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false, they do degrade with age and use. source: https://youtu.be/_nbQsnUvlo4
Hitting old foam with a hammer so that it shears apart is dramatic, but that's not the kind of force that it sees in actual use, and not a scientific test.
Here's a study on old used bicycle helmets which use the same materials: https://pubmed.ncbi.nlm.nih.gov/26902784/
It found no difference in impact attenuation properties.
The point in the video that you are missing is that the material when new, has oils that makes it spongy and more effective. Those oils evaporate with time. The demonstration with the hammer is just to show very casually how brittle the material becomes compared with a new one, and the difference is evident.
The study you linked, as yourself said, is for bicycle helmets. They are not designed to protect you against the same amount of force as a motorcycle helmet.
edit: typo
But they're made of the same material so it shouldn't make a difference. They also didn't hit the foam with a hammer in the study, by the way.
To the point of FortNine's accuracy in the figures, Ryan says himself that he's not aware of a proper study performed on used motorcycle helmets and he has his own personal formula, so... reasoned but not a source of scientific truth.
But they’re made of the same material so it shouldn’t make a difference
They are not designed to protect you against the same amount of force as a motorcycle helmet. That study just proves that expired bicycle helmets are still good for bicycle accidents, not motorcycle accidents. I rest my case.
You rest your case? You haven't presented any real evidence in support of it.
You are free to throw your helmet away every couple years if you want to.