After understanding Springs and Dampers, and before personalizing your motorcycle suspension to your taste and needs (Lower? Firmer Spring? Height adjustable?, you must also understand Shock Pre-load and how to make Shock Sag Adjustments. In this Part 2, Progressive Suspension explains what you should know before choosing between all custom suspension options.
3- Shock Preload Adjusters. All of Progressive Suspension’s shocks are Preload Adjustable. This allows the user to easily tune the shocks for their specific combination of bike and rider weight. Although we design a shock to fit and perform on a specific motorcycle, we have no way of knowing if the rider will be 140lb or 240lb and the addition of cargo and passenger further complicates the issue. An obvious solution is an adjustment on the shock that allows the rider to quickly set up his shocks to match his bike/rider weight combination. This is where Preload Adjustment comes into play. By compressing or uncompressing the spring a small amount, a shock can be perfectly tuned to suit the conditions for which it will be used. In addition to this level of tuning, most PSI shocks are also offered in Heavy Duty applications.
4- Shock Sag Adjustment. OK, so now you know your shocks can be adjusted to fit your bike and weight, but where do you start? Naturally from the beginning and with an understanding that shocks work in both the up and down movements of the bike. If all pavement imperfections where bumps, shocks for the most part would focus on compression. Unfortunately, there are pot holes and other sorts of gaps that require a shock to function in the other direction, as well. To achieve this, a shock compresses a small amount under the weight of the bike and the rider, before it has even been asked to do any work. This is called Sag, and is adjusted via the handy Preload Adjusters we include on each PSI shock. There are two key Sag measurements, the first is Free Sag, ideally measured from the center of the rear axle to an arbitrary point directly above the axle (a fender bracket, for instance). This is done off the stand and with a helper holding the bike straight up. A solid lift of the rear end (simply pick up the bike by the fender or rack as far as you can) will unladen the shocks and a Free Sag measurement can be taken. The next critical measurement is Rider Sag and this is done with the rider on the bike (hands on the bars and feet on the pegs) and a helper or two supporting it. A measurement is again taken from the center of the rear axle to the same point as before. The difference between the two measurements is your final Sag and for cruiser and touring bikes is generally expected to be .75” to 1.25” (depending on application). If your number is less, then the bike will require less preload, and if it is higher, more preload is needed. Twin shock bikes should always be adjusted with the equal preload on both sides. (Source Progressive Suspension)