Home / Downhill Bikes / Bos and 27.5″ wheels in Demo 2013!

Bos and 27.5″ wheels in Demo 2013!

At first glance it looks like a regular specialised demo: one of many in Poland, but this one is extraordinary. It belongs to Michał Martyka from BartBass GoodBike dh-team. Full specs inside.

This setup is based on 2013 Specialised Demo 8 Carbon frame. The front fork  is rare for this frame: Bos Idylle RaRe FCV 37. Under this long and boring codename lies a high tech weapon from Bos for 2015 season. The Bos is cooperating  with 27.5 new wheel size and has many changes when compared with previous RaRe. If we talk about its performance, it is superb and you can easily feel difference between this one and older model with 36mm stanchions. The rear damper is new air Bos Void 241mm. Demo and air spring? Michael says this mixture can work pretty well. Void is smooth linear characteristics with progression at the end of its travel. Normal, spring dampers require rubber bumper, here it is a no-no. Great adjustments which are very sensitive when compared to different air springs. Is this ideal rear shock? We will know this at the end of this season, but in meantime Michał is astonished by characteristics of new Bos. The only drawback of this small miracle is its price: 3500 PLN.

The rest of components comes from: Cloud, GubBike, Sram X-0 drivetrain with polish sprocket from Vicere, X-0 derailleur and x-9 shifter. Chain guide is a true classic:  E-13 lg. Brakes are the new Hope E4, which are very very light even though they have 4 pistons.


We intentionally didn’t spend time on describing these components, as they were described and tested many times before. A tasty thing in this bike, which we should mention is the wheel setup. They are regular t350 hubs with WTB Frequency i-25 rims, but they are 27.5”! They are fitted into Demo 2014, so to a frame which is supposed to be ridden on 26”.

We were curious how would this experiment finish.  Our bet was that, it will have terrible geometry, or it will be impossible to fit these wheels. Surprisingly, Michal was able to fit these wheels, but he has just 0.5 cm  of clearance between Maxis DHF 2.5 and rear triangle of the frame, so in muddy conditions it is a major problem.  After all, the wheel base is close to 650B rivals: 1223mm for M size frame with head angle of 63 degrees. The support height is 34.8 cm. 

As we can see, if you buy 27.5” fork, you can upgrade your frame to 650B standard. Is it a good deal?  It is a discussion, which is online for few months now. Below we present opinion of Michał:


I was quite sceptical about the new 27.5” standard., so I decided to test if I was able to mount 27.5” wheels on my Demo ’13 frame. I was also curious if the new 650 B standard was just a marketing move. The experiment turned out to be successful in just 50%: I did fit 27.5” wheels without any problems, but I didn’t prove myself that, 27.5” is just a marketing move. The new standard does make a big difference. I didn’t feel any lack of steering or any other kind of discomfort. The bike became much more stable and gained in the aspect of carrying the speed through obstacles. Rocks and roots, that were slowing me down on smaller wheel size, now disappears. The bike has more momentum so it is harder to slow the bike down: by rocks, but also by brakes. The bike does brake worse. The tire price is the same for 26” and 27.5”, so it is additional positive to switch for new standard.

Weight 16.88kg

Frame Specialized demo 2013 Carbon M

Fork: BOS Idylle FCV 37

Rear shock: BOS Void 241

Handlebar: Cloud Perform 790×25 mm

stem: Cloud Perform 45/55 mm

Grips: GUBbike

headset: GUBbike -1 degree

Seat: SLR 135g

Seat post: GUBbike PRO Carbon

Seat clamp: GUBbike

cranks: SRAM X.0 165 bb30

Support: X.0

Sprocket: Vincere Sumo

Chain: Yaban S1010cr

Pedals: CB Mallet DH

Chain guide: E-13 LG1

Derailleur: SRAM X.0

Shifter: Sram X.9

Cassette: SRAM X.0

Front wheel:

Rim: WTB FREQUENCY i-25  27,5

Spokes: HT 2.0

Hub: DT 350

Tire: Maxxis DHF 2,5 ST  27.5

Tube: Rubena  27,5

Rear wheel:

Rim: WTB FREQUENCY i-25  27,5

Spokes: HT 2.0

Hub: DT 350

Tire: Maxxis DHF 2,5 ST  27.5

Tube: Rubena  27,5

Front brake: Hope E4

Rear brake: Hope E4