As general rule, static stretches before exercise aren't particularly effective, and most studies show that they can be detrimental to muscle performance (the muscle is in a weakened state after a static stretch), and can even increase the risk of injury. Static stretches are what we think of as regular stretches - stretch a muscle and hold for x seconds.
Save static stretches for AFTER your workout.
Do dynamic stretches/warmup before an exercise. Dynamic stretching/warmups are when you slowly take a muscle/joint through a range of motion, gently increasing the range as you progress.
The best dynamic stretch are the ones that most closely mimic the actual activity you'll be doing. So, for biking, that might be the squats, jump squats, lunges, and other leg-pressing motions. Although honestly, as an avid biker myself (70+ miles a week), I usually let the bike itself be the warmup/stretch.