With Quickstep Grenadiers, Castelli set out to make a better seamless arm warmer. Others on the market are made with a nylon/Lycra blend that tends to hold moisture, but they knew they could get better moisture management with a polyester yarn. Working with polyester makes it more difficult to get the right stretch, return and manufacturing consistency, though. It took a year longer than they thought, but we're all glad they stuck with it. These warmers are a pro rider favorite because they're crazy light and they're extremely warm for their weight, yet even in milder conditions you don't overheat. They're so stretchy you hardly feel them on, yet when you take them off they occupy a fraction of the space that normal warmers would take in your pocket.