Before you even hit the parking lot to test ride a suspension bike, most shops will walk you through a careful sag and damping adjustment. But few of them will tell you that there is a whole other dimension of control inside your fork or shock’s air spring. By inserting or removing volume spacers, you can make your suspension more or less resistant to bottom-out. In turn, that may allow you to run more or less preload. This deceptively simple adjustment has gotten a reputation for being only for racers, or nerds, or nerdy racers. But Travis Engel believes everyone can benefit from volume tuning. So, he has this quick explainer on what it can do for you, and how you can try it for yourself.
Suspension is not just a means to an end. It’s not just a way to be faster, or smoother, or more comfortable. It’s a way to add another element of personalization to your bike. In the same way that you have a preferred handlebar sweep or saddle angle, you might like your suspension firm or soft or its damping fast or slow. But compared to other means of customization, suspension is uniquely interactive. It makes your bike not only feel like your bike, but also behave like your bike. And it can behave however you choose, especially nowadays. A bike’s intended use was once determined by its travel. But today, long-travel bikes can be efficient, and short-travel bikes can be capable. That gives you even more freedom to mold those bikes into the shape that fits you best. And volume spacers are an oft-ignored way to do it. So, before we dive into the surprisingly simple “how,” let’s dip our toes in the “why” and the “what.”
There are way too many misleading charts in bike-performance analysis. But this one is just a guideline. An illustration of what volume spacers are doing to your suspension feel. The most obvious use for volume spacers is to add bottom-out resistance for aggressive riders. And while that’s totally valid, it’s not that simple. Another way to use a volume spacer is how I used it when testing the SCOR 4060 ST. I wasn’t necessarily bottoming the shock out, but I wanted it to have a little better initial small-bump performance. I could get that by dropping the pressure, but then I would have a problem with bottom-outs. Adding a volume spacer, I was able to drop that pressure very slightly, and still have the sport later in the shock stroke. It’s not because I’m a shredder or a jumper or anything. I just wanted something different than what the bike offered out of the box. Similarly, on my monster-gravel hardtail, I found the fork’s volume reducers were preventing me from using all of its limited travel. I’m not bashing that thing, so I don’t need much bottom-out resistance. So, I removed some volume spacers and now I’m activating more of the travel more easily. The idea is to think about what your suspension isn’t doing for you. Within reason, adding or removing volume spacers should be able to bring you closer to getting it.
A volume spacer, often called a volume reducer, is a piece of plastic specially made for a particular model fork or rear shock. It clips (or sometimes screws) inside the air spring, reducing the volume of air by replacing it with non-compressible plastic. It works a bit like filling a plastic bottle part way with water. Screw on the cap and squeeze the sides, and you’ll find it gets hard to compress more quickly than when squeezing an empty bottle.
The “bottle,” of course, is the air spring. And you’ll need to open it up to install a volume spacer. But the process is not nearly as involved as servicing or rebuilding your fork or shock. The air spring is really just an empty chamber, with maybe a little grease or lubricating oil pooled at the bottom. You don’t need to keep track of seals or O-rings, or worry too much about spillage. It’s a surprisingly beginner-friendly job. Still, I’d recommend having your fork or shock’s service manual handy. Below are links to major suspension manufacturers’ service pages that will cover specifics like torque specs, which volume spacer you’ll need, or the maximum number of spacers your fork or shock allows.
Fox (Scroll to “Rear Shock Service” or “Fork Air Spring Service,” click on whichever link associated with your fork or shock that mentions “volume spacers.”)
RockShox (Click through until you find your fork or shock, click on any link with “Bottomless Tokens” in the title.)
Manitou (A PDF about Manitou’s novel IVA and IRT volume adjustment systems.)
Öhlins (Find the owner’s manual PDF for your fork or shock and scroll to the volume spacer explainer.)
MRP (Find the owner’s manual PDF for your fork and scroll to the “Ramp Control” or “Huck Puck” explainer.)
- Appropriate volume spacers for your fork model
- Shock pump
- Torque wrench
- Chamferless* six-sided socket (Fox and other brands)
- Cassette lockring tool (modern RockShox and Öhlins)
We’ll start with front suspension so that the hardtail owners out there don’t have to sit through a bunch of full-suspension nonsense. But also, the process is a little simpler. You can easily do it with your bike leaning up against a workbench, and you’ll be using tools that are already pretty familiar. But before you start, wipe down the area around the top of your fork’s air cap so you don’t brush dirt into the spring after removing the cap. Once it’s clean, you’re ready.
First, if you don’t already have your preferred fork pressure memorized, hook up a shock pump and write down what it reads. Then, use the bleed button on your pump to slowly release the pressure. If your pump doesn’t have a bleed button, you can release pressure directly at the valve, but try to do it slowly. Most air springs have a self-equalizing negative chamber, but sudden decompression could lead to a temporary imbalance that could complicate re-pressurizing the fork.
Though some modern RockShox fork air caps can be removed with a cassette lockring tool, most forks require a “chamferless” six-sided socket. The sockets you already have are probably slightly tapered on the surface that first impacts the bolt or nut so they self-center to make work quicker. But a fork cap is so thin that any chamfer will prevent the socket from getting a good bite. It’s technically possible to just use a regular open-ended wrench, but the hex on the cap is thin, shallow, and aluminum. It’s very easy to damage the cap, but you should be able to get a chamferless socket for about $15. To figure out the size you need, either look up the specifications of your particular fork in its service manual, or measure it with a caliper ruler. It should be 24, 26, 28, 30 or 32mm across.
Again, making sure the fork is depressurized, loosen the air cap and remove it. There actually may already be a volume spacer attached to it. Depending on what you’re looking to accomplish, you can either add or remove spacers, but be sure to check the service manual for the maximum number of spacers for your fork travel.
Before re-installing the cap, wipe off any debris it may have picked up while being removed. Once you’re installing it, be very careful not to cross the threads. They’re fine and aluminum, so it’s very easy to start them crooked. You should be able to get it most of the way down with your fingers. Tighten it to about 25 nm.
When pumping up the fork from zero, that negative chamber will again come into play. Once the fork is back to full height, you’ll want to stop every couple dozen pump strokes and compress the fork a few times, which equalizes the positive and negative chambers. Continue this as you approach the pressure you started with. But remember, you just changed your shock’s air volume, so you may end up wanting a different pressure. For your first few rides after changing your volume, bring a shock pump with you so you can add or remove to your heart’s content.
- Appropriate volume spacers for your shock model
- Shock pump
- Strap wrench
Rear shock volume spacers can be a little trickier. Fair warning, there’s a step near the end that takes a fair bit of strength, so it may help to have a second pair of hands at the ready. Also, you’re working in the confined space of your front triangle. Or worse yet, you may have to remove your shock completely if it’s tucked away like a Santa Cruz or Scott. If you’re not comfortable with removing a shock, that’s a sign you may want to seek a shop’s help in this process. So, I’ll cover the simpler, on-bike procedure.
Start by cleaning the area around your rear shock to avoid getting dirt inside it. Might not be a bad idea to give the whole bike a wipe-down, ‘cause you may need to get creative in hanging or leaning your bike, and there’s no telling which way will be “up.” The best scenario is clamping your bike in a work stand. Or, you can hang the nose of your saddle (with dropper post extended) on something sturdy. You may even be able to do this with the bike upside-down. The idea is to keep it from resting on its wheels, because once you’ve let the air out of your shock, it’ll compress and you won’t have much room to work in. I’ll demonstrate using the upside-down method.
As with the fork, check your pressure to give yourself a ballpark to return to when the job is done. Then release the air slowly, either through the pump’s bleed valve or carefully at the shock valve itself. For this next step, it’s especially important to check your shock’s service manual, because some brands access their air cans differently. The Cane Creek Double Barrel Air or Fox Float X2, for example, are opened by releasing a large C-clip and/or removing a locking pin. But most popular shocks from RockShox and Fox simply unscrew.
Or sometimes not so simply. It often takes a lot of torque to unscrew the air can, and there may not be a lot of friction underneath your strap wrench. Make sure the surface of the strap wrench and the surface of the air can are perfectly clean and free of oil residue. And there’s a way to set the strap tension that maximizes the constricting force around the air can. Instead of pulling the strap tight and setting the wrench arm perpendicular to the surface of the air can, loosen it a bit so that the tension peaks with the arm parallel or “tangental” to the surface of the can. If it’s still sliding, a lap of fabric tape will usually add enough friction to break it loose.
Here, I’ll turn you back to your shock’s service manual, because every brand does their volume spacers differently. RockShox uses stacked spacers of a fixed size, Fox uses a single spacer that varies in size, or they may use multiple spacers of different sizes, or they may use two-piece spacers that clip together. Plus, it’s not hard to insert something part-way or on the wrong side of an O-ring. Basically, be careful. Like the fork, there will probably already be a spacer inside the shock, which will help illustrate how it should look once you’ve added, removed, or swapped to your heart’s content.
Make sure the area around the shock is free of dust, especially the threads. Next is the part that sometimes takes some strength to pull off. There’s always some pressure left in the negative chamber, which you’ll have to fight to get the threads to link up. Good news is, it’s pretty much impossible to cross-thread them, because everything will be guided by the internal pistons. Just have a good wind-up as you’re pushing the two ends together. Once you catch a thread, you’ll be able to spin the can on. Once it’s tightened flush, use the strap wrench to snug it up the last bit, and you’re ready to repressurize the shock. Like the fork, though, stop every once in a while and compress the bike’s rear end to equalize the positive and negative chambers. And again, don’t stress on getting it exactly to the same pressure you had at the start. After the changes you’ve made to the volume, you may want it a little harder or softer. Bring your shock pump with you on your first few rides, and do some experimenting.
With how homogenized mountain bike design and technology have gotten in recent years, it’s easy to forget how much control we still have. You don’t have to accept your bike’s suggested frame size or tire spec or terrain category. And you don’t have to accept its suggested suspension feel. Hopefully, you’ll be ready to keep tinkering as you chase down what’s best for you. You might as well. You just spent, like, $30 on the tools.