Wheel alignment: camber, caster, toe, and reading a tyre that is wearing wrong

Three angles decide how a car tracks, how it steers and how long its tyres last, and the useful thing about them is that the tyre keeps a record. A tyre that is wearing wrong is telling you which angle is out, in a way no other symptom does — you just have to know how to read it.

The original defines the three angles carefully and never once says what a wrong one does. That is the half this adds.

The three angles

Camber is the wheel leaning in or out, seen from the front. Lean the top inward and camber is negative; outward and it is positive. A little negative camber keeps the tyre flat on the road while the body rolls in a corner, which is why almost every car has some.

Caster is the tilt of the steering axis, seen from the side — the same idea as the angled forks of a bicycle. Positive caster is what makes the steering self-centre after a corner and what makes a car feel stable in a straight line. It is the one angle whose wear pattern is nothing, because caster does not scrub a tyre; it is felt, not seen.

Toe is whether the wheels point slightly toward each other (toe-in) or apart (toe-out), seen from above.

And toe is the one that destroys tyres, which is the practical hierarchy the original never draws. A camber error wears one edge of a tyre gradually. A toe error drags the tyre sideways along the road for every metre the car travels, and a few millimetres of it will scrub out a new tyre in a few thousand kilometres. If you fix one angle, fix toe.

Reading the tyre

Run a hand across the tread as well as looking at it — several of these are easier to feel than to see, and one is nearly invisible.

The feathering test is worth learning because it is the only one that identifies toe from the tyre alone, and because a toe problem is expensive and silent. Nothing about the car feels wrong until the tyre is finished.

Three corrections to the original

The inner wheel in a corner turns further, on a smaller radius. The original has this inverted — it says the inner wheels take a greater angle and therefore trace a larger radius than the outer ones. It is the other way round, and the reason is the point: the inner wheel is going round a tighter circle, so it must be steered to a sharper angle to follow it without scrubbing. That is Ackermann geometry, and it is built into the steering arms rather than adjusted.

Cross-caster is not a safety feature. The original says the small deliberate side-to-side caster difference exists so that a driver who loses control is pulled toward the verge instead of into oncoming traffic. It is really road-crown compensation: roads are built with a camber so they drain, that slope makes a car drift toward the kerb, and about half a degree of extra caster on one side cancels it out. Set up for one country's crown, the same car drifts the other way on a perfectly flat road — which is the plain evidence that it is a correction for the road surface and not a safety mechanism.

And front toe-out is not the general rule. It says negative toe is normally used at the front. What a car is given depends on which wheels drive it: rolling resistance tries to push undriven wheels apart, so a rear-driven car is usually given a little toe-in at the front to end up straight at speed, while drive torque pulls driven wheels together, so a front-driven car often gets zero or slight toe-out for the same reason. The static figure is set so that the rolling figure comes out right, which is why it must come from the manufacturer's data and never from a principle.

Before anyone aligns anything

An alignment measures where the wheels are pointing. It cannot tell you why, and it cannot fix a car whose geometry is not staying where it is put.

• Worn ball joints, track rod ends and bushes make the measurement meaningless, because the wheel moves under load to somewhere the machine never saw. Any play must be dealt with first, and a workshop that aligns a car with a knocking joint has sold you a number.
• A settled or broken spring changes camber, so the ride height has to be right before the angles can be.
• Set the tyre pressures first. They alter the loaded geometry, and they are free.
• The steering wheel must be centred while it is done, or you get a car that tracks perfectly with a crooked wheel — and the only fix for that is doing it again.

Four wheels, not two

The rear axle decides which way the car actually goes. If it is not square to the body, the car travels slightly crabwise and the front wheels have to be steered off-centre permanently to compensate — so the steering wheel sits crooked on a straight road even though the front alignment is perfect. That offset is the thrust angle, and it is why a proper alignment measures all four wheels and sets the front relative to the rear's actual direction rather than to the body.

On a car with a torsion beam or a simple rear axle the rear may not be adjustable at all. Measuring it is still worth it, because it tells you whether the front can be made right.

When it is not the alignment

A car that pulls is assumed to need aligning and often does not. Two checks first, both free:

Uneven tyre pressures pull a car, immediately and convincingly.

And a tyre can pull on its own. A belt that has shifted slightly inside the casing makes the tyre very slightly conical, and it steers. The test is elegant: swap the two front tyres side to side. If the pull swaps with them, it is the tyre. If it stays, it is the car. That takes twenty minutes and settles a question people pay to have measured.

A dragging brake also pulls, and it comes with a hot wheel and a smell after a drive — worth a hand near the wheel, carefully, after a run.

When to have it done

After any suspension or steering work. After hitting a kerb or a serious pothole hard enough to make you wince. When new tyres go on, because it is the cheapest moment to protect them. If the steering wheel is not centred on a straight road, if the car pulls, or if a tyre is wearing unevenly.

And not otherwise, on a schedule, because nothing about alignment drifts with time — it changes when something moves it.

Video guides

Video, for the parts of this that are easier watched than read.