HIGHLIGHTS
- 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.
WHEELS & TIRES
- All bikes come standard with tubes.
- If you order rim tape with your Mulefut rims, we will tape the rims and install the tires with tubes.
- HED Alloy and HED Carbon rims do not require rim tape for tubeless set up.
- Tubes must be removed and tubeless valves installed for tubeless set up. We recommended to have this completed by a professional bicycle mechanic.
BUILD SPECS
Eagle NX | Eagle GX | Eagle X01 | Eagle XX1 | |
Crankset | SRAM Eagle NX | SRAM Eagle GX | SRAM Eagle X01 | SRAM Eagle XX1 |
Crank Arms | 175mm Alloy | 175 Alloy | 175mm Carbon | 175mm Carbon |
Rear Derailleur | RD ENX 1 x 12 | RD EGX 1 x 12 | RD EXO 1 x 12 | RD EXX 1 x 12 |
Chain | KMC X12 | KMC X12 |
KMC X12 |
SRAM XX1 |
Cassette | PG 1230 11-50 | XG 1275 10-52 | XG 1295 10-52 | XG 1299 10-50 Gold |
Shifters | SRAM Eagle NX | SRAM Eagle GX | SRAM Eagle X01 | SRAM Eagle XX1 |
Brakes | SRAM - Select | SRAM - Select | SRAM - Select | SRAM - Select |
Chain Ring | SRAM 30T X-Sync | |||
Frame | Borealis Flume Alloy | |||
Fork | Borealis Carbon Rigid Fork | |||
Headset | FSA | |||
Hubs | Borealis: Front 15×150 Rear 12×197 | |||
Rims | 26" or 27.5" Sun Ringle Mulefut V2 |
|||
Spokes | Borealis Straight Gauge | |||
Nipples | Borealis Silver Brass | |||
Tires | Your Choice | |||
Bar | RaceFace Next SL Carbon 740x10mm OR Borealis Carbon (based on supply) | |||
Stem | Borealis 50mm or 70mm | |||
Post | RaceFace Next SL Carbon 31.6 | |||
Saddle | Borealis | |||
Grips | Borealis | |||
Bottom Bracket | SRAM DUB 100mm | |||
DT/CS Protector | Lizard Skins | |||
Weight | See weight calculator for estimated weight. |
GEOMETRY CHART
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.
S | M | L | XL | |
EFFECTIVE TOP TUBE | 585 mm | 610 mm | 630 mm | 650 mm |
SEAT TUBE CENTER TO TOP | 381 mm | 432 mm | 484 mm | 534 mm |
HEAD TUBE | 105 mm | 120 mm | 135 mm | 150 mm |
SEAT TUBE ANGLE | 73 deg | 73 deg | 73 deg | 73 deg |
HEAD TUBE ANGLE | 70 deg | 70 deg | 70 deg | 70 deg |
STANDOVER HEIGHT | Varies based on wheel and tire size | |||
CHAINSTAY | 455 mm | 455 mm | 455 mm | 455 mm |
BB DROP | 60 mm | 60 mm | 60 mm | 60 mm |
REACH | 407 mm | 423 mm | 438 mm | 451 mm |
STACK | 597 mm | 611 mm | 625 mm | 639 mm |
ASSEMBLY
If you choose "No Assembly" your bike will require complete assembly with an exception of the wheels.
If you choose "Assembled by Borealis", there is some assembly required upon receipt. Your bike will be completely assembled and tuned by Borealis, then partially disassembled for shipment. The remaining assembly will require installation of the rear derailleur, chain, fork, seat post, wheels, and handlebars. A multi-tool will be included with your order that has all tools necessary to assemble the bike, with the exception of a torque wrench. If you are not familiar with this level of assembly, we recommend having your local bike shop complete the assembly. Below is a video showing the remaining assembly once you receive the bike.
OWNER'S MANUAL
COMPATIBILITY
- 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”






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Customer Reviews
I've been riding a steel Surly and simply wanted to lighten it up. I knew of Borealis but not much more. I was able to go on to their website, choose building the bike myself. I select the items that were important to me for function and weight..
The ad stated it could take up to 2 weeks, I was very surprised it was ready in 4 days. I was able to drive down to Colorado Springs pick the bike up. They even opened the shop a little bit early for me.
Into the bike no more than 2 weeks I have used it to commute to work, ride green MTB trails and finally today packed snow.
I am thrilled with the fit. I am thrilled with the part selection, I am thrilled with the build. I feel I made a great choice. I also upgraded the fork to the Rock Shox Bluto which makes steering a little more simple on bumpy terrains.
Made mines into an ebike total cost including bike is $3500-$4000. Had a size too big and even after months past, they still allow me to exchange the frame for one size down no charge or hassle. Light sturdy and fun. Even with a bafang bbshd 1000w mid drive, the bike still feels balance and stable. Been on dirt roads, tiny jumps, thick tree roots, cobble stone road, even a trail full or acorn or some type of nuts on it, with the mastodon manitou fork plus chubby tires (bike is sensitive about its weight) I could barely feel any bumps. Love it. Thanks borealis crew.
The Flume is a monster. Its lighter than expected, powerful, and super fun. Geometry feels right, love the color, quality build.
I purchased a Flume bicycle in 2021 during Covid when there was very few bicycles available. I never had a Borealis bicycle, but I’ve heard a lot of positive things about them, so I figured it was a safe bet to pull the trigger on. Man, I’m really pleased that I did. This is quite the ride. The gearing on this bicycle is absolutely phenomenal. You can pretty much climb any hill out there with this bad boy.
Aside from being very stylish, it’s also a very comfortable ride as well. I love the 5 inch tires. I for the life of me cannot figure out why anybody has suspension on one of these bicycles lol, it certainly doesn’t need it (I’m a recreational rider, with hard core MTB, front suspension is a good idea) If you’re thinking about purchasing one, I suggest you go for it. You definitely won’t be sorry.
I go way back to the early days of mountain biking, no suspension forks etc. This Flume reminds me of those old bikes. I bought it to ride on very sandy trails along a river and its great. It handles really well in tight spots and is a ton of fun. If I had one complaint it would be that the rim tapping Job by Borealis was less than good with neither tire holding air. I have had to redo both wheels myself.