Carbon - So what's it all about?
Posted on April 24 2021
Of course, superficially we all know what it's about, it makes you run faster. Kipchoge broke 2 hours for the marathon in Nike carbon shoes and since then a whole slew of records have fallen prompting the World Athletics to introduce rule limiting the size of the stack height to 40mm and the maximum number of carbon plates to 1 for road events. For track and field events almost all carbon shoes are banned on the track at pretty much every distance.
Clearly there the impact on times is now accepted to be relevant and needing to be controlled.
This hasn't stopped almost every shoe manufacturer coming out with a carbon plated offering and in this article we look at facts behind the trend. Its not the intention to be a scientific review. There are many articles online if you want to dig deeper but we aim just to outline the main benefits (or not) of slipping into a pair of carbon plated beauties.
Fundamentally, we all want to run faster and the idea that just the choice of shoes will improve your time by 4% is a thing. If you are targeting a sub 3 hour marathon and your shoes make the difference between running 3:06 and 2.59 I guess you would be tempted.
From a brief reading of the literature of course its obvious there are two key components, the carbon plate and the foam. What maybe most people don't realise is the majority of the benefit comes from the foam, not the plate.
In one of the pivotal studies on the topic (which seems to have guided a lot of the thinking in Nike) was by Hoogkamer, Kipp and Frank et al entitled "The Biometrics of Competitive Male Runners in Three Marathon Racing Shoes" the study determined the benefit from the shoes was 4%, hence the commercial version of the Nike shoe was called the Vaporfly 4%.
However, other studies have found that only 1% of that benefit comes from the plate, the rest comes from the characteristics of the foam.
Nike use a PEBAX foam which for its volume is extremely light and consequently high stack heights (so lots of potential for energy rebound) can be constructed for very low weight.
All manufacturers keep their foam formulations extremely confidential with only a few details leaking out. New Balance use FreshFoam which is a nitrogen infused foam which reduced the weight also.
Apparently only Saucony use PEBAX in a similar way to Nike. It is extremely difficult to get reliable comparisons to validate the claims being made.
However on foam, let's look at a bit of history.
For decades, running shoes used EVA (ethylene-vinyl acetate) for the midsole which let runners pound out miles relatively pain free. It was thicker in the heel because that’s where most people land first, thus more foam was better, softer.
The material is still used today because it’s versatile, soft, flexible, and, importantly, cheap for manufacturers. The problem: it compacts fairly quickly and loses its cushioning power.
Then came the quest for “energy return.” In spring 2013, adidas moved away from EVA to a midsole made from thermoplastic polyurethane (TPU). Boost was rapidly introduced across the range. Its basically a slab of a bouncy foam made from plastic pellets that were fused together using heat and pressure. The result was a shoe that not only absorbed shock incredibly well but returned that energy with every toe-off (like a spring). The TPU-based midsole also proved more durable and its cushioning powers remained unchanged even on extraordinarily hot and cold days (EVA, by contrast, gets harder in cold weather).
The problem with TPU is that it is heavy but the quest for "energy return" had began.
PEBAX has been around for a long time and its usually made into stiff plate. Mizuno's Waverider, which has been around forever, uses a PEBAX "wave" inserted in the midsole.
PEBAX has a lower density than some thermoplastic alternatives, making it lighter, flexible, more impact-resistant, and capable of returning a whole lot of energy. When blown into a foam, those properties remain, meaning you can compress the foam more and still have it bounce back with more force than other materials we’ve seen used in running shoes.
In the same study referenced above, Nike Vaporfly 4% using PEBAX foam returned 87% of energy, adidas Boost, 76% and Nike Zoom Streak (more traditional EVA) 65%.
Apart from the energy return characteristics, the light weight of the PEBAX foam meant high stack heights could be created with very low weight. This allowed the carbon plate to be curved rather than flat. Studies have also found that a stiff, but curved carbon plate has the effect of flicking the back of your heel up when you roll off the front of the shoe effectively pushing you forward.
This is very pronounced in the HOKA Carbon X for instance.
The stiffness of the plate also means the foot is not actually bending at the toes resulting in more leverage from the calf muscles. This adds to the "flick forward" feeling, providing of course, the users calf muscles are strong enough to do that.
What is also clear is that the combination and execution of the foam and the carbon plate is not the same across all manufacturers. For a start, all foams are different and there doesn't seem to be much published, independent evaluation of the manufactures foams, so it's difficult to really assess the effectiveness of each.
On the use of carbon, some manufacturers are keen to use the carbon inserts to better represent the natural movement of the foot. adidas uses 5 carbon rods inserted into the foam that mimic the structure of the foot for a more natural feel.
So how do you go about choosing a carbon shoe?
First thing to accept is that they (and we) are all different.
Secondly, remember, the genesis of this latest trend was research focused on a carefully controlled specific event with world class athletes. The way they run drove the direction the shoes were developed.
The first, most important question is what is your running form?. Are you a heel striker, mid foot or forefoot striker.?
By their very nature, many of the shoes with high stack heights have very soft foam. If you are a heavy heel striker, some shoes are going to feel very unstable. The Nike Vaporfly, and the NB RC Elite are unlikely to feel stable for heavy heel strikers because the foam on the heel is so soft and provides little support.
These shoes are awesome for mid foot and forefoot strikers and that is where you will really feel the benefit.
As an aside, however you run avoid running downhill in them because inevitably you will heel strike. We have a lot of hills in HK which are hard to avoid, but stick to the flats in your carbon weapons.
If you overpronate, also these same shoes will fall in with you and will not provide very little support. I see so many runners when I am out running, wearing these shoes and rolling in madly. That is not a good long term solution and will create injury.
However, get up on your toes at a faster session and you will feel the pop from and realise why they are so good. I really love the NB RC Elite for my faster track sessions. They are awesome.
If you are a heel striker, fear not the HOKA Carbon X (and X2) could be the shoe for you. The foam is still up there in stack height but is much firmer than in the NB RC Elite. The Carbon X and X2 also have a very rigid plate and an inflection point that is quite a long way back on the shoe and that will certainly give you that heel flick when you run. This can be a bit disconcerting at first
Finally, manufacturers are starting to realise that not all runners are looking for highly tuned shoes that suit only one particular form of running but that these same runners would also like to come to the carbon party. New Balance have introduced the Lerato which is a carbon plated training shoe which does provides a lot more support missing in many other carbon offerings.
So the conclusion is recognise that not all carbon shoes are born alike and its very important to get some good advise to help you choose the most appropriate shoe for you and your running style. That way everybody can be chasing PBs and shaving off maybe more than the famous 4%.
If you would like to check out our Carbon Range you can go HERE
We are always available to give you the best advise we can to keep you running fast and injury free.!