Tyre problems: How to know what's wrong with your tyre

In your most helpful articles about checking and maintaining cars and inspecting a potential used-car purchase, you have mentioned “uneven” tyre wear. If the tread wear is uneven, what has caused it and why does it matter? 

Uneven tyre wear is an early-warning system for several different faults. Spotting them as soon as possible, and putting them right, is important to your motoring economy and ultimately to your safety.

The surface of your tyre tread (where the rubber meets the road) should be smooth, flat and even – from side to side and all the way round. If one or both sides is more worn than the middle, or the middle is more worn than the sides, or the surface of each tread lug is slightly “sloped” (so it feels catchy if you run the palm of your hand lightly over the tread surface), or there are “flat spots” in the tread, then something is wrong with your tyre pressures or your steering alignment or your brakes…or your driving.

When the car is parked, where the tread meets the road it should be flat and share the whole weight pressure of the car across the whole width of the tread, equally. This maximises the tyre footprint and optimises grip and balance. 

Pressures

If the tyre pressure is too low, even if the tread is “touching” all the way across, the load will be greater on the sides than in the middle. The sides will wear faster. If the tyre pressure is too high, the middle will “bulge” and bear more load than the sides, and the middle will wear faster. In either case, the overall wear life of the tyre will be much reduced – parts of it will suffer excessive premature wear and the legal limit for tread wear applies to the baldest part. So, a waste of money and a loss of grip and balance. 

Alignment

If pressures are right but one side of the tread is wearing more than the other, the “camber angle” is out of line. The wheel will be leaning off-vertical and imposing an asymmetric load.   

If the pressures are right and wear is equal, but the surface of the tread lugs is not flat (becomes sloped or “feathered”) then the wheel alignment is wrong, or part of the suspension is faulty or a wheel hub is slightly bent. The two front wheels are not running exactly parallel to each other and in the direction of travel. One or both are trying to go in different directions; even if only by half a degree.

Whether slightly or severely, this will compromise the integrity of steering control, increase the rate of tread wear, fractionally degrade performance and fuel economy, and cause an annoying “pull” to one side or another when driving in a straight line. 

A similar effect can be caused on the back wheels if the rear axle or one of the hubs is bent out of line. Because one or more of the tyres is “scrubbing” as it rolls, wear will be much more rapid and progressively misshape the tread lugs. Feathering. 

Wobbles and rocks 

Scrubbing will also be caused if there is any play in the suspension mountings or linkages, or the “castor angle” is maladjusted – one or more of the wheels might have a very slight wobble. That creates a degree of side-to-side friction (sensed as vibration), hence faster wear than a wheel that is rolling straight and true. 
If a car has been driven a lot on very stony roads, the whole tread surface could look as if it has been nibbled by rats. The stones have not just rubbed grains off the surface; they have bitten lumps out of it (most common on 4x4s and more often used to tackle “rockeries”).

Large rocks or other objects can also “bruise” the sidewall. Any bulge means some of the cords that strengthen the tyre casing have been detached or broken. The treads might be fine and the air pressure may be correct and steady, but there is now a weakness in the casing that, suddenly and severely, could rupture. There is no repair; the tyre should reassigned… perhaps as a few pairs of shoes or as a piece of equipment in a school playground or two feeder troughs for chickens.

Maladjusted brakes

If wear of any shape is greater on one wheel than the others, or more on one side of the car than the other, that could indicate maladjusted brakes, imposing uneven distribution of loads and friction during braking. In very hard or emergency braking, one or more wheels could lock asymmetrically and cause the car to veer, skid or spin. And that could cost a lot more than money.

Harsh driving

The rate and shape of tyre wear is also an indicator of how a car has been driven. Harsh driving can hugely increase overall tyre wear, especially on the sides from high-speed cornering, right across the tread from violent wheel-spin acceleration, and cause flat spots from lock-up braking. Once the slightest flat spot has been created, that is the point where the wheel will lock up on every subsequent stamp on the middle pedal.

These ill effects are not slight. I once spent a day racing at the old Embakasi track, in a heavy but underpowered car wearing brand-new retreads. The track had six right-hand bends and one left-hander and very abrasive tarmac. After two races the left-hand half of all the tyres was completely bald. I rotated them for the afternoon session, and at the end of the day, I threw them all away. In less than 100 km, the tread, all the way across, was giving way to string. 

So whether checking your car or assessing the treatment and condition of a used one, “Sherlock Holmes” the tyre wear. It can be a very informative clue. For that reason, if a used car you plan to buy has four brand new tyres, check the spare, and ponder why the vendor has made a major investment just before selling.