Hello marty,

I'm riding on elite level (not as a professional however), with weekly training of about 18-21h
I'm having excactly the same problem. I can climb stairs, walk, press weight etc without problems, but after everything from 5-30mins on a bike, the left knee becomes sore when bending, especially if I wear long tights (and sometimes in 2 layers - denmark isnt excactly warm these days) it seems like there's extra pressure on the knee. Furthermore, it seems to worsen if the knee becomes cold, and if I've been sitting with my knees bend more than a certain angle (the 45 degrees you mention sound spot on) the pain can also appear.

As you describe, the pain isn't intolerable, more like a tightening feeling like you've hit your knee on something (which I surely haven't).
I haven't got any answers as to what or why, but i do have numerous theories for myself:

- being uncareful with saddle height, i made a sudden change in saddle height going from the winter bike to the summer bike.
- excessive mileage did a 6h steady pace by myself when i normally was riding 4h interval in a small group which might just have made that difference in what i could handle
- Cleat position - my kneecaps are rather large and bony + my feet seem to point away from the body, which i dont think my cleat position supported. Therefore my legs tended to make an internal movement towards each other at each pedal stroke.

I've been treating with
Ibumetin (I suppose that's the same as your Ibuprofen
Ice
Stretches and exercises
A topical anti-inflammatory gel


as you can see, this post perhaps isn't much of an answer, rather than a search for answer