How Fox Did High-Speed Rebound Adjustment
A fork's high-speed rebound setting has a big say in how your bike reacts when your hitting things hard and fast, but it also plays a part in where the fork will sit in its travel while you're rolling (spring rate determines static ride height). My testing of the GRIP2 damper at the two extremes showed nearly a full inch difference in ride height, which I wouldn't have believed if I didn't see it with my own eyes. More on that later, though.
The anodized orange part is your rebound piston, but it gets weird from there. See that dark gray disk? The high-speed rebound shims rest between it and the piston, and the VVC system adjusts how they react.
It turns out that Lindsley was already adjusting high-speed rebound internally for of many of Fox's athletes: ''When you valve a fork for everybody, most forks are kinda over-valved for a lot of people, so you have this big range with the orifice adjuster. But if someone wants to improve their performance, the first thing I do is go in there and pull a valve or two off if they're lighter, and maybe add one if they're heavier, but that's pretty rare.
''Then it became really important and I was doing that for all those riders, and it worked really well for them, but wouldn't it be nice to be able to do that without having to take the fork or damper apart? So we wanted to do high- and low-speed rebound, but that adds complication. But the reality now is that, with a few clicks, I can basically get the rebound as close as perfect as you can without having to take the fork apart.''
It wasn't that simple, of course, and the 'but' in this one comes down to real estate. As you might expect, simply adding in another external adjustment isn't an easy task, especially when you need to cram it in there without adding more length or width to the whole thing. Don't forget that every single gram counts as well.
High-Speed Rebound VS Low-Speed Rebound
The obvious question is, of course, why the hell would a rider want yet another dial to turn? And what does high-speed rebound even do?
First, always remember that when we're talking about speed, we're talking about damper shaft speed, not your literal speed. That part is important. You could be riding slowly and get a high-speed compression from a big drop with a flat landing that would compress your suspension quickly, and Fox wanted a dial to control the resulting rebound speed from those type of hits, AKA high-speed rebound. But if that drop had a steep landing that eases your transition from being in the air to being on the ground, your suspension is going to compress relatively slowly, making for a low-speed compression and low-speed rebound control. There's a dial for that, too, but that's old news.
The silver part with the pinch clamp is actually the center bolt, and it's attached like that so it can be as small as possible. That silver tong sticking straight up (there's one opposite it, too) is the leaf spring. When you turn the adjuster, the wings on the leaf spring rotate around the spiral that's on the face of the dark-colored disc to change the leverage applied to it.
Traditionally, high-speed rebound adjustment was done via preloading that valve, but that can cause the orifice to choke up, Lindsley explained, and then more force is required to open those preloaded shims. ''You get this in-between shitty point and then finally the valves crack open. It does work, but it's not ideal,'' he went on to describe. For the record, this layout is what's used in many dampers.
It's all about that real estate. ''You're really constrained with the size because there's not a lot of room in there to do stuff, as far as diameter goes. And we wanted to keep the relationship between the center bolt [commonly known as the piston bolt] that holds everything together and the outside [diameter] of the valves. We want as much distance as we can there because that allows the valves to flex more.'' I'm a visual learner, so I pictured a frisbee (the valves) sitting dead center and face up on top of a skinny pole (the small center bolt) - you'd be able to easily flex the frisbee over the pole if you pushed down on it. Now picture the same frisbee on top of a larger diameter pole that covers nearly all of its footprint - it'd be much more difficult to flex.
The patent-pending VVC system sits at the bottom of the GRIP2 cartridge, and while it's actually pretty simple, the small packaging is a clever solution.
They needed a way to provide external high-speed rebound adjustment but had next to no room, didn't want to use an existing and, in their opinion, compromised system, and also wanted to let the valves flex as much as they needed to.
''This is something that Damon and Bill Brown, the Head of Engineering, came up with,'' Lindsley said while holding up Fox's Variable Valve Control (VVC) adjustable high-speed rebound system, otherwise known as The Answer. At first glance, the rebound piston looks a lot like, well, a rebound piston, but then you notice that funny looking center bolt that I just mentioned. And then the dark-colored plate that's between that and the high-speed rebound shims. And then there's the even stranger leaf spring thingy. What's going on in there?
Here's how VVC works: The dark brown disc the looks like a piston with a bunch of tiny holes in it isn't a piston at all, but rather the face for that double-ended leaf spring. On that face is a raised spiral that's been machined into the top of it, with each end of the tiny leaf spring resting on it.
When you turn the adjuster, the wings on the leaf spring rotate around that spiral to change the leverage applied to it. When you're in the softest high-speed rebound setting (least amount of rebound), the ends of the leaf spring are contacting the outside of the disc where the spiral is farther away and pushes on the outer end of the springs to provide more leverage on the disc. More leverage makes it easier to move the disc. Turning the knob clockwise to add high-speed rebound damping rotates the spring so that the ends contact the spiral closer to the center, meaning they're both shorter leverage arms and more force is required to get the valves to open, and therefore you have more damping. Pretty clever and very compact.
''It's not preloading the leaf spring, it's just changing the fulcrum point,'' Jordan underlined for me. ''So, instead of pushing on the leaf spring from the outside where it has the most leverage and is really easy to flex, it moves the fulcrum point closer to the middle, effectively making it stiffer.'' The key thing to remember is that a shorter lever, aka the leaf spring, means more damping and vice versa. Lindsley said that the dark colored disc that rides on the leaf spring will move evenly, and there's still a valve stack in there, albeit a fair bit lighter than it would be otherwise.
You've got four knobs to play with on your GRIP2 fork, and Fox includes recommended starting points not just for air pressure, but for damper setting relative to your spring rate.
Fox isn't the first to do adjustable high-speed rebound on a fork - Marzocchi had it years ago - but they were going for a more usable, more tuneable system that provides a nice, linear line on the dyno without any funny spikes. Marzocchi's wasn't as effective, they say, largely due to due there being barely any room from the outside diameter of the bolt to the outside diameter of the valve, so they couldn't get the flex that Fox wanted.
Eeesh, that's a buttload of tech to swallow, and we can talk about valves, piston bolts, and doodads all day long, but it doesn't matter if it makes no difference on the trail. So let's find out if this extra knob on Fox's new GRIP2 damper makes a difference in the real world.