Ever wanted a custom tune for your fork and shock, but have been too intimidated to open up that damper? Variable Valve Control (VVC) gives you the ability to have this same desired effect via an external adjuster on our GRIP2 high speed compression and rebound circuits in the 2021 34, 36, and 38 FOX forks.
But what in the world is Variable Valve Control?
To understand what VVC is and how it works, we’d recommend checking out two FOX Academy videos to get a better idea of what the parts of a shock are called with Getting to Know Your Shocks and Why You Need Shocks.
Here’s a quick summary of some background information that sets the foundation for understanding VVC:
Shims + piston = piston assembly.
Dampers (whether in a fork or shock) control oscillations by using oil to convert motion of the wheel to heat inside the shock and dissipate the heat into the atmosphere.
We have more explanation below, but if you’d like to jump right in and watch FOX’s ace MTB tuner Jordi Cortes disassemble a GRIP2 damper, here’s the video:
A valve is another way to refer to the shim and piston working together in the piston assembly, because it controls oil flow. If you allow oil to flow easily, the damper shaft within the fork or shock moves quickly. If you restrict oil flow, the damper shaft within the fork or shock moves slowly.
Another example of controlling liquid flow is the lever (or valve) on your kitchen sink faucet controlling water flow (on, off, fast, slow, etc.)
High speed vs. low speed compression and rebound don’t refer to the speed of you or your bike — although these can be correlated to one another.
High speed and low speed refers to how fast the shaft of the damper inside your fork or shock is moving.
While you’re more likely to have a high speed shaft movement when you’re bombing down a descent, you’ll still have a number of low speed shaft movements to control.
The same goes for when you’re moving slower; it’s still possible to have some high speed shaft movements: like if you encounter a square edged rock, log, or curb that forces your wheel to immediately move out of the way.
Over damped: too much damping
The fork/shock responds too slowly to the terrain, leading to harshness and not enough travel usage. Loss of traction and control can also occur.
Under damped: too little damping
The fork/shock responds too quickly to the terrain, leading to a vague feeling and too much travel usage. This can also eventually result in harshness because the fork/shock will (1) run out of travel or (2) be too deep into the travel to effectively absorb successive smaller bumps. Can also result in loss of traction and control.
The negative impact of being under damped can feel very similar to being over damped. Don’t worry, this can be confusing at first and does warrant its own video and discussion for further explanation. We’ll cover this in more detail at a later date.
Primary methods to control low- and high-speed damping
Low-speed compression and rebound is usually controlled by using an orifice valve. This controls small amounts of oil that is allowed to flow freely around the shim stack. Jordi discusses more about how a low-speed orifice valve works at 3:38.
High-speed compression and rebound is usually controlled by using preload. This controls oil flow through shims that block ports on the piston by changing the force required to initiate the use of this valve. Jordi discusses more about how preload works at 4:02.
The main issue with high-speed compression and rebound being controlled using preload are twofold:
- Preload adds harshness at lower shaft speeds (because it is over damped) and increases the damping force applied at lower shaft speeds, which slows down the fork and shock compression and rebound too much.
- It lacks support at higher shaft speeds (because it is under damped) and does not apply enough damping force at higher shaft speed, which allows the fork and shock compression and rebound to move too quickly.
How does this feel when you ride? An example: you feel like you’re bouncing off all of the small braking bumps, rocks, etc. and can’t get your fork or shock to feel compliant enough because it is just too firm. But in the same setting, you still blow through all your travel when you land a jump or encounter a big, square edged obstacle.
These two issues can be present at the exact same setting and the three graphs below will show how.
Notice that as you add preload, the entire graph moves. At every speed, damping force increases, but the slope of the graph does not change. Harshness is found at the beginning of the graph where the force it takes to initiate opening the valve (flexing the shims) is always increasing.
In order to have support and the right amount of damping (less and more) at the various shaft speeds you and your bike encounter, you need to actually change the slope of the graph, not just move it up and down. Typically, we change the amount of shims on the piston to change the slope of the graph.
But with VVC, you utilize the adjuster knob to change the leverage that the shim stack has over a leaf spring — or in Jordi’s words, the “wing”.
This is the key here: adjusting leverage instead of preload. Leverage changes the slope of the curve. Preload makes the whole curve move up and down.
In the video, Jordi does a good job at pointing out the helical (spiral) ridge on top of the “hat” that controls the flex of the shims. It starts at 4:40, if you want to jump directly to that:
To get an idea of what you feel on your bike, we’ll put these two graphs together. Notice the shaded areas above and below the VVC line. When you use preload to adjust high speed compression and rebound, you run the risk of being perpetually over and under damped. In fact, the one point of intersection between the lines is the one point where we would say the preload high speed adjuster has an optimal level of damping. Every other point is either too much or too little.
How does this feel when you ride? Let’s go back to our example:
Over damped — You feel like you’re bouncing off all of the small braking bumps, rocks, etc. and can’t get your fork or shock to feel compliant enough because it is just too firm.
Under damped — You still blow through all your travel when you land a jump or encounter a big, square edged obstacle.
Now you know why you can feel both of these things on the same high speed adjuster setting: it’s because your adjuster uses preload on the high speed valve stack, not leverage.
That is why VVC is so cool. You get the same effect as changing the tune on the shim stack without actually having to. Each adjustment fundamentally changes the curve of the damping for applied graph giving you optimal damping for any situation based on your expectations, riding style, terrain, and bike setup.
VVC technology changes the game and is available in FOX Factory and Performance Elite 34, 36, 38 and 40 forks, including e-bike versions. It is also now available on our FLOAT X2 and DHX2 rear shocks.
Like what you see? Check out all the FOX Academy videos for powered vehicles and mountain bikes here.
For the hardcore enduro rider or downhiller, check out DIALED.