The bike (bicycle), once just a mode of transportation, is also a piece of cardio equipment ideal for athletes looking to increase endurance.
Why the Bike: Bicycles are more common than other cardio equipment like rower, SkiErg, treadmill, or air bike, and can also be substituted for any of them if you need or want to modify a WOD. The bike is also a lot less taxing on the body—biking is almost no impact compared to running. While the bike isn’t something you see prescribed in benchmark workouts often, it has been featured in a number of CrossFit Main Site WODs.
Scaling: When you see a WOD that calls for a monostructural exercise (such as rowing, swimming, running, etc.) you can modify the workout and use a bicycle instead—you’ll get a very similar stimulus. For example, if the WOD calls for an 800 meter run but you’ve opted to use your bike, multiply the distance by 3x or 4x to get the same amount of work (source).