Excellent Ab workouts to help tone which you can achieve using a pull up bar, this will also help exercise your obliques and lower abdominals. 
Knee Raises
What is a Knee raises?
A knee raise is a basic pull-up bar abdominal exercises.
How to do it?
- Grasping the bar with an overhand,
- Shoulder-width grip, let your legs hang straight down.
- Keep your upper body still,
- Lift your legs whilst bending your knees and parallel your thighs to the floor,
- Squeeze your abs and hold,
- Lower your legs back down and repeat.
Leg-Hip Raises
What is a leg-hip raise?
A leg raise is basically a progression to the knee raise.
How to do it?
- Let yourself hang from the bar,
- Lift your legs and bend your knees
- Tuck your hips under your body.
- Lift your knees toward your chin and squeeze your abs forcefully,
- Slowly lower your legs and repeat.
What is a oblique raise? An oblique raise is simply in its name, it helps to work your obliques through your lower body. How to do it?
Oblique Raises