The Chainlink

Chainlink Bike Review: Borealis Flume

By Brett Ratner

We'll get straight to the point. The Borealis Flume is not only an outstanding fat bike, it's a fantastic bike, period. After a month with our matte black demo, the only thing we have to complain about is that we didn't get enough snow to enjoy it on.

The bright side to this story is that we did manage a couple of really good days on frozen tundra. Then when the powder melted, we got to see the Flume really surprise us in conditions outside of its element.

The bottom line is we now understand why people are stoked about fat bikes.

For a seasoned mountain biker, it's well established that fat bikes are great for extending the riding season and making winter something to look forward to. They are also becoming a go-to for "bikepacking" adventures where unpredictable weather and other factors may force you to navigate large stretches of mud, snow, or sand.  

But what about a pavement-oriented cyclist looking to get into mountain biking? Thanks to our time on the Flume, we'd now argue a fat bike is a better choice, regardless of the weather. Why? We think the extra stability and traction a fat bike offers will help a rider tackle tougher trails sooner, with more confidence, and have more fun doing it.

And thanks to its high quality, excellent performance, and relatively modest price, we think the Borealis Flume should be on your short list of fat bikes to look at.

The Borealis Flume makes winter fun...and provides a worthy alternative to a mountain bike the rest of the year.

Borealis is a three-year old company based in Colorado Springs. They only make fat bikes and fat bike components like wheels, hubs and accessories.

Their product line consists of two high-end carbon models (the Echo and the Crestone), and the aluminum-framed Flume.

The Flume retails for $2,199 (rigid fork) and $2,399 (Rock Shox Bluto fork). It features a proven, bombproof SRAM GX 1x11 speed drivetrain and SRAM DB5 hydraulic braking system. Other highlights include front and rear through axles (a must-have in our opinion), and handbuilt wheels using their house brand "Turnagain" rims mounted up with well-respected 45NRTH Husker Du 26 x 4.0" tires. Frames come in a standard size range from Small to Extra Large. The 80mm wide rims can accommodate 26" tires from 3.8-4.8" wide. The fork and the frame also can accommodate up to 4.8" wide tires.

The Flume's grippy tires, paired with a 30-tooth front chain ring and a 10-42-tooth 11-speed cassette give you plenty of firepower to crawl up steep, slippery inclines. 

To Borealis' credit, instead of cheaping out on components to keep the price point down, the Flume provides a solid value by coming complete with comfortable Ergon grips and saddle, Race Face Turbine stem and crankset, and extremely light carbon fiber Borealis-branded bars and seat post (we assembled the bike when it arrived, and were amazed at how little these components weighed). In other words, the typical post-purchase upgrades will not be necessary.

Speaking of weight, the complete bike comes in around 33 pounds. For a bit of context, my personal steel-framed hardtail mountain bike with a Rock Shox SID fork and higher-end SRAM XO1 drivetrain weighs around 25 pounds. Eight pounds isn't nothing, but considering how much extra metal is necessary to accommodate 4"+ wide tires, that's not too bad.

Some might be disappointed to hear that the rims and tires can't be set up tubeless. However, our demo shipped with 26 x 2.75" mountain bike tubes. This (I'm told) is a well-known fat bike trick to help keep the rotational weight down and improve ride quality. That setup proved easy to mount up, holds air well, rides nice and has been trouble free thus no complaints here.

We can babble on about specs, but the bottom line is that for a relatively low price, the Flume delivers a quality build and a platform that can accept some of the widest tires out there, enabling you to ride just about anywhere. To be honest, if we were building up a fat bike from scratch, this is pretty much how we'd build it. 

More importantly, the Flume rides great. In designing this bike, Borealis went with a pretty standard cross country mountain bike frame geometry. On paper, it's very similar to my Niner hardtail in terms of head tube angle, seat tube angle, effective top tube and bottom bracket drop. In addition to that, the outside diameter of the 26 x 4" tire is very close to that of a 29er mountain bike tire.

This, to us at least, gives the Flume a familiar feel where you can just hop on and go, and not experience any sort of learning curve. It's like riding the bike you already know and love, but with a TON more traction and stability.

A smart thing that Borealis did, however, is equip the bike with a trail/enduro-style shorty stem and extra wide bars. The short stem helps shift your weight a little more toward the rear tire (making the handling less nervous), and the wide bars seem to give you a little more stability and confidence.

As expected, the Flume excels in snow, whether that be groomed, fresh, or old and crusty. It's also floats atop sand and provides extra traction over loose pebbles and scree.

All of this - combined with a set of tires that strike a good balance between traction, weight, and rolling resistance - helps the Flume feel extremely planted on snow and mud, even while riding steep inclines, sweeping downhill curves, and off camber switchbacks.

At Interbike demo day last fall, we also enjoyed how much more grip the Flume delivers than a mountain bike on loose sand and pebbles. That was especially evident while doing laps of the Demo Day's "fat bike proving ground" where you could experience first hand how well a fattie can navigate loose terrain at odd and steep angles, strewn with jagged rocks and some railroad ties thrown in for good measure. Admittedly, the course was completely rideable on a standard mountain bike, but it required quite a bit more finesse.

With any fat bike, however, more grip and stability is something to be expected.

The Flume shines as an all-rounder too. Whether on a pump track, tabletop jumps, or skills courses, the bike truly disappears beneath you and takes whatever you throw at it.

What wasn't as expected, was how well the Flume would do when we took it to Ray's Indoor Mountain Bike Park.

I ain't gonna lie, the Flume required a tiny bit more effort to maintain momentum on the pump track and flow sections, and it didn't seem terribly interested in going airborne (compounded by the fact I'm not terribly good a dirt jumps anyway). But at no time was I wishing I was on another bike. I was having a blast.

The skinnies were no match for this fattie...nor were the teeter totters or rock gardens. The Flume gives you confidence to tackle stuff that might otherwise intimidate.

More importantly, the Flume was an absolute monster on the skinnies, rock gardens, and other skill sections. Things I typically struggle with were a breeze on the Flume. This gave me lots of confidence to learn, improve, and tackle increasingly difficult obstacles as the day wore on.

And when it comes down to it, we think confidence is what puts a smile on your face...and why we have loved our experience with the Borealis Flume.

Visit to learn more about Borealis fat bikes. (NOTE: Borealis is currently running a season end sale with some deep discounts of 2015 models). Below are some photos highlighting the Borealis Flume's features.

Perhaps what gives the Flume a familiar feel is its similarity to a 29er hardtail mountain bike. Same comfortable riding position, but with more grip and stability.

One thing we appreciate about the Flume was that Borealis didn't cheap out on stuff to keep the price point down. Their entry-level build included quality touches like carbon bars and seat post, Ergon saddle and grips, and a Race Face stem and crankset.

The Rock Shox Bluto fat bike fork didn't seem necessary on snow, but will be a welcome addition for use on rocky and/or rooty dry trails.

The SRAM GX 11-speed groupset is rock solid. Aside from a little extra weight, it performs just as well as its pricier XO1 and XX1 counterparts. 

Front and rear through axles come standard. Technically speaking, the SRAM DB5 hydraulic brakes are entry-level, but they feel great and have thus far performed flawlessly...even in single-digit temps. 

The Flume ships with 45NRTH's proven "Husker Du" 26 x 4.0" tires. We thought they were great, but if you want even more grip and/or float, the bike can accommodate up to 26 x 4.8" rubber.

About the author:

Brett Ratner ( has been a professional journalist for more than 25 years. He has contributed to dozens of publications, including The Chicago TribuneThe Nashville TennesseanThe Nashville SceneGuitar Player and Musician. Brett began commuting by bike in 2005. Shortly thereafter, his interest in cycling expanded to century rides, bike camping and trail riding. The competition bug bit in 2012 and nowadays he also races cyclocross, track, mountain bikes, criteriums and gravel for The Bonebell.


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