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Alignment basics

The handful of angles that decide how a car tracks, how its tires wear and where its steering wheel sits, and the signs Rim Reader uses for them.

On this page

Toe

Seen from above, toe is whether the front of each tire points in toward the middle of the car (toe-in) or out (toe-out). Total toe is the two wheels on an axle compared with each other; a spec sheet usually gives it. Each wheel’s own toe is measured against the thrust line. Toe that is off wears tires fastest, feathering the tread across its width, and it is the angle you adjust most.

Toe seen from above. Toe-in: the fronts of the two tires point toward each other. Toe-out: they point away from each other. Seen from above, front of the car at the top. Angles exaggerated, not to scale. Toe-in (+) fronts of the tires point in Toe-out (−) fronts of the tires point out

In Rim Reader, toe-in is + and toe-out is −. Above each wheel the app shows which way the front of the tire points, for example Toe 0.16° R: turned 0.16° to the right. On a left wheel R is toe-in; on a right wheel L is toe-in.

Camber

Seen from the front, camber is how far each wheel leans. Negative camber means the top of the wheel leans in toward the car; most cars run a little. Too much either way wears one edge of the tire.

Camber seen from the front. Negative camber: the tops of the wheels lean in toward the car. Positive camber: they lean out. Negative camber (−) tops of the wheels lean in Positive camber (+) tops of the wheels lean out Seen from the front of the car. Angles exaggerated, not to scale.

Thrust line and thrust angle

The rear axle steers the car too. The thrust line is the direction the rear wheels push the car: the average of the two rear wheels’ directions. The thrust angle is that line compared with the body’s centreline. If it isn’t straight, the car crabs slightly down the road, and the steering wheel ends up turned to make it go straight. Rim Reader reads the body at the front doors’ window glass, and calls a thrust angle within 0.10° straight.

Thrust angle. The rear wheels point the car slightly off its centreline. The thrust line, square to the rear axle, is at an angle to the body's centreline: that angle is the thrust angle. FRONT thrust line where the rear axle points the car thrust angle between the two lines dashed: the body’s centreline, read at the front door glass Seen from above. Angles exaggerated, not to scale.

Steer-ahead

Steer-ahead is the front wheels’ average direction compared with the thrust line, with the steering wheel straight. If it isn’t near zero, the steering wheel sits turned when you drive straight. It is the commonest reason a car comes back from an alignment with the steering wheel still crooked: the total toe was right, but it wasn’t shared evenly between the two front wheels. The app shows it as Steering, with L or R.

Steer-ahead. Both front wheels point a little to one side of the thrust line while the steering wheel is straight, so driving straight needs the steering wheel turned. dashed: the thrust line front wheels’ average direction, off the thrust line steering wheel sits turned when driving straight Seen from above. Angles exaggerated, not to scale.

Cross camber

The difference in camber between the two wheels on an axle: left minus right. A car tends to pull toward the side with more positive camber, so cross camber matters as much as each wheel’s own number.

Caster

Seen from the side, caster is the tilt of the steering axis. It gives the steering its self-centring feel and its straight-line stability. Rim Reader doesn’t measure caster yet; it is planned, read by steering each front wheel left and right.

Units and signs
  • Angles are in degrees to two decimals, for example 0.16°. Toe can also be shown, per vehicle, in degrees and minutes, millimetres or inches; millimetres and inches need the tire size.
  • Toe-in is +, toe-out is −. Camber with the top leaning in is −.
  • Thrust angle and steer-ahead are in degrees with L or R.
  • Two decimals is the display, not the accuracy. See Accuracy.
More terms
Rim lip
The flat outer ring at the edge of the wheel rim. The bracket’s prong noses rest on it.
Spec
The range a maker allows for an angle, often a preferred value with a tolerance.
Generic targets
Rim Reader’s ranges for a typical street car, used when a vehicle has no specs of its own. Never your car’s factory specs.
Tie rod
The rod that links the steering to each front wheel. Turning its adjuster changes that wheel’s toe.
Closing a lap
Visiting each wheel twice, so the app can measure the drift in between and remove it.
Confidence
High, Medium or Low, with the reasons: how much a result can be trusted.