When the mornings are light and warm enough that I don't have to bundle up like Bib the Michelin Man and carry 10 pounds of batteries for retina searing redundant lighting systems, then I put the pannier and handlebar bag equipped boat anchor commuter bike in the garage and ride a road racing bike to work. I use a 15 liter Rapha backpack with waist and sternum straps. The pack is stuffed to capacity with work clothes, street shoes, books, breakfast and lunch. It remains very stable on my back and doesn't shift from side to side on out of the saddle climbs. Shaves several minutes off each direction of my 29 mile r/t commute.