4. FENDER ARCH CLEARANCE
This measurement is most critical for the front wheels because they need additional clearance to swing outside the width of the fenders while steering. Measure from the axle centerline to the bottom of the wheel arch. In general, the radius of your tire (half its diameter) shouldn’t exceed this distance. This measurement, minus the radius of your wheel diameter, gives you an estimate of what sidewall thickness you can fit. If pushing the fitment of the wheel flush with the fender line, placing a square to mock the wheel diameter in reference to a plumb bob (or in our case, a heavy socket tied to a worn-out USB cable) is a way to double-check width and offset; this is often a required measurement in custom wheel ordering forms, including a measurement to the inner fender.
[ Two more measurements here: maximum tire diameter and width. We doubled the vertical measurement to see that our maximum tire diameter would be around 30 inches. With Los Angeles’ potholed roads for this daily driver, we wanted to err on the side of a tall sidewall and went with a 29-inch diameter front and rear. [ A plumb bob is crucial to the mock-up process if you’re slotting in wide rollers. Here, our square sits at a 17-inch diameter.