Race sag is the foundation every other suspension setting builds on. Get it wrong and the bike will handle poorly no matter how good your forks and shock are. The good news: it's simple to measure and a great first wrench skill.
What Is Sag?
Sag is how much the suspension compresses under the weight of you and the bike, in your gear. Setting it correctly puts the suspension in the right part of its travel so it can both soak up bumps and resist bottoming. Most MX bikes target around 100–105 mm of race sag — check your manual for the exact figure.
What You Need
- A tape measure
- Your riding gear on (boots, helmet — it's part of the weight)
- A buddy to hold the bike (or a wall to balance against)
- A hammer and punch, or a sag tool, to turn the locking ring
How to Measure
- Fully extended: put the bike on a stand so the rear wheel hangs free. Measure from the rear axle to a fixed point on the fender. Call this L1.
- Loaded: take it off the stand, sit on it in full gear in your normal riding position, feet on the pegs, and have your buddy steady it. Measure the same two points. Call this L2.
- Race sag = L1 − L2. Bounce the rear a couple of times and let it settle before each loaded measurement for accuracy.
Adjusting
Loosen the shock's upper locking ring and turn the main adjuster ring. Tightening the spring (preload) reduces sag; loosening it increases sag. Make small changes, re-measure, and lock the ring when you're dialed in.
One More Check: Free Sag
With race sag set, measure how much the bike settles under its own weight (no rider). If free sag is way off, your spring rate may be wrong for your weight — a sign you need a different spring rather than more preload.