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Fox Racing Shox Tech: How Terralogic’s Brass Mass Works

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First introduced in 2004, Fox’s Terralogic created a true, physical platform for mountain bike suspension forks. While the design did provide a firm, virtually locked out feel for climbing and sprinting, it suffered from a noticeable delay on initial impact, which translated into a lack of sensitivity on small bump performance and some harshness when first coming into a bumpy section.

In 2007, Fox pulled Terralogic back to the drawing board and removed it from it’s lineup.

Queue up Sea Otter 2010 and Fox debuted a totally redesigned Terralogic internal for their 2011 model forks. The new version has been getting some good remarks from other media, and our initial test rides on it left us very, very impressed. We’re getting a long-term test fork this fall, so we’ll have a full review eventually, but in the meantime we really wanted to know how it worked.

I sat down with two of Fox Racing Shox’s outside tech guys at PressCamp and picked their brains.  Click ‘more’ to see internal diagrams and a full explanation of how Terralogic works…

Shown at the top of the post is the lower section of the inside of 2011+ Terralogic forks. The blue knob adjusts the threshhold sensitivity (ie. how firm of a platform you want before the shock becomes fully active), and it sits on the outside bottom of the fork leg.  The Brass Mass is, as you may have guessed, the brass colored bit. The silver part with the prongs that’s covering the lower section of the Brass Mass is the timing circuit that keeps the mass “open” during active travel. In other words, the timing circuit keeps the mass from closing back up after the first hit so the fork remains fully active until you’re on smooth(ish) ground again.


Sorry for the tall photo, you may have to scroll up and down a bit on this post. Or, right/option-click on the image and open it in a new window, then read along with the image open at full size next to your browser.

Directly above the Brass Mass is the compression circuit. It uses various sized shims to control compression speed based on the size of the hit. This design means you give up external compression adjustments, but the threshold adjustment should let you tune the feel of the fork to suit your needs.  Like any fork these days, you can always send it off for tuning if you really want to change the compression damping.



In the new models, the BrassMass is twice as big as the original version, weighing in at about 64g. Here’s why that’s important: Terralogic works because the BrassMass wants to stay on the same horizontal plane even when the fork’s lowers move up in response to a bump. As the lowers move up, the BrassMass appears to move down, which opens up the flow of oil through the piston and lets the fork move freely through its travel.

In the graphic above, the path on the LEFT shows oil flow when the BrassMass is closed. Oil flows through the inside of the BrassMass shaft, down past the mass and through a bypass valve. The Threshold Adjustment knob sets the amount of pressure on the bypass valve. If you open it up, the fork will be fully active all the time with virtually no platform. If you close it up, it’ll feel “locked out” until you hit a bump.

The path on the RIGHT shows the oil flow after you hit something hard enough to “move” the BrassMass down into the timing circuit. The mass will actually move 1/2″ needs to move less than 1/8″ before uncovering the oil ports, and once it does, oil flows freely through the holes in the piston. In both instances, oil is flowing back up through the compression damping circuit, which is where the arrows stop.

Newton’s first law of physics says that an object at rest will stay at rest until acted upon. In the original model, the smaller mass meant it wasn’t heavy enough to not move, so on the first hit, it just moved along with the fork lowers rather than staying put and opening up the flow of oil. That’s why the first hit on original Terralogic forks was harsh, but they seemed to have nailed it with this new iteration.

Picture taking a paper towel tube and shoving a ping pong ball in it until it’s stuck in one end. Grab the other end and flick the tube and the ping pong ball isn’t likely to come out. Repeat that with a heavier golf ball and it’ll go flying. Same basic idea with using a heavier BrassMass.


So, once the BrassMass has moved down into the timing circuit, how does it stay there? You can see the spring under the mass that tries to push it back into the closed position, so why doesn’t it just immediately close back up?

In a nutshell, suction. The return spring is relatively weak, offering just enough force to raise the mass when it’s not under any real load from suspension activity. As the mass moves down into the timing circuit, it displaces the oil within the circuit. As it tries to rise back up, oil must flow through the narrow gap between the mass and the Timing Circuit and fill the circuit back up. Because the gap is so narrow, oil flow is restricted, and so the mass’s upward movement is slowed.

Imagine holding two plastic Solo cups under water and pushing one into the other, then trying to rapidly pull it back out. Initially, it’ll be tough as water tries to rush in to fill the void. If your mental imagery is any good, then you get the idea.

Within the Timing Circuit, there’s a blowoff piston that lets the oil move out of the circuit quickly so there’s no delay in letting the BrassMass move down into the circuit. It closes off when oil tries to get back in. This would be like poking a hole in the bottom of the outer Solo cup in our mental example to make it easier to push the other cup into it, then plugging the hole with your finger when you try to pull the cup back out.

The original Terralogic used a stronger preload spring to hold the mass in place, which compounded it’s inactivity on initial impact and made it want to close up faster, reducing sensitivity and overall plushness.


We took one ride down the “Medium” trails on a 2011 Fox fork with Terralogic and the new Kashima Coat. It was mounted up to a sweet Yeti dual boinger. It worked really well. Really, really well, actually, with no noticeable harshness even on mild bumps or drops. Standing up to hammer was pretty impressive as the bypass valve allowed the fork to track small bumps (read pedals and some downward pressure by the rider) without mushing through the travel.  Basically, it offered the benefits of a lockout without having to fiddle with a lever or remote. Keep in mind, this was one 25 minute ride. We’ll be getting in a TALAS with Terralogic for a long term review later in the year. Stay tuned…

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13 years ago


Does tghis shock come with a remote lockout?

Please advise

Johan K

11 years ago

2 years too late, but the answer is no – the brass mass *IS* the lock out.

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