01A short fall with a fixed bill to pay
Height and mass both scale the energy: double either one and the bill doubles. That arithmetic is not there for precision in joules; it settles scope. The insert has one fixed quantity to absorb, and every millimetre of travel is spent against it. Accept a fixed total and thickness stops being a matter of taste — it becomes a figure you can defend.
Run the same case at 0.9 m and again at 1.5 m. Travel required barely shifts, force required jumps. Cramming comparable energy into less distance always raises force, and the peak follows. An insert that reads calm at the low end sits near its ceiling at the high end. Nothing got worse; the arithmetic moved, and the same foam is now working against a different bill.
02Thickness buys travel, density buys force
Thickness and stroke are not the same variable. A 20–60 mm insert sets a ceiling on available travel; the 8–25 mm figure is what a single impact actually consumes. Thin and stiff means short travel at high reaction force. Thick and soft means long travel at low force — until the last of it runs out and the shell takes the remainder directly.
Across 25–60 kg/m³, each step up in density raises the force needed for the same deflection, while absorption does not improve automatically. Soft foam absorbs by deforming; stiff foam absorbs by resisting, and the peak climbs faster than the absorption curve does. Both routes land inside the 55–85% band, and each pays for its choice in a different column of the record.
03Contact share of 40–70% decides the local number
An egg meets the insert over part of its shell only, typically 40–70%. Spread the same force over a smaller patch and local pressure rises, so the shell cracks at its thinnest point. Peak acceleration describes the whole case. Contact share decides how much of that peak lands on this one shell, and it shifts the moment the egg settles at a different angle.
Raise the share and the peak drops, but the foam now needs relief cut to follow the curve. Deep relief removes effective thickness, and available travel shrinks with it. The useful question is not how neatly the foam hugs the shell, but how much material still stands between shell and wall once the pocket is cut, and whether that layer is thick enough to work.
04When travel ends, the insert behaves like floor
The 8–25 mm stroke is availability, not a promise. In a real impact, leftover velocity often survives until the foam is fully compacted, and past that point the collision turns rigid: reaction force climbs steeply and the peak clears the top of the 40–120 g band. The shell then absorbs everything after that corner, in a fraction of the time.
Rebound count is the fastest check for bottoming out. A clean absorption returns 1–3 bounces, each smaller than the one before. A bottomed sample usually bounces once, weakly, or not at all, because the foam is compacted and the set stays where it was pushed. Few bounces is not good news by itself; pair it with stroke to tell full absorption from no elasticity.
05Read the three together or the verdict inverts
A peak inside 40–120 g hides the stroke entirely; a sample that bottomed out can still post a respectable peak. An absorption figure inside 55–85% flatters that same sample, because compaction absorbs a great deal of energy. Put the three on one line: peak in the lower half of the band, travel not exhausted, absorption high. Drop one and a bad sample looks good.
Three columns go into the record — peak, stroke, absorption — filled per drop. The rule is equally fixed: a peak above the band, a stroke pressed to the 25 mm limit, or a single weak rebound fails the round on its own. Footage covers the fall; release depends on those columns and on how many rechecks were run, not on the clip.
| Foam insert thickness | 20–60 mm, matched to internal clearance |
|---|---|
| Insert density grade | 25–60 kg/m³ |
| Drop release height | 0.9–1.5 m |
| Peak acceleration on foam | 40–120 g |
| Insert compression stroke | 8–25 mm |
| Energy absorption | 55–85% |
| Rebound count | 1–3 bounces |
| Contact area share | 40–70% of shell surface |
FAQ
The egg survived — does that prove the foam is enough?
It proves this peak stayed under what that shell could take once. Shell strength varies widely within a batch, so no range can be guaranteed. Judge the insert on all three columns, not on one intact egg.
Is a thicker insert always better?
Past 60 mm the extra travel often goes unused while internal clearance disappears, letting the egg shift inside the cavity. What matters is the pairing: enough stroke to absorb the fall, low enough force to stay under the band.
Why did the same insert crack a different egg?
Placement moved the contact share from the upper end of the band toward the lower one, which raised local pressure. Check whether the egg can shift in the cavity before blaming the foam.
Can one filmed drop settle the question?
No. The clip records posture and process. The verdict comes from the formal record — peak, compression stroke, absorption, and how many rechecks were run. A video is not a protection rating.
