The bikes stay upright because the tires fit snugly between the wooden slats. This DIY bike rack by @hamsterwheelbikerepair is made with just wood and screws! The diagonal pieces hold the bike tires.

Creating your own DIY swiveling bike rack is not only cost-effective but ... supplies including two heavy-duty door hinges, a 2 by 6 wooden plank, and a sturdy bike hook, like these SMARTOLOGY ...