Due to the extreme flex and compliant nature of the Free sole technology, the Free RN mimics the experience of barefoot running since the foot is allowed to flex naturally with the ground topography.
According to Nike, the Free line was developed after learning that Stanford athletes were trading barefoot on the golf course.
Therefore the flex of the sole has been designed to specifically mimic the biomechanics of a barefoot running experience.
The result is a shoe that feels natural on the feet and and is a joy to run in. I tested the Free RN on different surfaces, including forest, basic trail, and asphalt.
Nike Free RN General Info
When I first laced them up, my first impression was that the Free RN feels more like a slipper than a running shoe.
In a way, it reminded me of the La Sportiva Vertical K, which feels a bit like a climbing shoe in the way it wraps around your foot.
The Free RN sole is very flexible, in this regard I compared it to a minimalist shoe, such as the Inov-8 Roclite (I run with the 285’s occasionally). However, unlike the 285’s, the Free RN has cushioning along the foot.
The Free RN weights in at 8.55 ounces (men’s size 10), which is lightweight, in-line with minimalist running shoes.
The extreme flexibility of the Free RN derives not only from the flexible sole, but also from the overall uppers and heel design. Unlike many running shoes, the heel of the Free RN is not stiff since there’s no plastic heel cup.
Instead the heel is fabric and foam, with a last running from the top of the heel to the midsole.
The shoe is so flexible that you can even pull the Free RN on like a slipper if the laces are tied. This complete design defines a running experience quite close to barefoot running on grass.
I see the Free RN (and associated Nike Free shoe designs) as an important part of the evolution of the minimalist and barefoot running shoes design from the past decade, which have included notable shoe designs from companies such as Inov-8, Merrell (http://www.merrell.com/en/barefoot/), and the Vibram Five Fingers (https://us.vibram.com/shop/fivefingers/men/) line, where the goal was to offer projection for running over different surfaces, while allowing the feeling of barefoot running.
This often meant reduced cushioning, and required some time for your feet to strengthen, since the minimalist shoes generally offer less support than traditional running shoes. This was largely due to the materials used and the sole design.
In order to mimic the barefoot feeling, sole designs focused on being very thin in order to reduce sole stiffness, thereby allowing better compliance between sole flexing and foot flexing.
Therefore, the high flexibility of the sole came at the expense of minimal cushioning.
Now, this would be ok for soft surfaces, but on hard surfaces it would be more uncomfortable and for me personally, would reduce the distances I could run, even though I very much like the barefoot feeling of running.
Transitioning to a minimalist shoe would also take some time, since a runner would need to build up their foot strength, or risk injury if they went from a high cushioned shoe to a minimalist and decided to run a marathon without allowing time to get used to the minimalist design.
This is where I see the interesting value that the Free RN brings, it combines the natural barefoot feeling with cushioning.
The combination of high sole flexibility with a light weight design and cushioning means you can really use the Free RN as a general running shoe and maintain the barefoot running feeling.
At first I didn’t have a lot of exceptions running on the Free RN on asphalt, as this is where my past experiences with minimalist shoes have come up shot.
However, I’ve been quite impressed by the ability of the Free RN to cushion on asphalt while maintaining freedom of movement.
On the trail the Free RN works well if you’re on a normal trail, but you can almost guarantee that small rocks will get wedged in-between the openings of the sole, and additionally you’ll feel larger rocks as you run on them.
Running over a soft forrest floor is a squishy experience, and there’s basically no ability to stop from sliding given the lack of treads on the sole (however, this was fully expected).
Nike Free RN Sole Unit
The softness of the Free RN sole makes the running experience a bit like walking on soft spring grass or fresh snow, in that you feel the ground below your foot, but it’s very soft and inviting.
Of course, since it is a shoe and not snow, the shoe moves with you while you run. Due to the softness of the sole and the lack of stiff treads, the Free RN is not an ideal shoe for mountains or climbing up steep trails.
You can run with the Free RN on moderate trails, but it’s more suited to flat ground including concrete and asphalt. The softness of the sole will likely not be durable enough for excessive abuse on hard trails.
Of course, due to the high sole flexibility, you really wouldn’t want to use the Free RN on hard trail adventures. Since the sole is made of foam, there is cushioning along the full sole, making it flexible and shock absorbing.
The flexibility of the sole is due to the engineered cutting patterns, which run along the sole in a triangular pattern (Nike calls it auxetic design).
Cutting material is an easy way to build flexibility into a shoe sole, and may often be found in the sole region, but not always in a place where a person can see it easily.
Basically it’s an easy way to engineer different regions of bending stiffnesses using one material, as opposed to trying to build a sole from multiple materials of different stiffnesses (which would also be much more expensive from a production viewpoint).
For example, the Salomon S-LAB X Alp Carbon GTX mountaineering boot (http://xalp.salomon.com/de/) includes a carbon chassis layer in the sole design, which includes cut in the material along the sole, which gives good torsional rigidity along with flexibility along the foot length.
This is also how you can use a laser cutter to make a wooden tie (http://www.thingiverse.com/thing:707915). Also, I’ve seen cuts engineered into Mammut trail shoes in order to localize insole flexibility. The point is, you engineer the desired material flexibility even with a stiff base material.
Now, as compared to those examples, the advantage of the Free RN is that the cutting regions define high flexibility in the foam of the sole.
The triangular pattern means that there is an even distribution in the stiffness reduction (due to the cutting lines) in the three primary directions of the triangular cutting regions, which allows the sole to flex along the natural ways your foot deforms over the ground.
So, overall the sole is a nice balance between price and flexibility. I’m a big fan of flexible soles, especially if they flex comfortably at the ball of my foot.
I have found that if the flex point of a shoe is too far forward, toward the tip of my toes, that it increases the risk of my toe nails being injured and falling off after distances beyond 30 km. So, overall I’ve quite happy with the flex profile of the Free RN and the softness of the sole.