Last winter, we reviewed the Borealis Flume fat bike. While skeptical of this whole fat bike fad, it only took a few pedal strokes on a snowy trail for us to be completely hooked. Then, after conquering the skinnies on the fattie at Ray's Indoor Mountain Bike Park, we came away thoroughly impressed with the extra confidence a fat bike delivers on challenging terrain, and blown away by its ability to bulldoze obstacles like rocks and logs.
We enjoyed the bike so much, in fact, we arrived at the following belief: for most novice-to-intermediate trail riders (like us), a fat bike will not only extend your riding season through the winter, but the confidence it inspires will possibly make it the best year 'round option too.
Shortly after our review, however, the guys at Borealis threw a wrench in the works: "Would you like to try a set of '29+' wheels with the bike?"
Of course we said yes, and a week later, a humongous box containing two unbelievably large-diameter wheels/tires showed up at our front door.
The Borealis Flume fat bike also accepts 29 x 3" wheels and tires (shown) as well as 27.5 x 3". NOTE: We set the tires up tubeless, but that big-ass seat bag is needed for carrying a spare 29+ tube in the case of a puncture.
For a little bit of background, a typical mountain bike has tires that are between 2.2" and 2.5" wide. Common mountain bike rim sizes include 26", 27.5", and 29".
A fat bike rim is typically 26" in size, but the actual tire measures between a massive 4" and 5" wide.
As you may imagine, a 2" tire will generally be lighter and roll faster on hard surfaces. A 4" tire will be significantly heavier and slower rolling, but provide a LOT more grip on slick surfaces. It will also float atop loose surfaces like sand, mud, and snow...conditions where a mountain bike tire will sink and bog down.
Naturally, given the choice, you'd choose a mountain bike for smooth, dry, and fast trails, and lean toward a fat bike for extreme conditions where you need maximum traction and/or float.
But in reality, trails often fall more toward the middle of this spectrum. This results in situations where a fat bike can seem tedious and slow, while a mountain bike may leave a rider wishing for a bit more grip, cushion, and stability. Examples may include what I like to call "greasy" trail surfaces, trails where you have a loose material (like sand, scree, or loam) sitting on top of a hard surface, as well as trails with lots of technical rock and root sections.
Enter the "+" size tire. Simply put, a "+" tire is 3" wide, a size that splits the difference between traditional mountain bike and fat bike. And for a lot of trails, we've come to believe it's like the Goldilocks of tire sizes...just right.
A 26 x 4", 29 x 3", and 29 x 2.2" tire side by side. The fat bike size is clearly best for snow and mud, and the mountain bike tire is fastest on most dry trails, but the 29+ strikes a perfect balance between grip, float, weight, cushion, and rolling resistance.
"+" tires come in two sizes: 27.5 x 3" and 29 x 3". As a result, the outer diameter of a 27.5+ tire is similar to a standard 29er. A 29+, meanwhile, achieves almost comical visual proportions.
Over the past year, I've witnessed more and more riders (particularly older riders who have been mountain biking for decades) quickly adopting the "+" size...often mounting them to hardtail fat bikes with rigid forks.
We've yet to put any serious saddle time on a 27.5+, but after an entire spring and half a summer on the 29+, it's become our new recommendation for novice-to-intermediate riders looking for stability, comfort, and confidence to tackle tougher terrain. Why?
29+ Rolls Over Practically Anything
The photo (and the iPhone camera's tendency to flatten everything in an image) doesn't do this obstacle justice. But it's deceptively tricky due to its location at the top of a steep incline followed by a sharp left. A mountain bike tire will slip in between the planks if you don't hit your line perfectly, while the wider 29+ footprint will offer a bit of forgiveness.
The Ride is Unbelievably Plush
29+ Tires Stick Like Velcro
Momentum for Days
Confidence to Push Yourself
The Borealis Flume in fat bike mode
The 29+ Flume ready for any condition short of a blizzard or sand storm.
And don't get me wrong, there are compromises to the 29+ wheel standard. Based on some rough measurements, this 29+ wheelset weighs a full 1.6lbs (726 grams) more than my similar-quality mountain bike wheelset. That's nothing, however, compared to the whopping 7.6lb (3.4 kilogram) penalty for going from the mountain bike wheels to the Flume's fat bike wheelset. Regardless, the weight penalty translates to slower climbing and acceleration...which would seriously add up in a race or any other situation where you're trying to keep up with faster riders.
On the other hand, many claim the larger wheel circumference results in slower steering on tight and twisty trails. But in my experience, I can carry more speed through the corners on a 29+, so believe it or not, I've set lots of Strava "PRs" (personal records) with these wheels. In other words, on relatively flat trails with lots of tight corners, I actually go faster on the 29+ thanks to its added grip.
So what about the 27.5+?
"It kind of comes down to personal preference," said Borealis' Logan Gocza. "The 29+ will go over anything and has a ton of traction. The 27.5+ still has a lot of traction but can change direction more quickly and feels more nimble and lively than the 29+. I personally like the 27.5 plus because I ride rocky, techy trails that are more easily tackled on a bike that can change direction more quickly and pick its way through a rock garden. That said though, they both offer more comfort, balance and forgiveness than a standard size (2.2-2.5) tire."
Visit https://www.fatbike.com/ to learn more about Borealis Fat Bikes.
About the author:
Brett Ratner (firstname.lastname@example.org) has been a professional journalist for more than 25 years. He has contributed to dozens of publications, including The Chicago Tribune, The Nashville Tennessean, The Nashville Scene, Guitar 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.