Multi-Mode Air Shock by Fox

A typical air shock, related to this publication, includes two positive air chambers, a first air chamber, and a second air chamber. The first air chamber is the large chamber, typically where we insert volume spacers. The second air chamber is a smaller volume area that may be accessed with a switch. Additionally, there is a negative air chamber, which can be substituted with a coil spring.

In this article, we’ll be discussing a Multi-Mode Air Shock by Fox, US publication 20200191227. The publication date is June 18th, 2020 and the filing date is Feb. 26th, 2020. This patent is a continuation of granted application No. 15/942,337 filed in March 2018, which is now patent number 10,578,179.

After exploring USPTO documentation, Fox have continued pursuing the 2018 application as of Feb. 2020 with the application language that was filed in 2018. Typically, companies may do this to take a broader scope on an idea because they believe in the idea.

9/6/20 clarification edit: This patent appears to be the Scott Nude TR shock, which has already been released. Originally, this patent was published in 2018 and resubmitted in 2020. The two applications have the same claims but the 2020 version has an additional paragraph [0067] that encompasses electronic sensors to influence the mechanics of the shock. Importantly, Fox does not include this in the claims (in this application). Check out the ‘Intended Novelty’ and ‘How’ section for the additional information.

Brief Summary (tl;dr)

Fox have created a three-mode air shock where a user can select between a short travel and long travel mode by adjusting the compression ratio by opening an air path between a first and a second air chambers. When this air path is open, a user may then further change the long travel compression ratio with another switch (this does not mean they’ll have two different switches in the final product) that opens an air path between a second and third chamber. So, as an end-user, you will be able to choose between a short travel and long travel mode, or between a short travel and a longer travel mode. I would also like to point out that the term ‘travel’ is used by Fox to describe this method, but the term ‘volume’ may be more appropriate. Therefore, this shock can vary between a small volume and a large volume air chamber, and a small and even larger volume air chamber.


Adjustability appears to be a focus of bike manufactures. From adjustable seat posts, damper controls, or even chain stay lengths, adjustments are an essential feature to many consumers. These adjustments allow an end-user to tailor a generic bike to their specific riding style, height, weight, or trail features. Fox released a two chamber shock many years ago with the DYAD shock. The DYAD has two air chambers, allowing for a short and long travel mode. Though the DYAD shock has separate damping circuits, and this publication shows a single damping circuit.


Fox states ‘The [first] air chamber 112 and the [second] air chamber 114 may be combined to provide two different compression ratios’. Additionally, Fox states that an air shock with two positive air chambers ‘allows for communication between the chambers to be selectively closed by a valve or some selectively adjustable flow path’. In other words, the first air chamber is connected to a second air chamber via a valve, to change the compression ratio of the shock. Fox does not expand much on the first to second air chamber mechanism, as this is not the subject of this patent. All they need to say is “the first and second chambers are switchably connected” and move on to the novelty.

Why would a shock manufacturer want two positive air chambers connected to each other with a switch? Fox states that ‘This allows for a decreased compression ratio in the air spring when the valve is open and combining the [first] air chamber and [second] air chamber and an increased compression ratio in the air spring when the valve is closed and sealing the [first] air chamber from the [second] air chamber. This allows for two different riding modes: short travel when the communication is closed and long travel when open.’

So, what if we only want to decrease the compression ratio in long travel mode? What if we want a short travel mode switched to a very long travel mode, rather than just a long travel mode? It appears as though Fox have come up with a clever solution.

Intended novelty

The intended novelty of this invention is to solve the problem of how to only decrease the compression ratio of the long travel mode in a shock without affecting the short travel mode. To do this, Fox has introduced a tertiary (referred to as “third” throughout the article) air chamber only accessible through the second air chamber. Fox states, ‘The additional volume (third air chamber) needed to decrease the compression ratio in long travel mode must be in communication with only the [second] air chamber else the [first] air chamber will also experience a decreased compression ratio when in short travel mode.’

In this publication, Fox is introducing a third air chamber connected to the second air chamber only, allowing for only a long travel mode to be adjusted. So, when a user wants to only change the compression ratio of the long travel mode, they can do just that. Therefore, there are technically 3 different selectable travel modes within a single shock.

Fox states that the third air chamber is ‘in fluid communication with the [second] air chamber but not in fluid communication with the [first] air chamber except via the [second] air chamber’. This just means the third air chamber is attached to a second air chamber only and cannot be reached through the first chamber. A side note: in the physical world, fluid and air are equivalent and the terms can be interchanged. So you may see ‘fluid’ or ‘air’, but they’re both the same thing.

9/6/20 clarification: Additionally, Fox have extended their explanation of electronic actuation of this system to include, for example, wheel speed transducers and piston rod transducers. The piston rod transducers would measure piston rod position and velocity. Fox also state:

Any other suitable vehicle operation variable may be used in addition to or in lieu of the variables [the transducers] such as, for example, piston rod compression strain, eyelet strain, vehicle mounted accelerometer (or tilt/inclinometer) data or any other suitable vehicle or component performance data.

I recommend giving paragraph [0067] a read. It’s too long to add to this article but it’s got some good information as to where Fox may want to go with this system.


Fox makes it clear as to why they have developed this technology. Fox states:

By utilizing the third air chamber and the ability to cut off communication between third air chamber and [second] air chamber the compression ratio in long travel mode can be lowered from approximately 3.6: 1 to 2.8: 1 or the like, to better suit longer travel shocks/bike.

A third air chamber may provide some distinct adjustability that may be beneficial for some riders. For example, a rider may have finished a long climb and may set the third air chamber closed for a flowing trail at the top of a ridge, or a trail successive ups and downs. Then, the user may open the third air chamber prior to a long descent. Personally, I used to live in the Charlotte area with a 160mm Specialized Enduro. I would frequent the Pisgah region regularly, where 160mm is beneficial. Alternatively, during short weekday rides on the flat metro Charlotte terrain, a slightly shorter travel mode would have been a nice feature.


First, Fox provides some background with ‘The [first] air chamber 312 selectively communicates with the [second] air chamber 314 by a valve 318…’.  This statement explains the initial air flow path from first to second air chamber, with a selection mechanism. This will adjust the shock from a short travel to a long travel mode.

Figure 5 shows an example of the addition of a third air chamber, creating a third riding mode, by implementing a different selectable air chamber. Fox states, ‘The third air chamber 334 may be formed by addition of a sleeve 330 together with seals and an outer wall 332 of the [first] air chamber 312.’ This statement is saying that the third air chamber may be located on the outside of the first air chamber via a sleeve that can be removed from the outer wall of the shock.

The third air chamber 334 is accessed via flow path 336. In an effort to broaden the scope of the invention from a legal perspective, Fox states that there may be more flow paths than shown in the figure: ‘The air shock 300 may include more flow paths 336 which may include multiple channels, passageways, and the like that are bored or machined from the outer wall 332’. Though, from a legal perspective, Fox may be limiting their scope to channels passing only through the outer wall.

Before we discuss the method of actuation, we need to see how the air will flow between the second and third air chambers. Figure 8 shows an array of holes located on the underside of a part of the tubular outer wall 600. When open, these holes allow air to flow between the second air chamber (inside additional structure 738) and the depression 621. An important line states, ‘…depression 621 is what creates the third air chamber 334 space when the air shock is assembled’. Therefore, we can state that the third air chamber is equivalent to depression 621 plus the sleeve.

Additionally, inside the third air chamber 334, there are examples of volume spacers 338. Figure 8 shows three volume spacers that may be used to reduce the volume of the third air chamber 334. So, Fox have made the third air chamber even more adjustable with the option to add volume spacers.

Next, Figure 11 shows a detailed view of the actuation. Lever 808 is adjusted to move cam 812, which moves cam follower 814. Cam follower 814 then moves plunger 1102 up and down, moving a shim 1120. Shim 1120 seals the holes in the closed position, preventing air flow between the second and third air chamber. Figure 11 does not make it clear, but the holes are drilled through the outer wall 600, from the second chamber 314 to the third chamber 334. The holes are in front of and behind the plunger 1102, not show in this cross-section. The line in blue represents a hole connecting the second and third air chambers.

When the plunger is actuated, the shim is released and the holes allow for the free flow of air between the second and third air chambers. And there it is, a third riding mode only accessible in a long travel mode.

Fox have added some additional details at the end of the publication, related to automatic actuation. A key item to note is that the author of this publication is using the term ‘dampening’ to refer to the air-side of the shock, not oil side. For clarification, the author states the system ‘…adds dampening (e.g., adjusts communication between the plurality of air chambers)…

Fox states that the system can

automatically increase dampening in a shock absorber in the event a damper rod reaches a certain velocity in its travel towards the bottom end of a damper at a predetermined speed of the vehicle.

New part of this app: Fox expand on electronic mechanisms for this shock. In the various examples based on automatic actuation, Fox are opening the possibility for this technology to be entirely automatic. Inputs such as wheel speed, piston position, and piston velocity can be used to calculate optimal performance of a shock for ‘…movement events such as cornering roll, braking and acceleration yaw and pitch and “g-out”’. Additionally, this technology can be used to protect your shock from damage in the case of overshooting or casing a jump.

The ending of this publication begs the question: Do we really need this technology in our bikes? Our cars contain technology such as this, but do you want your bike to ‘adjust’ itself based on what manufactures have decided is ‘too fast’ or ‘too slow’. Do you want to maintain manual control of your components or are you willing to give control to Fox?

A few more notes to consider, Fox has added an interesting line in stating, ‘The sleeve 330 may be removable. The sleeve 330 may include a plurality of sizes and shapes to accommodate various compression ratios as well as various fitments depending on the application.’ This line opens the possibility for multiple air sleeves with multiple size third air chambers. Therefore, a user may expand the third air chamber with a new sleeve or reduce the third air chamber with volume spacers. SRAM are currently doing this with their Megneg kit, but Fox want this technology on the positive side, not the negative side.

Additionally, Fox states ‘…by putting the variable volume of third air chamber 334 on the sleeve 330 a user is able to adjust the volume much more readily than if the user had to disassemble the shock to obtain access to the air chamber.’  As a method for user convenience, the third air chamber is very easily accessible, so the user does not need to disassemble the shock to access the first air chamber to make adjustments.   I currently run the Rockshox Megneg kit and I can say that the process of switching volume spacers without taking the shock apart is awesome.

In the end, Fox have developed a very interesting shock with an incredible amount of adjustments. With multiple damper settings, multiple air chambers, volume reducers, and different air sleeves, consumers will have a nearly unlimited array of shock setting options to match any bike, riding style, terrain, height, weight, etc. You could run this shock on literally any bike with the correct configuration of volume spacers.

If you think you see something wrong in this article, please email me. We can’t have misinformation out there. Even if you’re not 100% sure you’re right, I’d still like to hear from you.

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