The Evolution of Football Helmets, by Jose Diaz
Is the NFL becoming less popular? - Quora
The History of The Football Helmet timeline
Design and Virtual Testing of American Football Helmets–A Review
Past Time leather football helmets
The Football Helmet timeline
Applied Sciences, Free Full-Text
Researchers test old-style leather helmets vs. modern ones and found leather performed as well, better in some aspects. Conclusion: Football helmets need to be softer. : r/nfl