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Car Aerodynamics  2011
 

Pascal Hayek
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TIRES, NOT JUST RUBBER

 

If I want to write an article about all what tires are about, this magazine will not be enough. So that’s why I will explain the tire in a simple and short way. At the end of this small course the reader will be able to read and understand what is written on the sidewall of any tire. At the very least, you'll be able to sound like you know what you're talking about next time you go to buy some new tires.

 

Remember that the tire is the only part of the car that is in contact with the road surface, that is why it is one of the most important “organs”  as well as one of the major safety related parts of an automobile . A tire should always be able to withstand the carrying capacity demands of the vehicle for which it is designed. Furthermore, it must be able to transmit the highest possible tractive, braking, and lateral forces on dry roads (with the widest variety of surfaces) and in damp and wet conditions (aqua-planing).

 

Further demands from a tire are:

 

• Rigidity at high speed

• Durability

• Insensitivity to scuffing damage

• Abrasion resistance

• Low rolling resistance

• Low noise level

• Handling characteristics

• True running

• Easy fitting

• Low price

 

Tire construction

 

 The fundamental materials of modern tires are rubber and fabric along with other  compound materials. They consist of a tread and a body. The tread provides traction while the body ensures support. Refer to the cross section of the tire in the picture below in order to understand the different parts of the tire.

 

 



 

 

 

 

 

 

1.The inner tire. This is the tire's version of an inner tube and stops air escaping usually constructed from a simple rubber.

 

2.The casing ply. These are fine cords, usually made from Aramid fibers such as Twaron and Kevlar. They are laid in straight lines on almost all bicycle tires, many motorcycle tires, and many tires for large vehicles such as buses, heavy trucks and tractors are designed for use with inner tubes. Inner tubes are doughnut-shaped balloons made from an impermeable material, such as soft, elastic synthetic rubber, to prevent air leakage. The inner tubes are inserted into the tire and inflated to retain air pressure.


3. Bracing ply. The bracing ply is usually constructed of a very fine steel cord in a rubber sandwich. Two or more plies are stuck together across the tread, at opposing angles of around 60 degrees. When the tire is cooked the opposing steel cords form triangles which keep the tire rigid. The formation of this structure is the key to the tires characteristics as they have to be strong enough to resist the forces generated by driving but soft enough to absorb bumps in the road. To make this work, steel has to be bonded with rubber which is a very difficult process.

 

4. Safety ply. This cord keeps the tire in shape at speed and reduces the effect of friction heating.

 

5. Tread. This is the part that is contact with the road surface. Not only does the rubber have to be soft enough to generate grip by "keying" into the road and clearing water, but it also needs to ensure it doesn't heat up too much to avoid unnecessary wear. This thick rubber, or rubber composite compound is formulated to provide an appropriate level of traction that does not wear away too quickly. 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. Tread patterns feature non-symmetrical (or non-uniform) lug sizes circumferentially in order to minimize noise levels at discrete frequencies. Sipes are valleys cut across the tire, usually perpendicular to the grooves, which allow the water from the grooves to escape to the sides in an effort to prevent aqua-planing.

 

6. The shoulder / sidewall. Moving round to the side of the tires these further help absorb bumps and give the tire it's turn in characteristics. The sidewall is that part of the tire that bridges between the tread and bead. The sidewall is largely rubber but reinforced with fabric or steel cords that provide for strength and flexibility.

The sidewall transmits the torque applied by the drive axle to the tread in order to create traction. The sidewall, in conjunction with the air inflation, also supports the load of the vehicle.

Sidewalls are molded with manufacturer-specific detail, government mandated warning labels, and other consumer information, and sometimes decorative ornamentation. The shoulder is that part of the tire at the edge of the tread as it makes transition to the sidewall.

7. The bead. The bead is where the tire joins the wheel rim.The bead is typically reinforced with steel wire and compounded of high strength, low flexibility rubber. The bead seats tightly against the wheel rim to ensure that a tubeless tire holds air without leakage. The bead fit is tight to ensure the tire does not shift circumferentially as the wheel rotates. The width of the rim in relationship to the tire is a factor in the handling characteristics of an automobile, because the rim supports the tire's profile.

Applications

Specialized tires will always work better than general/all purpose/all weather tires when being used in the conditions the specialized tires are designed for.

Tires used for tarmac are of a different rubber composition than those used for gravel, likewise winter tires differ from summer tires.

Reading the tire sidewall

This is probably the number one question I get asked - "how do I read my tire?" It is confusing isn't it? All numbers, letters, symbols are mysterious codes. Actually, most of that information in a tire marking is surplus to what you need to know. So here's the important stuff:

 

 

 

Key information - 215/65 r15 96v

215

The width of the tire in mm

65

The aspect ratio of the tire as a percentage of the width. In this case the sidewall is 65% of 215mm, or 139.75mm

R

Radial construction, as all modern tires are.

15

The size of the wheel in inches

96

The load index of the tire in kgs. (see table 1)

V

The speed index of the tire (see table 2)

 

 

 

Date of manufacture

The recommend life for a tire is around 6 years (depending on storage conditions.) The first 2 numbers in the date code refer to the week the tire was made, and the 3rd and 4th numbers refer to the year.

Rotation direction

If the tire is directional this shows which way the tire should rotate when mounted.

Side

If the tire is asymmetric the tire will have 'inside' or 'outside' printed on the tire wall, showing which way round it should be mounted on the car.

 

Speed Rating

The speed rating is the maximum service speed of a passenger car tire. Here is a list of rating indicators and their mile-per-hour equivalents. This rating system applies to all tire manufacturers.

Rating

Maximum Speed

Q

99 mph

160 kph

R

106 mph

170 kph

S

112 mph

180 kph

T

118 mph

190 kph

U

124 mph

200 kph

H

130 mph

210 kph

V

149 mph

240 kph

W

168 mph

270 kph

Y

186 mph

300 kph

Z

Above 149 mph

Above 240 kph

 

 

 

Load Rating table

 

 

The load index on a tyre is a numerical code associated with the maximum load the tyre can carry. Below is a sample chart of few load indecies.

 

Load Index

Charge (KG)

Charge (LBS)

89

580

1279

90

600

1323

91

615

1356

92

630

1389

93

650

1433

94

670

1477

95

690

1521

96

710

1565

97

730

1609

98

750

1653




 




 

 

 

 

 

 

 

 

 

 

 

 

 

Tire Care and Tire Life

 

Tires are consumable items that wear during normal use and have a useful service life. With that in mind you might think tires don't need much attention, but how you treat your tires can drastically increase, or reduce the tread life. Current law requires that car tires must have a minimum of 1.6mm of tread in a continuous band throughout the central ¾ of the tread width over the entire circumference of the tire. Tires have wear markers at regular intervals in the grooves of the tread - once the main surface of the tire is level with these markers it is time to replace your tire.

Fortunately there are many ways you can increase the life of your tire and with a few weekly checks you can ensure you get the maximum from your expensive rubber

Tire pressures

Tire pressures should be checked weekly, when the car has been standing for at least an hour to ensure the tires are cool. Under inflation can cause excessive wear on the outside of the tires, heat build up in the shoulders and even sudden deflation (blowout) at high speeds. Over inflation causes excessive wear down the centre of the tires tread shortening the tread life.

 

 

Alignment

Alignment is a general term covering toe, camber and caster.

 

Toe is how parallel the wheels are if viewed from above. Usually cars have neutral toe, or a little positive toe to help straight line stability. If toe is out of alignment the inside or outside of the tires will wear extremely quickly.

Camber is how much the tires lean inwards from bottom to top. Like toe, too much negative or positive camber can cause excessive tire wear and upset the handling of your car.

 

Balancing

Wheel balancing might not increase the life of your tire, but it can help increase the life of your suspension components. When a wheel / tire is out of balance you will feel it as a vibration at certain speeds either through the steering wheel, or through the car itself. A quick visit to your dealer or local garage to have the tires rebalanced will usually cure this.

Other causes of excessive tire wear:

 

Driving style

How you drive your car can have a massive impact on tire wear. A tire that would last over 30,000 kms. if driven carefully up and down motorways could quite easily be on the wear blocks in 3000 kms. if slid around every round about you encounter!

 

Overloaded car

The larger the load you place on your tires, the greater the wear. Equally, if carrying a large amount of luggage and / or 4 or more passengers the tires pressure needs to be increased to cope with the extra weight

 

 

Wrong load index

Relating to the above, the load index of a tire is the maximum load rating the tire is designed to work with. Placing the wrong load index tire on your car can lead to excessive wear, and in certain cases dangerous handling.

 

The tire industry was first started in the early 20th century, and grew in tandem with the car industry. Today, over 1 billion tires are produced annually, in over 400 tire factories, with the three top tire makers commanding a 60% global market share.

 

To conclude, keeping your car’s tires in good running condition will decrease the number of road accidents each year. The tire plays a big role in the safety of the car and its passengers.

 

 

 

Written by: Pascal Hayek

 

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