What is Suspension Sag and Why Would I Want It?

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May 20, 2010 | By: David Zemla

If you’ve been reading my J&P blog posts about suspension and bike fitment, you might recall that your shocks can be adjusted to fit your bike and weight. You also learned that in order to achieve a proper fit in the two-wheel world, sometimes you need to swap out a part. But what if a simple adjustment will do? For that, you need to understand that shocks work with both the up and down movements of the bike. If all pavement imperfections were bumps, shocks would focus on compression. But life on the road isn’t just a series of bumps. There are also potholes and other gaps and depressions that require a shock to function in the other direction as well. To achieve this, a shock compresses a bit under the weight of the bike and the rider before it’s 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 (as well as on most stock equipment).

There are two key sag measurements and 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 lift of the rear end (simply pick up the bike by the fender or rack as far as you can) will unload the shocks and a free sag measurement can then be taken.

The next critical measurement is rider sag and as you might suspect, 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.  For cruiser and touring bikes that’s generally expected to be .75-inch to 1.25-inch (depending on application). If your number is less, the bike will require less preload, and if it’s higher, more preload is needed. Twin shock bikes should always be adjusted with equal preload on both sides. So it turns out that sag isn’t always such a bad thing and in fact, a correctly set up bike will ride sweeter, deal with road imperfections better and reduce rider fatigue.

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