Your bike's rear sag is very important.

Too much and you increase the rake of your forks causing your bike to push to the outside of the corners or "hunt" for a berm. Also, excessive bottoming may occur if you have too much sag.

Not enough and your bike will under-steer, possibly headshake and seem harsh in the back.

Also, once you have the correct race sag set on your bike you can tell if your rear spring rate is correct by the amount of static or free sag the bike has under it's own weight.

All but a few riders can really feel their bike bottoming. Most riders think the damping or the linkage is too harsh. But what really is happenning is they are running too little preload on the spring or to soft of a spring and using the last half of the stroke only...

Setting correct spring rates with correct preload and race sag is very important to your bikes setup...

Setting your race sag is easy, here is the procedure;

Race Sag is a measurement of how much the rear suspension sags under the riders weight compared to no weight on the bike.

The First measurement is taken with the bike on a center stand with the rear wheel hanging freely in the air. Measure the distance between the rear axle and some convenient point near the fender like a seat bolt.

The Second measurement is taken with the bike on the ground and the rider standing straight up on the foot pegs supporting himself against a wall or tree. Again, me asure the distance between the same two points used above. (Standing increases the repeatability of this measurement.

The Race Sag is the difference between the First and Second measurement.

To change the sag, use a long punch and a large hammer to loosen the spring locking nut on the shock. Then turn the preload nut to adjust rear spring preload. Note: Increasing spring preload will decrease Race Sag and vice versa.

Changing the Race Sage to 3 ¾" will raise the rear end making the bike turn sharper and reduce bottoming. Conversely changing the Sag to 4 ¼" will lower the rear and increase high speed stability.

Or watch it on video...