Part 2:
Cushioning Technology
for the non- Rocket Scientist
In our last installment of this series we looked at running shoe design and your feet. This time around we're taking a closer look at cushioning.
If you wanted to pick the star, the MVP of a good running shoe, it would have to be the cushioning technology in the midsole. That's why running shoes go for the big bucks. The midsole is that foam stuff that provides the much needed cushion for the runner between your precious feet and the road.
Midsole – Protection and rebound
The midsole provides cushioning and rebound. It helps protect the foot from feeling hard or sharp objects. It's often designed to provide stability against your foot rolling in – called "over-pronation". Finally and most importantly it provides impact absorption for the impact force of 2.5 to 3 times your body weight at each foot strike.
• Protects the foot from feeling hard or sharp objects
• Provides stability against over–pronation with dual densities and contoured foot bed.
• Provides rebound properties for take-off
• Provides cushioning for the impact forces of 2.5 to 3 times body weight at heel strike.
The midsole technology by itself should not be the only criteria when determining which shoes work for you. The amount of miles you can run in a shoe is determined more by your own wear pattern, foot-strike, weight and personal preference. For many runners it's just as often that you change a pair of shoes because the sole is worn, or the shoes are too stinky, than it is that the midsole has failed.
More than you ever wanted to know about midsoles.
There's EVA
There's that PU stuff .
Billions and trillions of running shoes have been made with these two foams, OK, so I exaggerate, but not by much you get the picture. Most running shoes use these two foam like materials foams in some form of the midsole. Each material has its own positive and negative attributes.
EVA is the darling of running shoes. - It is light, it is flexible - and it has a great disposition as a cushioning material. If it were human, you would invite it to your next after race party. But like any good thing, it just doesn't last forever. Over time and repeated impacts EVA tends to compress and lose some of its some rebound or resilience. Runners notice this and say their shoes feel "flat". The technical term would be the foam has taken a "compression set".
The EVA nerd version: EVA is a copolymer or cross-linked foam made of ethylene and vinyl acetate. It is made up of hundreds of thousands of foam cells that contain air or gas. When you land on EVA foam you compress the EVA and the gas gets pushed out and then sucked back in again. Over time these cells don't rebound as well (just like your legs after a long run) and don't take as much air back in. Now un-cross your eyes and read on..
EVA on steroids
To compensate for this issue, a great majority of good running shoes use a compression molded EVA midsole. By compressing EVA in a pressurized mold the midsole forms a thick skin which helps the EVA last longer. It also provides structural integrity to the midsole and allows shoe companies to add lots of neat looking technical looking designs to the side.
What is PU besides a bad smell and how does it work?
PU is like your cousin Vinnie – the strong heavy set one, the one that can work all day and party all night.
PU, short for polyurethane, can be heavier and denser than EVA. Polyurethane acts much like EVA, except it has stronger cell walls that take much longer to take a compression set. PU is excellent foam, but to runners it seems to have less "bounce" or rebound when running. Running shoes made entirely of PU last a long time, but they tend to be too heavy for most runners.
The best of both worlds: PU + EVA
Today's best running shoes are often made with a combination of the two foams, PU and compression molded EVA drawing the benefits from both materials into one midsole. By putting PU into the high stress and high impact area running shoes can be designed for the best of both worlds.
Pick a technology – any Technology:
Many shoe companies use similar foam compounds to make their midsole materials. So similar that many have thought it important to differentiate their product. It is important to understand that these added components or branded formulas are designed to convince you that their cushioning technology is superior. If you ask runners who wear different brands, you will find that just about all well made running shoes have midsoles that will perform well for hundreds of miles.
Next up:
Running Shoe designs – How much stability do you need?