[twowheels]

A couple points to add to Matt's description

Originally Posted by MattR

The low and high speed compression adjusters are slightly misleading. The high speed (larger adjuster nut) is not really "high" speed... but rather it controls motion in a "higher" low speed range or mid speed level. Actual high speed changes are really done by a suspension tuner who revalves the piston and staging of the shim discs. This is largely true. The compression adjusters are working on a valve dealing with fluid displaced by the shock shaft into the reservoir as the shock compresses. The compression stack in the shock body is dealing with fluid moving from one side of the piston to the other. The shaft and the piston obviously move at the same speed, but the fluid velocity is determined by the volume flow divided by the area the fluid travels through. Port sizes, both on the shock piston and between the shock and reservoir determine the actual fluid velocity through each component. The compression stack on the shock piston can have low, medium, and high speed damping elements, but they are not adjustable without disassembling the shock.

These two compression adjusters (low and high) are only changing the oil flow thru a bypass hole. So, when you turn the adjuster screw or nut fully seated, oil is being restricted from flowing thru this bypass hole. It now has to flow through the piston valving. Think of it as water faucet... when you turn the faucet handle closed/off, then the water stops flowing. For suspension, when you make the oil flow more slowly/more restricted... it makes the suspension motion slower and feels more firm. On recent shocks, the low speed compression adjuster is a needle jet, much like a fuel screw or the rebound adjuster. The high speed adjuster actually works on a spring against the compression valve shims, so adding high speed is actually preloading the valve stack making it more resistant to fluid flow into the reservoir.

How do you know if you should tune using the high or low speed compression adjuster? Consider the shock shaft speed, not the motorcycle speed. For a given forward speed, if the wheel has to climb this / the shaft speed is slower than if it has to climb this |. Rolling whoops and G-outs would be considered "low speed" events. Roots, square edged bumps, and jump landings are typically considered "high speed".

A note of caution: The displaced fluid wants to take the path of least resistance. If you crank down on your high speed compression, you're forcing more of the fluid through the low speed port, and you can easily end up with choked flow or orifice damping. The overall damping is a function of the interaction of both flow paths.