Once. If the wheels on your vehicle are correctly aligned when the vehicle is manufactured at the factory, they should not change car alignment until something in the suspension wears out or is damaged. Alignment doesn't change. The only thing that changes it is wear or damage. Hitting a pot hole or a thousand pot holes won't knock your suspension out of alignment unless you hit something hard enough to actually bend metal. That really doesn't happen very often, so having the wheels aligned periodically is a waste of money.
On the other hand, there are valid reasons for having the alignment checked periodically:
If your tires are wearing abnormally, alignment should be checked to find out why. Chances are something is amiss and needs to be readjusted or replaced. It only takes a 1/8 inch of toe misalignment to drag the front tires sideways the equivalent of 28 feet for every mile traveled!
If you're buying a new set of tires and want to maximize tread life, it's a good idea to have the alignment checked as insurance. Even if the factory alignment is within the acceptable range specified by the vehicle manufacturer, there's often room for improvement. Resetting alignment to the "preferred specs" (which means the midrange or optimum specs) will usually extend tire life -- sometimes significantly. Considering the high cost of many performance tires today, assuring maximum tire life with an alignment is money well spent.
If you're experiencing any kind of steering or handling problem, an alignment check may be necessary for diagnostic purposes. An important aspect of aligning the wheels is performing a preliminary alignment inspection of the suspension and steering linkage. This is necessary to determine if there are any worn, damaged or mislocated parts. It's impossible to realign worn or damaged parts so any such parts must be replaced before the wheels can be realigned.
Wheel alignment is also required when certain suspension and steering components are replaced. On most cars with MacPherson struts, the front wheels should be realigned if the struts are replaced (NOTE: This is not necessary on certain import vehicles that have replaceable strut cartridges). Alignment is also required if the tie rods, tie rod ends, idler arm, steering links, control arms or control arm bushings, steering knuckle or steering rack have been replaced.
Another benefit of having the wheels aligned is to assure optimum handling and traction for driving safety. Camber, in particular, is a very important angle with respect to keeping the tire's treads in full contact with the road. Tires that lean in or out ride on the shoulder and reduce traction, cornering ability and tread life. Camber can even affect braking. Uneven camber or caster side to side can make a vehicle lead to the left or the right.
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