Oct 13, 2026OEM Development & Quality

Foam Density vs Firmness: The Numbers Behind Cushion Comfort

Density and firmness are independent: one catalog ships 29 kg/m3 foam at ILD 35 and ILD 100. The four numbers to put in every foam spec.

Short answer: density and firmness are two independent numbers. One foam catalog proves it: the same 29 kg/m3 foam ships at both ILD 35 and ILD 100 - three times the firmness. For load-bearing cushions, check the support factor: PFA guidance is 2.0 or higher. Send us your rider weight and duty cycle and we will spec the foam.
Cushion sourcing arguments usually go the same way: someone says "high density foam," someone else says "it still feels soft," and nobody opens the datasheet. The datasheet has four numbers that settle it - and they are independent of each other. Here is what each number does.
Key takeaways
• Density and firmness are independent. In one foam catalog, 29 kg/m3 foam ships in both ILD 35 and ILD 100 versions; the softer 35 kg/m3 grade is softer than the firmer 29 kg/m3 one. Density is not firmness.
• Density buys durability; ILD describes feel. Compression set tells you whether the foam recovers: Rogers' PORON 4701-30 is rated at 10% or less compression set at 70 degC (ASTM D3574).
• Load-bearing cushions have a rule: the support factor (65% IFD divided by 25% IFD) should reach 2.0 or higher, per PFA guidance - below that, cushions bottom out under sustained weight.
• Bike seats live in a published band: patent data for bicycle saddle foams shows 25% IFD values around 50-80 lbf - spec outside that band and riders feel it immediately.
• Two numbers, one quote: ask every foam supplier for density and ILD together, plus compression set - a quote with one number is half a specification.
How we verified this
Manufacturer datasheets (The Foam Factory's three grades, Rogers' PORON technical data), industry association guidance (Polyurethane Foam Association on support factor), and patent literature for bicycle-saddle foam values. No in-house lab testing in this article - every figure traces to the References, and each source type is labeled. For the related question of open-cell versus closed-cell chemistry, that article is coming next in this series.
  1. The two numbers, side by side
One catalog, three grades - look at which columns move together, and which do not:
HD36 Regular: 29 kg/m3 density, 25% ILD 35.
Lux HQ: 45 kg/m3 density, 25% ILD 50.
Super Lux: 29 kg/m3 density, 25% ILD 100.
(Figure 1: Foam density and firmness - the independent axes. Chart pending upload; data listed above.)
The reveal is in rows one and three: the same 29 kg/m3 density ships at ILD 35 and ILD 100 - nearly three times the firmness at identical weight per cubic meter. Any conversation that starts "we need high-density foam" and ends there has specified durability and said nothing about comfort.
The working definitions:
• Density (kg/m3) - how much material per volume. Higher density generally means more polymer, slower wear and better long-term support.
• ILD (Indentation Load Deflection) - the force to indent the foam 25% of its thickness. It describes how the foam feels, not how long it lasts.
What it means for importers: write both numbers into every foam line of the BOM. Density alone under-specifies comfort; ILD alone under-specifies life.
  1. What lasts: compression set and the heat problem
A cushion on a parked e-bike can sit at 50-70 degC inside a cargo box in summer sun - that is a compression-set test running in the field. Compression set measures how much thickness a foam loses after being squashed for a long time at temperature; low is good:
• Rogers PORON 4701-30: rated at 10% or less compression set at 70 degC (ASTM D3574) - a performance-grade benchmark for hot compression.
• Bicycle-seat HR foams: commonly 40-60 kg/m3, with published resilience around 59% and compression set around 8% in the patent literature.
Two practical consequences:
  1. Heat is the real test for bike cushions. A foam that passes a room-temperature press test can still flatten after a summer of parked, loaded service. Ask for the compression set figure at temperature (70 degC per ASTM D3574 where available).
  1. Density protects against long-term sag. High ILD foam that is light on density feels great on day one and bottoms out by month twelve - the support factor, next section, is the guardrail.
What it means for importers: for cushions that hold riders in the sun, require a compression-set figure, not just a press of the thumb.
  1. The load-bearing rule: support factor 2.0
Soft foams and firm foams can both fail the same way: the first inch feels fine, then the foam bottoms out and the rider feels the seat base.
The number that predicts this is the support factor - the 65% IFD divided by the 25% IFD of the same sample. It measures how much the foam firms up as it compresses.
PFA guidance: load-bearing cushions should reach a support factor of 2.0 or higher. Below that, the foam carries light loads gracefully and heavy loads not at all.
For bike seats specifically, the patent literature puts typical 25% IFD values at about 50-80 lbf (Figure 2) - and rider-facing cushions should be specced by rider weight and duty, not by the word "firm."
(Figure 2: The bike-seat firmness band - typical 25% IFD 50-80 lbf. Chart pending upload; data listed above.)
What it means for importers: for passenger-duty cushions, add the support factor to the spec sheet, confirm the foam sample lands in the 50-80 lbf ILD band for adult riders, and test with the design load - not the light press of a hand.
  1. The foam spec, in five lines
Five lines make a complete foam spec:
  1. Density and ILD together - both numbers, every grade, in every quote.
  1. Compression set at temperature - 10% or less at 70 degC (ASTM D3574) is the performance benchmark.
  1. Support factor 2.0 or higher for anything load-bearing (PFA guidance).
  1. Rider duty in the brief - rider weight, daily hours, parking conditions; the factory uses these to pick density-and-ILD pairs.
  1. Sample verification - press-test the actual sample to the spec, and record thickness recovery after a loaded week in the sun.
What it means for importers: the foam conversation ends when five lines are on paper. "High-density" in a chat is not one of them.
Limitations
• The three grades cited come from one supplier's catalog; other makers publish different pairs - the independence of density and ILD is the transferable point, not these exact numbers.
• Compression-set standards differ (ASTM D3574 versus ISO 1856 conditions); compare figures only within the same standard and temperature.
• Patent-derived IFD values describe saddle foams historically, not a standard requirement.
• Our in-house foam testing is pending; results will be added when available.
FAQ
Is denser foam always firmer?
No - and one catalog proves it: the same 29 kg/m3 density ships at ILD 35 and ILD 100. Density speaks to durability, ILD to feel. Specify both, every time.
What ILD should a bike passenger cushion use?
Patent data puts typical bicycle-saddle foams around 25% IFD 50-80 lbf. For passenger cushions, stay in that band for adult riders and verify with the design load - and never settle the question by hand-press.
What is support factor and why does it matter?
It is the 65% IFD divided by the 25% IFD - how much the foam stiffens as it compresses. PFA guidance calls for 2.0 or higher for load-bearing use; below that, cushions bottom out under sustained weight.
Why do foam cushions flatten after a summer?
Compression set: foam squashed under load at high temperature loses thickness permanently. A cushion loaded inside a parked cargo box runs that test daily. Ask for compression set at 70 degC (ASTM D3574) - 10% or less is the performance benchmark cited here.
What should I ask a foam supplier for?
Five things: density, ILD, compression set at temperature, support factor, and a sample matching the spec for press-testing. A quote that carries one number is half a specification.
Specifying passenger cushions or seat pads for a cargo bike range? Send us the rider weight, daily hours and parking conditions - our team will match density and ILD pairs, verify compression-set figures, and sample in 7-12 days. See our materials and QC process at colorwaybag.com/bike-materials-and-qc, browse the seat cushion program at colorwaybag.com/products/e-bike-passenger-cushions-backrests, or contact us at colorwaybag.com/contact-us.
References
  1. The Foam Factory - HD36 / Lux HQ / Super Lux foam grades (density and 25% ILD values as published). thefoamfactory.com
  1. Polyurethane Foam Association - support factor guidance (65%/25% IFD ratio, 2.0 threshold for load-bearing). pfa.org
  1. Rogers Corporation - PORON 4701-30 technical data (compression set 10% or less at 70 degC, ASTM D3574). rogerscorp.com
  1. Bicycle saddle foam patent literature - typical HR foam 40-60 kg/m3, resilience and compression-set values, 25% IFD 50-80 lbf (US9670307 / EP1818347). patents.google.com