I assume that you’re aware that most cars have four tires. Beyond that, most drivers are out of their depth. The information in this section will take you as far past the shallows as you need to be. You’ll be able to speak intelligently with your mechanic and be able to identify potential problems with your car so you can get them looked at before they cause major headaches or unsafe conditions.
Tires are a critical component keeping you and your passengers safe. The most basic of tire checks is tire pressure. A common misconception is that tires should be inflated to the inflation pressure cast into the sidewall. This is the maximum inflation pressure. In theory, you could take the max load/max pressure listed on the tire, and interpolate the correct inflation by weighing each corner of the car. Sounds like a lot of work though. How ‘bout just inflating the tires to the pressure that your car’s maker recommends? It’s usually on a sticker in the driver’s door jamb and in the owner’s manual. Less frequently, it’s on a sticker in the glove box. Procure and use a good tire-pressure gauge, like the ones we give out at our Ladies’ Car Care Clinics. I’ve read tests that indicate that the gauges with the little plastic or metal thing that shoots out of them are more accurate than the dial types. That’s what we use at the shop.
Tread depth and tread wear patterns are easy to spot with a little training. Tires sold in this country have tread wear indicators, which are little bars molded into the tread area at intervals identified by little arrows on the top of the sidewall. These little cross bars are designed so that they’ll blend into the rest of the tread design when the tire has about 2/32” of tread depth. One problem with these indicators is that most drivers don’t look at their tires enough to notice them. A related problem, one that we see all the time at the shop, is that if they do look at the tires, they don’t look all the way across the tread – we see lots of tires that have plenty of tread on the outside edges, but are worn totally bald on the inside edges. A bigger problem is that in all the wet braking tests I’ve read, a tread depth of 2/32 of an inch is just not enough. In one test, a Ford pickup that stopped from 70 miles-per-hour in 4.8 seconds and 256 feet with new tires took 7.5 seconds and six inches shy of 500 feet to stop with 2/32” tread depth. When it passed the point at which it had stopped with the new tires, it was still going 58 mph! This is why many experts are recommending replacing tires when they’re at about 4/32” tread depth. At the shop, we use tread depth gauges to check tires, but there’s a quick and dirty trick that requires you to be solvent to the tune of 26 cents. Lincoln’s head on a penny is 2/32” from the edge, and good ol’ George’s head on the quarter is 4/32” from the edge. Just invert the coins in the tread, making sure not to put it on a tread wear bar, and check the depth at several spots around and across the tire. If for some reason you replace onlytwo of your tires together, always put them on the back. If you lose traction, it’s generally easier to control the car in an understeer situation (where the car is “pushing” the front tires as you try to steer) rather than an oversteer situation (where the rear of the car is trying to switch ends with the front).
Tread wear patterns are a great way to diagnose handling, steering, and suspension problems and worn out parts. The key is to identify them early enough that the tires aren’t totally destroyed. Unusual wear patterns include “cupping”, “feathering”, “heel and toe wear”, etcetera. The only thing to keep in mind is that if the tires are good quality tires of the correct size, weight rating, and speed rating for your car, and you’ve been good about balancing and rotating them, they should wear pretty evenly across and along the tread. If not, something is amiss and you should bring the wear to the attention of your mechanic.
In the last couple of years, nitrogen inflation has gained some traction (sometimes I make myself laugh) in the automotive world. It’s been used for a long time in aircraft tires, but not for the reasons sometimes cited. Nitrogen is used in aircraft tires only because it doesn’t support combustion, as air does. As I said, this is the only reason nitrogen is used in aircraft tires, despite the confidence of some of the assertions to the contrary. Nitrogen is pushed by some people for use in car tires for a couple of “reasons”. One supposed benefit is that because nitrogen molecules are larger than oxygen molecules, tires will lose pressure more slowly. However, when you stop to consider that air is about 78% nitrogen, and that with nitrogen inflation there’s no way to totally purge the air… I recently read a test in which 50 tires were inflated with air and 50 were inflated with nitrogen, then left alone for a year. When they checked the pressure in all the tires there was statistically no difference in the amount of pressure lost. No surprise. Some of the other “benefits” of nitrogen inflation are a better ride, less likelihood of corrosion of the wheel, less heat build-up, better ride, etcetera. All of these claims are nonsense. If you really want to fill your tires with nitrogen, knock yourself out. It doesn’t do any harm. Just so you understand the real story.
One last comment on tires – there are very cheap tires out there, and there are really expensive tires out there. I recommend staying away from the really cheap ones. The most expensive tires are usually the best available (true in many things), but most people would never notice the difference. There is a huge middle range that usually has a tire or two to suit almost any driver. Make sure you get tires of the correct size, speed rating, and load rating for your car. This info is usually printed on a sticker in the door jamb, and also in your owner’s manual.
Making sure that all four of your tires are pointing in the right direction is important, as you might imagine. For one thing, it’s easier on the tires. For another, it’s easier on the car – it’s much harder to roll a car when the tires are all fighting one another. Here’s an amazing fact – if your “toe” (we’ll get to what that means) is out by just 1/8th of an inch, your tires are literally being dragged sideways, perpendicular to the direction of travel, 28 feet every mile you drive! If toe is out just 1/16th of an inch, the tires are still being dragged sideways a quarter mile for every hundred miles you drive. You can imagine the cost in gas, in wear and tear on the engine, transmission, and drive train, and in tire life that has. We use an alignment machine to see which way the tires are pointing. We attach a target to each wheel, and the machine’s computer uses cameras to look at them. In addition to making sure that all the tires are rolling in the same direction, this machine lets us make sure that your car’s steering angles are correct. There are four main static alignment dimensions – caster, camber, toe, and thrust angle. Caster angle is the angle of an imaginary line drawn through the upper and lower pivot points of the steering axle, as seen from the side. Think of the fork of a motorcycle or bicycle, and how it leans back. When this line leans back, it’s called positive caster. This angle is what makes the car want to track straight instead of veering to either side. Camber angle is how far from straight up and down the wheels lean when seen from the front or rear of the vehicle. If they lean out, they’re said to have positive caster; if they lean in they’re said to have negative caster. Toe is nearly self-explanatory – it refers to whether the left and right tires on the same axle are closer to each other on their front edges or rear edges. Thrust angle measures whether the rear axle is perpendicular to the centerline of the vehicle, and whether the front and rear axles are parallel. Small changes in these angles can have a huge impact on gas mileage, how the car handles and feels to drive, and tire life. Most cars should have the alignment checked once a year or so.
Years ago, almost all cars and trucks used a shock absorber at each wheel to control bouncing. Without shock absorbers, the car would keep bouncing for a long time with each bump, and would be very uncomfortable and hard to control. Some modern vehicles still use shock absorbers, but mostly only on the rear. Most cars now have struts on one or both ends. A strut is a shock absorber inside a coil spring that has the additional task of serving as a structural part of the suspension. When used on the front, the strut serves as the pivot that allows the wheel to steer. Shocks and struts should be replaced if they’re leaking, or if the car bounces a few times when you push it up and down and let go. Sometimes they get noisy or creaky with age. Worn out shocks or struts can cause excessive tire wear, and make the car harder to control, especially if you have to suddenly brake or steer to avoid an accident or road hazard. They can also increase braking distance. A cool thing about replacing shocks and struts is that you never notice as they wear out, because it happens so gradually over the course of thousands of miles. When you first drive the car with new shocks and struts, it suddenly feels like it used to (like it’s supposed to!). It’s usually a good idea to replace the strut mounts when the struts are replaced. Most of the time there’s no additional labor charge and they’re probably worn out too.