Question: ab exercises you can do on a pull up bar?
Answer: Leg Raises. Take the simple knee raise (where you hang from a bar and raise your knees up towards your upper body) and bring it to the next level with hanging leg raises. ... 
Knee Raises. ... 
Around the World. ... 
Toes to Bar. ... 
The Dragon Flag. ... 
L-hang. ... 
L-hang siders. ... 
Windshield Wipers.