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LINEAR XL AIR SPRING
THE NEW LINEARXL AIR SPRING ISN’T JUST AN AIR SPRING. IT’S A WHOLE SYSTEM.
One thing we've learned over generations of suspension—performance comes from the whole fork. Not just a single technology. Not just the air spring. Not just the damper. The LinearXL air spring isn't just a stack of new technologies, it’s part of a system.
Our engineers ride the product themselves, debrief, iterate, repeat. Through this process, we’ve tweaked and shaped the air spring curve to be more linear and consistent, so you can predict exactly how the fork will feel regardless of where you are in travel.
LinearXL is designed to deliver the most linear-feeling air spring we've ever built. The twin tube design, AirAnnex, Breather Holes, and Adjustable Bottom Out are all new technologies, working together to provide you the support you deserve.
Let’s dive in.
Twin Tubes
First introduced in the 2024 BoXXer, we carried the twin-tube design forward to our latest generation of forks: BoXXer, ZEB, and Lyrik. With the move from BoXXer’s 35mm to 38mm upper tubes, it was nearly impossible to tweak the ratio from the piston and shaft diameters enough to make the spring curve match the ride feel the development team was looking for.
By adding another tube within the upper tube, we decreased the piston diameter, reducing the ratio between the piston and air spring shaft diameters. We further reduced the ratio by growing the traditional 10mm air spring shaft to 14mm, and made it hollow to expand the volume of the negative chamber. We also bumped up the top of the air spring piston to create a hollowed-out top cap, maximizing available negative air volume. Confused by all of this positive and negative air chamber talk? Check out DebonAir+ to learn more about air spring theory.
Introducing the twin tube design refined the shape of the air spring curve to be more linear from the initial engagement through mid-stroke, but we wanted to smooth out the end-stroke ramp so the spring felt consistent all the way through travel. So, we added the AirAnnex.
LinearXL AirAnnex
Adding AirAnnex accomplished two things: smoothing out the end-stroke ramp and improving speed sensitivity. The team had an idea to add more volume to the lower leg, so they cut a hole, attached an empty module, and tested it out. It worked.
Built onto the fork’s lower leg, what’s now known as the AirAnnex adds more volume to the lower legs, reducing casting volume ramp.
What is casting volume ramp? Glad you asked. Air molecules that live in the lower legs (because it’s a sealed system) can act as a secondary air spring as the fork moves through travel. Like an air spring, as the fork is compressed, that air becomes pressurized, causing it to ramp up at the end of travel—all within the lower legs. By adding the AirAnnex, we added more volume to the lower legs, decreasing the pressure hoisted onto those air molecules, drastically reducing that end-travel ramp.
Now, you might be tempted to take the cap off the AirAnnex, just to see what’s inside. It’s human to be curious. We’re here to say: Please don’t. It’s not that interesting in there, and it will negatively affect the performance of your fork. Look, we’ve taken it apart for you, and you can see what’s here. Now, leave it alone.
Breather Holes
Another way to impact the fork’s casting volume was to add drilled holes into the upper tubes, called Breather Holes. These holes connect the casting volume located between the bushings to the volume outside the spring cartridge (between the twin tubes). This allows the air molecules to move around more freely in essentially one giant chamber (the inside of the chassis) instead of getting trapped and bound up in the system, reducing speed sensitivity. So, regardless of how fast or slow a hit is, it delivers approximately equal force for the same displacement. Curious about the dimples drilled into the upper tubes? Read more about ButterWagonTech.
Adjustable Bottom Out (ABO)
The final piece to the LinearXL experience is Adjustable Bottom Out (ABO), a simple yet innovative solution that allows riders to set their bottom-out resistance. ABO offers 10mm of adjustable end-stroke control across 8 clicks, for easily tunable control. With the adjuster turned counterclockwise towards the "-", bottom-out resistance engages in the last 7 mm of travel. When turned clockwise towards the "+", support is extended to the last 17 mm of travel.
As you turn the ABO knob toward the “+”, the gold pedestal beneath the bottom-out bumper rotates up 1.25 mm through each click. The more clicks you add, the more it raises the pedestal—causing the bumper to engage earlier in the compression stroke, providing bottom out resistance sooner. The reverse is also true—removing clicks of ABO will lower the pedestal, so the bumper engages later in the compression stroke, providing bottom-out resistance later, closer to full travel.
Have a Select+ fork? You can adjust your bottom-out control, too. Once you drop the lower legs, you’ll find three notches on your spring-side shaft that allow you to increase or decrease resistance. [Learn more]
Wondering what happened to your spring-side Buttercups? They’re still there—integrated within the gold pedestal, absorbing trail chatter like they do. New to ButterCups? [Learn more]
