Pour-In Foam Analysis
Some Problems with Pour-In Foams
Cost & Time
Pour-in foams are more expensive, messy, and more time-consuming to install than a Crash Pad.
No Adjustment
Once poured, the foam is not adjustable for thickness or changes.
Closed Cell
Closed cell foam compresses air bubbles instead of expelling air and can create rebound that adds G-forces to an impact.
Wear Out
When closed cells pop, the characteristics and capabilities of the foam degrades.
Bag Pressures & Gaps
Pressure pockets in the bag during cure will vary the cell size, air pockets in the bag will leave gaps.
No Test Data
No comparative high-speed impact test dummy data is available for pour-in foams. In addition to this, each pour would test differently yielding inconsistent results.
We initially looked into pour-in foams to solve the spinal compression problem for ourselves, but rejected it after we found out more about it. There are many well intentioned suppliers of pour-in packages but below is a brief summary of why, after our research, we decided against the Pour-in foam idea for our racing and as the best solution to the spinal compression problem.
The compression resistance of a material, at very high speeds (one tenth of a second or less) is the characteristic that reduces impact force. Just like all shocks are different, all materials react different. There are so many variables in a pour, in the field, that there is no certainty what the characteristics of a pour will be. Testing is only relevant to the piece tested.
The Shock Absorbing Material used in the Crash Pad is factory made and is always the same.
Another important factor is that contoured material performed much worse when it was sculpted or molded to match the shape of the test dummy bottom. The thinking is that the preload by the butt on a flat sheet of material creates a uniform supporting force instead of uniform density such as you would get from a molded or contoured fit.
In tests, the preloaded flat sheet performed considerably better than a form shaped material like a contoured cut or a pour-in would be. The flat sheet approach quickly molds to the contours of the butt with more density in the more compressed areas. There are no pressure points due to the uniform resistance of the Shock Absorbing Material.
A pour-in should have similar density across the pour but that is where the problem starts with the pour-in materials. The characteristics of the pour can vary greatly depending on many factors including: uniformity of the mix, duration of the mix time, partial mix/partial pour times, temperature of the mix, humidity, barometric pressure, temperature of the seat, heat sink effect of the seat changing outer parts cure temp more than inner, cure of one area while adjacent is being filled, air pockets, bubble size, cell size, etc.
Preload is very important! - If bottom space is created, somehow, with a pour-in, the material fits the contours of your butt. This makes for thin foam at the parts that protrude most. Tests found that contoured materials lose substantial capability in absorbing impact compared to the same material as a flat sheet. The Shock Absorbing Material flat sheet material preloads to create a uniform supporting force with some areas compressed (preloaded) more than others. Contoured material (their or ours) instead ends up with a uniform density creating thinner spots with less material and less supporting force.
Contoured material does not work as well as flat sheet, in tests, by a substantial margin.
More Problems with Pour-In Foams
Inconsistent Results
Test results can vary significantly due to the inconsistency of conditions at the time of the pour. No certainty that a particular foam pour is good or bad for impact.
Not Portable
Pour-in foams are not always easy to remove. They also may not fit another seat once poured. Crash Pad can easily be adjusted, added, and removed to any seat.
Thickness & Temp. Differences
The foam cures differently in thick and thin areas. The metal of the seat acts as a heat sink to draw heat from only one side of the pour, varying the cure across the thickness.
Bottom Space
The pour does not create space under you. If supports are used to raise you during the pour, then when they are removed, there is no material in those voids - exactly where it is needed the most. And, the supports can distort the shape of your butt for molding.
Weather and Time Changes
Barometric pressure and humidity change the cell size, ambient temperature changes cure rate and rigidity. Mix time, ready time, and pour time will be different for each application.
Mix Consistency Changes the Cure
Cell size varies with mix consistency, and time until poured. Everyone will do things a little different. Power blenders can introduce air bubbles and stir sticks take longer and may not create a uniform blend.
Conclusion
After we researched the foam pour-in material characteristics, uncovered the many installation issues, and had no-good high-speed impact performance data, we rejected the pour-in foam approach for the spinal compression problem. Otherwise, pour-in foams are good for containment, as the body's needs are different in all other directions than vertically down.
The Shock Absorbing Material we use is factory made under well-controlled conditions, is uniform, and within a narrow specification tolerance. It is ALL the same, and ALWAYS the same. The U.S. military could have anything they want to reduce spinal compression injury. We chose what tested best for them.
The pour-in foam is very good to fill the gaps around you in the seat, for a good fit. It works well to distribute sideways forces over a broad area. However, it is an unknown when it comes to compression space needed under your spine and there are too many variables and uncertainties to insure performance.
Some racers have put Crash Pad raw material in the bottom and lower back of their seat and then poured-in for a good fit in the rest of the seat. Others have done a cutout at the bottom of their existing pour-in and used the Shock Absorbing Material for the bottom and lumbar areas.
To reduce spinal compression forces, the flat sheets of Shock Absorbing Material used in the Crash Pad tested best in testing for the U.S. Military. If there was something better, we would use it.