Are abdominal crunches bad for your lower back?You have to work on your core strength.
If you want to do abs you have to do lower back as well.
Do the plank or bodybridge whichever one you want to call it. Once your body is straight, concentrate on pulling your bellybutton up towards the sky, by sucking it in.
A very good tip is to put your feet flat on a wall with knees bent (not quite 90degrees) for crunches.
Its also important to make sure your hamstrings are strong and flexible, as this is proven to contribute to lower back pain.
Another exercise is to lie flat on your back place your hands under your backside this eases strain on your lower back, keeping your legs straight, as you progress and your core gets stronger you will be able to remove your hands from under your back and rest them on the floor beside you.