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Tyres
Tyres are made of vulcanized rubber which is inflated with air to support the vehicle. Vulcanisation involves curing of the rubber with a curative like sulphur at high temperatures. This makes the rubber harder, more durable and resistant to chemical reactions. Tyres grip the ground when the vehicle changes speed or direction. The part of the tyre which is in contact with the road is known as the contact patch and the friction between the road and the tyres ensures the car obeys the driver's inputs.
Approx 85% of the car's weight is supported by the air and just 15% is supported by the tyre, hence maintaining the correct tyre pressure is important. Tyre pressure is not just important for a safe ride, but also for tyre life, handling, fuel economy and tyres also minimise the side-to-side movement of the car, which can be felt in trains.
(Tyre diagram showing the various parts)
Cross ply or bias ply:
Tyres where the fabric cords run from one bead to another at an angle with respect to the center line of the tyre are
called cross ply or bias ply tyres.
Radial Tyres:
Radial tyres don't have belts that cross over each other like cross ply tyres do. Here, parallel plies radiate from one bead to another.
They are softer and offer a more comfortable ride. As the ride is soft the sidewalls are weak and there is no directional stability, and so
stiff belts of steel or fabric run around the circumference of the tyre between the plies and the tread.
Tube Tyres:
Tube tyres have an inner tube in between the rim and the tyre. It's this tube which has the air filled in it. When the tyre is punctured,
it's the tube which loses air immediately and the tyre goes flat. This tube is doughnut-shaped and made of rubber. It has a valve which protrudes
through a hole in the rim. To repair a punctured tube, it has be taken off from the tyre and rim completely. If a nail punctures the tyre tread,
then the tube could have multiple punctures as the tube gets deflated and rotates within the tyre.
Tubeless Tyres:
Tubeless tyres don't have tubes like the tube tyres. The tyre is sealed to the rim, to lock all the air between the tyre and the rim. If a nail
penetrates the tread, the air in the tyre doesn't escape as quickly as it would in a tube tyre. The air will only rush out when the nail is removed.
Tyre Markings:
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Tyre Size details:
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185 stands for the tyre's width in mm from sidewall to sidewall, or as viewed from above |
65 stands for the aspect ratio of the tyre |
H is the speed rating of the tyre |
R is the type of construction: in this case, radial type |
13 is the diameter of the tyre in inches |
Load Index:
Load Index (LI) is the maximum amount of load your tyres can carry. This is valid only up to the speed of 210kph. At 210+kph the load-carrying
ability of the tyre reduces.
The load index is easy to calculate. All you need to do is know the weight of your car and distribute the weight equally among the tyres.
Let us consider an example, if the car weighs 1000kgs, then divide it by 4(4 tyres). So 1000/4= 250kg. So the Load Index of the tyre is
60 (see table.) It is better to have a tyre with a high load index if you carry a lot of things in your car. Normally, tyres for cars have
a high load index so that even at high speeds they do not undergo much stress and do not fail or rupture. They will fail theoretically at speeds
and loads which cannot be attained by the car. Most tyres have a load index between 75 and 105.
(Load Index Table)
LI |
KG |
LI |
KG |
LI |
KG |
LI |
KG |
LI |
KG |
LI |
KG |
| 50 | 190 | 70 | 335 | 90 | 600 | 110 | 1060 | 130 | 1900 | 150 | 3350 |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 51 | 195 | 71 | 345 | 91 | 615 | 111 | 1090 | 131 | 1950 | 151 | 3450 |
| 52 | 200 | 72 | 355 | 92 | 630 | 112 | 1120 | 132 | 2000 | 152 | 3550 |
| 53 | 206 | 73 | 365 | 93 | 650 | 113 | 1150 | 133 | 2060 | 153 | 3650 |
| 54 | 212 | 74 | 375 | 94 | 670 | 114 | 1180 | 134 | 2120 | 154 | 3750 |
| 55 | 218 | 75 | 387 | 95 | 690 | 115 | 1215 | 135 | 2180 | 155 | 3875 |
| 56 | 224 | 76 | 400 | 96 | 710 | 116 | 1250 | 136 | 2240 | 156 | 4000 |
| 57 | 230 | 77 | 412 | 97 | 730 | 117 | 1285 | 137 | 2300 | 157 | 4125 |
| 58 | 236 | 78 | 425 | 98 | 750 | 118 | 1320 | 138 | 2360 | 158 | 4250 |
| 59 | 243 | 79 | 437 | 99 | 775 | 119 | 1360 | 139 | 2430 | 159 | 4375 |
| 60 | 250 | 80 | 450 | 100 | 800 | 120 | 1400 | 140 | 2500 | 160 | 4500 |
| 61 | 257 | 81 | 462 | 101 | 825 | 121 | 1450 | 141 | 2575 | 161 | 4625 |
| 62 | 265 | 82 | 475 | 102 | 850 | 122 | 1500 | 142 | 2650 | 162 | 4750 |
| 63 | 272 | 83 | 487 | 103 | 875 | 123 | 1550 | 143 | 2725 | 163 | 4875 |
| 64 | 280 | 84 | 500 | 104 | 900 | 124 | 1600 | 144 | 2800 | 164 | 5000 |
| 65 | 290 | 85 | 515 | 105 | 925 | 125 | 1650 | 145 | 2900 | 165 | 5150 |
| 66 | 300 | 86 | 530 | 106 | 950 | 126 | 1700 | 146 | 3000 | 166 | 5300 |
| 67 | 307 | 87 | 545 | 107 | 975 | 127 | 1750 | 147 | 3075 | 167 | 5450 |
| 68 | 315 | 88 | 560 | 108 | 1000 | 128 | 1800 | 148 | 3150 | 168 | 5600 |
| 69 | 325 | 89 | 580 | 109 | 1030 | 129 | 1850 | 149 | 3250 | 169 | 5800 |
Speed Ratings:
Tyres are rated with a speed rating alphabet with these ratings indicating the maximum speed up to which the tyres can survive a ten-minute
test without actually undergoing any structural failure.
(Speed Rating Table)
| Speed Symbol | Max Speed Cability Km/h | Speed Symbol | Max Speed Cability Km/h |
| L | 120 | S | 180 |
|---|---|---|---|
| M | 130 | T | 190 |
| N | 140 | U | 200 |
| P | 150 | H | 210 |
| O | 160 | V | 240 |
| R | 170 | *ZR W | 270 |
| Y | 300 | ||
| Z | 240+ |
ZR Notation:
Tyres which have a speed rating of above 240kph may have a ZR notation on them. But the tyres having a speed rating of above 300kph will possess a ZR notation along with them. If the tyre service manual description isn't apparent, then ask a professional to obtain the manufacturer's recommendation.
Tyre Profile:
Tyre profile is also known as aspect ratio. We have probably heard people talk about low profile tyres, specially the ones who like performance modifications. If the aspect ratio is 50, then it means that the sectional height of the tyre (From the rim to the tyre tread) is 50% of the sectional width of the tyre.
Types of Tyres:
Performance Tyres:
Performance tyres are designed for sports cars or for people who drive swiftly. These tyres are softer and have lesser life compared to normal tyres.
These tyres are suited for dry conditions as the treads of these tyres don't dissipate water to the extent that normal tyres do. As they are softer,
they grip better (in dry conditions) and are better at cornering. The best example of performance tyres are slicks which are used in racing.
The slicks have no tread on them. The contact patch of the tyre with the road is more resulting in better grip. But if these very tyres are used in
wet conditions, they won't be able to pump out the water on a wet road . A Formula 1 slick tyre is very soft and will not last beyond 200-300km.
These tyres grip more at higher speeds and are able to withstand higher temperatures. Performance tyres are differentiated by high (usually “H” or
higher) speed ratings and low aspect ratio (profile).
All Weather Tyres:
All weather tyres are used by manufacturers on most of their cars. These are the normal tyres found on all cars which are intermediate. There are a
compromise between slick tyres and wet weather tyres. The tread design is also intermediate to let the water through when driving on a wet road.
A performance tyre is softer compared to this tyre and corners better, but the common man won't feel that difference while cornering. These tyres
are harder and have a long life.
Wet Weather Tyres:
Wet weather tyres are softer than performance tyres. These tyres need to get up to a certain temperature to grip well. They are the best tyres for
rainy weather. The treads are designed to diffuse maximum amount of water coming in its way between the tyre and the road.
Snow & Mud Tyres:
Snow tyres have a deeper tread and suitable for snowy or icy terrain. The deep tread is for more grip as other tyres will lose traction on normal
roads. These tyres make a lot of noise due to their tread design and wear out the tyre and the road if driven quickly on dry surfaces.
All Terrain Tyres:
A tyre commonly used on SUV's / 4WD vehicles and is designed to provide good compromise between off-road capabilities and on road manners.
Mud Tyres:
These tyres have extra deep treads which help them grip on a muddy track with loose mud where the normal tyres loose traction. These tyres cannot be
used on normal roads as the treads and the road will wear out and the tread noise will be excessive.
