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So do you attach the phone to the wheel somehow?



The phone doesn't necessarily has to be mounted against the wheel at all times. You can hold it against the wheel, take a measurement and move on.
My wheels happen to have a nice flat spot in the center so it's very easy to hold the phone against it.

For camber-adjustments it would be nice to fix the phone on the wheel while adjusting.
For caster-adjustments you need to keep one of the phone's axis as good as vertical, while holding the right side of the phone against the wheel for the measurement. So a fixed phone could work as long as you can reposition it upright after turning the wheel.

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