PThe amount of preload has nothing to do with the proper springing of a motorcycle (as long as the spring is not near coil bind or block length). That said, it is usually best to run as little preload as possible while still attaining the appropriate sag numbers and spring rate.
Most of the time it is necessary to cut a new preload spacer for inside the fork. Most high quality units will have external adjusters for preload adjustment, but many do not. Also, It is good practice to ensure that proper sag numbers can be archived with only the first 1/3 of an adjusters range being used.
The easiest to obtain material of choice is standard white cold water plumbing PVC (Schedule 40 PVC Pipe) that is very close to the same dimensions as the spring diameters. The best way to cut and trim the spacer is on a lathe. this keeps each end square to the axis. If you lack a lathe, I have found that a carpenters chop saw will give adequate results. Completely clean and deburr the spacer before use.
- 1/2" IPS PVC - .840" OD x .622" ID - 21.3mm OD x 15.8mm ID
- 3/4" IPS PVC - 1.050 OD x .824" ID - 26.7mm OD x 20.1mm ID
- 1" IPS PVC - 1.315" OD x 1.049" ID - 33.4mm OD x 26.6mm ID
- 1 1/4" IPS PVC - 1.660" OD x 1.380" ID - 42.2mm OD x 35.1mm ID