I bought a new pair of 99 Rockshox Sid XCs to replace my ancient Mag 21s that I’ve had forever. They feel light on the front end of my bike (a Marin Pine Mountain), but they also feel harsh. They’re OK when Im working them really hard, but over smaller stuff, like bridleways with loads of horses hoof prints, I get lots of judder. What should I do to make them feel better? I weigh 10.5 stone. I put 60psi in the main and the negative air chambers.
The 99 forks are adjustable in six ways. Most adjustments can be made externally, allowing you to adjust your forks up without ripping them apart. The most important thing is to make a note of the set-up you’re using, and record all changes. Never change more than one thing at a time.
Try increasing the air in the negative spring (thats the valve at the bottom of the left leg). For your weight, try 90psi in the negative chamber and leave the main chambers at 60psi. This should give you a plusher ride. Back off the compression damping as well the knob at the bottom of the right leg. Pull it out to adjust the compression damping, and turn it in single click increments.
Find a rutted piece of ground that you can ride over repeatedly to gauge the effect your adjustments are having. The compression damping affects how quickly the fork can react to being compressed if its too high, the fork wont take up small bumps. Turn yours anticlockwise to reduce the compression damping.