Tech Article: How Rockshox Motion Control Works

I took a little tour of Rockshox’s Colorado Springs, CO, facility yesterday and had Sander Rigney, RS’s product manager, run through their Motion Control compression damping system for me. If you recall, we posted a tech article on how Fox Racing Shox’s Terralogic works recently, and it’s really cool to see how different companies use completely different mechanisms to accomplish a similar end result (and by similar, I’m not comparing performance, just the fact that all suspension brands want damping, etc.).

The parts above are what make up the Motion Control for their XC oriented forks. Motion Control DH is a little different and we didn’t take a look at that on this visit (update will come if we can get photos of that system, but I’ll explain how and why these versions are mainly for XC use versus DH).

For XC / Trail / All Mountain type riding, you have two options depending on how much you wanna spend on your fork: Motion Control and Blackbox Motion Control. Technically, you also have XX Motion Control, but it is essentially the same as Motion Control without the adjustable compression damping. Again, this is explained better in a sec.

Jump past the break to see how they work to control the compression…

Unlike many other suspension forks, Rockshox’s Motion Control mostly eschews shim stacks in favor of simple ports.

Shown directly above is Motion Control’s compression damping port fully open (left) and closed (right). They’re fuzzy because we used a very linty rag to wipe the oil off before we photographed them, they don’t look like that normally. So, when you turn the blue compression knob at the top of the right fork leg, you’re changing the size of the oil flow port. The more you close it, the more restricted the oil flow is and the slower your compression damping becomes. Simple, right?

With Blackbox Motion Control, you get Dual Flow compression damping. Shown on the right, this means you get a slightly differently shaped valve cover (the bronze piece) that lets oil flow through multiple ports. Why? Because there is a small shim stack behind the ports to control high speed compression damping (for when you drop off something or hit something really hard). The multiple ports let the oil flow through the shim stack evenly rather then coming at it all from one side. During slow speed compression the oil simply flows around the shim stack…it only comes into play on large hits.

The silver part is the shim stack.

So, the big difference between BBMC and MC is the secondary compression damping via the shim stack. That, and a titanium compression tube versus a thermoplastic one.  These tubes are what control the lockout blowoff in conjunction with the Floodgate:

The other feature that’s built into the the Motion Control assembly is Floodgate, which is what sets the blowoff threshhold when the fork is locked out.  And that’s where that technically named piece called “the poker” comes into play.

When you have the fork locked out, that essentially means the compression oil ports are closed, so no oil can flow and the fork can’t move. Well, in the real world, the fork should still move slightly to allow it to track well. The ti or plastic compression tubes will compress slightly even when the fork is locked, allowing several millimeters of travel. When you hit something really big while locked out, the tube will compress enough for the poker to, literally, poke the compression valve cover out, as shown above. This fully opens the oil flow for one hit, allowing the fork to soak up the hit, then a spring pushes it closed again (shown below).

Turning the Floodgate knob sets how much the tube will compress before it pokes the valve cover out. It only moves about 3mm total, which tells you how little the compression tube actually compresses to do its job.

With the XX Motion Control, you get the thermoplastic compression tube and the hydraulic remote that either fully opens or fully closes the ports. So, you don’t have any compression adjustment externally.

That said, the XLoc has a Floodgate control knob on it, and if you dial it all the way open, it provides something similar to a platform with good suspension motion, but it still won’t be quite as plush as when the lockout is turned off.