The rule used to be, start with the shell on, but take it off after warming up and before you start sweating. The rule can now be broken, thanks to the Assos SJ.BlitzFeder Long Sleeve Shell. The fear was that once you started sweating, the shell wouldn't breathe, and your baselayer and jersey would be soaked through. Then once you unzipped the shell, you'd freeze.No longer. This long sleeve shell is made of Assos' new Foil043 fabric. It's feathery light, a claimed 96g for a size medium shell. It's ultra-thin; ball it up and it easily fits in a jersey pocket without sticking out. There's even a stuff sack sewn into the jacket. Assos calls the material a 'natural windbreaker,' which is their way of saying that there's no membrane creating the windproof qualities of the material; the structure of the threads and the density of the weave make it so. A treatment is added to give the fabric water repellency. And, as Assos cares about aesthetics, this fabric was engineered not to rustle in the wind. Assos claims that the value of waterproofing is overrated in many riding conditions. They theorize that if you're going hard enough, or if it's warm enough, a waterproof shell will cause you to overheat and create a nasty, sweaty terrarium inside the jacket because the sweat vapor can't evaporate nearly quickly enough. It's a theory that has many adherents, and we can see where it makes sense for shorter rides with the occasional shower.As with all Assos garments, it's designed to fit closely, and it fits best when in the riding position. There is elastic at the wrists and waist and elastic mesh inserts on the sides of the back and under the arms to allow some stretch.The Assos SJ.BlitzFeder Long Sleeve Shell is translucent White. There are reflective tabs on the shoulders and on either side of the waist. Sizes run from X-Small to TIR.