How to time your laps on circuit with an iPhone
You do not need to wait for a timing box to know your lap time. A closed circuit, a session and an iPhone that can detect the line are enough to start.
What you actually need
A circuit lap timer does not start with hardware. It starts with a closed track, a session, and a phone that can record position consistently enough to detect a line crossing.
ST1 Track uses iPhone GPS to start timing automatically. You pick your circuit, go out, drive. The app detects laps without a button press at every line.
Before you leave: prepare the session
Enable location. Give the iPhone a clear view of the sky — an inside pocket or a stable mount beats the bottom of a bag. Charge the battery. Choose the circuit in ST1 Track, or let the app suggest tracks around you.
If the layout is not in the database yet, you can still drive: a few laps are often enough to record the circuit for your next sessions.
During the session
The right habit: launch ST1 Track before you leave the pits, then forget the phone. Automatic timing is useless if you stare at the screen under braking.
At every crossing, the app stores the lap. Sectors fill in. You can read the session afterwards: where you gained, where you lost, and how the next lap can change.
After: read the time, do not just display it
An isolated lap time says little. Progress comes from sectors, history and comparison between sessions. That is where ST1 Track stops being a simple stopwatch.
Go further with the sectors and theoretical lap guide, then the GPS accuracy page if you want to know what an iPhone can — and cannot — measure.