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Dirty Runner: Montrail FluidFlex Trail Shoe Review

Posted on April 29 2013

If you’ve read any of my past reviews you know that I have a certain ideal shoe that I’m searching for.  My perfect trail shoe would be one with a minimal upper that fits like a glove over the forefoot, a wide toe box, excellent drainage, great grip, soft ride, 0-4mm heel/forefoot differential, and enough cushioning to run 100 miles.

Two years ago that shoe seemed impossible to find.  Luckily, it seems like I’m not the only person who is after this style of shoe as more and more shoe companies seem to be coming out with what I call “middle-imal” trail shoe options.
One such company is Montrail, and the introduction of the new Fluid Flex seems to meet every single one of my desires (well, in a shoe at least). I’ve been super excited to try it ever since I first caught wind of it.

Montrail describes the Fluid Flex on their website as follows:  “A 4mm offset, articulated midsole and toothy microlug outsole, the FluidFlex™ guarantees a close-to-ground feel that both flexes with the natural movement of your foot, and protects it at the same time.” They list it as weighing 7.6 oz and having an 11mm forefoot height and 15mm heel height. They also state that it has a “secure fit and close fitting upper.” I will specifically address this shortly. 

The kind folks in Montrail’s PR department had tried to get me an early production pair in November, but the limited run ended up being spoken for and to my disappointment I ended up waiting what seemed to be forever to finally get a pair. As I was pestering them relentlessly, Pete surprised me with a pair 3 or 4 weeks ago (purchased from Running Warehouse). They were a size 11 and I wear a 10.5 in most shoes. When I tried them on they seemed ok, so I ended up putting 57 miles on them before Montrail sent me a pair in my normal size. (Disclosure: the latter pair were provided as a media sample – no charge).

I’m glad I got the 10.5’s. Although the 11’s felt good just putting them on, when I actually tested them there was a lot of slop in the fit, and I kept jamming my toes against the front on long descents. The 10.5s fit much better. With all that said, lets talk about the shoe!
The upper is very minimal, with almost no structure at all.  The only exception is an interesting padded section on the left and right side of the heel cup.  These “bump outs” sit right below my ankle bones and do not produce any discomfort.  With such a minimal heel I would guess they are there to keep your heel locked into the shoe and thereby prevent slippage, which is exactly what they do. I’m pretty sensitive to these type of things and once on the move I can’t even feel them.  And my heels don’t slip, so I would call it a successful design. 
The size 11’s weigh in at 8.25oz and for some reason my size 10.5’s come in at 8.3oz.  But…I just weighed them and they have some dirt and grit on them.  Either way, that’s pretty light for a “middle-imal” shoe.

The toe box is hard for me to describe.  Although it feels roomy, the angle at which it tapers puts a little bit of pressure on my pinky toe.  It has not caused any blisters (I have since put over 50 miles on the size 10.5’s), but the longest I have run in them is only 5 hours. I don’t know if they would cause me problems over the course of 100 miles.
The Fluid Flex foam is incredible – I love the way these shoes feel underfoot. They have a flexible, soft ride that doesn’t beat up my feet like true minimal shoes do. The entire sole is made of the Fluidflex material with high-wear areas covered by small, square, and grippy rubber blocks. Even though they don’t have a rockplate, I have found them to be more than adequate over sharp rocks and roots. And when you have to run on the pavement they are a pleasure to be in. After 50 miles they show almost no wear. The traction has been very good, and I have tested them in everything from mud to rocks to ice and water.  

Speaking of water, these shoes drain incredibly well.  I have completely submerged them many times and within minutes they feel like they are dry.  There is no pooling of water within the shoe at all. Considering that you can actually see through the the upper material, it’s no surprise.


There are a few things that make me hesitant to wear the Montrail FluidFlex for a 100 mile race, although I haven’t ruled it out yet.  First, the minimal upper doesn’t wrap my foot tight enough. The asymmetrical lacing system is very comfortable, but I just can’t get it tight enough to prevent my toes from hitting against the front of the shoe on long descents. I do have a low volume forefoot and have had this problem in many shoes. I don’t think this will be an issue for most people. 

The other issue, which is very easily fixed, is the laces. They are terrible. They’re way too long and do not stay tied. At all. Especially when wet. I have reverted to tucking them into the front and then tying them again to keep them from coming undone.  No biggie. Just replace them. 


So, did Montrail come up with my dream trail shoe?  Close, damn close!  If I could just get a tad wider toebox (or maybe just a slightly different shape, more like the Altras) and get the upper to fit tighter around my forefoot it would be perfect. They feel like running on a cloud. Those of you looking for a more minimal Hoka with a much better upper will be very excited – they have that same running on a cloud feeling without all the bulk and excess squish.

The Montrail Fluid Flex is available for purchase at Running Warehouse, Zappos, and

Nate Sanel is an ultrarunner and author of the Dirty Runner column on Runblogger. You can find more of Nate’s writing on his personal blog, Biker Nate, or follow him on Twitter.

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