Funny story. In 1922, a Bavarian named Franz Xaver Kugler built a bar at the end of a popular bike trail in Germany. After some 13,000 cyclists rode into town demanding beer (natch), Kugler realized he didn't have nearly enough booze to satisfy the thirsty German cyclists.
Panicking, Kugler improvised by mixing half his inventory of beer with thousands of bottles of clear lemon soda from his cellar. The 50/50 beer-lemon concoction was an instant hit, and Radler (meaning "cyclist" in German) was thus born.