Vacuum Bleeding

It's time we dig just a bit into vacuum bleeding. We have a lot to cover but we'll try to keep it simple 🤓

First, we need to look at how a damper works in a shock. The damper is a series of components that control the oil flow inside your suspension. It provides support while compressing and extending, preventing your bike to bounce uncontrollably.

Most high performance suspension dampers are completely filled with oil, and there's a floating piston or bladder keeping that oil under pressure.

For a damper to work best, there needs to be a very low amount of gas inside the oil chamber, and that's where the vacuum pump kicks in, it sucks the air out through a bleed port, then switches to pressure filling, the damper gets cycled in the process to make sure there's no trapped bubbles within the system.

Fluids are considered incompressible, in theory. But a fresh bottle of suspension oil can have ~10% gas dissolved in the liquid. The vacuum pump is in charge of reducing that to a minimum when filling.

But what happens when I have an aerated damper?

Oil is incompressible, but gases are not. In a freshly bled shock, an input on the shaft will start pushing oil through the circuit virtually immediately, and will keep the bike supported and controlled at all times.

When you have a high amount of gas inside the oil, the gas begins to compress before oil can flow through the valves, giving you an "undamped" moment while compressing/extending.

Also, the air bubbles in the oil reduce the consistency in viscosity, which translates into inconsistent damping. 🙅

When servicing, an aerated shock will behave just like a shaken can of soda, when the oil is still under pressure it behaves like a liquid, but release the IFP or bladder charge, and the drop in pressure allows the dissolved gas to boil out and make the foamy mess we all know 💨

How does it happen? Well, "no seal is a perfect seal", the gas in the reservoir (or air spring) can work its way into the damper overtime and with riding.

So what does this all mean? Follow the manufacturers service intervals to keep you and your bike happy 🖤