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

  CORRESPONDENCE

   

  Original tire size : 145 155 165 175 185 195 205 215 225 235 245 255 265 275 285 295 305 315 325 335 345 30 35 40 45 50 55 60 65 70 75 80 85 R 13 14 15 16 17 18 19 20 21 22 23 24	New dimension : 155 165 175 185 195 205 215 225 235 245 255 265 275 285 295 305 315 325 335 345 30 35 40 45 50 55 60 65 70 75 80 85 R 13 14 15 16 17 18 19 20 21 22 23 24
  Enter a speed to know the gap : Km/h

         

  tire width :	cm.	new tire width :	cm.
  tire height :	cm.	new tire height :	cm.
  tire diameter :	cm or in.	new tire diameter :	cm or in.
  at Km/h the new tire diameter is equivalent to Km/h, a difference of %.

   

• PRESSURE

    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.

• DIMENSIONS

    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: 235/60/15

  235 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:

  1949 to 1964	1965 to 1970	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

  This is the famous DOT followed by a 4-digit number.

  

  Example here: 2602 DOT tire made in the 26th week of 2002.

• 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.

   

• SPACING

  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

  

• GEOMETRY

  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.

   

• HANDLING

  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.