BOS Dizzy - 2015 Fork

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Review by Brandon Turman // Action photos by Courtney Steen

BOS is a company whose suspension products have an almost Ferrari-like reputation - high-end, foreign, and ahead of the curve in many ways (and expensive). With a good history in the gravity racing scene and a no nonsense, performance first approach to marketing and design, the French brand is now looking to bring their experience and know how to the cross country scene. The new Dizzy fork was designed to "combine the plushest, most controlled travel available with superior pedaling prowess for the ultimate XC race fork." We've spent the better part of two months pounding out miles in the mountains of British Columbia and Arizona to see if it's up to the task.

BOS Dizzy Highlights

• 100/120mm travel options for 29-inch and 120/140mm for 27.5-inch wheels
• Air sprung
• Closed cartridge design
• External rebound and S-M-H compression adjustments
• Tapered steerer
• 15mm QR through axle
• 32mm Al 7075 stanchions with BOS extra low friction coating
• AL 7075 crown
• Magnesium lowers
• Anodized parts
• 160mm post mount disc brake tabs (180mm max rotor size)
• 45mm offset and axle-to-crown length of 505/525mm on 29-inch models
• High quality seals and bushings
• Contains BOS high performance and eco-friendly Bio Oil
• Claimed weight: 1,580g (3.48-pounds)
• Measured weight: 1,644g (3.62-pounds, 29-inch fork with 120mm travel and uncut steerer)
• MSRP: $1,080 USD

As it should be for any high end XC race fork, it's clear that low weight was a major design goal. From the slimmed down magnesium lowers to the 32mm aluminum stanchions and 15mm quick-release through axle, grams have been shaved on every part of the Dizzy. At a measured 1,644g it's in the same class as the RockShox SID (claimed 1,440 to 1,588g) and the new inverted RockShox RS1 (claimed 1,666g), while besting the FOX 32 Float 29 by a few hundred grams (claimed 1,796g). While weight was certainly a big consideration, what of actual suspension performance?

The Dizzy contains an all-new damping cartridge that's also loaded into the highly regarded BOS Deville AM fork. Externally it offers a simple three position S-M-H (Soft, Medium, Hard) compression adjustment: "Soft provides the plushest travel for rough, flat terrain. Medium will take care of most situations, providing a good platform for pedaling efficiency while retaining the ability to absorb bigger hits. Hard eliminates pedal-induced fork movement, enabling you to lay the power down on fire roads or smooth singletrack."

Rather than simply adjusting low-speed compression as many quick adjustments on forks do, switching between the three modes adjusts both the low and high speed rate curves. You can see in the effect in the graph above.

Soft provides very light hydraulic support. Medium is the best compromise between comfort, chassis support, and grip. Hard considerably stiffens the fork in an effort to limit suspension oscillations during pedaling.

Compression is adjustable within the Medium setting, letting you tune the feel according to your preferences and trail conditions. There is a pre-set notch that can be repositioned by removing the compression dial. You can set it anywhere between the Soft and Hard positions.

How does the compression adjustment work? When you turn the compression dial, you operate a longitudinal translation of the compression needle shaft (a). That makes the needle go inside the piston support to adjust low speed compression. At the same time, the compression needle shaft (a) activates the high speed spring seat (b). This compresses the high speed spring (c) and changes the force on the shims to adjust high speed compression.

Interestingly, the air/oil tank is not pressurized. BOS has another system inside to prevent cavitation but they weren't willing to disclose details. They did indicate that it makes service easier, though it still needs to be performed by a BOS service center.

The air spring side uses a combination of an air and coil negative spring. The coil spring acts on the first 3mm of travel to remove the preload at the beginning of the stroke and avoid topping out during rebound. The air negative is always active, and allows the fork to be smooth regardless of the fork's air pressure.

Setup & Initial Impressions

As expected, installation went well with no issues. Cutting the steerer tube took more effort than usual, but we see that as a good thing. Mounting the 180mm front disc brake required the use of 20mm post mount adapter. The cable is held in place with a zip tie. There's also plenty of clearance with a large (for XC) 2.3-inch Maxxis High Roller II tire.

BOS introduced a new quick release through axle design with the Dizzy. An angular adjustment on the axle nut ensures that the lever closes in the right position. At the lever end the combination of a cone expander and large lever blade make installation and removal of the axle very easy. The axle itself is machined with a taper in the center to reduce weight.

Recommended pressure settings are as follows:

After airing up the fork it's necessary to cycle it three or four times to distribute air between the positive and negative chambers. You'll then need to recheck pressure to be precise. At 175-pounds we initially aired up to 165psi, which yielded just 16% sag in Soft mode while standing on the pedals in the attack position. The fork felt quite firm at this pressure. In the same setup guide, BOS also recommends ~17% sag for non-technical trails/climbs and ~21% for rough terrain.We dropped the pressure to 150psi (21% sag) given our typically chunder-filled ride plans, which brought us to what felt like a much more reasonable, active feel for our weight.

Cycling the fork for the first time were very impressed with how smooth it was, with no noticeable stiction or dead spots through the entire range of travel. We experimented a bit by pumping the fork up to the maximum 200psi, and even at high pressures the initial feel was still impressive, indicating that the negative spring is designed well.

The compression dial was easy to adjust while seated thanks to a generously sized lever, and changing between the settings yielded three very different feelings. It's possible to adjust the dial anywhere between the three settings on the fly if you choose. The rebound knob is located at the bottom of right leg and is neatly tucked away. Adjusting the rebound dial doesn't make much of an audible noise, but the detents do have a pretty distinct feel.

On The Trail

As most know, we're not of the usual XC breed here at Vital. We have roots in downhill racing, prioritize performance over weight, and will happily run tires with some real meat on them versus the fastest rolling treads on the block. Heck, we don't even own a single pair of spandex shorts. That said, we do know a thing or two about good suspension, how to get down a hill fast, and we can stomp on pedals long enough to know if a setup is efficient or not.

The 120mm fork was tested on a 2015 Banshee Phantom, a 105mm travel aggressive XC/trail 29er that has more in common with an enduro race machine than a flyweight XC race rig. The bike's relatively slack 68-degree head angle and rowdy disposition provided a great platform to really push the fork into some taxing situations. We replaced the Phantom's lowered 120mm RockShox Pike with the Dizzy, dropping 280g and a few millimeters of axle-to-crown height in the process.

Test rides included several trails in the Whistler Valley (rooty and steep) as well as Flagstaff (rocky and fast), Prescott (typical smooth and fast XC race conditions), and Sedona, Arizona (extremely rocky).

We spent a handful of days dialing in air pressure. After floating around the 135-150psi mark, we can't imagine ever riding at the race recommended 165psi (save perhaps a fire road short track race). When fully open it would be very rare to use even close to full travel. Wanting to actually take advantage of the three compressions settings and full range of travel, we found it ran best about 20psi below the chart's suggestion. When outright pedaling performance is the foremost priority and you're really stomping on the gas, simply flip to the Hard compression setting for all the efficiency you could want.

For general use the fork feels best in the pre-set Medium position. Beyond a 1/4 turn it has a distinct platform feel. Part of what lets you get away with lower pressure is the fork's magic ability to stay high in the travel while feeling completely plush. We never feel as though we were diving through the travel in the Medium setting, and it provides ample front end traction at the same time. The fork ramps nicely with a controlled bottom out.

In the Hard setting it's close to a lockout, but does offer more give in a smoother, less notchy fashion than most XC race forks. While some may be quick to balk at the lack of a true lockout, we're of the opinion that a fork should be able to move regardless of the setting. This removes the overly jarring feel of unexpected bumps and helps keep you on course, especially when you're flat out or near exhaustion.

On slower, jagged and flat terrain, the Soft compression setting does a good job of reducing vibration and arm fatigue as it flutters along the surface.

While most XC forks have us shouting about stiffness concerns, it seems a non-issue with the Dizzy. There's very slight binding under big torsional loads, but it still glides into the lowers and remains notch free even then.

Things That Could Be Improved

If you take a quick glance at any bike on the starting line at a World Cup XC event or high level Pro national race, you're very likely to see at least one remote lockout on the bars. To our knowledge BOS does not offer it for the Dizzy.

The detent on the compression adjuster could be more noticeable, especially for those times during a race when you're lucky to throw a hand down and make the adjustment.

The air valve is recessed quite far under the air cap, and it can be difficult to thread on some air pumps.

While awesome from a functional standpoint, the large size of the quick release through axle lever seems out of place on a fork where weight was a priority.

We've heard that some Specialized bikes may be incompatible with early Dizzy forks due to the length of the tapered portion of the steerer. BOS is addressing this in later production runs.

Long Term Durability

After two months of submitting the fork to conditions that typically exceed those intended for a XC race fork, we have nothing but good things to say with regards to durability. It's creak free, isn't leaking, and feels just as buttery as day one. It's also backed by a one year warranty should any issues arise.

Know that because the Dizzy uses a closed damper, oil changes must be done at a BOS MTB service center. They recommend an oil change once or twice per year, which isn't much considering there are just 15cc of oil in the lowers of each fork leg. BOS says full service is needed once a year for racers and once every two years for recreational riders. These are very relaxed service intervals.

Speaking of service, BOS is opening a new Las Vegas, Nevada BOS sales/service center in late November, 2014. They expect to have full stock in early December, and will offer a risk free trial program in the USA on the Deville and Dizzy range. In the meantime you can reach them at info@bosmtb.com.

What's The Bottom Line?

In a race scene where every gram is heavily considered, the BOS Dizzy fork competes head to head with the best. What sets it apart is not the weight, however, but actual suspension performance that will impress even the most demanding riders. Every detail is well thought out, meets precise tolerances, and yields velvety smooth operation across the board. Easy to activate efficiency is available for those looking for it, and for the XC racer who truly gets after it on the descents (and maybe even drops the saddle a bit), know that this fork provides the same level of downhill performance as BOS's longer travel forks, which says a lot. Performance was so good, in fact, that we'd be quick to recommend the fork for trail riders looking to drop weight while maintaining a high level of bump-eating ability.

Visit www.bosmtb.com for more details.

About The Reviewer

Brandon Turman likes to pop off the little bonus lines on the sides of the trail, get aggressive when he's in tune with a bike, and to really mash on the pedals and open it up when pointed downhill. His perfect trail has a good mix of flow, tech, and balls-to-the-wall speed. He loves little transfers, rollers, and the occasional gap that gives him that momentary stomach in your throat kind of feeling. Toss in some rocky bits with the option to double over them or risk pinch flatting and you've got a winner in his book. In 14 years of riding he worked his way through the Collegiate downhill ranks to the Pro level. After finishing up his mechanical engineering degree, his riding focus turned to dirt sculpting and jumping with the occasional slopestyle contest thrown in for fun. Nowadays he's Vital MTB's resident product guy, putting in saddle time on nearly every new platform and innovation the bike industry has to offer.