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.

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.

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”

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. 

Assembly Video

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

SIZE CHART

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.

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.

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.

RIDER HEIGHT RECOMMENDATIONS
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.

GEOMETRY & SIZING
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.

• AL-6061 construction
• 34.9mm clamp
• Works with high direct mount front derailleurs
• 34.9mm adapters include shims to allow the adapter to be clamped to a 31.8mm seat tube diameter

Weight with Borealis Hubs 150-197XD, no valves, sealeant, or tubes: 

26x80: 2590g / 5.70lb

27.5x80: 2710g / 5.97lb

HED Big Fat Deal 26x85 or Big Half Deal 27.5x85 Carbon rims hand built on our Borealis Gen 4 Tool Free Hubs with 72 points of engagement. 

Complete Wheelset Weights with 12x197XD rear hub:

26x85mm: 4.12lb

27.5x85mm: 4.25lb

Rim weight is 500 grams and 85mm. This one-piece, full-carbon rim will change the way you ride. Big Deal utilizes a ribbed, single-wall design to maximize stiffness and minimize rotational weight, on a tubeless-ready platform.