Copied from Sonic Springs webpage. I have used this method for a number of years and bikes and it has been pretty solid.
The purpose of the spacers is to provide the proper amount of pre-load on the springs. In general, street bikes need about ¾” of pre-load, assuming that the spring rate is correct for the weight of the bike and rider. So the question is, how do we determine the right length for the spacer?
Take the fork cap, and if it has a pre-load adjustment, set it in the middle. Now measure the depth of the cap from the bottom to the base of the portion that sits against the top of the fork tube. Include any stock washer that will sit on top of the spacer. Basically we’re measuring the length of the portion that fits inside the fork tube.
Say for example that this length is 2”. If we cut the spacer so that with the spring, washer and spacer installed it came up flush with the top of the fork tube, when we installed the fork cap the spring would be compressed 2”. Too much. So we need to cut the spacer shorter to get the proper pre-load. In this case 1 ¼” shorter. Easiest way to measure this is to put the spring, washer and uncut spacer in the extended fork leg. Mark the spacer tube even with the top of the fork tube. Pull the spacer out and measure 1 ¼” down from the mark. Mark and cut from that point. Now, when you put the spring, washer and spacer in, the top of the spacer should be 1 ¼” below the top of the fork tube. When the fork cap is installed, the 2” section inside the fork tube will compress the spring ¾”. Voila, correct pre-load!! Just make sure you use the right numbers for you, not the ones in this example. Also, back the adjuster all the way back out before re-installing the fork cap, to make it easier to get the threads started. Re-set the adjuster to the middle position after everything is bolted back up.
