SUSPENSION AND TIRES
by Mark Davidson

SUSPENSION OVERVIEW
The suspension of a vehicle is used to allow the tires to react to changes in the road surface and support the chassis of the vehicle. The suspension allows the wheels to move up and down while dampening the transfer of this movement to the rest of the vehicle, including the occupants. Independent suspension is used in the front ends of passenger cars and most light trucks. Some vehicles use four wheel independent suspension. Independent suspension allows one wheel to react to surface variation without affecting the other wheels. Each wheel is attached to the chassis using individual suspension components.

TIRES
The tires of an automobile provide traction for starting, stopping and driving. They are used to support the weight of a vehicle and as an aid to the operation of the vehicles suspension. Passenger car tire sizes in the U.S. today, are rated in the P-Metric system. The rating can be found on the side wall of the tire. This is an alpha-numeric rating system that uses a letter or number to represent size, usage or construction of a tire. An example is P215/75R15. The first letter is an indication of the usage of the tire, P would stand for passenger car, LT would be a light truck. 215 is a rating in millimeters for the section width of a tire. The number after the slash represents the aspect ratio of the tire and the tires sidewall height in relation to its width. The R designates the construction of the tire, in this case it stands for radial. If a letter were to preceded this letter, it would indicate a speed rating of the tire. The last number indicates the rim size that the tire is intended for.

Other information that can be found on the tire side wall is the Uniform Tire Quality Grading designation. This is a government tire rating system used by manufacturers to provide tire comparison information. With the UTQG rating system, information is given to compare tire treadwear, traction and temperature resistance. Treadwear is a measure of tire durability. 100 would be a tire rated for standard resistance to treadwear. A rating of 300 would be a tire with three times the treadwear resistance of a standard tire. This rating should be used to compare tire treadwear within a manufacturer, not between manufacturers.

The traction rating is a grade classifying a tires ability to stop in a straight line on wet surfaces, concrete and asphalt under test conditions. An A rating means the tire meets maximum DOT standards. A B rating means the tire meets medium DOT requirements. A C rating indicates a tire meets minimum requirements. There are three grades used to measure a tires resistance to generation of heat due to friction. Grade A indicates a tire that can withstand temperatures generated at 115 mph for 30 minutes. Grade B indicates a tire that can withstand temperatures generated at 100 mph for 30 minutes. Grade C indicates a tire that cannot withstand temperatures generated at 100 mph for 30 minutes.

Tire wear is affected by driving conditions, wheel alignment, balance and inflation pressure. Proper maintenance is the best way to insure longer tire life. Inflation pressure should be checked weekly. Tire rotation schedules should be followed as recommended by the manufacturer. Wheel alignment should be checked if uneven tire wear is noticed or after running over a large hole or object. Aggressive driving should be avoided to extend tire life as well.

(Mark gave up on sports when the Browns left Cleveland and now spends his Sundays working under a shade tree in the back yard tuning his son’s soap box derby car.)