Tuning and dialing in the rear shock on your mountain bike can play a huge role in how the bike performs out on the trail. With an air shock, not only can you adjust the air pressure inside the air chamber but you can also change the volume of that air chamber. Decreasing the volume of the air can will make the spring curve more progressive and give you more bottom out resistance towards the end of the stroke. Increasing the volume of the air can will give you a more linear spring curve, making it easier to use full travel. There is no one size fits all for size or number of volume reducers to use inside your air shock. Every bike and shock combination out there is different. Today we are going to be learning how to change the volume reducers inside a Fox Float DPX2 rear shock. The DPX2 can be fitted with only one volume reducer at a time and that's why Fox offers a DPX2 Volume Reducer Kit with 5 different size volume reducers. Finding the best volume reducer for your bike, your setup, and your riding preferences will help you get fully dialed in.
We don't really need many tools for this How To. A standard set of hex keys, a high pressure shock pump, and the Fox DPX2 Volume Reducer Kit will get the job done. This volume spacer kit includes 5 different spacers: 0.2, 0.4, 0.6, 0.8, and 1.0. These five different spacers cover the needs for a variety of different suspension designs. The bike we are working on here is a Santa Cruz Hightower V2 and is using the light blue 0.6 volume reducer.
In the graph below, you can see what adding a larger volume reducer does to the spring curve on your bike. On the Y axis, you will see Spring Force. This is the force required to continue to compress the air spring. On the X axis, you will find Shock Travel or stroke. As you continue through the travel, the spring force increases. By adding or removing volume spacers, you can change the spring force required to compress the air spring as you move through the shock travel. Keep in mind that the most progressive setup isn't always the best one. If you aren't using full travel at the recommended sag, chances are you might need to use a smaller volume reducer, increasing the size of the air spring volume.
It's not 100% necessary to remove the shock from your bike to change the volume reducer inside the DPX2, but this will make things a bit simpler and easier for you, especially if this is your first time. It is handy to know that you can make changes to your shock's spring curve while the shock is still mounted to the bike. All you need is your shock pump, a small hex key, and the volume reducer kit and you can be tuning out on the trail.
Before you can unthread and remove the air can, you first need to release all the air inside the shock. This is a good rule of thumb to have when servicing any air suspension. Use a small hex key to depress the air valve slowly.
The air can threads directly on to the head and body of the shock. Grab a hold of the oil reservoir with one hand and on to the air can itself with the other hand. The air can uses a traditional thread so lefty loosey righty tighty.
Once you have unthreaded the air can, you can slide the air can down the shaft of the shock. This will just get the air can out of the way so you can get to the volume reducer inside.
Next you can slide the o-ring and shim down from the top of the shock. Using a small hex key, unclip the volume reducer from the damper shaft inside the shock. The plastic volume reducer just clips in so you might need a small tool to snap it out of place.
Find the right volume reducer you want to use and snap it in to place. Remember that adding a larger volume reducer decreases the air volume inside the air can and increases mid stroke support and bottom out resistance on your shock. Installing a smaller volume reducer or removing it completely does the opposite and allows you to use full travel more easily.
Once you have the volume reducer snapped in and the shim and o-ring up against the bottom of the volume spacer, it's time to slide the air can back in to place. Slide the air can back up the shaft of the shock and thread it on the head of the shock. You will feel the threads bottom out when they do. Tightening the air can by hand is plenty tight for this application.
Now that you have the shock reassembled, air up the shock with your shock pump. Keep in mind that when you change the air can volume, you will need to set the sag again. Be sure to equalize the positive and negative air chambers by cycling the shock through its travel as you add air to the shock.