BOS Idylle Rare Air FCV Fork

The Damper and the FCV System

As mentioned above, the Idylle’s damper is an open bath system housed in the right leg of the fork.

The most interesting component of the damping system is the FCV unit, which I’ll get into below. The primary damper doesn’t appear to be anything way out of the ordinary, but it is well executed. I never noticed any spiking or weird performance quirks while riding.

Since it’s an open bath unit, there’s a fair amount of oil sloshing around in there. While this adds a bit of weight, it also means that the unit dissipates heat pretty well. Even on long runs, I never noticed the performance of the fork appreciably change.

As noted above, the external damping adjustment all worked as expected, and I felt that I could pretty easily dial in the fork’s performance. In my initial rides on the fork, I felt that it needed a bit more support through the mid-stroke. Adding some air pressure and a few clicks on the high speed compression knob took care of this, although I ultimately ended up adding even more pressure, and backing the high speed knob off a bit. This isn’t something I’ve been able to do on other forks since it tends to just ruin the small bump sensitivity, but on the Idylle that’s not a problem.

It’s a bit tough to tell, but this also seems to be a place where the FCV system helped out a bit. The FCV (Frequency Control Valve) is a valve in the damping mechanism that, according to BOS, can differentiate between force inputs from the rider vs. force inputs from the terrain. So, theoretically, the fork resists diving due to braking, or bobbing while sprinting, but it remains active when the fork smacks a bump.

To some extent, the FCV system bears some similarities to an inertia valve, like the “brain” used on some Fox-equipped Specialized bikes. Basically, the valve inhibits oil flow on the compression stroke until a sizeable bump is hit, knocking the valve open, and thus allowing for increased flow. This is noteworthy in that it’s the actual force of the bump that opens the valve, as opposed the buildup of oil pressure from the suspension compressing.

From riding it, it’s clear that the FCV is nowhere near as severe as a brain; while the brain effectively acts as a very noticeable lockout, it’s not immediately obvious that the FCV system is doing anything. The fork feels plush and smooth, and it actually took me a while to realize that the FCV was working.

Early on in my time on this fork, I noticed that I was using more travel than I felt like I should. Even on trails that weren’t overly rough, I’d use 50-60% of the fork’s travel pretty regularly. For comparison, on my previous Boxxer on the same sort of trail, I might expect to use 40%.

And, at least with my Boxxer, it would have been a problem if I’d been using that much travel, because it would mean the fork was too soft and was diving in corners and under hard braking. But even though I was using more travel on the Idylle, I never felt like it was diving or running low in its travel when I didn’t want it to. Instead, it was just absorbing bumps more completely, but without the negative attributes of running really soft suspension.

As best as I can ascertain, I think the FCV gets credit for this. The Idylle absorbs bumps in the trail better than pretty much any other fork I’ve ridden, but it doesn’t dive or do anything goofy in corners or while on the brakes. To be clear, that’s not to say that there isn’t any brake dive (there is). It just means that if the Idylle is setup to have the same amount of brake dive as a Boxxer or 40, it remains much more supple to absorb both small- and medium-sized hits.

What all of this adds up to is a fork that, at the same time, is more supple and more supportive than any other fork I’ve ridden.

As I said in my review, the Canfield Jedi is a great bike for maching through really rough trails, and the Idylle is an impressive fork in that setting. It keeps the front end very composed when smashing through rocks and roots—it has that effortlessly smooth feeling that I wish more forks had.

All of that suppleness also just makes the fork a lot more pleasant to ride day in and day out. After a few long days of hammering out lift-served laps, it was pretty obvious that the Idylle keeps my hands and forearms happier. Even running consecutive, long lift-served days on rough trails, my hands stayed in decent shape. Doing that same thing in years past on other forks, my grip strength definitely deteriorated over time.

Maintenance and Support

This is where things get a bit tricky when it comes to BOS suspension. BOS is a French company, and supposedly getting support in Europe is do-able. For the last couple of years, BOS has provided support in North America as well, but there’s been mixed reviews as to how reliable that support really is. Added to that is the fact that BOS equipment is fairly rare, so getting their special oil isn’t as easy as sauntering down to your local shop with a crisp benny in hand.

While a full rebuild is problematic for the home mechanic (more on that below), changing the oil is a pretty simple affair, which is a definite upside to the open bath damper. Pour the old oil out, do a bit of pumping and squeezing to make sure it’s all out, and then pour new oil in. Pump and squeeze a bit more, measure your oil level, and seal everything back up. Not too bad, especially when compared to bladder systems like in the Boxxer and 40.

The other upside is that the service intervals on the Idylle are few and far between. Even with frequent use, BOS only recommends an oil change twice a year and new seals once a year. That’s quite a bit less frequent than, for example, a Fox 40 which has a recommended service interval of 125 hours. Admittedly, it’s a bit tough to compare this directly since I don’t know how many hours BOS expects a person to ride in a year.

In practice, BOS’s service intervals seem to be plausible. I have around 27 days on my Idylle and it’s still perfectly smooth and feeling good. Maybe not quite as smooth as when it was new and fresh, but I’d guess that it still has quite a while before it needs a rebuild. I’ve talked to some other Idylle owners that have more time on theirs, and it’s the same story. Some are over a year without a rebuild, and everything feels good.

Issues

The Idylle has run pretty flawlessly for me in terms of suspension performance, but, aside from the lack of knobs on the damping adjustments, I have two main complaints about it.

First, the crowns creak. Maybe BOS just took a bit too much material off of them, or maybe this is one area where their tolerances weren’t perfect. Either way, after a day of hammering down rough trails, the crowns start creaking really loudly. Greasing the bolts and the interface between the crowns and headset didn’t help, nor did slightly over-torquing the bolts. Fortunately, this is a pretty easy fix: loosen everything up, give it a quick wiggle, and re-torque the bolts. It’s not a deal breaker, but it’s annoying.

The second issue really just comes down to the fact that this is a somewhat exotic, uncommon fork that isn’t intended for the home suspension tuner. Parts are hard to come by (at least in North America) and everything is expensive. BOS says that you can only use their special race-grade oil, and they don’t tell you what weight it is (internet speculation pegs it to be a high grade 7.5wt).

BOS also doesn’t make seal kits available for purchase by the public. You can buy the special oil and dust wipers off of BOS’s website, but BOS offers neither the parts nor the instructions to a full rebuild on the fork. BOS simply says that you have to send your fork to an authorized service center for that kind of work, which is problematic if you (a) prefer to work on your own equipment, or (b) you don’t want to be without a fork for a couple weeks while it’s being serviced. In North America, there’s only one service center (in Las Vegas), and as of this writing, information about that service center is somewhat hard to come by, as they apparently forgot to pay their web hosting fees.

Added to that, even though BOS provides some basic instructions for servicing the Idylle, BOS says that if you work on your own fork, it voids the one year warranty. Realistically, that’s probably only enforced if you screw something up, but still, that’s not a particularly user-friendly policy.

Bottom Line

So, were the Canfield guys right? Is this thing “insane”?

It took me a bit to arrive at a conclusion, but yeah, the Idylle Rare Air FCV is a damn good fork. It’s not the lightest, nor is it the stiffest, but I’m reasonably comfortable saying that it has the best action of any fork I’ve ridden, DH or otherwise. It hits that perfect balance of early stroke suppleness, mid stroke support, and progressive bottom out resistance.

Whatever it is that BOS did inside this thing, it works. I never felt the fork bind or spike under harsh loads, and it had just the right amount of support through corners and when on the brakes.

But that performance comes at a cost, both in the monetary sense, and the fact that the fork isn’t particularly user friendly for servicing. This is sort of the Apple vs. PC debate: Apple makes a minimalist product with basic adjustments that costs a lot and works very well, but isn’t really customizable. The PC is less expensive, more versatile, and more customizable, but if you don’t know what you’re doing, it can turn into a bit of a disaster.

So the Idylle is a high-end Apple, and something like a DVO Emerald (which is really optimized for tinkerers) might be a high-end PC. Boxxers and 40’s fall somewhere slightly down the food chain; both are functionally very good, but their performance doesn’t match the Idylle, and they’re not as easy to tune as an Emerald.

Ultimately, the Idylle isn’t for everyone. Its price alone pretty clearly puts it out of reach of a good number of riders, and the fact that it’s not very user friendly in terms of service will likely turn some garage mechanics away.

If you can live with the Idylle’s few shortcomings, its performance is at the top of the game. It handles every obstacle I’ve thrown at it better than any other fork I’ve ridden.

It somehow manages to simultaneously be more comfortable and forgiving than the competition, but also stiff and supportive when it counts, which is about the best thing I can say about a DH fork.