Montrail FluidFlex Review

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Montrail FluidFlex Review

Occam’s Razor, a principle used in logic and problem solving, states that the simplest explanation is usually correct. Montrail may have been employing this principle when designing the Montrail FluidFlex ($90) trail/road hybrid running shoe. If you have a material that is simple yet effective, in this case Montrail’s FluidFoam, why muck it up by overbuilding a shoe around it? In response to this question, Montrail designed an elegantly simple hybrid shoe, without any superfluous materials, which highlights the fantastically bouncy and resilient FluidFoam technology.

Montrail FluidFlex

The Montrail FluidFlex


The FluidFlex defines breathability with an open Airmesh upper throughout the full length of the shoe. The Airmesh is reinforced by a durable nylon lining on both sides of the midfoot inside the shoe, and sewn overlays are present only around the toecap. This gives the upper a great deal of flexibility and can give the shoe a sloppy feel to first-time wearers despite the shoe being true to size. Several runs in I found that the upper was more than supportive enough to keep my foot firmly on top of the cushy midsole. Runners looking for a shoe to wear in wet conditions will appreciate the drainage capabilities of the FluidFlex due to the almost entirely mesh upper. My feet were dry within 10 minutes after crossing creeks and even when running in the rain, the shoe does not absorb water or become squishy.

Montrail FluidFlex - lateral upper

The Montrail FluidFlex’s lateral upper.

Asymmetrical lacing provides additional support through the midfoot, and Montrail reinforced the eyelets to add durability. I found that I could lace up the FluidFlex very tight or very loose depending on the type of trail conditions and workout I was running that day. A flexible and minimal heel cup is given extra comfort by the placement of two soft pads on either side of the ankle that I expect did their job as I never experienced any rubbing or discomfort in the heel. The FluidFlex does have a pronounced heel cup which cradles the heel up to the achilles tendon snugly. This design took awhile to get used to, but improves the lockdown of the shoe, especially while running uphill. Again, after several runs I only noticed this when I first slipped the shoe on before running, and I haven’t experienced any discomfort.

The toe box of the FluidFlex is wide enough, but undermined by a bit of a tapered shape which took my feet some time to get used to. Barely noticeable, this taper may place some pressure on runner’s pinky toes, and while I didn’t have any problems I would hope that this design feature is improved in future editions.

Montrail FluidFlex - medial upper

The Montrail FluidFlex’s medial upper.


Segmented FluidFoam runs the entire length of the shoe with a 4mm heel drop. The segmentation increases the flexibility of the already malleable FluidFoam, but is nowhere near as deep as something like a Nike Free. There is almost no torsional rigidity in this shoe (twists very easily) and the FluidFlex really has the feel of a racing flat.

I’m going to do my best to explain the feel of FluidFoam, but remember that this is my subjective opinion. FluidFoam is a very soft, very bouncy, EVA foam whose closest comparison on the market is Nike Lunarlon foam, which is present in Nike’s Lunar shoe series. However, FluidFoam seems to be even lighter, and it is absolutely impressive how much foam Montrail is able to put underfoot while keeping the weight of the FluidFlex at 7.6 oounces (men’s size 9). FluidFoam is also one of the softest foams on the market, which had me worried that it would compress and loose its bounciness. Well, I can resoundingly say that after 400 miles the FluidFoam still feels great and this minimal shoe has held up incredibly well.

It is important to point out that there is no rock plate present in the FluidFlex. Maybe this was done to keep the shoe as flexible as possible, and for the most part a rock plate is not needed due to the substantial cushioning of the FluidFoam. There have only been a few occasions where I felt discomfort from a rock through the FluidFoam and for the most part this design works fine. Also missing (or welcomed depending on the runner) is noticeable arch support. The inside of the FluidFlex is not contoured which means that your feet do more of the work.

Montrail’s Gryptonite outsole pattern is placed on the forefoot and the heel of the shoe leaving the FluidFoam exposed at midfoot. Initially, I thought that I would wear the exposed foam down as a midfoot striker, but it seems to be holding up. The Gryptonite on the forefoot is very tacky and works great when ascending and rock hopping. Additionally, some foreward facing grooves on the heel provide a bit of breaking during descents.

Montrail FluidFlex - outsole

The Montrail FluidFlex’s outsole.

Overall Impression

I have been wearing the FluidFlex for several months now and, initially, I focused on the low weight and wearing the shoe for speed workouts and hill repeats. In the last several months I’ve extended this shoe up to five hours running on mountain trails and it continues to impress me. One of my favorite aspects of the FluidFlex is that I can wear it on the road with the same degree of comfort as my road shoes, and this is the best hybrid shoe on the market in my opinion. For the runner looking for a lightweight shoe to run trails, roads, races, and long runs in, this is your shoe.

Whether the FluidFlex could be a great shoe for races over 50 miles is in the eye of the beholder. Personally, I enjoy wearing the FluidFlex for faster outings on less technical trails when I want to push the pace. The FluidFoam firms up great at a fast cadence. Certainly there is no shortage of cushioning in the FluidFlex and I never finished a run feeling like my feet were beat up or sore. This shoe is a great choice for runners looking for a minimal feel with maximal cushioning.

Tom Caughlan

is iRunFar's Minimalist Gear Editor. Tom’s passion for trail running and specialty running retail experience shine through in all of his highly technical reviews, which do range outside minimalist shoes.

There are 7 comments

  1. David

    Nice review, thanks. How would you rank order the protection factor on technical trails between the FluidFlex, Salomon Sense Ultra and PI Trail N1?

    1. Tom Caughlan

      Great question, and I feel qualified to answer because I run regularly in all three shoes. In order of most protection to least:

      1. PI N1

      2. Sense Ultra

      3. FluidFlex

      This is very close between the Sense Ultra and the FluidFlex, but for technical trails the Sense Ultra provides a more locked down feel and a thin rock plate. While I didn't experience much poke through on the outsole of the Fluid Flex, the very flexible and soft nature of the midsole/ outsole make it a better shoe for mellower trails. When I think of technical trails I think of lots of talus/ broken rock or constant rock hopping.

        1. Ben

          I've been running in the FF since they came out in Feb/March and when I hit rockier trails, I throw in some SOLE Customer Footbeds which add some protection as well as additional structure. Magical!

          Spot on review Tom!

  2. Nathan

    Thanks for the review. Is this shoe a replacement for the Rogue Fly? That is great shoe with a similar upper and outsole.

  3. Tom Caughlan


    The FluidFlex is not a replacement for the Rogue Fly and they have quite a different feel to them. The fluidfoam EVA on the Fluidflex is quite a bit softer and it has a lower drop (4mm) vs the Rogue Fly (10mm). Both transition well to the roads when needed.

    I haven't heard anything about Montrail discontinuing the Rogue series and it is great to have more choices coming from Montrail.

  4. ziel

    good review! here's my 2 cents:

    i've just worn out a pair of these (i' am through the rubber and into the foam) and the uppers still look like new. i don't track miles very well but they've got around 7-800 on them. typically i am hard on uppers as i run in oregon rain, gritty dust and leave shoes laying around baking in the sun way too many hours. serious kudos to montrail on the uppers! they are nailing it with materials and simplicity of design!

    the shoe is designed to be a hybrid trainer and it functions well for that. it has a bit of a "cadillac" feel with it's big airy midsole. fluid foam is an appropriate name. i think of racing flats as light but more stripped down and responsive than these shoes. the rogue fly seems closer to that feel than these.

    the comment about the toebox taper and the pinkie toe is spot on! i've felt this a bit and hope for a little more splay room up front in the future.

    if you run a lot of paved and/or gravel road miles, or even hard packed trails, this is a really nice trainer. there might not be a better hybrid out there though many manufacturers seem to be giving this segment a bit more attention lately. the quest for the shoe that does everything pretty well instead of one thing very well makes my wallet happy.

  5. Mike B.

    Good shoe that could be great (for me) if they changed the toe box. It tapers too much and puts some pressure on the side of my big toe.

  6. Jim H.

    I live and run in Tucson (think lots of vertical and very rocky trails). Recently, I've been running in Rogue Racers, Rogue Flys, Vertical Ks, Helios and the Montrail FluidFlex. I got the Helios and Fluidflex the same week in February and have been alternating running in the two. A few initial observations on the Fluidflex, really good durability – I probably have 500 miles on them, twice what I would get out of a pair of Rogue Racers or Rogue Flys. Cushion is still very good, and the tread is just now starting to show significant wear. I'm amazed at how much more durable the tread has been from the similar forefoot tread on the Rogue Fly/Racer. The Fluidflex uppers are also holding up well, with a few small tears/holes just now starting to develop. I like the Helios, but their cushioning is long gone with similar miles to the Fluidflex. Also, although the Helios has a thin rock plate, I actually feel like I get greater rock push through protection from the foam in the Fluidflexes. Unlike others, I haven't had any problems with the Fluidflex toe box, but I do have moderately narrow feet. With the thin, relatively unsupportive upper of the Fluidflex, I do find myself cranking the laces down tighter than I would on the other shoes, but this hasn't resulted in any pain across the top of my foot, maybe due to the asymmetrical lacing? The other good thing about the Fluidflex? The price!

  7. Dave F

    PERFECT timing on this review, as I have been looking at these shoes for the past few weeks online. I normally run trails in the Brooks Puregrit and did my first 50 miler in those with good results. Looking at a 75 in November and figured maybe I would get something still 4 mm and in the lightweight/'minimal' category but with a bit more cushioning just to be easier on my feet for the added distance. I think I might be sold on these as long as I can find a pair locally to try on to make sure the toebox works for me.

    Thanks for the great review.

    1. Tom Caughlan


      There is a similar feel between the Puregrit (not Purgrit 2) and the Fluidflex on the trail. I will say that the FluidFlex doesn't have that contoured footbed feel of the Puregrit. If you can run 50 in the Puregrit you should be just fine over 100k in the FluidFlex as there is a bit more cushion.

  8. Astroyam

    Liked the feel of these shoes, but i started bleeding around the heel/achilles within 5 miles. Seems I'm the only one though… Foam was very nice.

    1. @schlooie

      Ugh. Sorry to hear that. The heel layout is very different in this shoe. I'm guessing a thicker wool sock is needed. I'm always aware of the weird rubbing in the back, but haven't experienced pain or blistering yet.

  9. Ben Z

    I love these shoes but also wish for a less tapered toe box — starts to be a bother on longer runs. Otherwise these are really the perfect shoe for me during non-technical outings, especially those that involve a mixture of roads and trails. Also kind of impressed by how durable and effective the little lugs are.

  10. rms

    I'm also disappointed at how quickly the midsole broke down in the Helios. OTOH the MT1010v2 lugs peeled off after 3 runs. Still looking for a daily mid-distance shoe; perhaps a PI N1 or N2

  11. Tony

    Put 400 miles on my first pair and just bought a second. A great hybrid shoe. Just finished a Michigan trail marathon on primarily packed sand, clay, mud bogs, along with plenty of roots and ruts (barely any rocks). No issues with hot spots or painful toes as I've had in the past. I plan on using these for a 50 miler next month. I really enjoy the cushion feel of the FF. I hope this doesn't change in the next version. My runs compose of approximately 20% pavement, 40% dirt-road and 40% trail.

  12. Dave J.

    Have you run trails with Hoka One One trail shoes? And how might these compare with the Montrail? I haven't tried either, but am looking for better cushioning on my beat up feet. I run with Kinvara 4s and their trail version, but not quite satisfied for races longer than 20 miles. Thanks for your great review.

    1. Anonymous

      I have both shoes! I use the Hokas for long runs and long races…the Montrail I go for Tempo runs and speed workouts….both shoes have 4 mm of drop and they make you go for midfoot stride. By far the Hokas protect much more….

  13. Anonymous

    I own both the fluid flex and the pi trail n1. Both are excellent shoes with amazingly seemless transitioning from trail to road. I prefer the pi however because they offer a much stiffer midsole that for me is much more conducive to running longer than an hour. These shoes are really quite different from on another because of the contoured footbed feel of the pi verses the complete lack of contour of the ff. Both are great, just have to try them out.

  14. pacer1

    I wear these on trail only. I Love them, and can't give enough praise to Montrail on this design. I have skinny feet and love a wide toe box – Not easily accomplished; the lace design allows to tighten the mid section of the shoe and leave the toes with more room. The lightness and comfort make this an excellent shoe. Not for rocks or mud, but the ultimate lightweight comfortable trail running shoe!

    I now own 3 pairs which all look ridiculous off my feet – the shoe molds to the foot, so the toes on mine now point straight up.

  15. Dave F

    I grabbed a pair of the Fluidflex on Thurs and gave them a break in run this morning. Fairly noncomplicated single track with some uneven sections and sandy sections. I did 15 miles total and so far, am really digging these shoes. Due to the toebox, I went half size up but still feel I can get a good 'locked down' feeling. I also had two stream crossings with water halfway up my lower legs and the Fluid flex drains amazingly well. They didn't retain water at all and in minutes practically felt back to normal. No squishing, no nothing.
    I need something like a four hour plus run in them to really get a sense of them for distance but so far, I give them very high marks (except for the laces – waaaay too long and they are super thin, almost flimsy. Those are getting swapped out asap).

  16. TheWind

    I just bought some Hoka Mafate 3s last week, ran in them three times both knees are swollen, I never have knee problems, but have had some Achilles problems from some 5 fingers…..I hope the Montrails are not like the Hokas, the Hokas put way too much stress on knees with the stiffness of the soles and the movement in the shoe from all the foam…….I was so disappointed with the Hokas, I have a pair for sale if anyone want them, I did some cool Maori graphics on them, so they are not ugly at least! The FF look and sound cool, but I worry about that soft foam now……..any comments?

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