There seems to be a lack of useful data on foams used for MYOG padding (hip belts, shoulder straps). What ASTM data is available, does not seem to correlate well with my personal experience with the foams I have tried. ASTM test procedures also don’t seem to represent how the foam is actually used. Before I bore anyone to death, here are results and methodology for Minicell T200 from foambymail, FY20 from owfinc and EVA50 from nunatuk (? I may remember incorrectly).
Here is a picture of the test setup. 1″ wide strips of 10mm / 3/8″ thick foam, a 1″ wide block pressing on the foam. Record deflection from full thickness vs time for 8 hours.
Stiffness of the foams at 13 seconds is 48 psi/in for T200, 131 psi/in for FY20, and 57 psi/in for EVA50. As time progresses, things change.
Why 5psi? Seemed a reasonable number based on some rough calculations for carrying 50lb on the hip belt. I understand that’s not ‘backpacking light’, but it’s a realistic peak load for me.
Rationale:
I used the Minicell T200 for a hip belt and found it became uncomfortable fairly quickly in use. So I ordered different foams and found an improved experience, but wanted to know why. There is also quite a difference in cost and availability for the different foams, so it seemed useful to understand things a bit better.
Stiffness (published as pressure to deflect 25%) of the T200 seemed appropriate, and quick & dirty testing showed it matched the provided data. The ASTM recovery test for 25% deflection (I clamped it in a vise with some .25″ spacers for 24 hours) also looked good. Actually, better than the replacement foams, despite the replacements performing better in actual use. This suggested the published ASTM data may not be useful for selecting a foam for use in a backpack hip belt.
Loading in a hip belt is of course not a constant deflection, but a constant pressure extended over time, which the ASTM test data published for the foams don’t seem to cover. And I couldn’t find any data on FY20 at all.
The test results seem to match my in-use experience of the different foams. Use at your own risk.
