Tires perform several important functions and are the only part of your vehicle that comes in contact with the road. The average "footprint" of a tire in contact with the road is similar to the bottom of an average man’s shoe sole. The entire weight of your vehicle and its contents is supported by the tire and air inside the tire.
Due to this critical role of tires, they are extremely complex in their design. While designs vary by product and manufacturer, an average tire contains over 100 separate components. Tires can include natural rubber, synthetic rubber, steel, nylon, silica (derived from sand), polyester, carbon black, petroleum, etc. The combination of ingredients and processes used by different manufacturers leads to different performance characteristics for every tire in the market today. Understand the basics of a tire and rely on your owner�s manual and professional tire retailer to explain the differences between tires and recommend the right tire for you.
A huge range of tire sizes exist with many combinations of speed rating, load range, UTQG, etc. This plethora of sizes results in thousands of unique tire size combinations from multiple tire manufacturers. In this section, learn more about tire size options as it relates to your vehicle application.
If you do not know where to find the informations about your tire size, the best place to look is your vehivle itself. Modern vehicles offer many tire and wheel options so it is always best to check your vehicle itself to determine what tire you need. The first thing you need to your tire information placard. This placard is usually located on the door jamb of the driver’s door.
Next thing you need to know is your tire size and service description. This you will find on the tire itself.
One important thing: the size from the placard should match the one from the tire. The "service description" won’t match if the tires are not original.
There are two important things you need to know when changing your tires: the size and the service description.
Size: made up of three numbers: width (first number), aspect ratio (second - the sidewall height listed as a percentage of the width) followed by a "R" and third the rim size (rim diameter, listed in inches). When you need to change your tire this number shoul match exactly.
Sercive description: made up of two parts. The first is a two or three digit number indicating the "load index" of the tire: determine the weight the tire can cary. And second the load index number, a letter called "speed rating" - the top speed tire can handle. When replacing tires you should select a tire with the same or higher load index and also a speed rating that indicates the same top speed or higher than what is listed ones your tire information placard
Passenger Tire Sizes
The passenger vehicles come equipped with P-Metric and Euro-Metric tire sizes. With the P-Metric system, Section Width is measured in millimeters. To convert millimeters into inches, divide by 25.4. The Aspect Ratio (Section Height divided by Section Width) helps to provide more dimensional information about the tire size. The Euro-metric system originated in Europe and is also referred to as the European Metric System. Most European tire manufacturers build tires that conform to this system. The Metric System is very similar to the P-Metric System, except in terms of load carrying capacity. Load carrying capacities of Metric and P-Metric tires are sometimes not the same, even for two tires of the same size designation.
Alignment is one of the key maintenance factors in getting the most wear and performance from your tires. Wheel alignment provides safe, predictable vehicle control as well as a smooth and comfortable ride — free of pulling or vibration. You will get the best performance and wear from your tires when you routinely align your wheels. Poor alignment is a result of the suspension and steering systems being out of adjustment.
There are three types of wheel alignments: front-end, thrust angle and four-wheel alignment. The most complete method of alignment is four-wheel alignment. Four-wheel alignment combines front-end with thrust angle and includes a check on the rear wheels as well. Front-end alignments check only the front tires. Thrust angle checks that the wheels are "squared" to each other, preventing your car from going sideways on the road.
There are five factors involved in setting the alignment to specification: caster, camber, toe, thrust and ride height.
Caster is the angle of the steering axis (the part of the suspension that supports the wheel and tire assembly). Caster is important to steering feel and high-speed stability.
Viewed from the front of the vehicle, camber describes the inward or outward tilt of the tire. The camber adjustment maximizes the tire-to- road contact and takes into account the changes of force when a vehicle is turning. Camber is the one adjustment that can be set according to driving habits. Generally, if you drive more aggressively when cornering, more negative camber can be set. If you drive on highways and do very little hard cornering, more positive camber can be set.
Toe describes whether the fronts of the tires are closer (toe-in) or farther (toe-out) apart than the rears of the tires. Toe settings vary between front and rear wheel drive vehicles. In a front wheel drive vehicle, the front wheels try to pull toward each other when the vehicle is in motion, which requires a compensating toe-out setting. A rear wheel drive vehicle works just the opposite, necessitating a toe-in setting. Stated differently, toe is set to let the tires roll in parallel (at zero toe) when the vehicle is in motion.
Ride height is the distance between the vehicle’s frame and the road. This is the reference point for all alignment measurements. Vehicle customizing very often will include raising or lowering the vehicle.
The definition of balance is the uniform distribution of mass about an axis of rotation, where the center of gravity is in the same location as the center of rotation. A balanced tire is when the mass of the tire, when mounted on its wheel and the car’s axle, is uniformly distributed around the axle.
Balanced tires can be the difference between a good or bad driving experience. Some cars (and drivers) are more sensitive to an out of balance tire than others, but no one is happy with a vibration.
The vehicle vibration has two main causes. f it is speed dependent, increasing as your speed increases, and becoming especially noticeable around 40-45 mph, it is most likely a balance-related vibration. The second possible cause of vibration is that the tire and the wheel assemble isn’t exactly round. When the high spots on the tire and the wheel match to each other, it doubles the amount of runout , or "hop." If there is a hop the vibration will not end when you rebalance your tires. A hop can often be fixed by simply loosening the tire on the wheel and turning it 180 degrees, reinflating the tire after relubricating the bead. If the problem persists, rotate the tire another 90 degrees, and again 180 degrees if there is still vibration. Doing this allows for the high spot to be tried at each quarter of the wheel, and at one of the points, the tire should be round. From here, rebalance the tire and test drive to check for remaining vibration. If you still feel it, the problem is either in the tire itself of elsewhere in the vehicle.
Serious injury may result from explosion of the tire/rim assembly due to improper mounting - never exceed 40 psi to seat beads - mount only on designed diameter rims - only specially trained persons should mount tires.
Tire rotation can be beneficial in several ways. When done at the recommended times, it can preserve balanced handling and traction and even out tire wear. It can even provide performance advantages. High performance tires should be rotated every 3,000 to 5,000 miles, even if they don’t show signs of wear. Tire rotation can often be done with oil change intervals while the vehicle is off the ground anyway. Tire rotation helps even out tire wear by allowing each tire to serve in as many of the vehicle’s wheel positions as possible.
Usually the tires on the front need to accomplish very different tasks than the rear tires. The tasks encountered on a front wheel drive car are considerably different than those of a rear wheel drive car. Tire wear experienced on a performance vehicle will usually be more severe than that of a family sedan. Each wheel position can cause different wear rates and different type of tire wear.
Four Tire Rotation
When you replace tires in complete sets, you can maintain the original handling balance. If you replace only some of the tires, the tread depth will vary and handling may not be optimum. In addition, when you replace tires in sets you have the option of improving your tires. Tire manufacturers are constantly adding new features and introducing new tires to the market. You can take advantage of the new products when you replace all four tires at a time, rather than trying to find a match for the tires you already have on your vehicle. While beneficial in many ways, tire rotation cannot correct mechanical problems or problems caused by incorrect tire inflation.
On front wheel drive cars, rotate the tires in a forward cross pattern (A) or the alternative X pattern (B)
On rear wheel or four wheel drive vehicles, rotate the tires in a rearward cross pattern (C) or the alternative X pattern (B)
If your car has directional wheels or tires, rotate them like in D.
If your car has non-directional tires that are a different size from front to rear, rotate them as shown in E.
Proper air pressure makes tires wear evenly, prevents tire failure and increases handling and traction. While air pressure is responsible for the great task of supporting the weight of your vehicle, it is an easy aspect of your vehicle to maintain.
Correctly inflated tires receive appropriate support from the contained air pressure to provide an even distribution of load across the footprint and help stabilize the tire’s structure. And while most drivers recognize that this has a significant impact on tire wear, rolling resistance and durability, only a few realize underinflation also has a noticeable influence on how quickly and precisely the tires respond to the driver’s input.
Since tires generally lose about one psi per month, it is important to check them regularly and often. You should be able to find the recommended tire pressure for your vehicle in the owner’s manual or on your tire information sticker. Keep in mind that the tire pressure listed is a "cold" pressure. This means it should be checked in the morning before the car has been driven. Also, remember if you check your pressure inside an attached or heated garage, you will lose pressure when you enter the colder air outside the garage. Add 1 psi for every ten degree difference in advance to account for the temperature change.
Air Presure Adjustment for High Speed Driving
the tires on the vehicle should be properly sized, inflated and inspected if you plan to drive fast because the tires will be subjected to tremendous stresses.
Because of the weight they bear, pneumatic tires’ sidewalls bulge and their treads flatten as they roll into contact with the road. This results in dimensional difference between the tire’s "unloaded" radius (i.e., between the center of the axle and the top of the tire) and its "loaded" radius (between the center of the axle and the road). The engineer’s call the difference between the two radii "deflection." Increasing vehicle speed will cause the tires to deflect quicker and increasing vehicle load will cause the tires to deflect farther (if tire pressure isn’t increased).
If a vehicle’s horsepower and top speed are increased significantly, confirm that the required adjustments are made to the tire pressure and/or load conditions beyond the vehicle manufacturer’s recommendations using the following charts.