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A Unique, Premium Ride That Is Rugged and Capable for XC & Trail Riding in Any Weather
For some, trail or XC mountain biking is a fair-weather-only activity. Fighting the muck after some rain might not sound like a joyride. For others, local trails may not be the most conducive to riding if you live in a sandy, gravelly, or snowy locale. However, a cure exists for these particular ailments in the form of true fat tire eMTBs for XC and trail riding.
The Borealis Keystone operates in this niche, and it was impressively outfitted to thrive in different riding scenarios.
Borealis has been making fat bikes for a long time—as evidenced by their owning the domain fatbike.com—and their first take on an electric bike is an impressive one. The Keystone has the modern geometry associated with trail and XC riding, is spec’d with a host of desirable Shimano and SRAM parts (hello, electronic shifting!), and lets the rider pick the tires ranging from offensive lineman to sumo wrestler sizes.
One of the most surprising takeaways from our time on the Keystone was how simple it is to ride. The bike weighs 60 lbs, but it feels light by fat bike standards—there are several in the 70 – 90 lbs. range, after all. It was surprisingly easy to maneuver around on our local trails, and we even found it worked well with the motor off.
That’s not to say we didn’t like the motor, though—quite the opposite, in fact. Many fat bikes supercharge the motor, making the bike a bit hard to control on technical terrain. Others don’t bring enough punch and are lacking enough power to charge a hill. This was a third bear “just right” scenario.
The Keystone felt versatile and enabled us to ride trails we’d otherwise pass over because they were too dusted with sand. The fun factor of a bike is high when you feel less restricted.
The bike also impressed us with its excellent performance in our standardized tests. Read on below for all the details.
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E-Bike Category:Fat Bike / eMTB
E-Bike Class:
Class 1 (pedal assist to 20 mph)
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Borealis Keystone Video Review
ELECTRICAL SPECS & FEATURES
Battery:Shimano 630Wh Battery Display: Shimano SC-EM800 Motor: Shimano EP8 Drive Unit Headlight:N/A Taillights:N/A Pedal Assist: 3 PAS level assist, Torque sensor – Eco, Trail, Boost Claimed Range: Throttle:N/A App:Shimano E-Tube UL Certification:N/AWeight & Dimensions
Claimed weight: 56 lbs Tested Weight: 60 lbs Battery Weight: 8.8 lbs. Stack: 702mm Reach: 445mm Top Tube Length: 660mm Seat Tube Length: 482mm Seat Tuble Angle: 73° Head Tube Length: 175mm Head Tube Angle: 69° BB Drop: 60mm Chainstay Length: 480mm Wheelbase: 1223mmComponents & Accessories
Brakes: 4-piston SRAM G2 RS hydraulic brakes, 200mm front / 180mm rear Fenders: N/A Fork: Manitou Mastodon PRO EXT 120mm, 51mm offset Frame: 6061-T6 Aluminum Frame Drivetrain:SRAM GX Eagle, 12-speed 10-52T cassette, 36T chainring Grips: Ergon GP1 Saddle: WTB Volt Medium Steel Seatpost: KS Lev Integra 125mm (SM, MD) / 150mm (LG, XL) Handlebar: 760mm Borealis Carbon Stem: Borealis 50mm or 70mm Kickstand: included Cranks: Samox 170mm Pedals: included, alloy Tires: (As tested) Terrene Johnny 5 26×5.0 light Rims: (As tested) SunRingle Mulefüt SL Hubs: Borealis Gen 4 15×150 / 12×197 Spokes: Straight Gauge 2.0 Black
The Keystone is a true adventure bike. There are very few places it CAN’T go.
The Shimano EP8 motor felt like a very fitting drive unit. Responsive, powerful, and intuitive to ride.
The battery is large enough to supply hours of riding even at full power.
Borealis went with a tried and true setup in the Shimano EP8 motor and accompanying Shimano battery and display.
The EP8 produces 250 watts of nominal power, can support a 500W peak, and tops out at 85Nm of torque. The motor isn’t as powerful as some others on the market, but that’s not a complaint from us.
Many fat tire bikes lean into hyper-aggressive motors – but that’s not necessarily what you want from a trail bike. Power is only as good as its distribution; in this case, it delivers the juice you need in a great fashion.
As you pedal and engage the motor, you can sense the torque progression—even if you take a hard first step, the motor will ramp you up in a manageable way.
The Shimano system operates in my personal preferred way, with just three riding modes: Eco, Trail, and Boost. The low, medium, and high settings feel appropriately spaced in terms of the assistance they provide, meaning you can dial in the assist to your needs or what the trail dictates.
The unit works well together. The battery is large compared to the motor’s needs, meaning you’ll get longer rides in (more on that in the range section).
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The combo of the fork and wide tires inspires confidence in different riding scenarios, from surfing across dirt or sand to managing slick rock descents.
They Keystone comes in four frame size options.
The Shimano dislay is fairly minimal, but has all your essentials.
With the Borealis Keystone, you get not one but two apps to use. The bike pairs with Shimano’s E-Tube app. This offers a greater display and the nice ability to customize how much data you get at a glance if you choose to mount it to your handlebar.
The Shimano app has a lot of data, and you can pick the focus of what is displayed from options such as: your heart rate (when using a compatible heart rate monitor), calories burned, cadence, and more.
The second app ties into the drivetrain. SRAM’s AXS app is pretty easy to use. It’s fairly minimal, and there are micro adjustments available for setting it up to your preferences, like assigning the long press button or inverting the shift buttons entirely.
This particular app is the definition of optional—it’s not really a need, but it’s great for those who like control on everything, and it’s worth an install just to keep firmware up to date and an eye on when to charge/swap the batteries.
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For this test, we tested the bike in its max power setting to see the floor value in terms of mileage, and the results stood out in our database.
The Keystone went 38 miles on max power. Only about a dozen other bikes amongst fat tires and eMTBs have topped that result. However, every bike that did beat it had a battery with 14-38% higher capacity than the Keystone’s 630 Wh unit.
This proved to be an impressive result, and it will likely translate to several hours of use regardless of where you’re riding or how much power you’re using.
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One of the highlights of such massive tires, you can ride around in snow, sand, mud and other things average tires struggle with.
The dropper post was nothing fancy, but it worked as it should. What more can you ask for?
The 12-speed cassette has 520% gear range which is nice for finding your own cadence on a fat tire bike.
The Keystone holds its own on hills, as evidenced by our two different hill tests. The first one was done at our designated testing spot of all bikes: Hell Hole Trail. While it’s more eMTB-ish, nature earned it a trip to the Watertower hill climb as well, where we test off-roaders in their more natural environment.
At Hell Hole, the Keystone posted a time right around our current average of all the bikes we’ve tested there. It ascended the hill in a minute and thirty two seconds; the current average time for bikes stands at one minute and twenty nine seconds.
Nothing wrong with hitting the average time for a bike that has the widest tires at 5” that we’ve ever tested. It bears a drag penalty that it largely overcame.
At the water tower it held it’s own too. It was slower than most other bikes, but again, not expected. The point is it didn’t feel burdensome to ride on those tires, but rather, it felt like it floated over the rocks instead.
The Mastadon fork has pre-load and rebound adjustment. It is a nice specialty fork for fat tire use.
The full carbon bar was a nice touch, and the grips are great for a nice soft wide platform to rest your hands on.
There is some seriously chunky tread on the tires, which help tremendously across all sorts of ticky, loose terrain.
We were somewhat surprised to learn.
The Keystone is a different breed. You can enjoy riding on any good hardtail-friendly MTB trail with versatility beyond that that. Think OHV areas, gravely, sandy stuff. Situations where you need a tire to float, not shred.
Borealis lets you pick your tire and wheel size. With mountains of sand here in our local Southern Utah desert, we opted for the widest Terrene Johnny 5 tires and thoroughly enjoyed riding over sandy trails.
The parts picked for the rest of the bike are a cut above most in the fat bike world.
The Manitou Mastadon fork is a special fat tire fork optimized for extreme conditions and use. The 120mm of travel is plenty since the tire volume further softens the ride. You won’t be doing intense downhill on this setup, but if you want enhanced comfort on something that can tackle diverse terrain, it more than checks that box.
The SRAM G2 brakes had great stopping ability and gave us a controlled ride.
The electronic shifting of the SRAM AXS system isn’t necessary for having a fun experience, but it’s undeniably cool and something many mountain bikers will envy. As a bonus, you can shift with abandon on the trails; it’ll guide you through shifting under load to help best preserve your drivetrain.
Overall, the Keystone was a playful adventure bike. It was fun to romp around on, and it handles well in different settings. There was a lot of evidence that it was made by people who know a thing or two about fat bikes.
There wasn’t much to criticize except maybe the lack of accessories available to pair with the bike at purchase. That’s about as first-world as problems get, though, and you can find what you need to add for overlanding, hunting, or other adventures elsewhere online.
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The SRAM brakes provided reliable stopping power
The SRAM GX Eagle drivetrain works well and has a fitting gear range.
Wide-winged grips
The Manitou Mastadon fork works well, and has unique claims about working in weather extremes.
The Keystone is a great example of how fun and capable a trail or XC-oriented fat tire e-bike can be. There was little about the experience that needed changing – a better checkout experience with more accessory options perhaps?
But in terms of the ride it had great control, power, feel, and a high fun factor.
Happy Riding! Make sure to let us know if you have any questions or if you think we left anything out in this review of the Borealis Keystone down in our comments section.
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