Getting around by bike can be a sweat-provoking task. I wear a backpack; not only my undershirt can get sweaty, but also whatever's atop it. I only own a couple of tallitot k'tanot, and I wash them by hand. I don't want to buy more tallitot k'tanot, or to have to wash them more often, unless there's a good reason to.
During daylight hours, when should a commuter cyclist wear a tallit katan? When shouldn't he?
Please do not answer unless you are citing one or more authentic Torah sources.