Car Tyre (Tire) rotation
Tire rotation or rotating tires is the practice of moving automobile wheels and tires from one position on the car, to another, to ensure even tire wear. Tire wear is uneven for any number of reasons. Even tire wear is desirable to maintain consistent performance in the vehicle and to extend the overall life of a set of tires.
By design, the weight on the front and rear axles differs which causes uneven wear. With the majority of cars being front-engine cars, the front axle typically bears more of the weight. For rear wheel drive vehicles, the weight distribution between front and back approaches 50:50. Front wheel drive vehicles also have the differential in front, adding to the weight, with a typical weight distribution of no better than 60:40. This means, all else being equal, the front tires wear out at almost twice the rate of the rear wheels, especially when factoring the additional stress that braking puts on the front tires. Thus, tire rotation needs to occur more frequently for front-wheel drive vehicles.
Turning the vehicle will cause uneven tire wear. The outside, front tire is worn disproportionately. Cloverleaf interchanges, and parking ramps turn right in left hand drive (otherwise known as right hand traffic) countries, causing the left front tire to be worn faster than the right front. Furthermore, right turns are tighter than left turns, also causing more tire wear. Conversely the sidewalls on the right tire tends to be bumped and rubbed against the curb while parking the vehicle, causing asymmetric sidewall wear. The symmetric opposite occurs in countries that drive on the left.
In addition, mechanical problems in the vehicle may cause uneven tire wear. The wheels need to be aligned with each other and the vehicle. The wheel that is out of alignment will tend to be dragged along by the other wheels, causing uneven wear in that tire. If the alignment is such that the vehicle tends to turn, the driver will correct by steering against the tendency. In effect the vehicle is constantly turning, causing uneven tire wear. Also, if a tire is under or over-inflated, it will wear differently than the other tires on the vehicle. Rotating will not help in this case and the inflation needs to be corrected.
Car manufacturers will recommend tire rotation frequency and pattern. Depending on the specifics of the vehicle, tire rotation may be recommended every 8,000 km (5,000 mi). The rotation pattern is typically moving the back wheels to the front, and the front to the back, but crossing them when moving to the back. If the tires are unidirectional, the rotation can only be rotated front to back on the same side of the vehicle to preserve the rotational direction of the tires. Most unidirectional tires can be moved from side to side if they are remounted; tires with asymmetric rims are a rare exception. More complex rotation patterns are required if the vehicle has a full-size spare tire that is part of the rotation, or if there are snow tires.
To clarify; the pattern for asymmetrical tires to be rotated, or positioned, is for the tires on the driving axle of two-wheel drive vehicles to remain on the same side of the vehicle as they are moved to the non-driving axle, and for the tires on the non-driving axle to cross over to the opposite side of the vehicle as they are placed onto the driving axle. For all-wheel drive (AWD) vehicles and four-wheel drive (4WD) vehicles, it is recommended that the tires from each axle cross over as the fronts move to the rear and rears to the front. For the 4WD configuration, it is dependent upon how much 4WD driving is actually performed, whether it's controlled via mechanical/computerized devices or in vehicle controls, the recommendation will likely be found in the Owner/Operator manual or can be obtained by speaking to the manufacturer or dealership.
Current thinking stresses the desirability of keeping the best tires on the rear wheels of the vehicle, whether it is front, or rear wheel drive. The reason for this is that if the rear wheels lose grip before the front ones, an oversteer condition will occur, which is harder to control than the corresponding understeer which will happen if a front wheel is lost. This is also the case if a tire blows out, so the intuitive belief that the front steering/driving tires need to be the best quality is not the case. A rear tire blowout will cause the vehicle to become very difficult to control, especially at highway speeds. It also greatly increases the risk of rollover due to yawing, a condition where the rear of the vehicle swings out and becomes perpendicular to the direction of travel. Yawing will cause the tire to separate from the rim, the rim can then dig into the pavement, or dirt and grass if the vehicle is no longer on the road, which will trip the vehicle and cause a rollover.
In some cases (for example, BMW), automobile manufacturers may recommend performing no tire rotation at all. Additionally, some vehicles are designed (or retrofitted) with front and rear wheels of different sizes and unidirectional rotation treads, making rotation impossible.