Suspension Terminology

---

Cavitation happens when oil pressure inside the shock drops so low that tiny vapor bubbles form. Those bubbles collapse as pressure rises again, reducing damping and making the suspension feel less controlled. This can happen when you makes the top subcompression valving soft enough to where it can not hold the oil in the cartridge. It can also happen when you stiffen main the compression to the point where it pushed oil out of the cartridge thru the subcompression.

One condition that can contribute to cavitation is when the subcompression valve is tuned too soft. If it cannot maintain sufficient pressure within the cartridge during high shaft-speed compression events, oil may not adequately fill the low-pressure area behind the piston, increasing the likelihood of cavitation.

The opposite extreme can also create problems. If the main compression valve is excessively stiff, it can generate very high pressure across the piston. This increases the demand for oil flow through the subcompression valve. If the subcompression circuit cannot supply or replenish oil fast enough, pressure in parts of the cartridge can drop enough to promote cavitation.

In simple terms, the main compression valve and the subcompression valve must work together. If the subcompression is too soft or the main compression is too stiff, the pressure balance inside the cartridge can be upset, increasing the risk of cavitation and causing the shock to lose consistency during repeated hard impacts.

In a modern Showa or KYB closed-cartridge motocross fork, the cartridge continuously recirculates oil with the outer chamber through the subcompression (subtank) valve. Under normal conditions, the amount of oil entering and leaving the cartridge remains balanced, allowing the cartridge to stay properly filled and maintain consistent damping.

During very high shaft-speed impacts, the hydraulic bottoming system can pump additional oil into the cartridge as the bottoming cone displaces oil. If the cartridge already contains excess oil, or if cavitation has occurred and the cartridge is trying to recover its oil volume, internal pressure can rise enough to force oil out through the subcompression valve into the outer chamber.

Cavitation occurs when pressure inside part of the cartridge drops below the oil's vapor pressure, allowing vapor bubbles to form. As those bubbles collapse and the cartridge refills, the fork may expel oil through the subcompression circuit to re-establish its normal oil volume and pressure balance.

For a suspension tuner, seeing oil blow off through the subcompression valve is often an indication that the cartridge is no longer operating in its ideal pressure balance. The root cause may be excessive cartridge oil volume, cavitation caused by insufficient cartridge pressure, or a mismatch between the main compression valve, the subcompression valve, and the fork's overall oil management.