The Science of Flats

In depth look at the 2018 Specialized 2FO

The Science of Flats

Climate control in the shoe can be one of the most difficult projects to undertake. After all a dirty great big British puddle will challenge even the most waterproof of shoes. In many ways trying to keep the water out is not so much an issue rather than trying to get out what is inevitably going to get in.

Hydrology matters aside, grip, comfort and durability are also key to a good flat shoe. How much grip, what kind of shape and density should the sole have? Keeping the shoe locked on but also isolating the rider from impacts as much as possible whilst still maintaining a good degree of feel is a difficult balance which few have succeeded in.

For ten months we’ve been riding different prototypes of the 2FO shoe from Specialized in their search for the ultimate flat pedal shoe. Over the years many brands have advanced the flat pedal shoe, no more so then Five Ten, however nobody has ever ticked all the boxes and made the perfect flat pedal shoe.

There is so much to discuss here, but come on how difficult can it be? We talked to Nate Riffle of Specialized to get into the nitty gritty of a flat pedal shoe.

The all new 2FO

Dirt: Should the flat shoe and pedal be a system?

Nate Riffle: The shoe and pedal need to work together, both need to be adaptive enough to be compatible with many different products (the shoe needs to work with many pedals, the pedal needs to work with many shoes). This “adaptiveness” is also required to allow for the following:

  • Rider preference on foot/pedal placement – forefoot, midfoot, rearfoot
  • Rider preference on toe-in/toe-out rotation
  • Rider’s ability to change depending on different terrain

Is this shoe built for pedalling or riding or drinking lager? And is there a difference?

Ha! Awesome question. These shoes are built for ripping trail. We like to think that they’re comfortable and stylish enough to wear to the bar, but providing the best riding experience was the overriding objective for every decision made in designing these shoes.

Do you have a theory of where the best flat pedal foot placement is? And do you have any evidence?

We think this comes down to rider preference. For the most-part, riders prefer forefoot placement while pedalling since this improves the effective of ankling, and prefer midfoot placement while descending since this reduces the strain on the Achilles and gastrocnemius when the body is absorbing impacts from rough terrain

How do you decide how much surface area of the pedal the shoe takes?

We feel that this is a balance of a few factors but is driven by an effort to provide the largest connection area possible without becoming cumbersome or inhibitive.

How much flex and stiffness is needed in the shoe?

This was something that we looked at very closely for this shoe. When we started the project, we were hearing a lot of feedback from riders that they preferred a shoe that would allow them to “wrap” their foot around the pedal. We also received feedback that this lack of stiffness results arch cramping and foot pain, especially on long descents and rough terrain.

This duality is what led us to develop the midsoles we have for the 2.0 and 1.0. The 2.0 is optimized for those riders who are going big. It has a super-soft EVA behind the rubber to allow the pins to penetrate deep into the shoe for a solid connection. Then it has a 2nd layer of higher-density EVA against the rider’s foot. This prevents excessive flex, protects the rider’s feet from hard riding forces, and gives the rider a solid platform to stand on, providing them more control of the bike through their feet. The 1.0 shoe is designed for lower ride forces and uses only the single-density, super-soft EVA designed for pin engagement, and pedal connection.

The original 2FO flat shoe and the latest to be launched in August. Both have many attributes in common no more so than exceptional durability, comfort and made for purpose.

How do you decide the density of the sole?

We did this through multi-phase, extensive wear-testing, along with pressure data capture and analysis of flat-pedal riding.

We started this entire process by consulting with our tire team, footwear engineers, and rubber chemists to tune our SlipNot compound. We reviewed and set objectives for the rubber (What does the rubber need to do? What is the purpose of the rubber?) and from there developed 8 different rubber compounds to take into testing, each with specific durometers, tensile strengths, tear elongation percentages, tackiness, and abrasion resistance properties.

From there, we also knew we needed to tune the material that supports the rubber – the EVA. Soft rubber doesn’t guarantee great ride quality if it’s support structure is too firm. The rubber needs to be soft for pin penetration, and the EVA behind it needs to be soft to subsequently accept the rubber deformation due to the pins. It’s important that these two materials work together for the best pedal connection. This was tuned by first, analysing rider pressure data to see what types of forces to expect, rate of force increase, rate of force decrease, and impact frequency (think of this like your suspension, EVA durometer is like setting the amount of pressure in the air chamber, rate of return and frequency is like your rebound control). From this analysis, we developed 3 EVA compounds to take into testing.

Once we felt comfortable with these variables, we started with a qualitative test with a few of our most sensitive riders (notably Dylan Dunkerton and Curtis Robinson of the Coastal Crew). We spent a few days doing the same run over and over, swapping out shoes for different constructions on each lap, and gathering feedback for how each rode. The shoes were scored against rubber hardness, pedal feel, pedalling stiffness, pin engagement, and pedal connection over varying terrain (big jumps, small jump, brake bumps, rock garden/root section).

From the qualitative testing, we found there were 3 clear front runners that were very close in performance. These are what we took into quantitative testing and had around 20 different riders start logging hours on the shoes. From this testing, we got to our 1 best construction.

What impact does this have on ride quality and damping?

It has a major impact! Our testing (both through data collection and by gathering rider feedback) showed to us that the properties of the EVA (density, thickness, rebound) is the single most-important component to good flat-pedal ride quality. This totally debunks the industry myth that it is all about the rubber.

One of the first prototypes of the new 2FO was good but was weak in the toe area and carried the same tread pattern as the originals

Is there a difference in sole density between flats and clips and why?

Yes. With the flat shoe, we need to go ultra-soft with the sole for pin engagement. This comes at a cost – increased wear rate. With clip shoes, the cleat is what’s holding you to the pedal, so we can go firmer with the midsole and rubber to increase the durability.

What is the profile between heel and toe and how does this affect rider position and fatigue?

Not sure I fully follow the question, but I think you’re asking about heel drop? We’ve been referring to this as “degrees of hangulation” and an extreme amount of hangulation helps in a few ways specifically with flat pedals

  1. It helps keep you on the pedals – there’s a lot of lateral rearward force being applied to the rider over bumpy terrain. More hangulation means the shoes are oriented more-perpendicularly to this force and are less-likely to skip forward on the pedals.
  2. More hangulation also stretches the gastrocnemius muscles towards their limit of extension. This means the rider can rely on the resting tension of the gastrocnemius and Achilles tendon, as well as the range of motion in the skeletal system, to stabilize their foot, and the muscles won’t need to fire constantly to sustain a specific amount of flexion. The result – less muscle fatigue.

Is it a dry weather or wet weather shoe?

Riders have asked for an all-conditions shoe, that’s what we’ve built. Rubber blends can vary if you’re targeting a purely wet or dry environment, but we’ve asked for the help from our tyre team to build a mixed blend that works well in both. Grip and durability being the focus, we feel we’ve made a compound that will impress in both conditions.

How have you decided on compound and depth of tread pattern?

I wrote a little about compound earlier so won’t double up here. For the tread depth and spacing we started by looking at the pedal. One of our designers (Jon Takao) created an awesome template which overlaid data gathered from these pedals (pin location on the pedal, pin width, pin height). From this, he designed the tread pattern so that there would be a large enough gutter between each of the lugs, enough girth to the lugs so they aren’t fragile and prone to tearing, the right height of the lugs to allow for pin-penetration into the net rubber before contacting the pedal body, and enough variability to the way that he overlay the pedal template to ensure that the rider would have a lot of foot placement options and the lug pattern wouldn’t be overly specific for foot rotation (0d, 3d, 5d, 7d, 9d, etc) and fore-aft placement (mid-foot vs fore-foot)

The new 2FO is all change form the ground up. New compound Slip Not tread, an even stronger upper, and well thought out density to the mid sole to minimise fatigue
Totally re-worked heel area
Designed to take a beating
Less permeable upper, tough laces
The relatively deep mid sole can be seen here and stands this shoe apart from many of its competitors. Not to hard not too soft, but cushioning the rider from hits as much as possible

Where are the pressure points on a flat pedal shoe?

Compared to clip in shoes, where there’s a heavy bias for forefoot pressure, we see more pressure directly where the pedal is placed on the shoe (not surprisingly). What was surprising for us was the force difference between left and right. We theorized that force would be similar between left and right, since a force difference would mean the pedals would turn. This was not the case and we see slightly higher force being applied through the leading foot while descending, and more wear occurring in the rubber of the trailing shoe. Our explanation for this: the leading foot absorbs more of the impact riding over rough terrain, since the rider is using this foot to also prevent them from going over the bars as the bike transmits energy up and back. The trailing shoe then sees more rubber wear because it isn’t as weighted in this scenario, and the pins can scratch at the rubber like a cheese grater when they aren’t forced fully into the rubber.


The pressure profile for Dylan Dunkerton looks like this. This is indicative of what we saw for most of our testers. While the left foot has more force dispersion, the forefoot and big toe measured slightly higher pressures for Dylan who leads with his left foot.


And here’s the bottom of some old 2FOs after having been worn by Curtis Robinson for a few months. Curtis leads with his right foot, and you can see slightly more wear on the left shoe.

If a good flat pedal shoe is about 100mm why is the shoe roughly 100mm front and yet only 80mm rearward of axle?

I’m not sure I fully understand the question here, so let me know if this answers it for you: we came to this new 2FO shoe with a very different approach on tread from our previous model. Our goal with the new shoe was to optimize the lug pattern, and then cover as much of the shoe as possible with these lugs to allow the rider the most foot-placement variance possible. We’ve changed the lug pattern only slightly in the heel and toe area to provide extra off-bike traction when heel striking climbing down something and for toe-off when pushing up an incline.

The early prototypes (left) were similar to the old design and let in a lot of water, whilst the new 2Fo (right) lets in less

What part do socks play?

We were not able to measure any variance due to sock choice. But I could certainly see a loose-fitting sock resulting in poor control of the shoe.

How important was durability?

Durability is paramount. We have added a few features to the shoes to add protection in high-wear areas (particularly in the toe and heel), placed high-abrasion materials where damage from rock strikes or crank rub is most likely, and validated with hundreds of hours of wear-testing that the shoes were holding up to the expectations of riders. We’re confident that the shoes will hold up through the harshest conditions and their integrity and grip.

Are there different insoles available?

There are. These shoes will work with all our aftermarket Body Geometry SL footbeds with varying amounts of arch support, and with a custom molded footbed like our Retül ones.

 What is meant by body geometry in a shoe?

There are 3 “Body Geometry” features in all our shoes (including the 2FO). These features represent a small amount of biomechanical correction that we’ve found is beneficial for most riders. For best results, we recommend riders consult with a Body Geometry Fitter for a prefit assessment, Body Geometry fit, and personalized amount of support.

  1. Forefoot Varus Angulation – the medial side of the forefoot is 1.5mm thicker than the lateral. This angulation helps correct for Valgus forefoot collapse, which eventually works up the chain and results in the knee falling into valgus, which often results in knee pain.
  2. Longitudinal Arch Support – Arch collapse has similar results to forefoot collapse. The collapse works its way up the chain and causes the knee to fall medially. Our shoes ship with our “red” insole which is our lowest amount of arch correction. While many riders will need more arch support, this ensures that all riders can use the shoes out of the box.
  3. Metatarsal Button – This is the little bump in the middle of our insoles. This works to help spread the metatarsals apart and prevent toe numbness that may happen when the shoes are tied too tightly.

What is faster down a hill, flats or clips? Do you have hard facts?

Oh man! That’s the age-old questions! I think you’ll be as hard-pressed to get a definitive answer on this as you would be to get one on wheel and tire size. I think my answer would be: do what you like and what you’re comfortable with and you’ll be plenty fast.

My personal preference: flats


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