Variable Friction Tuning for Shock Absorption by Fox

In this article, we’ll be discussing a Variable Friction Tuning for Shock Absorption by Fox Racing, US publication 20200292024. The publication date is Sept. 17th, 2020 and the filing date is May 29th, 2020. This patent is a continuation of application No. 15/490,794 filed on April 18th, 2017, which is now patent number 10,670,104.

Today, we’re talking about shafts rubbing against sidewalls. nice.

It’s very possible this patent has already been implemented in the automotive field. I’m having a hard time finding anything about variable friction shocks in anything Fox is currently making, but I could be wrong. If anyone has seen this, let me know. That being said, this is still a cool idea and I wanted to share it, especially the last part.

Brief Summary (tl;dr)

Fox have developed a simplified shock design that incorporates varying levels of friction in the sidewalls and/or the piston shaft. As the piston moves inside the shock, the seal on the piston makes contact with different areas of the sidewalls, where each area has a different coefficient of friction. Alternatively, the piston shaft can have varying levels of friction and have similar results.

In an example scenario, the first 50% of the shock’s travel would be in a very low coefficient of friction zone, for small bump sensitivity. The next 25% of the shock’s travel would be in a slightly higher coefficient of friction zone, to assist the shock in high travel conditions. There can also be a third section for the last 25% of the shock’s travel that is a much higher coefficient of friction that assists in bottom out conditions.

A Live Valve variant is also introduced, where a controller applies a varying level of voltage to the piston based on input from a bunch of sensors around the bike. As the voltage is applied to the piston, the actual friction coefficient of the piston changes. The walls of the shock do not have a varying surface treatment. The variation is determined by the voltage applied to the piston.


Variable shock absorbers (dampers) are not a new idea. Any shock that can change its characteristics without an input can, technically, be considered ‘variable’; this would include active shocks such as the Live Valve system. For example, Lord Corporation created a variable shock absorber all the way back in 1991. This system uses multiple elastomer coatings on the sidewalls of the shock.

Fox are one of the leaders in shock surface treatment. With their release of Kashima coating in the late 2000’s or early 2010’s, they’ve proven that they can develop shocks with an impressively low coefficient of friction. I currently run a Fox 40 on my downhill bike and the initial stroke movement is so smooth. But that is an outer coating, where the stanchion meets wipers and seals, and get covered in garbage. So, what if Fox applies this same knowledge of surface treatments to the inside of a shock?  

We’ll be talking a lot about the coefficient of friction. In short, friction is broken down into two items, static and kinetic friction. Static friction is the ‘stiction’ between two components before any movement, which is generally higher than kinetic friction. Kinetic friction occurs as objects rub against each other.


First, nowhere in this patent does it say that this is for a bike, but there is no reason this idea can’t be used in a bike. This patent refers to the application of this shock being for a ‘vehicle’, which is very broad. Last I checked, a bike is a vehicle.

This primary idea of this patent is a passive variable shock, meaning the shock can change its characteristics but only results in a singular output, where the feeling of the shock is consistent every single time it’s used. The coefficient of friction changes as the shock piston travels through the shock tube. This is a function of various surface treatments on the inside of the shock, where the piston meets the sidewalls of the inside of the shock.

In another variation, Fox also introduce an electrically controlled system that changes the coefficient of friction between the piston and the sidewall with a special coating on the piston, sensors, and a controller… Live Valve.

Intended novelty

The intended novelty of this patent is the use of multiple zones of different coefficients of friction inside the damper tube, where the piston meets the sidewalls of the damper tube.

The electrically controlled part of this patent is not claimed as novel in this particular document, but that doesn’t mean it won’t come out later.


Someone at Fox said “Hey, we want a shock that behaves differently at different travel locations, but we don’t want to add any electronics. How do we do that”, and this is what they came up with.

Fox state that one reason they are doing this is for a simplified shock.  

…the damper can operate with far fewer valve architectures and even be simplified to only a main piston valve or base valve configuration.

The electric version of this shock is another Live Valve idea, so they’re just opening up the possibility to control shocks with a simpler system than opening and closing damper valves.


Figure 5 shows the first example of the shock in question. Sections 130, 132, and 134 represent different surface treatments with different coefficients of friction. As the piston moves, the different surface treatments will provide a variety of shock characteristics.

In an alternative example, Fig. 6 shows the same concept but applied to the piston shaft. The variation in friction comes from the meeting between the shaft and either the shaft guide 136 or the shaft seal 138. Fox don’t say, explicitly, which one provides the countering friction, which appears to be an oversight in the document and should have been included.

Fox state that both the damper tube and the piston shaft can have different surface treatments at the same time:

In some examples, both the damper tube and the piston shaft include [different] surface treatments.

This last example is kind of mind-blowing to me, and shows this system can be an active system, rather than the previous passive systems in this patent. Fox have a variable friction shock but with no surface treatments. Instead, they’re applying variable voltages to the damper piston, which then changes the friction coefficient between the piston and the sidewalls of the damper.

This is where Live Valve would come into play. If you read my article on Live Valve, you’ll remember that the current Live Valve functions by varying the movement of the damper fluid with a valve. I also state that the Live Valve patent opens the door for different shock variation methods. This patent shows another possible example of how Live Valve would work.

Here’s the Live Valve line, stating the controller receives input from sensors, sends a voltage to the piston, and the coefficient of friction changes.

The voltage is set by a user to one or more predetermined values. Each of the predetermined values corresponds to a desired level of friction. A control system is provided to regulate the variable voltage based on various parameters associated with vehicle operation or shock absorber characteristics. For example, a controller receives a plurality of signals from sensors including a temperature associated with operation of the shock absorber, a cavitation measurement, piston velocity, piston position, a vehicle speed, or other signals. In one embodiment, the controller is integral with another controller such as a vehicle master controller or engine control unit (ECU). Based on the data, the controller generates a voltage or current value to be applied to the damper piston.

Fox also have an interesting line that shows how the system can compensate for temperature changes:

…in colder weather and/or after a prolonged period of rest, many shock absorbers experience higher levels of friction between the damping piston and interior surface of the damper tube. Hydraulic damping fluid increases in viscosity as the temperature decreases. Damper tubes also decrease in diameter as the metal contracts due to lower temperatures. These and other natural phenomenon result in reduced ride quality, harshness, unwanted noises, increased component wear, and other undesirable side effects. The control system compensates for the temperature of the shock absorber by decreasing the coefficient of friction. For example, when the sensed or modeled temperature of the damper tube is below a threshold temperature, the controller begins to apply voltage to decrease the coefficient of friction.

Lastly, Fox state this system can vary based on terrain conditions and maneuvers.

The voltage V is increased or decreased during normal temperature operation of the shock absorber as well to compensate for a variety of conditions in which increased or decreased damping forces are desired including but not limited to: steady-state high vehicle speed (highway driving), off-road situation-specific events (rough road, low vehicle speed rock crawl, jumps, landings), evasive maneuvering (rapid turning events), body roll, body pitch/heave, body yaw, and the like.


Fox state an example scenario of how they’ll determine what levels of friction are in what positions inside the shock:

…in a ride zone or first portion of the damper tube, it is desirable to include a first surface treatment with a first coefficient of static friction that is the lowest of all and a first coefficient of kinetic friction that is lowest of all… a second coefficient of static friction that is higher than the first coefficient of static friction and a second coefficient of kinetic friction that is higher than the first coefficient of kinetic friction… The end portions of the damper tube repeats the same tuning process for selecting the third surface treatment

Fox don’t say much on the actual process to achieve different coefficients of friction on the surfaces, but they do state a few surface treatment process that can be used:

Examples of surface treatments which are used to create the surface treatments 130-134 include but are not limited to coatings, vibro-rolling, chemical etching, abrasive machining, honing to generate micro-grooves, reactive ion etching (RIE), high energy chemical plasma, photolithographic techniques, abrasive jet machining (AJM), excimer laser beam machining (LBM), vibro-mechanical texturing (VMT), laser surface texturing (LST), electro-plating, electric- field-induced polyelectrolyte coatings, and evaporative deposition.

As far as the electrical part of this patent, Fox state that the piston has a polyelectrolyte coating, where a voltage is applied and the surface ‘sticks’ to another surface. In other words, a voltage is applied and the coefficient of friction changes.

…a damper piston that is selectively charged with a variable voltage… At least one of the interior surface, the damper piston, and the wear band includes the surface treatment which includes a polyelectrolyte coating… A variable voltage is applied to the damper piston using alternating current. As the voltage is varied, the coefficient of friction of the engaging interior surface and damper piston or wear band varies.


Like I said, this is not bike-specific, but it there’s no reason it can’t be implemented in a bike. Variable surface coating is a pretty cool solution to simplify a shock, though we then have to wonder if service intervals would be increased?

The Live Valve variant in this patent is also nifty, and seems much simpler and more reliable than the current live valve system with fluid and a valve. There are zero extra moving parts and it’s possible the system could react quicker than a mechanical valve. But, I don’t think this system would be useful for explicit lockout scenarios, but more useful for just slight adjustments to suspension characteristics.

Let me know when you think about this one, I really like it and I’d love to see it in action.

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