Muscle tightness is indicative of muscle weakness. And I'm not talking about the muscle that's "tight".

The tight muscle is protecting your joint from reaching a range of instability.

All the stretching in the world won't increase your quad flexibility. In fact, stretching pre-ride and during the ride can do you a disservice. The discomfort you feel during stretching is the feedback from your muscle to your brain. It's saying "that's far enough". The farther you go, the louder it talks to you. Do it long enough, and the muscle begins to shut down. This is the lengthening you get with passive stretching.

Ask yourself this: do you want to shut down your muscle when you need it most?

Radical? Yes. This is totally the opposite of what has been drilled into our heads since phys-ed. But it's true.

You need to figure out what your quad is compensating for and concentrate on re-activating it. Your quad will relax accordingly.