Winter tire review; Tire rotation

Tire rotation or rotating tires is the practice of moving automobile tires from one wheel 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.

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 has 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 right hand drive 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.

Manufacturers will recommend tire rotation frequency and pattern. Depending on the specifics of the vehicle tire rotation may be recommended every 12,000 km (7,500 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.

Current thinking stresses the desirability of keeping the best tires on the rear wheels of the vehicle, whether it is front or rear 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 actually the case.

In rare cases, automobile manufacturers may recommend performing no tire rotation at all (eg BMW MINI).


Snow tires for bmw; Siping

For the Chinese city, see Siping (city)

Siping is a process of cutting thin slits across a rubber surface to improve traction in wet or icy conditions.

Siping was invented and patented by John Sipe in the 1920s. Sipe worked in a slaughterhouse and grew tired of slipping on the wet floors. He found that cutting slits in the tread on the bottoms of his shoes provided better traction than the uncut tread.

The process was not applied to vehicle tires on a large scale until the 1950s, when superior tread compounds were developed that could stand up to the siping process. On roads covered with snow, ice, mud, and water, sipes usually increase traction. A US patent to Goodyear claimed sipes improve tire traction as well, and tend to close completely in the tire “footprint” on the road. A 1978 study by the US National Safety Council found siping improved stopping distances by 22 percent, breakaway traction by 65 percent, and rolling traction by 28 percent on glare ice[1].

Tire tread block shapes, groove configurations, and sipes affect tire noise pattern and traction characteristics. Typically, wide, straight grooves have a low noise level and good water removal. More lateral grooves usually increase traction. Sipes are small grooves that are cut across larger tread elements. Up to a point, more sipes give more traction in snow or mud.

As is often the case, there are compromises. Winter tires, and “mud and snow” tires, may have thousands of sipes and give good traction. But, they may feel “squirmy” on a warm, dry road. Treadless racing “slicks” on dry roads give maximum traction. These have no sipes, no grooves, and no tread blocks. They also have very poor traction on wet surfaces. Tire manufacturers use different tread rubber compounds and tread designs for different tires’ usages.

Large sipes are usually built into the tread during manufacturing. Sipes may also be cut into the tread at a later date, called “microsiping”. Bandag developed a machine for microsiping which places a curved knife blade at a slight angle on a rotating drum. The drum is placed so when it is pressed against the tread the tire is pressed into an exaggerated hollow, as if driving down a rail. The drum is lubricated and rotated and the knife makes a series of diagonal cuts across the tread. For improved traction, the tire may be siped twice, leaving diamond-shaped blocks. A significant problem with field siping is that the tread picks up rocks, glass, and other hard road debris in use, and even with thorough cleaning the knife service life is often poor.

Microsiping can dramatically improve tire traction in rain and snow. However, microsiped tires may also have increased road noise and tire wear when operated on dry surfaces.

In Massachusetts in the 1970’s, it was legal to operate a school bus with bald tires, provided they were double microsiped.

Both Bridgestone and Michelin sell snow tires that are siped at the factory, while Saf-Tee Siping and Grooving sells machines that can sipe most standard vehicle tires. Siping can also be done by hand. Be aware that siping the tires can void the manufacturer’s warranty.


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