Suspension school · Damping Adjustments

High-Speed Compression (HSC)

Square edges and big hits, and the difference between support and spike

High-speed compression damping (HSC) takes over when the damper shaft moves fast: square-edged rocks, roots at speed, landings, and hits that arrive in a few hundredths of a second. In most dampers these events force oil against a stack of thin sprung steel shims that blow open under pressure, and the HSC adjuster changes how easily that happens.

The bike speed distinction matters here from the other direction. A 60 cm drop to flat rolled at walking pace produces a far higher shaft speed than a fast run down a smooth trail. Flat landings at low speed are among the hardest things suspension ever absorbs, which is why casing a jump slowly can hurt more than clearing the same jump quickly.

Support Against Spike

Get HSC right and big hits feel deep but controlled: the wheel swallows the rock, the travel absorbs the landing, and nothing clangs at the end. Too little HSC and medium hits use nearly everything, leaving no reserve and producing harsh metal-on-metal bottom-outs on the genuinely big stuff. Too much and energy cannot get into the travel fast enough, so the hit passes through the chassis to your hands and feet instead. That sharp jolt is what tuners call spiking.

Too much HSC has a second cost that is easy to misdiagnose: deflection. A wheel that cannot compress quickly enough gets knocked off line by square edges rather than absorbing them. If the bike started pinballing through a rock garden after you firmed things up, open HSC back out before blaming tyres or line choice.

Reading a Bottom-Out

• An occasional soft bottom-out on the biggest hit of the ride is correct. Travel exists to be used, and never reaching the end means carrying suspension you paid for but never spend.
• A regular, harsh clang on ordinary medium hits means you need more resistance: one or two clicks of HSC, or a volume spacer if the pattern below fits.
• Never getting near full travel on your hardest local trail means too much resistance somewhere: open HSC, remove a spacer, or check the spring is not over-pressured.

HSC, Volume Spacers, or Pressure?

Three tools overlap at bottom-out and they behave differently. Air pressure sets support everywhere and fixes sag, so it is not a bottom-out tool unless sag is wrong. Volume spacers steepen the last third of the air spring's ramp and care only about position in the travel, not speed. HSC resists fast movement at any point in the travel.

The sorting rule: bottoming on fast, sharp hits (square edges, flat landings) is a damping problem, so add HSC. Bottoming on slow, deep compressions (big berms, G-outs, steep transitions) is a spring problem, so add a volume spacer. If both, fit the spacer first, because spacers add nothing to small-bump harshness, then trim with HSC.

The eMTB Angle

Kinetic energy scales with mass, so a 24 kg eMTB arriving at the same compression at the same speed as a 15 kg bike simply brings more energy for the suspension to deal with. Add the extra descending an eMTB racks up per ride and the high-speed circuit works harder for more of its life. If you have come to a full-power eMTB from an unassisted bike, expect to run slightly more bottom-out resistance than you are used to, whether via HSC or one extra volume spacer.

For reference, Fox's baseline on the GRIP X2 fork damper is 5 clicks of HSC from closed out of 8. On the 2025 onwards Float X2 shock, Fox keys its recommendations to air pressure, with compression settings firming as pressure rises, so heavier riders start with more of both.

Testing It Properly

HSC is the hardest circuit to assess in a car park, because you cannot generate real shaft speed by pushing on the bars. Pick a short rock garden or a drop you know well, ride it at your normal pace at the baseline setting, then bracket two clicks either side and compare. Judge it on three things: how harsh the hit feels through your contact points, whether the wheel held its line, and how much travel the o-ring shows afterwards.