A fork oil viscosity change is the simplest way to tune your fork outside of the built in adjustments. This was the way it was done before fancy adjustment knobs. This type of tuning has limited range compared to valving and shim stack changes, but it will extend adjustments to a degree for when the knobs don't suffice.
REFERENCE
suspension tuning
"The amount of damping is dependent on the weight of the oil and the size of the damping holes. Many street bikes are underdamped and can benefit from slightly heavier oil - but it's easy to go too far. If you've changed the oil and now find you've got a harsh ride over sharp bumps, you've probably gone too far.
One point is often overlooked: stiffer springs need stiffer damping. When I raised the spring rate on my SV I found the 10W oil wasn't damping enough, and a change to 15W made a noticeable difference.
High end forks and shocks as found on some sport bikes have adjustments for compression and rebound damping. These will allow fine tuning, but they're useless unless you have the right weight oil to start with."
Fork oil, by design, is incompressible. Hence, whatever the fork oil weight is, piston movement that displaces oil, will move move a shim stack in the exact same way.
What changes is that different oil viscosity will flow less or more given the size of the ports or the deflection of the shims. Comparatively, changing the shim stack to thinner shims will allow the shim to deflect more and allow more oil to flow.
Both tweaks have their limits. The thinnest shims will only allow oil to flow as much as the port it covers. The thinnest oil is also governed by the same physics.
Not to get too deep into the weeds, adjusting the "plushness" or "riding height" of your fork is a relationship between fork oil weight (viscosity), port size and shim stack set up.
Changing one can affect the performance to a degree but is still dependent on all the others.
Between the available "knobs" and the oil, you should have a significant amount of tunability to get the fork to feel just right. Ofcourse, this is dependent of the user. If you now what to look and feel for, and how to address it, you'll figure it out.
Fox says the same thing for shims - Tuning Shim Valves
If oil and knob adjustments are still unable to give you the feel youre looking for, it's time for custom tuning.
Note: Basic motorcycle forks with no knobs relies simply on fork oil weight and height adjustments. Early MTB forks were the same.
Caveat - Changing fork oil will affect rebound speed. They use the same oil reservoir. If the rebound speed goes out of range, too fast or too slow, and you are unable to adjust it well with rebound knob, you can look into getting the fork custom tuned.
REFERENCE
suspension tuning
"The amount of damping is dependent on the weight of the oil and the size of the damping holes. Many street bikes are underdamped and can benefit from slightly heavier oil - but it's easy to go too far. If you've changed the oil and now find you've got a harsh ride over sharp bumps, you've probably gone too far.
One point is often overlooked: stiffer springs need stiffer damping. When I raised the spring rate on my SV I found the 10W oil wasn't damping enough, and a change to 15W made a noticeable difference.
High end forks and shocks as found on some sport bikes have adjustments for compression and rebound damping. These will allow fine tuning, but they're useless unless you have the right weight oil to start with."
Fork oil, by design, is incompressible. Hence, whatever the fork oil weight is, piston movement that displaces oil, will move move a shim stack in the exact same way.
What changes is that different oil viscosity will flow less or more given the size of the ports or the deflection of the shims. Comparatively, changing the shim stack to thinner shims will allow the shim to deflect more and allow more oil to flow.
Both tweaks have their limits. The thinnest shims will only allow oil to flow as much as the port it covers. The thinnest oil is also governed by the same physics.
Not to get too deep into the weeds, adjusting the "plushness" or "riding height" of your fork is a relationship between fork oil weight (viscosity), port size and shim stack set up.
Changing one can affect the performance to a degree but is still dependent on all the others.
Between the available "knobs" and the oil, you should have a significant amount of tunability to get the fork to feel just right. Ofcourse, this is dependent of the user. If you now what to look and feel for, and how to address it, you'll figure it out.
Fox says the same thing for shims - Tuning Shim Valves
If oil and knob adjustments are still unable to give you the feel youre looking for, it's time for custom tuning.
Note: Basic motorcycle forks with no knobs relies simply on fork oil weight and height adjustments. Early MTB forks were the same.
Caveat - Changing fork oil will affect rebound speed. They use the same oil reservoir. If the rebound speed goes out of range, too fast or too slow, and you are unable to adjust it well with rebound knob, you can look into getting the fork custom tuned.
