Ok, here it is and it's not pretty! I had to replace the main timber when my cars got wider............
This is the nail which creates the baseline, just touching the rim. It's not quite at 3 o'clock because wheels have got a lot bigger, but it's close enough.
So first measure here, then slide the frame behind the wheel and repeat. It really does work. Assuming the wheels run concentric!
![[Linked Image]](https://tm-img.com/images/2022/06/17/P1060183.jpg)
![[Linked Image]](https://tm-img.com/images/2022/06/17/P1060184.jpg)
The other side I just use a metal tape, a rule would do the same thing. I work generally to 1/16" or up to 2mm. The measurement at the front should be bigger because I measure from the outside.
![[Linked Image]](https://tm-img.com/images/2022/06/17/P1060185.jpg)