45NRTH’s Flow & Dunder ♦ These Beisties Bite!

Dunderbeist on the left, Flowbeist on the right

Beasties In the Wild

This is a short-term/initial thoughts review. I have only gotten about a month of time on these tires but luckily it was long enough to get in rides with a wide enough range of conditions from mid-winter to spring snow. This was more than enough time to get a good feel for them. If you are curious about weights, dimensions and other info, Gomez wrote up a great intro post which is available here.

Just like Gomez, the Flowbeist and Dunderbeist (henceforth known collectively as “The Beasties”) easily set up tubeless for me. I am using Surly Daryl rims and super wide “stans style” strapping tape for my tubeless conversion. That means I don’t have the benefit of a true tubeless rim shape and tires tend to be a bit looser fitting which can complicate tire installation. Even under these challenging rim conditions I had no issues setting them up. I am so stoked that tubeless ready fat tires are now available.

Before getting into specifics about the Beasties, I think it makes sense to explain my impressions of the “optimized for groomed singletrack” marketing that 45NRTH has used with these tires. Generally, with mid-winter snow conditions on groomed trails you have a firm and compacted base but the top layer of the snow is a bit softer. Either the top layer is soft because it is fresher and hasn’t been compacted as much as the base, or it has been loosened up by use. This is true for official groomed fat bike trail as well as the “snowmobile singletrack” that a lot of riders enjoy all winter. The Beasties have tall, widely spaced knobs that punch through the loose top layer and find purchase on the firmer snow underneath. The combination of the knob height and spacing, in tandem with the medium sized casing width gives you just enough float to roll well when groomed conditions are softer, but not so much girth as to be slow rolling. As I will explain later on, the tires are great in a lot of conditions but 45NRTH has really seemed to hit the sweet spot of optimal tire characteristics for groomed trails.

Wonder Dunder

The Wonder Dunder:

The Dunderbeist is all about drive traction. If it can possibly dig down and find something to hang onto, it will. For instance, it does well punching through a bit of fresh snow to the old hard pack snow below. In spring slush it will dig down to something solid below. On groomed trail it will sink through top layer of churn left over from other trail users and grip on the hard underlayer. The flip side of all this is that it will also saw through weak crust and dig a hole faster than any tire I have ever been on.


In slippery spring slush the paddle wheel type tread is great but it can exhibit some side slip if there is camber or ruts/ridges to the surface. This characteristic isn’t any worse than other tires but it does come to light due to how good the drive traction is. I found myself rototilling spring snow while going up hills that others couldn’t ride, only to be slowed or stopped when my rear end kicked sideways on off camber and rutted sections. I didn’t notice this characteristic in colder snow.


There is little to no ramping of the knobs which increases rolling resistance, but ups the traction. This wide spacing and lack of ramps does produce a lot of vibration when riding on hard pack conditions or the road. The great knob separation and smooth rubber between knobs helped the tire to shed snow incredibly well and even in really wet snow I ran into no cases where the tires packed up. The braking traction seemed to be excellent.

One thing of note is the directionality of the tire. The directional arrow indicates it should run in a lower rolling resistance direction but it looks like it would have even better traction running the opposite way. The recommended direction has the skinny side of the trapezoidal shaped knobs forward instead of the big backside of the knob. Because of my limited time on the tire I never turned it around and rode it but it looks like it may be the velcro tire of choice run backwards if you knew you were going to be in soft conditions and didn’t care about rolling resistance.

While this tire is great on any bike, if I had a “ skinny” rear ended, 170mm spaced fat bike that couldn’t handle much more than a 4” tire, I would definitely look into squeezing this tire into the frame. Hands down, this tire has the best drive traction of any tire out there except for the Lou in certain conditions that require more float.

Going with the Flow:

The Flow is a is a very different beast (get it?) than the Wonder Dunder. To start with, they have very different profiles with the Dunder (shown on left) having a very rounded shape while the Flow has a denser pattern of knobs in the middle, then a lot of widely spaced knob in the intermediate zone and aggressive, tall side knobs rimming the sides of the tire.

Dunderbeist on the left, Flowbeist on the right
Dunderbeist on the left, Flowbeist on the right

Sometimes when a tire goes from densely packed tread in the middle to very spaced out knobs on the sides you can get weird handling characteristics. This can take the form of squirminess or side slip when the bike is more upright on the snow and then the traction kicks in when you get the bike leaned over. These tires didn’t seem to have that issue at all. In fact, this was the most “normal” feeling fat tire I have ever been on when initiating a corner. It showed no signs of self steer or other weird handlebar inputs even when run at stupid low pressures (3-4psi). You just lean it over and rail.


The knobs are heavily ramped on one side to reduce rolling resistance while leaving nice vertical faces to assist in braking traction. Unlike the Wonder Dunder, the front tire rolls fairly smoothly on hardpack conditions.


All of the knobs except for the furthermost side knobs have sipes to create more biting edges. When combined with the tall, stiff side knobs you get a tire that inspires a lot of confidence when railing corners.

The cornering traction of the Flow seems to surpass any tire I have ridden. Braking traction also seems very good. It is not going to work miracles for you if the conditions are really slippery, but it is going to provide as much cornering traction as possible.

Overall Impressions:

The Beasties are a super fun set of tires. They have as much or more traction as any other tire available but 45NRTH has also minimized the negatives such as slow rolling and heft that generally come with great traction. The Beasties allow you to be more playful on trails, letting you throw the bike around more and pushing the envelope just a bit further.

You may not want to use them for racing because there are faster rolling and lighter tires out there. You also may not want to pick them for big adventures in the wilderness where you may need more float. However, if you are looking to maximize your fun on snowmobile or groomed trails, these will produce the biggest grins of any tire I have ridden.

KBS Beastie Rating: 5 out of 5 Pints!



  1. Those look like a tire for 4 seasons. With those I wouldn’t have to go through the tubeless shuffle of changing tires.

    Ride groomed did them justice, showing they can actually be performance ridden with a shit eatin’ grin!

  2. Great write up thanks!!! What are your thoughts be on running to Flow front and back for a little more speed and weight savings if the ultimate in drive traction is not needed? Maybe even reverse the Flow on the back if needed?

  3. I’ve been running these tires since last fall on my Rock Mountain 50 and I have to admit they’re the best fat tires I’ve ever used. They’re so good, I use them year round ….. and they’re really durable as well!

  4. If you had to choose, Bud and Lou tubed or the Beist tubless? Im stuck with bud/lou that dont like tubless at all (below 6psi sealant blows out the bead on the smallest bumps). I dont wan tto run tubes over the winter.

  5. It depends a lot on where you ride, the kind of trails you ride and how much you weigh. If I was worried about the possibility of pinch flats due to rocks and roots buried in the snow or was riding where thorns or dunegrass were an issue, tubeless would be a no brainer. At my weight (about 200lbs in winter gear), if I rode more groomed trail than ungroomed in a high snow area and I wanted to be able to be competitive with fast guys I would also probably go with the Beasties and tubeless. If I rode a lot of snowmobile trails or fresh snow and needed as much float as possible, I would probably go with the Bud/Lou and tubes. If you are lighter than me, the beasties may offer plenty of float for all round winter riding.

    Sorry for the lack of black/white answer. Hopefully what I wrote helps.

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