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Independence Day SALE - 20% OFF COMPLETE BIKES & FRAMES ENDS 7/15 - Discount automatically applied in cart!

0% APR up to 24 months with Affirm! Financing Options Available at Checkout.

$1,699.00 USD

Size Chart

bike size chart

Shipping and Returns

PO boxes not accepted. Shipping discounts or exclusions only apply within the lower 48 states of the US. Contact us if damage occurs during shipping. 

Borealis offers a 30-day money-back guarantee. If you are unsatisfied with your purchase for any reason, you can cancel, exchange, or return your product for a full refund, minus any associated transaction fees and shipping costs. Customer is responsible for return shipping costs. Returned and exchanged items must be in sellable condition. Affirm transactions are subject to a 10% non-refundable transaction fee.




Original Owner
BOREALIS offers a LIMITED LIFETIME warranty to the original owner of each new BOREALIS bicycle, fork or frameset that the bicycle frame, fork or frameset when new is free of defective materials and workmanship.

Normal Conditions
The limited warranty is conditioned upon the bicycle being operated under normal conditions and use, and properly maintained.

This limited warranty does not apply to paint/finish or components attached to the bicycle/fork/frameset such as rims, drive train, brakes, seat post, handlebar and stem, or any suspension related parts or components.

Paint/finish, components attached to the bicycle/frameset such as rims, drive train, brakes, seat post, handlebar and stem, or any suspension-related parts or components are covered under the limited one (1) year warranty.

Load Weight
The total cargo load for BOREALIS frames and forks is 300 pounds. The total cargo load includes the rider and any associated gear warn by the rider or attached to the frame or fork.


Subject to the following limitations, terms and conditions, BOREALIS warrants to the original owner of each new BOREALIS bicycle that this bicycle, when new, is free of defective materials and workmanship. This warranty shall expire one (1) year from the date of the original purchase from an authorized BOREALIS dealer and is conditioned upon the bicycle being operated under normal conditions and use, and properly maintained. This warranty is void if the bicycle was not purchased new or not properly assembled by an authorized BOREALIS dealer.
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Customer Reviews

Based on 8 reviews
Jessica M
Hard to beat value!

I opted for the Borealis Flume after getting several recommendations for the brand in an online mountain biking forum. After much research between many of the quality brand fat bikes out there, the Flume fit my needs. I also loved the look of the frame and matte black color option. Quite a light bike for not being a carbon frame and the welds look amazing - so smooth. The bike arrived early and in great shape. I’ve just recently completed the build and am looking forward to its maiden voyage this weekend! I anticipate being thrilled.

anthony lopez
Entry Level Fat Bike

If your looking for an awesome entry-level FAT bike to get started, this is the bike for you! Great quality for the price but most importanly the service is second to none. You will not regret the purchase...

Dan Pingel
Fat biking is more fun now

I love this bike! My goal was to find a bike that felt more like a nimble mountain bike and less like some of the slow, lumbering fat bikes I've ridden. My new Flume fits the bill perfectly. In the past, I never liked fat biking quite as much as mountain or gravel biking....this bike has changed that. It's a blast to ride and I can't wait to get out and explore more with it. Lastly, it's a huge win to have the flexibility to choose my wheelset and components to create a truly custom build to fit my needs.

OMG great bike choice

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.

Great customer service!

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.


  • 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.


  • 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.


Eagle NX Eagle GX
Frame Flume (Aluminum) Flume (Aluminum)
Fork Borealis Carbon Rigid Fork Standard Option
Bottom Bracket SRAM DUB 100mm
Crankset SRAM Eagle NX Alloy 175mm w/ SRAM X-SYNC 30T SRAM Eagle GX Alloy 175mm w/ SRAM X-SYNC 30T
Rear Derailleur SRAM Eagle NX 12 Speed SRAM Eagle GX 12 Speed
Chain Eagle GX Eagle GX
Cassette PG 1230 11-50 XG 1275 10-52
Shifter SRAM Eagle NX SRAM Eagle GX
Brakes SRAM Level T 2-Piston Hydraulic
Rotors SRAM Centerline 160mm
Headset FSA
Hubs Borealis GEN 4 Black 15x150 Front / 12x197 Rear
Rims Sun Ringle Mulefut V2 26x80 or 27.5x65
Spokes Borealis Straight Gauge
Nipples Borealis Silver Brass
Tires Your Choice
Bar Borealis Carbon 35x760mm
Stem Borealis 50mm
Post Borealis Carbon 31.6x400mm
Saddle Borealis
Grips Ergon GA20
DT/CS Protector Lizard Skins
Weight See weight calculator for estimated weight.


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




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. 

Assembly Video


Download Owner's Manual



  • 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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