Toe in vs toe out: what's the difference?

What is toe, exactly?

The angle each wheel makes with the car’s centre line, seen from above. A wheel whose front points in towards that line has toe in, or positive toe; one pointing away has toe out, or negative toe. The settings are tiny, usually a fraction of a degree, or a millimetre or two across the rim, and they are measured with the car at its normal ride height and the steering straight.

Why are the wheels not set dead straight?

Because a rolling wheel does not stay quite where it was set: the bushes and joints in the steering and suspension give a little under load. Front wheels that are pushed along, as on a rear-wheel-drive car, are dragged back by their tyres and tend to splay apart, so they are set with a little toe in. Driven front wheels pull the car, which tends to draw them together, so many front-wheel-drive cars are set near zero, and some with a touch of toe out.

Toe also shapes how the car feels. Toe in makes it settled in a straight line, at the cost of a slightly slower response when you start to turn; toe out makes it turn in eagerly but more inclined to wander.

How does wrong toe wear the tyres?

A wheel with too much toe rolls slightly sideways all the time, so its tread is scrubbed across the road on every turn. That wears the tread blocks into a saw-tooth: run a flat hand across the tread and it feels smooth one way and sharp the other. The feathered feel is the sign of a toe problem, and because the scrub never stops, toe wears tyres far faster than the same error in camber. The other wear patterns are in what causes uneven tyre wear.

An error on one side shows in the steering too: you steer to make up the difference, so the steering wheel sits off centre when the car runs straight.

How is toe set?

At the front, on the track rods (US: tie rods). Each one screws into its track rod end, so turning the rod changes its length and steers that wheel slightly. In the app’s MacPherson strut model, the track rod’s own description says it plainly: turning it in the track rod end sets the toe. Which way gives toe in depends on whether the rack sits behind or ahead of the wheel centres, and a lock nut holds the setting.

On a four-wheel alignment, with sensors or camera targets on every wheel, the rear wheels are measured first, because they set the line the car actually follows, the thrust line. The front toe is then set to that line, both sides equally, so the steering wheel ends up straight. Many rear axles have no toe adjustment: on a twist beam it is built in, while multi-link axles usually have adjusters.