Until the mid-1980s, most motorcycles featured cheap damper-rod forks with overly soft springs and damping that worked only at specific suspension speeds. Tuning in those days was pretty much limited to fitting progressive-rate springs and changing the fork oil to broaden the performance window.
However, sportbikes switched over to cartridge designs in the ‘90s. These offer much more consistent performance, separate rebound and compression damping functions and can be tuned more precisely over a larger range. If you are serious about racing, the internals can be changed for more precisely machined parts working in a different range to reflect the use you’ll be putting them to, hustling the motorcycle around a track lap after lap.
Cartridge forks work on the principle of a variable orifice, giving more damping control over a wider speed range. The damping is controlled by stacks of shims built on compression and rebound pistons. The two pistons are enclosed in a cartridge that looks similar to a large bicycle pump.
Fork emulators are small aftermarket valves replace the fork caps on damper rod forks. The provide a cartridge-fork-like compression damping curve that’s adjustable, and separate compression from rebound damping so each can be independently tuned.
Shims cover ports (holes) in the piston. At low fork speeds, oil pressure in the ports bends the shims fractionally, so the exposed part of the hole is small. At high speeds, the shims bend a large amount, exposing the entire hole.
A bypass hole around the piston can be fitted with a needle to give a range of adjustment. Turning in the needle closes the hole, making the damping stiffer; turning it out makes it softer. The shim stack itself can be altered to change the overall damping level to make the fork softer or harder. This is generally termed re-valving.
Compression and rebound damping. This affects the speed that the forks compress under braking and when they hit a bump. Rebound damping affects the speed that the forks extend again, when you release the brakes or bounce up from a bump If you prioritize ride comfort, you’ll end up with a softer setting. Conversely, if you want to accelerate and brake hard, you’ll end up with a firmer setting.
Springrate is a linear spring compresses at the same rate throughout its travel, while on a progressive unit this will alter. Many current motorcycle especially tourers, still have progressive springs to cover all bases. The key is to have a linear-rate spring that is matched to your weight and riding style.