One lap, step by step
1. A rest on the ground gives the gyroscope its zero
Every lap starts with the bracket lying still on the ground for ten seconds. No phone gyroscope reads zero when it is still. That small leftover would add up during a walk round the car, so the app measures it while nothing is moving and subtracts it from everything after. The same rest tells the app which way is down. A hand can’t give this zero: even a steady hand moves about 1° a second.
2. Every wheel is read twice, with a lift between
At each wheel you hold still for a reading, lift the bracket right off, set it back and hold still again. Over a few seconds the gyroscope can’t drift by any amount that matters. So if the two readings disagree, the bracket didn’t go back on the same spot, and the app asks for that wheel again straight away. You find out at the wheel, not a lap later.
You don’t tap anything. When the bracket is level and steady, the reading is taken by itself and the phone beeps and buzzes.
3. Going back to each wheel measures the drift
The app sets the order: rear left, rear right, rear left, rear right, then front right, front left, front right, front left. Each visit is two readings, so a full lap is 16. The wheels don’t move between visits, so any difference between a wheel’s first and second visit is drift that built up while you walked. The app measures that drift for this lap and removes it from every reading.
Each axle is closed this way on its own. If an axle’s drift is too large to trust, the app asks you to read that axle again. After two redos it says the lap would not close, and shows no numbers. An honest refusal beats a wrong alignment.
4. The rear axle sets the line the car drives along
Measuring the front wheels alone tells you only their total toe: the two front wheels compared with each other. To know what each front wheel is doing on its own, you need the direction the car actually travels, and the rear axle sets that. The average direction of the two rear wheels is the thrust line. Each front wheel’s toe is measured against it, and so is steer-ahead, which tells you whether your steering wheel will sit straight. That is why a full lap reads both axles.
5. The front doors give the thrust angle (optional)
The thrust line shows which way the rear wheels push the car, but not whether that is straight compared with the car’s body. Tick Thrust, and the app also reads each front door’s window glass: the bracket lies flat in the bottom back corner of the window. The two doors are mirror images, so the curve of the glass cancels out, and together they give the body’s centreline. The thrust angle is the rear axle’s direction against that centreline, and it shows which rear wheel is off.
A thrust angle within 0.10° is reported as straight, because a door can hang slightly differently from its twin (we estimate up to about a tenth of a degree). If a door reading fails, only the thrust angle is dropped. The wheel numbers are kept.
6. Camber, read on the bracket’s edge and corrected for the floor (optional)
Camber is read after the toe lap has closed, on a second trip round the car, so handling the bracket for camber can’t disturb the toe numbers. At each tire:
- Two floor readings. The bracket lies flat on the floor beside the tire, then is turned round where it lies. Together they measure the slope of the floor there.
- One top-of-rim reading, as for toe, which tells camber which way the wheel faces.
- Two side-of-rim readings, with a lift between. The bracket stands on its edge against the side of the rim, and the two readings must agree.
The app corrects each axle’s camber for the floor slope it measured beside the tires, so a garage floor that isn’t quite level is accounted for. Reading the top of the rim as a reference made camber about 3.5 times more repeatable in our tests than gravity alone, the way a simple digital gauge reads it.
