Read any good tires lately? Reading tires may not be as exciting as reading a good action/adventure novel, but there are similarities. There's a hero (you). There's a story plot to unravel (Are the tires worn? If so, what does the wear reveal?). And there may be one or more villains (worn tie rod ends, misaligned suspension components, bad bushings, etc.) that you have to track down and eliminate. Every story is different, but all can have a happy ending if you just take the time to read them. Your reward will be a satisfied customer and hopefully a sequel adventure as word of your prowess spreads far and wide.
When you read a set of tires, ask yourself, "What can these tires tell me about how the wheels are aligned, the condition of the steering and suspension, and how they've been maintained? If you see even wear across the full width of the treads on all four tires with no shoulder wear, uneven spots, cupping, feathering or other problems, it's a short story that says everything's fine. But if you find uneven or abnormal wear on one or more tires, you've got a novel that may take some in-depth analysis to comprehend.
So let's consider some of the common story lines you're apt to
encounter as you read through volume after volume of worn tires.
Normal vs. abnormal
Normal is a subjective term because what's normal wear in one situation may not be normal in another. On front-wheel drive cars and minivans, for example, it's normal for the front tires to wear at a much faster rate than the ones on the rear.
As a rule, all tires should wear evenly across the full face of the tread as long as wheel alignment is correct, the tires are maintained at the recommended inflation pressure, and the vehicle is driven sensibly. Abnormal wear would usually be the reading if you found the tires were worn unevenly, cupped or feathered. But there are exceptions to every rule.
Take heavy outer shoulder wear, for example. It often tells you the vehicle has a camber or toe problem. But if everything checks out fine, it would tell you the driver has an attitude and enjoys pushing the limits of adhesion. Aggressive driving and hard cornering can produce the same kind of outer shoulder wear that might be mistaken for camber or toe misalignment. So what appears to be abnormal wear may actually be normal wear depending on what caused it.
So what should you do in a situation like this? Since there's no way to "correct" the condition short of recommending a kinder, gentler driving style (or a driver's education class), all you can do is make sure the alignment settings are within acceptable limits, the tires are properly inflated, and recommend rotating the tires every 7,500 miles to help even out the wear. Then do yourself a favor. Enter the customer's name on your direct mail list for future tire promotions because he or she is obviously going to go through a lot of tires!
Rapid shoulder wear on the front tires may also be "normal" on some 4x4 trucks, vans and minivans because of the vehicle's steering geometry. All vehicles are supposed to have a certain amount of "Ackerman" built into the steering linkage so the front wheels will toe-out with respect to one another when they are turned. This is necessary to compensate for the different turning radius or path that the inside and outside wheels follow then rounding a corner. If there isn't enough Ackerman or toe-out, the tires will scrub every time the vehicle turns a corner.
The amount of Ackerman or toe-out that's provided depends on the placement and angle of the steering arms and steering linkage. Some vehicles are better engineered than others in this respect. Those that aren't wear out the front tires rather quickly, especially when driven in an urban environment that involves a lot of turning.
Caster can have an effect here, too. Caster causes the wheels to lean or tilt when steered, which changes camber (called "camber roll"). A lot of caster may therefore cause an increase in shoulder wear. This type of condition is common on vehicles like Mercedes and BMW that have high caster settings.
Measuring toe-out on turns while doing your alignment checks will
tell you whether or not the vehicle is within the factory specs
(assuming specs are provided, which they sometimes aren't -- in
which case all you can do is check to see that the toe change
is equal side to side when the wheels are steered). If toe-out
is not within specs, you're probably dealing with a bent steering
arm. But if it is within specs, the only way to overcome the lack
of sufficient Ackerman is to avoid making turns or trade the vehicle
for one that provides adequate Ackerman. Since neither suggestion
is very practical, the best you can do is recommend rotating the
tires more frequently to spread the wear around.
Camber or toe wear
Shoulder wear on the inner or outer edge of a tire is often interpreted as camber misalignment. Too much positive camber can accelerate wear on the outer shoulder of a tire just as too much negative camber can accelerate wear on the inner shoulder. Underlying causes can include bent struts, mislocated strut towers, bent spindles, collapsed control arm bushings, even weak springs. But camber misalignment usually won't wear a tire as much or as quickly as toe misalignment will. Why? Because toe misalignment scrubs the tires as they roll. Only 1/8 inch of toe misalignment front or rear will scrub the tires sideways 28 feet for every mile traveled. That may not sound like much, but over thousands of miles it can really wear away the rubber.
On bias ply tires, toe misalignment will usually reveal itself as a feathered wear pattern which you can feel when you run your fingers sideways across the tread. The direction of the feathering tells you if the tires are toed-in or toed-out (rough edges towards the inside signal toe-in while rough edges to the outside indicate toe-out). But on radial tires, toe misalignment tends to roll the shoulder of the tire under as it scrubs producing wear on the inner or outer ribs only. Toe-in will wear the outer rib while toe-out will cause wear on the inner rib. In both instances, wear can be aggravated even more if the tires are underinflated.
Causes of toe misalignment include worn tie rod ends, worn idler arms and pitman arms, center links, inner tie rod sockets (R&P steering), bent tie rods or steering arms, or an incorrect toe setting. But toe wear on the front tires can also be caused by misalignment at the rear wheels.
If rear toe (or axle alignment) is off and creates a thrust angle to either side of the vehicle's centerline, it will cause the vehicle to lead towards the opposite side. To keep the vehicle going straight down the highway, the driver has to steer slightly off-center. This, in turn, causes the front wheels to toe-out a bit (Ackerman, remember?) which creates toe wear on the front tires. So the tires themselves won't tell you the whole story. You also need to check for a steering pull or off-center steering, which means interviewing the customer or test driving the vehicle yourself.
Toe wear on some vehicles has been blamed on factory specs that allow too much latitude. Setting toe to the preferred specs rather than just checking to see that it falls somewhere within the maximum limits can minimize toe wear problems.
Chevrolet, for example, issued a technical service bulletin (#86-90) that recommended resetting the toe to zero to cure an outer shoulder wear/cupping problem caused by excessive toe-in on 1985 & '86 Caprice, Monte Carlo and light trucks.
Honda issued a similar bulletin (TSB #90-030) to counter a inside shoulder wear problem on 1989-91 Civics. The problem here was not enough toe-in, so Honda advised resetting the front toe to -3 mm.
It's also important to make sure you're using the latest alignment specs. From time to time, the OEMs will issue technical service bulletins revising their recommended alignment specs. If you're using computerized alignment equipment with a built-in database, the latest revisions will usually be included as part of a periodic update. But if you're pulling the specs out of old manuals on the shelf or an outdated alignment spec book, be careful because the specs may have changed.
A case in point is 1982 to '87 Chevy Camaros. The original factory
specs for toe alignment called for 0.30 degrees of toe-in with
plus or minus 0.20 degrees either way. An outer rib wear problem
on these cars caused Chevy to issue TSB 87-35 that revised the
alignment spec to 0.10 degrees of toe-in plus or minus 0.20 degrees.
Uneven wear
If you read a tire and find more wear in the center area of the tread than the side ribs, the tire may be too wide for the rim or it may be suffering from chronic underinflation. A low profile tire that's too wide for a rim may not make full contact with the pavement at the side ribs, causing the center of the tread to wear more rapidly. Likewise, a tire that is too narrow for a rim may suffer more wear on the outer ribs of the tread than the center.
Proper inflation is absolutely essential not only for good tread life but also for a smooth ride and good fuel economy. When a tire is overinflated, it tends to bulge out and experience more wear in the center area of the tread. Underinflation shifts the weight carried by the tire to the edges of the tread causing the shoulders to wear more than the center. Underinflation causes the tire to squat more, which increases rolling resistance and increases the flexing of the tire's sidewalls. The added friction produces heat which may damage the tire or cause it to fail. So anytime you read tires, always check to see that they are properly inflated. And if they're not, advise your customer to check them periodically (once a month is a good recommendation). Tire pressure should be checked when the tires are cold before the vehicle has been driven since driving raises the internal pressure.
Sometimes you'll read a set of tires that show uneven wear, yet are aligned and inflated properly. These tires are the "mystery novels" of the genre because they exhibit weird wear patterns for no apparent reason. But there is a reason, and it sometimes has to do with the construction of the tire itself.
Some low profile tires (mostly performance tires), tend to develop uneven wear patterns because of their belt and tread design. As the tire rolls along, the deflection and squirming of the tread produces harmonics that cause certain areas of the tread or ribs to wear more than others. The wear pattern can be further compounded by other factors such as toe misalignment, wheel imbalance, wheel runout, underinflation or lack of rotation. After awhile, a "heel and toe" or "washboard" wear pattern may develop that generates annoying noise and vibrations at speeds above 40 mph. Once the wear pattern is established, it's too late to reverse it by rotating the tires. So the only cure you can recommend is to replace the tires and possibly switch to another brand or style that is less "quirky."
If you find a diagonal wipe wear pattern on the rear tires of
a front wheel drive car or RWD car with an independent rear suspension,
it usually indicates an unequal toe problem. Because there's no
steering linkage, the rear wheels can't self-center themselves
if one has more or less toe than the other. So one or both tires
are forced to drag along producing a diagonal wear pattern on
the tread.
Cupped wear
A cupped or scalloped wear pattern on a tire may be caused by by wheel imbalance, excessive wheel or tire runout, weak shock absorbers or struts, or worn suspension parts such as control arm bushings or ball joints. The tire develops a cupped wear pattern because it bounces up and down as it rolls down the highway. The motion may or may not generate enough force to be noticed by the driver, but eventually forms a cyclic wear pattern on the tread.
You can check the wheels for missing weights, but rebalancing them will be a waste of time because the damage has already been done. The tires will likely have to be replaced.
You should also check tire and wheel runout, especially if you notice any signs that would indicate a close encounter with a curb or pothole when reading the tires. Scuff marks or cuts on the sidewall, or hubcap or rim damage would all be clues that might tell you the wheel is bent.
A bounce test and/or a short test drive should tell you if the dampers are providing adequate ride control. If the original shocks or struts are weak, it's time to replace them.
Don't forget to check the control arm bushings and ball joints
as play or looseness here can also allow unwanted gyrations in
the wheel and tire as the vehicle travels down the road.
Final chapter
When reading tires, don't forget to note tire sizes, tread styles and brands on both sides, front and rear. Intermixing different types of tires (radial and bias) on a vehicle is not recommended. Nor is intermixing different sizes or tread styles side-to-side because differences in diameter and rolling resistance may create a steering pull. This can even happen when different brands of tires that have the same size and tread style are installed on opposite sides of a vehicle.
Anytime you're replacing tires on a vehicle that's equipped with ABS, recommend the same size as the originals. Changing tire sizes can upset the calibration of the ABS system and is NOT recommended by any car maker. Tires should also be lubricated when mounted to minimize the risk of bead damage, and then balanced. Don't use too much lubricant on alloy wheels, though, or the tires may slip on the rims throwing them out of balance.
Also, don't forget to read the spare - even if it's a temporary spare. Because it's tucked away in the trunk or underneath the vehicle, the spare is often overlooked. But it might be low on air, flat, not the correct one for the vehicle, or missing altogether.