The word has gotten out by now that a back protector is a great thing when mountain biking precisely because of the increased risk of crashing. Just like a helmet, it offers protection where it is needed most in the worst case scenario. If the piece is going to be worn, then it should be so lightweight that I hardly notice it and I can add it with a quick reach into my EDGE TWENTY backpack ... so what's the real reason to pursue the trails without one? There's not one for me!
Material name by manufacturer: Viscoelastic Soft Foam
