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Borealis Flume Eagle 12 speed

Borealis Flume Eagle 12 speed
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With premium frame quality and only the best components, the Borealis Flume offers the best value on the market. This expedition-ready fatty mirrors the Crestone geometry, but does so in lightweight, 6000-series aluminum. Up mountains, down glaciers, and across deserts - the Flume is ready for action. There are all kinds of alloy fat bikes on the market, but none with the Flume’s purebred heritage and durability.


Standard features include: SRAM Eagle NX 12-Speed drivetrain, SRAM Level T brakes, rigid carbon fork, carbon handlebars, carbon seat post, durable Sun Ringle Mulefüt rims and your choice of tires.
Frame geometry provides responsive handling, offering maximum control for varying conditions in snow or sand.
Tire clearance up to 26x5” and 27.5x4.5”.
Compatibility for 100-120mm of suspension travel for increased comfort and control.
Lightweight, 6000-series aluminum frame comes with 2-3 bottle cage mounts, rear rack mounts on seat stays and Lizard Skin protectors on both the down tube and drive-side chain stay.

Rockshox Bluto Fork tire size limitations: 26x5” or 27.5x4”
Mastodon Pro EXT Fork tire size limitations: 26x5” or 27.5x4.5”
Borealis Rigid Carbon Fork tire size limitations: 26x5” or 27.5x4.5”
Dually 29x45 and 27.5x45 Rims – Max tire width: 3”
All rims 65mm to 85mm wide are compatible with tire widths of 3.8” to 5”
HED Carbon 100mm Rims – Max tire width of 4.8”

Based on our research, riders at the heights below will usually take the corresponding frame size. If you’re at a transitional height that spans two sizes, look at the geometry chart to get the Stack and Reach you like based on other bikes you are comfortable.

Borealis offers four frame designs with unique fit and geometry to suit your every Fat Bike need. Begin by taking a rough size approximation on the chart below that is based on height, then select the corresponding bike with your fit needs by using the geometry chart further below. We recommend a similar fit to you mountain bike, so grab a measuring tape and dial it in.

EFFECTIVE TOP TUBE 23.02 in 24.02 in 24.8 in 25.59 in 585 mm 610 mm 630 mm 650 mm
SEAT TUBE CENTER TO TOP 15 in 17.01 in 19.06 in 21.02 in 381 mm 432 mm 484 mm 534 mm
HEAD TUBE 4.13 in 4.72 in 5.31 in 5.91 in 105 mm 120 mm 135 mm 150 mm
SEAT TUBE ANGLE 73 deg 73 deg 73 deg 73 deg 73 deg 73 deg 73 deg 73 deg
HEAD TUBE ANGLE 70 deg 70 deg 70 deg 70 deg 70 deg 70 deg 70 deg 70 deg
STANDOVER HEIGHT Due to various rim and tire options stand over is not identified.
CHAINSTAY 17.91 in 17.91 in 17.91 in 17.91 in 455 mm 455 mm 455 mm 455 mm
BB DROP 2.36 in 2.36 in 2.36 in 2.36 in 60 mm 60 mm 60 mm 60 mm
REACH 16.04 in 16.65 in 17.22 in 17.76 in 407.3 mm 423 mm 437.5 mm 451.2 mm
STACK 23.50 in 24.05 in 24.61 in 25.16 in 597 mm 610.9 mm 625 mm 639.1 mm


Frame Borealis Alloy
Fork Borealis Rigid fits tire up to 26x5" or 27.5x4.5"
Headset Borealis
Rear Shock N/A
Rims/Wheels MuleFut 27.5x65
Hubs Borealis: Front 15x150, Rear 12x197 w/XD Driver
Spokes Wheelsmith Black
Tires 45N Dillinger 27.5x4.0 Tubeless Ready Folding
Crankset SRAM Eagle NX
Chainwheel 30 Tooth
Bottom Bracket SRAM Dub 100mm
Chain KMC X12
Front Derailleur N/A
Rear Derailleur SRAM Eagle NX 1 x 12
Rear Cogs SRAM Eagle NX 12spd 11-50t
Shifters SRAM Eagle NX
Handlebars Borealis Carbon SL
Tape/Grips Borealis
Stem Borealis Alloy
Brake Levers SRAM Guide RS G2
Brakes SRAM Guide RS G2. Rotors: Avid Centerline 180/160
Pedals N/A
Saddle Borealis
Seat Post Borealis Carbon 31.6
Seat Binder Borealis Alloy
Accessories & Extras Downtube and Chainstay Protectors
Color(s) Seafoam
Sizes Small, Medium, Large, XL
Weight See Borealis Weight Calculator

* Subject to change without notice.


Getting FAT | A Fat Biking Story

6 years ago, Dave Ochs, John Norton, and Steve Kaczmarek helped found the Borealis Fatbike World Championships. Today, it's in its 5th year with over 750 riders having participated in the elite division and over 1,400 competitors in all of the various races during worlds. This is that story.

Part Numbers

Option MPN Store SKU
Small 13857 BIFB0040001
Medium 13858 BIFB0040002
Large 13859 BIFB0040003
X-Large 13860 BIFB0040004