Tech Tuesday: Volume Spacers and Spring Curves

It's been ten years since the last Tech Tuesday article, so it's probably time to get this educational series rolling again. All of the previous articles can be accessed here, and it's worth taking a few minutes out of your day to watch 16-year-old Mike Levy explain how to fix a flat. —Kaz


Words: Samuel McMain
Photos: Ryan Palmer

How to fine-tune your ride.

Dialing in your suspension is a time-consuming task. We get it and we’ve all been there too. Setting your sag is one thing (that we covered here), but diving even deeper to optimize your spring curve is a task that involves more than just a shock pump, a measuring tape and a wall to lean on. But as with most things suspension-related, an increase in investment correlates strongly with an increase in performance and comfort on the trail. The purpose of this edition of Tech Tuesday is to demystify the use and principles of volume spacers and arm you with the know-how to continue down the path fine-tuning your ride.

Speak the language:

Air chamber – Your suspension’s spring, this is a chamber filled with pressurized air that is compressed as your suspension moves through its travel

Air volume – The size (volume) of your air chamber

Spring rate – How much force is required to compress your air spring (air chamber)

Spring curve – A representation of how your spring rate changes throughout suspension travel

Volume spacer – An object that can be added or removed from your suspension’s air chamber to increase or decrease the air volume

Bottom-out – To fully use your suspension’s travel

Bottom-out resistance – How hard it is to achieve bottom-out

Mid-stroke – The middle part of the suspension travel

Supportive – A characteristic feeling of a suspension setup that resists moving too easily into travel

Progressive – A spring curve that increases in spring rate through travel

Linear – A spring curve that keeps a constant spring rate through travel


What the heck is a spring curve and why is it important?

Before we can start shoving those colorful little volume spacers in our forks and shocks, it’s important to understand why we would even want to do such a thing in the first place. In fact, by the end of this post, some of you might actually want to remove some spacers. Intrigued? Read on.

To start with, your spring curve is a representation of how your spring rate changes as your suspension moves through its travel. With a coil spring, the spring rate remains unchanged throughout the travel, while air springs are always progressive. As you move through travel in an air spring (be it in an air fork or air shock), the air chamber (volume) shrinks while the amount of air in that chamber stays constant—in other words, the air is forced to compress and increase in pressure. The more air is compressed, the more force it takes to compress it further. This principle is why air springs work as well, springs.

When we talk about air suspension being progressive, we mean that the spring curve of an air spring increases in rate the farther you move into travel. The last centimeter of travel is harder to move through than the first centimeter.

So, how can we use the progressive nature of air springs to our advantage on the trail?


The basics of using progression

Hold on to your butts; this is going to be a quick and dirty explanation. In a few words, if you’re bottoming out frequently, but your sag is correctly set, add a volume spacer to reduce air chamber volume and increase the rate of progression in your suspension. Reducing volume makes your suspension firmer sooner in the travel, increasing the spring rate in the mid and end-stroke.

On the other hand, if your sag is set correctly and you can’t seem to use your travel fully, removing a spacer and increasing air chamber volume makes the middle and end of suspension travel softer and easier to move through. This would be changing your spring curve to be more linear instead of progressive.

Sound simple? Well, there are some caveats to it all (that we can harness for performance profit!)



Air chamber volume and its effects on spring curve

Before we really start to unpack this, we need to talk about the part of the spring curve that an air chamber’s volume affects. Because in the real world, changing air chamber volume only affects part of your spring curve—the mid-stroke and end-stroke.

The initial part of travel, say, roughly the first 35%, is only affected by the air pressure. See also, sag. This is slightly a simplification of the actual physics of it all, but for our purposes let’s say that the first part of your suspension travel is only affected by the PSI reading on your shock pump.

But that leaves the majority of your suspension up in the air (pun intended) to be modified by adjusting volume. Here are the basics:

Reduce volume by adding spacers: slightly increase spring rate in the mid-stroke, greatly increase spring rate in the end-stroke.

Increase volume by removing spacers: slightly decrease spring rate in the mid-stroke, greatly decrease spring rate in the end-stroke.

Take a minute to think about the implications here; these basic statements aren’t as basic as they may seem. Here are some quick tips you can apply to your suspension tuning:

Add volume spacers to reduce the frequency of bottom-outs
Remove volume spacers to make it easier to use your full travel
Add volume spacers and decrease air pressure to make your suspension more supple while maintaining bottom-out resistance
Remove volume spacers and increase air pressure to make your suspension more supportive initially while maintaining bottom-out resistance


Hold up, where did those last two options come from?



Adjusting air pressure when adding/removing volume spacers

Yeah, this is totally a thing you can do. Adjusting air pressure while also changing air chamber volume allows you to fine-tune the two extreme ends of the spring curve independently of each other. You might think of it this way, adjust your air pressure to adjust how you want the initial part of your travel to feel, then add or remove volume spacers to achieve the desired bottom-out resistance.

What about mid-stroke support? People are always talking about mid-stroke support these days. Well, in short, you can’t have your cake and eat it too—usually. It’s very difficult to achieve exactly the amount of mid-stroke support you want while also dialing in the initial and end-strokes. Want more mid-stroke support? You can either bump up pressure or decrease volume (both of which affect one end of the spring curve as well as the mid-stroke).

There are ways to get around this, but they have to do with how your particular fork or shock was designed and often involve the size of your negative air chamber. For example, Rock Shox’s MegNeg air can for the SuperDeluxe rear shock increases the negative air chamber volume, which increases mid-stroke support by offsetting positive air chamber forces farther into the initial part of the travel, allowing you to run a higher positive pressure. There’s more going on there, but that’s for a future post.

Adding/removing volume spacers on your fork/shock

For the most common forks and shock out there, Fox and RockShox, the process of adding or removing volume spacers is pretty straightforward. Each company uses plastic “pucks” for their forks that are attached to the bottom of the top cap of the fork’s air chamber. Rear shocks will differ slightly, but most use a similar plastic puck that slots upside the air chamber of the shock near the top of the shock or clips around the air can.

The procedure for adjusting volume spaces in suspension can be broken down into a few key steps, which we’ll outline below. However, note that there may be significant differences between various shocks and forks, so be sure to check your user manual for specific instructions. Some brands, like Cane Creek, DVO and Ohlins, don’t actually use plastic pucks in their forks and adjust air chamber volume differently, and the process for doing such on those suspension designs isn’t going to look like what we outline here. Again, check your user manual.


Step 1: Note your air pressure! Before you do anything to your suspension, make sure you know what pressure (and the rest of your settings) you are running. This doesn’t mean just sticking a shock pump on, as the increase in air volume from the hose of the shock pump will actually slightly decrease the pressure of your air chamber. This is especially important in air shocks with small air chambers running high pressures—for example, connecting a shock pump to a DVO Topaz T3 caused the pressure to drop from 114psi to 109psi. Take your reading after filling your suspension, and write it down somewhere for future reference.

Step 2: Depressurize your system! This is a HUGE safety step. Never, ever attempt to disassemble your suspension with it pressurized. I’ve seen top caps go through ceiling tiles and riders go to the ER with oil sprayed at high pressure into their eyes. Seriously, make sure there is no air in the fork.

To do this, use your shock pump’s air release valve to let the air out of your suspension. Remember that there’s a pressurized negative chamber as well, and you’ll need to cycle your suspension while slowly releasing pressure to equalize both chambers for depressurization. On a rear shock where you will be removing the whole air can, a pressurized negative chamber will make it very difficult (and possibly dangerous) to actually remove the air can.

Step 3: Remove the top cap or air can. On most forks, you’ll need a dedicated non-chamfered socket to remove the top cap. Do not use a standard chamfered socket as you’ll round the flats of the top cap. A top cap-specific, champferless socket is the best tool for the job, but a pair of Knipex pliers work well too. Crescent wrenches should be avoided, but can be used in a pinch. Remember, that top cap is made of soft aluminum and easily damaged. Some forks, like Rock Shox and Ohlins forks, actually use a standard cassette tool to remove the top cap.

For rear shocks, assuming you’ve fully depressurized the system, most air cans will unscrew (lefty loosey) from the shock body. Some other shocks, like a Fox Float X2, Cane Creek DB IL or DVO Topaz T3, slide open, not unscrew. Refer to your user manual if you’re unsure what part of the shock is the air can, as well as how to get into it.


Step 4: Note the current volume spacer(s) already in your suspension. Again, this is super important. Make you know what your baseline is before making any changes; it’s impossible to make comparisons to the previous set-up if you don’t know what that set-up was.

Step 5: Add or remove spacers. Most fork volume spacers clip or screw together, while some, notably Cane Creek and MRP, use self-contained systems. Refer to your user manual to see how your fork gets things done.

On rear shocks, the process is similar but with some additional rules. First and foremost, avoid using a metal tool to remove spacers. You run the risk of scratching internal surfaces and ruining seals/leaking oil/pressure. If you can’t get a spacer out with your fingers, find something plastic to do the job. Unless you’re a master at Operation, try to avoid using a metal pick. Additionally, note where and how the spacers are located and oriented. Some rear shocks allow for the adjustment of both positive and negative air chambers, so make sure you’re adjusting the right one! Again, refer to your user manual.

Step 6: Re-install the air can or top cap. Be especially careful not to cross-thread or damage o-rings. Additionally, rear shock air cans generally only need to be hand tight if of the screwed-on variety, like a car’s oil filter (but refer to your user manual). Top caps need to be a bit tighter, but use a torque wrench and be very careful not to over tighten things. See also: Very expensive parts to replace if damaged.

Step 7: Inflate. Reconnect and inflate your suspension back to your original pressure. After the first 50psi, stop and cycle the suspension—give it a good compression, but it doesn’t need to bottom out—to equalize the negative and positive chambers—you can damage things if you skip this step! When you arrive at your desired pressure, cycle the suspension a few times to make sure things are fully equalized. It can take 10-15 cycles in some cases. Then check the pressure again, as it will lower slightly when the pressure equalizes between chambers. In some cases, it might need a little top-off.

Step 8:: Go Ride!


Testing your new setup!

Rule one of suspension set up reads thus; Thou shall not change more than one setting at a time. This means you should not make an adjustment in pressure at the same time as adding or removing volume spacers or changing damping. Why? If you change more than one variable, how do you know which variable caused a change in how your suspension feels?

With that in mind, if you changed air pressure or volume, chances are you will need to tweak another setting, but resist doing so until you’ve done a lap or two on your test loop so you get a feel for what needs a tweak.

Speaking of test loops, pick one that is representative of the riding you normally do. It should include the following aspects: high-speed chunder (to test initial suppleness), big but slow compressions like tight berms, G-outs or hard braking segments (to test mid-stroke support) and a few hard hits (to test bottom-out resistance). You don’t need to go out and find the gnarliest huck-to-flat as that’s probably not something you’ll ride every day, and you’ll end up with a set-up that’s too stiff. Likewise, don’t sell yourself short or you’ll end up with a setup that blows through travel too easily.

Your test loop should also be easily accessible, fun and highly repeatable. Say, it should be doable to run it 2 or more times in an hour. It’s hard to compare runs that are hours or days apart.

Keep a page of notes on your phone or notepad with your impressions each run and/or at different settings. The farther you go into fine-tuning your set-up, the more you’ll rely on your notes to recall past settings and how they felt.

On a final note, the weather will significantly affect how your suspension feels. Riding in near-freezing temps will make your suspension perform differently than on a hot summer day. Keep things consistent!


Final considerations

The fine-tuning of suspension takes time, especially since you need to partially disassemble your suspension to make changes. Patience is key, and keep in mind that the process is as much about finding what doesn’t work as it is about finding what does. And on that note, what works will change from venue to venue—your ideal setup for a mid-summer bike park day will be different than what you’ll run mid-winter in sloppy, slow roots and mud. This highlights the importance of keeping notes of your settings.

However, that’s not to say that you can’t find a set-up that more or less suits 90% of your riding, compromising a bit on the outliers.

Our last piece of advice is this: try new things. No suspension setup is ever dialed and humans are remarkably good at adjusting and adapting. What you might think is a perfect setup one day might actually feel terrible after playing around with settings, but you’ll never find that out if you don’t change things up.

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