Tyre Nomenclature – Guide to understanding your tyre better

Tyre Nomenclature – Guide to understanding your tyre better

Who doesn’t like a sweet set of rubbers on their ride? When it comes to getting a new set of tyres, most factors are overlooked. Let’s understand tyre specifications. To do so, have a look at the side of any tyre and you’ll find a code similar to one mentioned below.

 

Nomenclature:                  

205 / 50   R 19   97   W

 diag_tire_width

205  :   Width of the tyre (mm).

50    :   Section height of tyre/ Aspect Ratio

R      :   Structure of tyre (radial).

19    :   Diameter of Rim (inches).

97    :   Load Index.

W    :   Speed Rating of tyre.

 

Width of tyre: Takes no genius to figure this out; wider tyres, more ground contact and thus resulting in improved surface hold. The simplest way to improve traction is to get a wider set of tyres.

Aspect ratio: The number 50 here states that the sidewall (section height) is 50% length of the tyre width.

Structure of tyre: A tyre comes in a lot of different structures. Radial tyres have become most common lately.

Diameter of rim: One of the most aesthetically improvable factors that every person agrees to, is a good set of rims.

Load Index: A rating that implies the load carrying capacity per tyre. In simpler words, how much weight the tires can carry over them.

Speed rating of tyre: Shows the maximum speed that a tyre can safely run on.

 

Nobody using their automobiles likes to get smaller and narrower tyres. It damages the aesthetics of a vehicle terribly. We all admire a wide set of tyres containing a huge rim size. The everlasting battle between performance and efficiency still goes on. The width of a tyre plays a major role in finding the sweet spot between performance and efficiency.

Let’s say that vehicle A and vehicle B of the same make and model are fitted with 250mm and 230mm wide tyres respectively (both tyres have the same structure and diameter). The contact surface area between the tyres and the road in vehicle A is higher. Which means it will face higher amounts of rolling resistance. The rolling resistance is a vital factor that affects the handling, acceleration, braking and efficiency of a vehicle. Higher the amounts of rolling resistance, better is the grip. As for vehicle B, since the contact surface area is lesser, it goes through relatively lower amounts of rolling resistance and thus is more efficient.

Vehicle A is a better handling machine compared to vehicle B. It carries higher corner speeds, provides more traction, has better high-speed stability and better road holding capacity. It might have a minute decrease in acceleration because more rolling resistance infers that the engine will have to put more effort for accelerating when compared to vehicle B. Also, higher the contact surface area, more is the amount of road surface felt by the occupants. And more surface means more undulations-resulting in a harsher ride quality. On the other hand, vehicle B has higher efficiency, a slightly quicker acceleration time, better ride quality and lower road noise. To summarize the differences :-

Vehicle A – 250mm tyres Vehicle B – 230mm tyres
Better Handling Higher efficiency
Higher cornering speeds Lower road noise
Better braking Better ride quality
Better high-speed stability Higher acceleration
More traction  

 

A lot of automobiles we see today are modified purely for aesthetics. Few car owners get their tyres so wide that they stick out of the side body. This might look fancy but has an immensely negative impact on the car’s aerodynamic performance. These tyres directly create dead zones at high speeds resulting in increased amounts of air resistance. For this reason, it is always good to get tyres that are only wide enough to be properly aligned with the fenders or the side body of a vehicle.

The section height and rim size combined, make up the overall diameter of a tyre. Why are they important? Here’s the thing. A vehicle’s odometer, speedometer, torque and gearing settings are all based on how much distance it travels when its tyres complete one revolution. In other words all these components and their settings are based on the tyre’s circumference, which is directly dependent on the tyre’s overall diameter. Changing the overall diameter might change all these settings. Hence it becomes very important to make sure that the overall diameter remains the same as specified by the manufacturer. Generally, a 2.5% increase or decrease from the original overall diameter is tolerable when looking for a new set. A lot of websites are available today that help you know whether a new size is good for your vehicle or not. Like this one:

http://www.etyres.co.uk/how-to-change-your-tyre-size

Cornering stability is one factor that gets affected by the section height of a tyre. Lower the section height, lesser is the amount of sagging in it (side-wards motion within the sidewall) and better is the cornering stability. Different section heights are selected in vehicles depending on use (road or off-road), cushioning and cornering stability. For example, an SUV is required to have a large section height for off-roading when compared to a sedan.

Below is the chart showing Load Index and Speed Ratings to give you a lot better idea. The one thing which has more or less always remained the same since the inception of automobiles, are tyres. And the reason is that these factors massively affect the automobile’s characteristics.

 

Load Index Max Weight/tyre (kg) Load Index Max Weight/tyre (kg) Load Index Max Weight/tyre (kg) Load Index Max Weight/tyre (kg)
61 257 76 400 91 615 106 950
62 265 77 412 92 630 107 975
63 272 78 425 93 650 108 1000
64 280 79 437 94 670 109 1030
65 290 80 450 95 690 110 1060
66 300 81 462 96 710 111 1090
67 307 82 475 97 730 112 1120
68 315 83 487 98 750 113 1150
69 325 84 500 99 775 114 1180
70 335 85 515 100 800 115 1215
71 345 86 530 101 825 116 1250
72 355 87 545 102 850 117 1285
73 365 88 560 103 875 118 1320
74 375 89 580 104 900 119 1360
75 387 90 600 105 925 120 1400

 

Load Index Rating

 

 

Code Mph Km/h Code Mph Km/h
A1 3 5 L 75 120
A2 6 10 M 81 130
A3 9 15 N 87 140
A4 12 20 P 94 150
A5 16 25 Q 100 160
A6 19 30 R 106 170
A7 22 35 S 112 180
A8 25 40 T 118 190
B 31 50 U 124 200
C 37 60 H 130 210
D 40 65 V 149 240
E 43 70 Z Over 149 Over 240
F 50 80 W 168 270
G 56 90 (W) Over 168 Over 270
J 62 100 Y 186 300
K 68 110 (Y) Over 186 Over 300

 

Speed Rating

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