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85 – the load index, a maximum of 515 kg (1,135 lb) per tire in this case; H – the speed index, this means the maximum permitted speed, here 210 km/h (130 mph) The tires on a Hummer H1 might be labeled: 37X12.5R17LT. 37 – the tire is 37 in (940 mm) in diameter; 12.5 – the tire has a cross section of 12.5 in (320 mm)
These represent the tire's resistance to the generation of heat at speed. Tires graded A effectively dissipate heat up to a maximum speed that is greater than 115 mph. B rates at a maximum between 100 mph and 115 mph. C rates at a maximum of between 85 mph to 100 mph. Tires that cannot grade up to C or higher cannot be sold in the US. [9]
The tire is indexed into the test station and placed on the lower chuck. The upper chuck lowers to make contact with the upper bead. The tire is inflated to the set point pressure. The load wheel advances to contact the tire and apply the set loading force. The spindle drive accelerates the tire to the test speed.
The wet grip index (WGI) is calculated from the results of two tests specified in the regulations. The first test measures the maximum achievable average deceleration of a vehicle as it slows from 85 ± 2 km/h (52.8 mph) to 20 ± 2 km/h (12.4 mph). The second test (the "skid trailer" test) is usually performed using a tow vehicle and trailer.
The inner width of the rim on which the tire is mounted should be about 65% of the tire's nominal section width for tires smaller than 30 mm and 55% for those larger. The section height of a tire is usually identical to its section width (for tires less than 28 mm, 2.5 mm have to be added to the width to get the height).
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