Bos Deville 35 fcv- Mtbr.com
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  1. #1
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    Bos Deville 35 fcv

    Got this fork in a couple weeks ago in 170mm and got a few rides in on it and did some tinkering, seems like there is so few of these forks out there, and no information on the forums so i'll post some here. It definitely has some quirks, but its pretty easy to correct these quirks, and afterwards, its goooood.

    Quirks:

    The airspring is strange, i think the 90's called when they designed it. The equilization port is activated right at top out where the air piston sits right against the seal head at the end of the stanchion, like there is no space between the piston and the sealhead at top out. The negative chamber literally consists of the hollow air shaft volume which has an ID of 8mm (OD is 12mm) thats it, its tiny. It takes some force to get the fork moving as there isn't really enough negative volume to counteract the positive volume and essentially its like the fork has preload on the spring. The airspring does remain pretty linear through its stroke though, almost feels like the spring rate drops a bit near bottom out. Feels like it gives lots of support in the beginning of the stroke because of this but it bottoms out easy, a backwards step from previous air springs for sure. I have a couple year old idylle and the airspring on that has a massive negative chamber (about 5 inches long) including using all of the shaft space, and the equilization port is about 20mm from top out, feels just like a coil spring, so i'm not sure why BOS went this route with the deville. Easy fix though, couple hours with the lathe and in went a coil spring.

    Notes about air spring:
    All of the pieces are very well made, and everything is anodized very nicely. The seal head uses an o ring, and also a wiper seal with a bushing in between. The wiper is to help prevent oil migration from the bath oil into the air spring. The wiper seal appears to be meant for pneumatic application because it has a very thin lip area on it and adds almost no friction from itself. Main piston seal is an o ring with a very thin backup ring on either side, much thinner than usual it seems. Airspring lube from the factory was grease. Overall friction from this system is just a bit less than the system in my mattoc and mezzer but not by a huge amount, One thing that i don't get is it seems that airsprings have gotten to a bit of a plateau in the friction department now as the squeeze and compression on the orings/quad rings cant really be lowered much more while still sealing well enough. I really think dedicated pneumatic seals in forks would improve the friction to be much closer to a coil but i guess cheap is the name of the game here. Mr. Kapfinger is the only one who has this figured out so far for production forks. Until this happens, I will keep converting all of my forks to coil.

    Bottom out bumpers are the other quirk. They are 20mm long and sit in the bottom of the lower casting and are some sort of foam They look identical to the foam used in the Formula Neopos, not sure what the foam is exactly, if anyone knows what this stuff used for neopos is or which company or companies make it / what its actually known as outside of the bike industry, I would like to know so I can get some to experiment with. The foam works really good as a bottom out bumper making the fork feel like it has endless travel, that is until about the third bounce in the parking lot as the bath oil is absorbed by the foam and they then become hard as a rock as the oil has to be squeezed out when the fork legs make contact, not so good. I trimmed these down as I would much rather run more than 1ml of bath oil in the fork and plan to add hydraulic bottom out in the future.

    Damper notes:

    I had the damper apart briefly tonight to have a look at it. it appears to be a DBC type damper with a spring backed ifp. Build quality is outstanding, even a step above manitou. Everything is hard anodized in there, including piston holders. Some interesting things going on though, the pistons themselves appear to be made of stainless steel, both the base valve and the rebound piston. The rebound piston is interesting, i don't know if its intentional or not, but it has smaller round ports and larger parabolic ports, the smaller round ports are used for the flow on compression instead of the larger parabolic ports which go to the rebound shims. The od of the piston at the rebound shims is 15mm, while it is 16mm on the midvalve side, the midvalve shim is 16mm diameter and is flush with the od of that part of the piston and it has about 0.5mm float. The rebound shims are delta shims and they do overhang the edge of the piston, they appear to also be 16mm od while the part of the piston they rest on is 15mm od. Full od of the piston/ID of the damper tube is 18mm. The bushing in the sealhead for the damper shaft is long, its about 15mm and that is for a series of progressively smaller holes in the side of the damper shaft which is for the hydraulic top out. No hydraulic bottomout, there isn't enough room for it, although the cup and cone from the mattoc do thread right onto the base valve/rebound piston holders there isn't enough free space in the damper to house them.

    Base valve looks like every other base valve, a piston and some shims on top of it. I need to take this apart to understand whats going on with it. There is LSC/HSC and a lever to firm the fork up on top of the leg. I'm not sure what goes on for the HSC adjustment because the shims on the base valve piston are not preloaded, the inertia valve is in the way. It seems all of the adjustments alter the free bleed in some way, this will be a winter project to look at and see just what is going on here. I must say that don't really like the idea of inertia valves, there is a lag time for them to react, but from riding impressions whatever BOS has done with this damper is pretty amazing.

    Chassis notes:

    This is undoubtedly my favorite chassis out of every single crown I've owned. There is the exact right amount of clearance in the bushings straight from the factory, not enough to cause knocking but plenty enough to allow oil to get in there easily, no need for sizing right off the bat. The main seals are the exact same trelleborg low friction seals as the mattoc, just 35mm instead of 34. The axle is 15mm except for one side is 20mm, it threads into one side of the fork with the hub and holds it tight to that leg while the other leg is free to float on the 20mm diameter area which is then clamped with a pinch bolt. It allows the fork legs to always be in perfect alignment and never squeezed together if the hub is on the narrow side of tolerance. There is no binding at all anywhere in the stroke and with the damper/spring top caps unthreaded, it readily sinks through all of its travel immediatly. There is 120mm of bushing overlap and they aren't the same bushings used by all the other manufactures, they have a black surface on them instead. There are no foam rings in this fork, just seals and then straight away the top bushing. It feels about as ridgid as my mezzer, its a stiff fork. Offset is 44mm, and I am unable to discern any difference between the mezzer at 37mm offset which was the same when i went from a 44mm offset mattoc to the mezzer.

    The ride:

    I'm about 145lbs, and ride my enduro bike like my dh bike, no mercy, all of the hucks to flat. Havent had the fork to whistler but what i ride locally is similar to whats found at whistler.
    Spring is 40lbs/inch.
    Lower leg lube is motorex 5w40 (stock is bos bio oil, don't know the specs)
    Running lots of lube in the lowers to create the needed progression/bottomout resistance, bandaid fix for not currently having hydraulic bottomout. Due to damper space constraints it will have to be included on the spring side, some machining will be required.

    Despite the wacky air spring, in stock form its really good, very rideable and no harshenss, provides lots of traction, the damper does a really good job holding the fork up in the travel, but i still think its let down a bit by the airspring being a too stiff at the beginning of the stroke and too soft at the bottom out. Basically ran almost no sag at all with the stock air spring and the largest volume setting on the negative spring. Still even so, its much better than RS/Fox, but stock, out of the box, not as good as the mezzer. However, with a coil, its an entirely different animal. The small bump sensitivity for little rocks, roots, braking bumps is insane, there is pretty much no stiction at all and it just eats them up giving literally all of the traction. There is not much dive during braking or lower speed maneuvers either, stays up in the travel really nicely, but gets out of the way and moves very easily when needed on all sizes of bumps at all speeds. The damping curves that BOS has used in this fork just seem to work so good for me, I actually prefer it more than my mezzer that i ran for a few weeks with a coil spring, i found it to offer even less dive than the mezzer and was a bit comfier on the bumps, and that mezzer with a coil is wicked good.

    Bottom line is this fork WITH a coil in my opinion is the best single crown fork i've ridden, it does what a good fork should do, suck up bumps without using too much travel, keep the wheel glued to the ground and, and provide a comfortable ride. Slightly better than the mezzer with a coil. However, you will either need to have a lathe, or have a buddy with a lathe to do the coil conversion, which is a rather large drawback for most people. Comparing completely stock deville vs stock mezzer, its fairly close, but because of the stock HBO in the mezzer it would get the nod.


    Well thats a lot of words, hopefully some of it makes some sort of sense and i haven't repeated myself too many times. Silly me, i didn't take any photos.

  2. #2
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    Excellent writeup, although I dont have a BOS fork I found this extremely informative and interesting. Thank you.

  3. #3
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    Great write up. How would one get more info on your Mezzer coil conversion?

    Thanks

    Ken
    Transition Scout Carbon

  4. #4
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    Great write-up.

    Regarding the foam, I thought NEOPOS looked like EVA foam which is closed cell. But sucking up oil makes your bumpers sound more like microcellular urethane (MCU) which is used by almost everyone for coil shock bumpers. Especially automotive.
    Owner of www.shockcraft.co.nz, Mech Engineer, Tuner, Manitou, Motorex, Vorsprung EPTC, SKF, Enduro
    www.dougal.co.nz

  5. #5
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    Interesting write up!

    I saw something about adjustable negative spring volume with the FCV forks, with an interchangeable rod accessed from the bottom, did this fork have that?
    www.thesuspensionlab.nz
    Servicing in Rotorua, NZ/Vorsprung Elite Tuning Centre/DVO service centre/Insta @thesuspensionlab

  6. #6
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    Dougal,

    Thank you, after some searching it appears to be microcellular urethane, i found a picture of a brand name called PORON that looks identical to these bumpers.

    Johnny,

    Yes it does have an adjustable negative chamber volume, there are three different settings using different length spacers inside of the air spring shaft. The foot nut that threads into the bottom of the air shaft has a small external thread, looks like m5 or m6, that the spacers thread onto. The spacers are made of plastic and have an oring to seal the inside of the shaft. I was using the fork with the smallest spacer for the largest negative volume.

    Ken,

    I had to machine 2 pieces for it, a spring seat to sit on top of the piston and a top cap to allow enough space for the spring, its possible to fit the spring in the leg without a new top cap but that would have required more machine time to fit the spring in which i was to lazy to do at the time lol. I removed the coil spring until i get some shrink wrap as i didn't have enough at the time and the spring was rattling around in the cavernous space inside the stanchion quite a lot. I'll try and get some pictures up in the mezzer thread this weekend.

  7. #7
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    Added to post below

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