"scrub line' is the term you are looking for. In order to be safe in the event of a flat tire, the rim must contact the ground before any of the chassis or frame. in other words, with a bare wheel bolted on, the rim itself must be the lowest point of the chassis and frame. To get more clearance, run a larger diameter wheel. I have concerns with the dropped spindles and dropped springs that i wouldn't clear with 15 inch wheels. the cure, 17, 18, or 20 inch wheels. 17s would give me one more inch or clearance, etc. awesome work ramblebee. JP in KS.