I like hanging leg raises, but they may not be the best fix if your hip tightness is caused by over use from cycling, planks should certainly help though, I would also look into single leg glute bridges or something like that. Working on getting into, and holding a very, very deep squat could also prove useful.
Also, here's a test to look at the ankle issue that people have brought up, I guess take this into consideration along with the hip test I wrote about earlier.
Stand barefoot facing a wall, your feet parallel, shoulder width apart, toes pointing forward.
Your toes should be as far from the wall as the length of your hand from palm to tip of middle finger.
Bend forward at the ankles until your knees hit the wall. Do your heels raise?
If so, stand just slightly closer to the wall and repeat the exercise until you can touch the walls with flat heels, how far forward do you have to move?
If you can touch the wall from the first position, try this to add the hips into the mix, with your knees touching the wall, slowly lower your hips until your heels begin to rise. How far can you make it down until you're in a heels up position?
If you can touch the wall with your knees, heels down from the first position, you should have more than enough ROM in the ankles to squat to any depth, and you're back to the hips again.
I know this stuff is just very general, but it should give you a decent way to look at the different systems involved in the squat, you'll just have to do some research on how clear up whatever issues you might find.