Your long-distance racer, improved. A softer, lighter foam is coupled with the same durable rubberized foam outsole to give the runner a smoother and more pleasant ride. The upper is tweaked for a better lockdown and extra breathability while re-tooling the heel collar to take pressure off the Achilles while also locking the heel in well. Durable enough to train in and fast enough to race in, the Carbon X 2 is a welcome improvement on the already well-received version 1!