tire pressure at 20 degree Celsius to 32 PSI, it will be 33.75 PSI (Slightly less) at 36 deg. C. or 9 deg. C add around 1 PSI of pressure.

Changing tire size -> Bigger Tire Size

What happen if I select a bigger tire size for my car?

BiggerTireThe car motor apply a momentum and torque at the centre of the wheel, the wheel rotate in the ‘+’ sign direction and this to be happen without skidding, the road should make a reaction force ‘F’ in the opposite direction of car movement. When we increase the tire size ‘R’ the torque created at the centre (opposite to motor torque) increase with ‘R’ and the car acceleration will be less than a smaller tire size, but once the speed is constant (no acceleration) the car speed it is increase as ‘R’ increased but the kilometre or millage in front of you in speedometer or millage-meter still unchanged, but the car run slightly faster by ‘R’ / ‘r’, where ‘r’ is the initial car tire. Also you should note that the brake should be stronger than ‘r’ case to let the car to stop at the same distance, unless the stop distance will be also increased. So do not increase your vehicle tire size if you not need to use it for off-road driving.

Tires type

1)  Mud and Snow (M+S) Rating

All-season tires are branded with the M+S symbol, meaning they have met the Rubber Manufacturers Association’s (RMA) requirements for an all-season tire. These tires are designed to provide year-round traction on wet and dry roads (including light snow).  If you’re looking for all-season traction, make sure your tires have the M+S symbol on the sidewall.

Mud and Snow

Mud and Snow

2)  Severe Snow Use Rating

A step up from the M+S rating, these tires have met the RMA’s requirements for use in severe snow conditions.  Tires rated for severe   snow use will have a ‘mountain and snowflake’ image imprinted on the sidewall.

Sever Snow

Sever Snow

3) Winter rating

As the temperature drops below 40-degrees Fahrenheit, tire rubber hardens, significantly reducing grip with the road.  The good news is that winter tire technology has come a long way in recent years to provide excellent winter traction, ice grip, and ride comfort. Rubber compounds in winter tires are designed to stay softer at cold temperatures.  This gives the tire much needed flexibility to ‘grab’ the road with the thousands of ‘biting edges’ provided by the sipes or cuts in the tread.  Winter tires can be categorized in three groups:

  • Studdable Winter Tires:

    Studded tire

    Studded tire

  • Studless Winter Tires:
  • Studless winter tire

    Studless winter tire

    These tires are designed to provide the best balance of winter traction with ride comfort.  These tires rely on the cuts, or sipes, in the tread to provide thousands of ‘biting edges’ when the tire comes in contact with the road.

  • Performance Winter Tires:
  • performance winter tire

    performance winter tire

    Designed for maximum winter traction while maintaining high-speed performance characteristics.  These tires provide superb steering response, ride comfort, and grip in all types of winter conditions.

To help water evacuation, you should be aware of tire pressure, a good pressure help water to disperse. Under-pressure let tire tread to become obstacle for water evacuation and over pressure reduce stability of the vehicle.

Tire water evacuation

Tire water evacuation

Water evacuation

Tire Water evacuation

Summer Tires:

Maximum traction on wet and/or dry roads is what summer tires are designed for.  These tires may be used in the spring, summer, and fall, but are not intended for use in cold weather (including snow and icy conditions).  Summer tire looks smooth compared to a winter or all-season tire.  This enables the tire to grip more of the road surface, but hinders it from gripping well on snowy or icy surfaces.  If you drive your vehicle in winter conditions, you should have a set of winter or all-season tires to compliment your summer tires.      

Summer TireSummer Tire

Reference: http://www.sullivantire.com/tires/tire-classroom.aspx#all season

Symmetrical and asymmetrical tires

Thread Types

Thread Types

Symmetrical (tread)  tires are typically quiet, and it is better in hydroplaning, where the water evacuation will be symmetrical and less annoying to other cars, and each tire is subjected to the same force from water. The images above are symmetrical type.

Asymmetrical tireAsymmetrical tread tires are used for sport cars. Asymmetrical tires have large blocks of tread on the outside to increase  cornering stability and narrower blocks of tread along the inside of the tire to  aid winter or wet weather driving. But the asymmetric lead to non uniform force distribution in hydroplaning (aquaplaning)  (driving in watered road or heavy rain) for each tire, but it will be balanced by the 4 tires. So never mix symmetrical with asymmetrical tires.

Asymmetrical tire

 Directional tread usually takes the form of v-shaped grooves that help to disperse water from the centre to the edge of the tread. And the rib sometimes can be like a teeth to increase traction or breaking with ice. And in another case it is used to increase tire stiffness or rigidity

Ice tread

Ice tread

Tread type

Treads shape are important for traction, braking, cornering, hydroplaning.

  • -The fine lines (or zigzag) are made to reduce noise of tire on road.
  • – The big channels are for water evacuation to reduce the hydroplaning on road.
  • -Increasing number of tread, increasing the road adherence in snow or wet road


Low noisy tread
Low noisy tread

For more about tread shapes…

Aquaplaning or hydroplaning by the tires of a road vehicle, aircraft or roller coaster occurs when a layer of water builds between the wheels of the vehicle and the road surface, leading to a loss of traction that prevents the vehicle from responding to control inputs. (Wikipedia.org)

Aquaplaning Hydroplaning

Aquaplaning Hydroplaning

To reduce aquaplaning effect, and reserve car stability, make sure you appropriate inflate your tire, and tread should be new. As velocity increased over water the aquaplaning increased, and the adherence between tire and the road decreased, once this occur it is difficult to re-maintain car stability.

Refer to http://www.formula1-dictionary.net/aquaplaning.html and check the water layer below tires at different tread and different tread thickness.

For an idea how water evacuate from wheel, refer to http://www.etyres.co.uk/glossary-tyre-terms?term=aquaplaning

Aquaplaning Hydroplaning

Aquaplaning Hydroplaning

 Check on youtube aquaplaning effect when the car loss adherence (contacts) with the road. http://www.youtube.com/watch?v=mERAaeCrj0E
For water evacuation efficiency with pressure, check http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=3
underinflated tire

underinflated tire

Good inflated tire

Good inflated tire

How to select the right thread type?

For passenger cars it is good to select the low noisy treads, in another hand the sport car doesn’t need that due to exhaust system sound is higher.  Select tread types by knowing the asphalt quality, low quality need more number of thread and avoid smooth treads. Road shape, if a pool may form in your road, you should select the big channels to help evacuating water. For off-road drive you need a thick treads and coarse one to help drive on gravels or rock.

Increasing channels (fine or thick) help to dissipate heat from tires. To let the tread work its job, you should inflate tire with the appropriate pressure. The tread on the tire edge are important for off-road or parking near (or contacting) the edge of pedestrian sidewalk.

Illustrations and explanation for tire wear

Under Inflation

Under Inflation

Under-inflation has caused this tyre to wear on the outer edges of the tread, leaving the central tread area far less worn. The tyre inner-liner can also degrade.

Over Inflation

Over Inflation

Over-inflation has resulted in the central tread area being forced into contact with the road causing rapid or crown wear.



A typical example of the wear pattern caused by front wheel misalignment, (Toe-in or toe-out). The edge of the tread is “feathered” and worn progressively from one side.

Camber Wear

Camber Wear

Excessive wheel camber has caused sloping wear on the outer edge of the tread on one shoulder of this tyre.

Illegal Wear

Illegal Wear

This tyre has been used well after reaching the legal minimum pattern depth of 1.6mm across the central ¾ of the tread, going around the complete circumference of the tyre.

End of Life

End Of Life

This tyre has reached the legal minimum pattern depth of 1.6mm.

Emergency Braking

Emergency Braking

An emergency braking manoeuvre with this tyre has caused the tyre to rapidly wear through the complete casing causing the tyre to deflate.



Sharp objects can cause considerable damage rendering a tyre unserviceable.

Studded tireImpact Damage

Impact Damage

This is damage caused by an impact to the sidewall. The bulge or “egg” indicates localised casing damage.

tire underinflated / overloaded

tire underinflated / overloaded

Reference http://www.kwik-fit.com/changing-tyres.asp

Wheel bolts fixing order / wheel installation

Start fix wheel bolts diagonally not in order.


General references: