This is Elliot's bike mocked up at ride height. His 4.00x18 Avon SM on a 18x2.50 rim, tire inflated to 35 lbs., the chains adjusted, and the fender spaced off the wheel with an old 530 Oring chain where it needs to be. The ground clearance will be about 4 3/4". Now I can figure out how much we need to take out of his springer.
I had guesstimated we'd need to take out 6" before I mocked it up. But, you also have to take into consideration the "sag" in the front end, with the weight on it to determine the final length of the fork. I took a very scientific approach to that - I put the front axle in the rockers, blocked up under the front axle, then I let the jack down until the springer settled as far as it would normally set on the spring resistance, with the major components in place. It settled down approx. 1". After that, I took Elliot's bike back up to a little more than level in the front. Totally level just didn't jibe with the lines of the bike.
Next, I figured out the angle of the rocker (19 degrees), and the length of the rocker from the centerline of the back leg, straight across to the centerline of the axle. I set the front wheel we're using in place, and I rolled it in until I had the distance from the centerline of the back leg and the center of the wheel bearing at 4". Then I centered the rear rocker bearing on the back leg, made sure the rocker angle was 19 degrees, then slightly moved the wheel and rocker around until they lined up again.
I marked the centers of the rocker bearings on the front and back leg centerlines, then took a measurement from the center of the original rocker pin holes up the legs to my marks. 8 1/4" difference on both front and back legs. Done deal. We'll cut out 7".....8 1/4" minus the 1" for "sag", and we'll leave in the extra 1/4" in there for any additional weight like fuel and oil after we're done.
Can you see through that plastic shipping wrap over the front wheel? Take a good look....it's a NOS THREE SPOKE 21x2.15 Invader wheel. It's never been on a bike.....until now.