Ohlins has built a transparent shock and put it on a dyno, so you can easily see the effects of cavitation.
If you’re an experienced mountain biker and enjoy tweaking your suspension settings, you’ve probably come across the term cavitation. But what does it mean, and why so suspension manufacturers go to such great lengths to stop it happening? Well, thanks to an ingenious new see-through shock built by Ohlins USA, it’s easy to see how cavitation happens, and why it has such a negative effect on suspension performance.
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The Ohlins team has based its transparent shock on a motorcycle unit, but the design, construction, and effects are basically the same as you’d find in a modern mountain bike shock. It uses standard shock internals encased in a perspex body, which is then mounted on a shock dyno used to replicate the forces encountered when hitting a bump or landing a jump.
In the video you can see the main piston going up and down inside the shock body, forcing the oil inside through the valving. In the remote reservoir on the right you can also see the volume of damping fluid fluctuate, regulated by what’s known as the Internal Floating Piston. This is, as the name suggests, a floating piston inside the piggy-back pressurised by a gas (usually nitrogen). The gas can compress, which lets the fluid volume change under pressure, so that air can’t be introduced into the damping circuit.
Inside the Fox Float X shock. The IFP is in the remote reservoir
When the technician releases the air pressure from the IFP, the turbulence immediately starts to occur in the damping fluid and tiny bubbles start to form within the main piston. This makes the damping performance inconsistent and unpredictable. Tuning the IFP pressure (in this case 10 bar) will also have an effect on the damping curve, with higher pressures increasing the spring pressure of the system and making it harder for the shock to compress initially. Of course a lower IFP pressure risks allowing cavitation. To help solve this, suspension have various solutions, one of which is the RockShox Countermeasure, a negative coil spring that presses on the IFP.
Another point to take away from this video is that there’s a lot going on inside your shock. Seeing that kind of action over the course of a year or more will inevitably see wear and tear on seals and damping fluid, which is why it’s so important to get your suspension serviced regularly.