I have a hardtail mtb frame with a suspension fork I want to change to a rigid fork. How do you design a rigid fork for the conversion? Should I sit on the bike(to preload the fork) and then measure the trail and rake? I don't have much to go on, any comments appreciated.
You dont need to measure trail directly (it is tricky do accuratly do), the rake of the fork does not change with suspension movement so rake of an exiting fork is much easier to measure. To accurately calculate fork trail, you need to know the fork rake and the head tube angle (HTA), so you do need to get a measurement of the existing HTA of your 25% sagged suspension.
see fork rake calculator; http://yojimg.net/bike/web_tools/trailcalc.php
Critical to building or selecting the new fork are both the crown-to-axel (A-C) length (which effects the HTA) and the fork rake. If you have any desire to change the fixed HTA on the ridged front end, now is the time to do so by selecting the approporate A-C length fork, figure about 1° of HTA change (steeper) for each 20mm reduction of fork A-C length. Once you have the HTA fixed via the A-C measurement, then you need a fork with the approaprote rake to get to a reasonable fork trail value. The rake of a a fork limits the range of usefull frame HTA that it should be paired with, in order to not wind up with trail that is too far out and thus effects handeling.