Dear Geek:
What is static sag and how do I set it?
Nigel Godfrey, Melbourne, Australia
1 Static sag is a measure of how much your bike settles on its suspension with you on board. We generally recommend the Race-Tech method of measuring sag, which takes into account any stiction in the suspension components. With a couple of friends helping, fully extend the front suspension and measure from the fork seal to the bottom triple clamp (for conventional forks) or from the seal to the fork bottom (for inverted forks, as shown here). This measurement is L1.
2 Put on your regular riding gear and hop on your bike, assuming your normal riding position. Have one friend hold the rear of the bike so that you can keep both feet on the pegs. Your second pal needs to take the same measurement as in step one-once after pushing down on the front end and slowly letting it settle up (L2), and again after gently lifting the front end and letting it settle down (L3). L2 and L3 would be identical in a perfect, frictionless world.
3 Static sag is calculated as: sag= L1-(L3+L2)/2. Averaging the two measurements with you sitting on the bike negates any stiction that may be present. Aim for approximately 30mm of static sag for street riding and 25mm for the track. If you have too much sag, tighten up the front preload. Too little, and you'll have to loosen the adjusters accordingly. One turn on the adjuster will usually change sag by about one millimeter.
4 Moving to the rear of the bike, extend the suspension and measure from the axle to a solid point directly above. Try to avoid measuring to the bodywork, as it tends to move about when you lift the rear end or sit on the seat. Instead, measure to a point on the subframe (or something equally solid). As before, this measurement is L1.
5 Repeat steps two and three, with one person holding the front of the motorcycle while the other takes the two measurements at the rear. As with the front end, 30mm of sag is a good starting point for street riding, 25mm for the track.
6 Calculate the static sag and adjust the rear preload accordingly -one turn of preload usually equates to between two and three millimeters of sag. Here's a tip to check that your rear spring rate is in the ballpark. With your bike unladen, lift the rear end until the suspension tops out, then gently let it settle-how much it drops is the free sag, which should be approximately five millimeters. If the suspension doesn't drop at all, you've dialed in a lot of preload and should consider a stiffer spring. Too much free sag is a sign that a softer spring may be in order.
This article originally appeared in the August 2003 issue of Sport Rider
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