RIMS, TIRES and GEOMETRY
Knowing to "read" a tire is important when you want to change it. Replace 14-15" wheels by 17" or more is mechanically simple, with just a few screws or nuts apart. But choosing the right tire size and just the right tire is not as simple. We must know the characteristics of the original tires and also the performance of the vehicle.
- THE TIRE
Debatable subject because it is more a matter of personal taste and felt the steering wheel as true of a scale or other, but it should still be taken to control the data base constructor. Personally I put between 2.2 and 2.5 bar.
In 60-70 years, indicators of tire size was not as currently understood. Manufacturers were using letters to denote the widths of tires.
Exemple in US size: GR70-15 (equivalence 225/70/15)G is the load code.R is for Radial structure.70 is the height of the sidewall in % of the width.15 est le diamètre interne du pneu (ou celui de la jante) en pouces.
Exemple in metric: 235/60/15235 is the width of the tire in millimeter.60 is the height of the sidewall in % of the width, here 141mm.15 is the inside diameter of the tire (or the outside rim) it's in inche, here 381mm.
Correlation table between tire size European/US size:
1970 to 1980 since 1960 since 1980 NUMERIC (in inches) ALPHA NUMERIC EURO-METRIC P-METRIC EURO-METRIC or P-METRIC serie 80 à 90 serie 50 - 78 serie "82" serie 75-80 série 70 serie 65 serie 60 serie 50 5.60-14 6.00-14 145R14 175/65R14 6.00-14 6.45-14 155R14 175/70R14 185/65R14 B 165R14 P175/75R14 185/70R14 195/65R14 205/60R14 6.50-14 6.95-14 C 175R14 P185/75R14 195/70R14 215/60R14 7.00-14 7.35-14 E 185R14 P195/75R14 205/70R14 225/60R14 245/50R14 7.50-14 7.75-14 F 195R14 P205/75R14 215/70R14 235/60R14 8.00-14 8.25-14 G 205R14 P215/75R14 225/70R14 245/60R14 265/50R14 8.50-14 8.55-14 H 215R14 P225/75R14 5.60-15 A 155R15 P155/80R15 185/65R15 195/60R15 6.00-15 B 165R15 P165/80R15 185/70R15 195/65R15 205/60R15 225/50R15 6.50-15 6.85-15 C 175R15 195/70R15 205/65R15 215/60R15 7.35-15 E 185R15 P195/75R15 205/70R15 215/65R15 225/60R15 6.70-15 7.75-15 F 195R15 P205/75R15 215/70R15 235/60R15 7.10-15 G 205R15 P215/75R15 225/70R15 235/65R15 245/60R15 265/50R15 7.60-15 H 215R15 P225/75R15 235/70R15 255/60R15 275/50R15 8.00-15 8.85-15 J 225R15 P225/75R15 235/70R15 265/60R15 9.15-15 L 235R15 P235/75R15 255/70R15 275/60R15 295/50R15
Note one important thing about the tire width. It is not because a tire has a size 295/50/15 that is necessarily 295mm wide on the ground. For example, my rear tires were 295/50/15 but the tire surface in contact with the ground measured only 250-255mm! This is due to the shape of the tire sidewalls that have a very round profile (or called "balloon"). Modern tires have a profile "square" and therefore, a 245/45/17 tire has a footprint of about 240mm (depending also the width of rim).
- INDICES (UTQG*)
*U.T.Q.G. (Système Uniformisé de Classement de la Qualité des Pneus)
On a tire, there is much information that it is good to know. Most of us just look at that size because it is the only thing easy to understand (and again). Few know where and what a load index or speed, much less the hardness or adhesion. Do not panic! I'll try to explain it all clearly.
First, the speed rating. It expresses with letters. Here are the most commonly used.
Speed Rating Speed in km/h S 180 T 190 U 200 H 210 V 240 ZR >240 W 270 Y 300
Second, the load index. This time, they are numbers that represents it. Below, the most common. It corresponds to the maximum permissible load per tire according to a speed set.
load index 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 load in kg 450 462 485 487 500 515 530 545 560 580 600 615 630 650 670 690
load index 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 load in kg 710 730 750 775 800 825 850 875 900 925 950 975 1000 1030 1060 1090
Other important indicators are on one side of the tire, ie the hardness of rubber, the grip index and temperature index.
These are the names appear in english TREADWEAR, TRACTION and TEMPERATURE.
TREADWEAR : Treadwear index is the rate of wear of a tire. This index is obtained by a test circuit on a distance of 9600 km, under controlled conditions. This index is between 60 and 620 and the reference value is 100. For example a tire with a Treadwear of 50 will wear two times faster than a 100 tire when a tire with a Treadwear of 420 will wear 4.2 times slower.
TRACTION : The Traction Index is a tire grip on wet and straight. This index is expressed by the letters AA (highest index), A, B and C (lowest index). The index C is the minimum acceptable.
TEMPERATURE : The temperature index is the tire's resistance to heat and its ability to dissipate heat. This index is obtained by a laboratory test on a test wheel. Excessive heat can cause degradation of the tire. This index is indicated by the letters A (the highest score and best), B (average index) and C (lowest index). The index C is the minimum required by law. The temperature index is applied to a tire properly inflated (or underinflated, nor over-inflation) under conditions of use "normal" (no excessive speed and not overload).
- DATE OF MANUFACTURE
- THE RIM
DIAMETER AND WIDTH
Some technical terms have to know about the rims.
First, the diameter. It is expressed in inches (1 inch = 25.4 mm) and diameter "D" does not include the edge of the rim. We talk about the diameter "seat" the tire is where the tire is received.
The width of the rim is also expressed in inches and just like the diameter, the edges are not included.
Nothing like a little drawing to show how to measure the spacing on a rim. Think cross-tightening!
- OFFSET-ET and BACKSPACING
The offset (offset-ET) is simply the distance between the timeline of the wheel relative to the longitudinal axis of symmetry of the latter. This value can be positive or negative. When the value is positive, the timeline is shifted outward. It has very little "edge" outside. When it is negative, the timeline is shifted to the inside of the rim. it was a "big edge."
The backspace (or withdrawal) is the distance between the timeline and the edge of the rim to the inside (under the rim flange itself).
Click on the diagram to better understand
CAMBER: This is the angle between the vertical axis of a wheel and perpendicular to the ground. The camber can be positive or negative depending on the nature of the angle.
CASTER: This is the angle between the steering pivot the wheel relative to a perpendicular to the ground. Hunting is negative when the angle is directed to the rear of the car.
THE TOE: This is the angle between the longitudinal axis of a wheel and the chassis. The top of this light is directed toward the front of the car. Conversely, the opening is the angle between the longitudinal axis of a wheel and the chassis. The top of this light is directed toward the rear of the car.
UNDERSTEER: It's when the car is a tendency to pull straight into a turn. This is due to lack of grip of the front and / or a bad front axle geometry.
OVERSTEER: This is when the vehicle is do a spin or donut (very common on our cars US). It is mainly caused by poor distribution of weight (too much weight on the front) which leads to a precarious grip of the rear wheels, especially on a drive. To remedy this, we must increase the support on the rear axle through do a fin for example, soft rubber tires and weight in the trunk. That is why often the battery is relocated to the rear.