Do you weight less than 70 kilo? More? Don't know? Well, if you aren't average (weight-wise, speed-wise or height-wise), then the spring of your box-stock motorcycle is very likely not right for you.
How can this be? That's simple. Honda, Kawasaki, Suzuki, Yamaha and other manufacturers didn't built the motorcycle for you – they built the motorcycle for a mythical person. That person is what they call the target audience. At Suzuki, that target audience is much lighter than the one that Yamaha aims for. But all manufacturers in Thailand don't specifically target foreigners living in the Kingdom.
So, it is likely that your shock spring was designed for someone else, somebody much lighter then you. Have no fear. We can tell you a simple way to determine whether your shock spring is too soft or too hard.
Setting sag is something that every rider thinks he can do but usually does wrong. Yes, you still measure from the fender to the rear axle (with and without your body on the motorcycle) – and the difference between the two numbers is the sag, but here are some more tips. 1) Do not measure the sag with the rider standing up. 2) Bounce on the suspension before measuring the sag. 3) Sit where you actually ride, not some dream position. 4) Measure in line with the arc of the rear wheel.
Once you have the sag set for you weight and riding position, you need to get off the motorcycle an put it on the stand again. Once on the stand, take a new measurement of the distance from the rear fender to the axle (it should be the same as your original measurement, but it never hurts to double check. Now, that the motorcycle off the stand.
The static sag is the important number. Static sag is a measurement of how much the motorcycle sags under its own weight. Static sag can only be checked after the sag is set. With the motorcycle sitting under its own weight on level ground, take another measurement from the fender to the axle.
Now, subtract that unladened measurement from your motorcycle-on-the-stand measurement. The difference will be the static sag number.
Static sag should not be more than 40mm on bike motorcycles. If you have more than 40mm of static sag, your shock spring is too stiff. That may sound wrong, but it is right. If you have less than 30mm, then it is likely that your spring is too soft. It's that simple to determine whether your shock spring is right for your body.