Say you have perfectly set up your suspension , you had the sag correct and the damping adjustments all correct.

Then you lost a lot of weight which nescesitated lowering the spring rate to get the correct sag. Am I right in thinking that generally speaking, the rebound damping would need to be reduced, while the compression damping increased, to accomodate the lower spring rate?

Ride it and see?

I’d have thought both would go down if anything.

Yes, your thinking is basically right.

The lower spring rate is less force to push through the damper. The damping was previously set to retard against a larger force, with a new lower force, the damping will in effect be stronger.

No. You would need less compression damping too.

The simplest method – use lower weight oil.

Wot Glenh said. The spring holds up the bike, and the damper controls the spring. So less weight = softer spring + less damping. The compression / rebound ratio doesn’t really change

You will need to speed up the rebound, compression damping, just leave it where it is, though it may need a little lowering.

When you are moving the fork, all that is happening is energy conversion. Kinetic energy is converted by the spring in strain energy in the spring material, or we can consider it and potential energy, the energy the spring converts is not lost from the system, it is there ready to be converted to kinetic energy again to extend the fork. The damper disperses the kinetic energy, the damper converts the kinetic energy to heat energy and is lost from the system.

Bit horrible that paragraph.

Kinetic energy converted to potential energy (spring, stored) + heat energy (damper, lost).

So softer spring, lower load, less stored energy extending the fork again, less damping required to disperse the stored spring energy on extension.

The compression side of things, less kinetic energy compressing the fork, less kinetic energy needing to be dispersed, less damping required i reckon.

F=MA

As the sprung mass (thats you and the main bits of the bike (not the wheels, they are “unsprung”)) has fallen you will need to reduce both the compression and rebound damping if you want the system characteristic frequency to be back to the previous value.

You have already found that to get to the same sag, you have had to reduce the spring force, and so you will require a lower rebound damping effect to match that lower force (assuming your unsprung mass hasn’t been changed (same wheels as before etc). But, as you are now lighter, if you kept the same comrpession damping as before, the overall system would be overdamped (assuming it was just critically damped previously) So you need to reduce the compression damping too.

One of the side effects of getting lighter, is that the ratio of unsprung to sprung mass is reduced, and things like damper friction start to play a larger part in the overall damping system.

I find that a good starting point for rebound is to set it to the point where, having pushed down hard on the suspension the system is able to extend and keep the tyre touching the floor faster than you can try to lift the bike off the ground. This gets you somewhere close to a system that will be optimised for “traction” (keeping the tyre in contact with the ground) however, i generally end up a few clics eitherway from this point to optimise handling.

Thanks all, i was partially correct then

I lost about a stone recently, and had to lower the spring rate to get back to the correct sag, but i didnt alter the damping, and it felt like the fork was packing down when riding fast over rough ground, i just wanted to check my logic and what i thought i felt was sound ish before i reduced the rebound damping by a click