Reinforced rubber holds up jetliners. It seals power plants. Tires grip highways at 100 mph. For nearly 100 years, this workhorse material has powered modern industry without anyone fully grasping why it performs so well. That changed this week. Engineers at the University of South Florida pinpointed the mechanism: a Poisson’s ratio mismatch between rubber chains and embedded carbon black particles. The discovery, detailed in a Proceedings of the National Academy of Sciences paper published April 13, 2026, unifies decades of rival theories into one clear picture. David Simmons, the senior author and USF engineering professor, put it bluntly. “How is it that we’ve been using this for 80, 90, 100 years and haven’t really known how it works? It’s been through enormous trial and error,” he said in a USF statement . Tire makers buy grades of carbon black—fancy soot—and test endlessly to find what sticks. Rubber starts as long, entangling polymer chains. Stretch it.…