As the name suggests the Columbia Montrail FluidFlex F.K.T. II is a smooth, flexible and comfortable shoe. There are a few minor refinements that could make this shoe incredible, but as it is, it is still a very good shoe. The upper fits comfortably, but what really stands out is the midsole. For such a lightweight shoe, it is very well cushioned, responsive and flexible. Unfortunately, it seems, that this comfort comes with a price and that is a lack of durability. After an average testing of about 50 miles there was noticeable compression of the midsole. The upper and the tread seemed to hold up really well.

The FluidFlex performs really well on dry, hard packed mild to moderately technical trails.  The tread pattern is unique and grips well but is not designed for steep technical trails. It works great as a door to trail shoe as it is equally smooth on the roads. With the large amount of exposed foam on the bottom, the FluidFlex does hold on to mud and small pebbles. While this generally does not affect the performance of the shoe, at times, mud can become stuck to the bottom of the shoe. You can also hear the pebbles stuck in the bottom clicking on asphalt, but this did not detract from the comfort or from any other score.

Unlike most shoes in this test, the FluidFlex is more designed for comfort over speed. They are not mutually exclusive but here it seems that Columbia Montrail chose comfort over speed. That being said since this shoe is fairly lightweight, and a lower drop, it is capable of going fast. The most significant limiter on its speed is the lack of reliable grip.

Comfort

The Columbia Montrail FluidFlex F.K.T. II was designed to to be a comfortable, lightweight shoe. And overall they have succeeded in this. It has a light, breathable upper that is flexible, and fairly secure. The tongue is nicely padded and a nice soft material, but did cause some discomfort based on its length and where it hit against your ankle. Underfoot, the FluidFlex midsole is soft and flexible, yet responsive. The FluidFlex F.K.T. II is enough shoe for long hours out on the trail but also fast enough for shorter distances. Some testers did note that they were already starting to feel the shoe “flatten” as there was already noticeable compression of the midsole. The midsole material is used across several different shoes and has stayed relatively unchanged for a number of years

Speed

The FluidFlex F.K.T. II is an interesting shoe.  When initially put on, it does not look fast and since it is so comfortable your initial thought is that it is just a comfortable, everyday trainer. But, this shoe was one of the fastest we tested. The fact that it is very light (8.9 oz Mens size 9, and 7.3 oz Womens size 7) makes it feel fast, and the low 4mm drop helps with a quicker turn over. One reason why this is not considered a race day type of shoe is the fact that it is so soft. There is some energy lost in the cushion that other faster shoes do not have. It doesn’t have the raw speed characteristics that the race day shoes have, but would be ideal for longer distances from the half marathon up to 50k.

Security of Fit

Security of Fit is really where this shoe shined. With its very breathable, lightweight upper, it has well placed overlays that hold the midfoot very securely.  The heel collar is well padded and securely holds your heel in place. Even on steep descents, the secure fit inspires confidence. The FluidFlex F.K.T. II also runs true to size. Since it is a fairly snug fit, if you have wider feet or prefer a wider toe box, this shoe may not fit you well.

Agility

When it comes to agility, the FluidFlex F.K.T II struggles. The main component that is lacking is the outsole. The small lugs, shallow depth (4mm) and a lot of exposed midsole foam do not make for a shoe that can really carve it up on the trails. This shoe was not designed to be a technical mountain trail shoe and is really at home on gravel paths and buttery single track. The soft midsole that makes the shoe so comfortable really detracts from its ability to cut around switchbacks.  But, the lightweight and low drop make it nimble enough that you can still bounce around rocks and roots on the trail.

Responsiveness

This category really had our testers split. Some felt that the FluidFlex F.K.T II was very responsive while others felt just the opposite. The ones that felt it was responsive attributed it to a smooth transition and good toe off. While the ones who felt it was not very responsive attributed it to the softness and lack of energy return from that softness. The main problem here is that the midsole may be too flexible. Depending on your foot strike, a very flexible shoe can feel too flat if you land more on your heel, or it can help get you up on your toes if you already land on your midfoot.

Protection

As with the previous categories, the protection of the FluidFlex F.K.T. II is adequate. Since this shoe is mainly designed for nontechnical trails, there is no rock plate or a lot of attention paid to protection (however Columbia Montrail does say that there is a trail shield in the forefoot, but was not noticeable). The midsole is firm enough that it stops minor rocks from getting through, and the toe bumper protects against minor stubs. With the thin upper there is little lateral protection from side hits.

Out testing spans the course of several months. Our testers spend a few weeks running at least 50 miles (typically much more) in each shoe. They live all over the country so each shoe can be put through our testing protocol in a variety of terrains and conditions. Each tester is encouraged—to the best of their ability—to test the shoes in as many different terrain environments as possible: in the mountains, on dirt roads, buffed single track, technical terrain, etc. In addition to varying terrain, our testers have to test each shoe at different efforts including a long run, easy running and fast running. As mentioned above we test for six different criteria: comfort, speed, security of fit, agility, responsiveness and protection.