Last week’s Tuesday Tip “The Improper Sag Effect” sparked this question in twmx.com reader Tyler Hayes:
“…I ride a YZ125 and I weigh 95 pounds. When I first got my bike I felt some bad headshake, so my dad called a suspension shop and they told us to put more sag and get softer springs. So we gave it more sag and replaced the stock springs with a softer set, but I still have headshake. Will adjusting the sag help?”
Tyler raises another great question about suspension — how to determine if you have the right springs. So once again we turn to Sobe No Fear/Samsung/Honda’s Todd Brown for the answer…
Now that you’ve obtained your rider sag, it’s time to measure your free sag to determine if the springs on your bike are right for you. Place the bike on the same level spot on the ground and measure between your two reference points without the rider on the bike. The difference between your extended measurement (on the stand) and this new one will represent your free sag.
If your free sag measurement is 20mm or less, I recommend going to a stiffer spring rate. A spring that’s too soft forces you to add excess preload to get the desired rider sag, and as a result the rear end is raised, which can cause it to unload too much in the air and top out as the travel rebounds, resulting in a harsh feel.
If your free sag measurement is 35mm or more, I recommend dropping to a softer spring rate. A spring that is too stiff will not allow for maximum traction under acceleration, and thus you’ll feel every bump on the track.
Note: If you determine that a stiffer spring is required on the rear end of your bike, then more than likely you’ll also require a stiffer set of springs in your forks, as well.