The Rule of Thumb for Measuring Foam Cushioning Properties in Footwear
To see whether a pearl is real, we rub it against our teeth. To determine if the bathwater is too hot for an infant, we test it with our elbow. Why don’t we use our fingers? Because our fingers are not sensitive enough to feel the scaly bumps on the nacre of a pearl, or to accurately sense temperatures that would be too hot for an infant yet suitable for an adult.
The same is true for sensing the comfort level of foam cushioning. Footwear designers and consumers alike often try to feel whether a foam will be comfortable underfoot by squeezing it between their thumb and forefinger. Pinching foam can be a first evaluation for foam softness, but to understand the cushioning properties of the foam for underfoot uses, you need to dig further.
First, we squeeze too hard with our pinch.
The pressure exerted by an adult male pincher (thumb and forefinger) is typically around 17 psi. A person standing on two feet puts about 3.5 to 4.5 psi of pressure on a hard surface. A person exerts even less pressure on a conformable surface such as a foam. So we tend to over-squeeze the foam.
Second, we don’t pinch long enough.
Generally when we talk about “comfort” in a shoe, or say, furniture, what we really are talking about is a lack of discomfort. Discomfort in these applications comes from cutting off the blood flow in capillaries of soft tissue, which happens at about 2 psi pressure.
For example, try pushing your finger into the back of your other hand. When you remove the pressure your skin is white because there is no blood near the surface. As the blood returns, it re-gains the red of its color.
After about 15-20 minutes of no blood flow, soft tissue cells start to die, sending a signal of discomfort to the brain. As a result, we shift our weight to allow blood to flow there again.
That is why an airline seat might feel really comfortable when we first sit down — we’re taking uncomfortable pressure off our feet. But after about 15-20 minutes, we may start to squirm and shift our weight in our seat. This is also why we tend to shift frequently in our sleep while laying on an uncomfortable mattress.
How do you perform a pinch test correctly? You need to pinch with less than a quarter of our strength and hold that squeeze for 20 minutes. Nobody does that!
So what would be a better test to measure the cushioning properties of foam used in footwear?
At the PORON Comfort Solutions Center, we use a couple of test methods to measure a foam’s cushioning properties:
1.CFD: Compression Force Deflection: CFD measures the amount a foam will deflect as more pressure is put on it. We can use this to characterize different foam materials and their ability to compress at body weights, as well as their ability to bear the load of your body without fully compressing (also known as “bottoming out”) to cause capillary-collapsing high pressures. We take these data points and map them on a CFD curve. For example…
2.Asker C: This is a quick lab test which also measures the push-back force that a foam gives to a weight placed on it. See our video at:
3.Foot testing: When comparing foam samples, we encourage designers to physically test the samples by standing on them. When you do this comparison, stand on each sample on a hard surface and feel them with your feet. Then linger on the materials to give discomfort a chance to sink in.
Also, ask your neighbors to join in on the comparison! You might be surprised by your findings. In a recent blind foot testing study at the PORON Comfort Solutions Center, 91% of testers ranked four very similar materials (some indistinguishable by CFD or Asker C) in exactly the same rank order.
Each of these test methods are far more accurate than a pinch test in gaging the cushioning properties of foams for footwear, and they take a similar amount of time for evaluation.