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Q: I have a 2005 VW Passat wagon. My right front tire has a slow leak. I have been filling it about once a week. Lately, I notice a shimmy at about 40 miles per hour. I can feel the steering wheel shaking back and forth. Below that speed, or above it, the shimmy seems to go away. Also, when I put air in the tire, the shimmy goes away for a while and then returns. Can low pressure cause a shimmy? I have been thinking about getting new tires, so maybe fixing it is not worthwhile? - Suzanne B., Minneapolis

A:The shimmy is probably caused by that wheel being out of balance. Both an automotive tire and your car's wheels have minor variations in weight as you move around their circumference. When you spin the tire-mounted wheel, these weight variations cause it to wobble. The way to eliminate the wobble and make the wheel spin smoothly and evenly is by balancing it, which a tire service shop does by placing small weights where needed along the wheel.

Your symptoms suggest that the right front wheel is out of balance and therefore wobbling. Tires with lower pressure have a lower resonance frequency. When your right tire becomes under-inflated, its resonance frequency occurs at about 40 miles per hour. That natural vibration is combining with the unbalanced wheel to create a noticeable shimmy at that speed. (Shimmy that you feel in the steering wheel usually indicates a problem at the front wheels, whereas shimmy you feel in the car seat usually means a problem at the rear wheels.) Raising the pressure in that tire raises the resonance frequency, causing the noticeable shimmy to disappear.

Depending on how long that tire has been going low, it may be wearing differently from the driver's side tire. If you're thinking about replacing the tires anyway, this suggests they're near the end of their service life. Rather than repairing and balancing a worn tire in wintertime, when good traction is most important, you should replace at least the front two tires. The service station that replaces them will balance them at the same time. Then your shimmy problem should be cured.

Have the shop that puts the tires on check your front steering and suspension components to make sure everything is in order there, too.

Winter is hard on wheels and tires for many reasons. Potholes flourish, gravel and debris gets strewn on the roads for traction, curbs are harder to see when there's snow, and the temperature drop from summer highs causes reduced tire pressure. The best thing you can do for your tires is to check the pressure at least once a month with a good gauge, rotate them about once a year according to manufacturer's instructions and monitor them for sidewall or other damage, such as punctures. Your car will ride better and your tires will last longer with routine checks and even pressure. Use the pressure recommended in your manual or on the sticker in the driver's door opening.