I knew the topic had been disscussed here before in cop with the physics analysis from a lot expert here. But after rewatch MK's FOG at 2003 worlds gala, I just notice that she did a backwards inside to outside change edge spiral at the end. Just a thought about the difficulty of spiral ranked with edge. Start with the least difficult one.
Forward outside edge: since I've seen most lower rank skaters always do this type spiral. And considering the body mass center and lean position, I would say an outside edge spiral need less speed to balance out the center force.
Forward inside edge: since most skaters only got inside edge after they got the outside edge. And considering the body position and its mass center skater needs more speed to balance out the center force.
COE Forward inside to outside: I rank this before the backward one because I don't see too many skaters try the backward one.
COE Backward inside to outside: I only saw MK did in FOG.
I guess the most difficult one should be COE: outside to inside (forward or backward). Because usually skater enters spiral with the most speed and exit spiral with reduce speed. Like I said before an inside spiral need more speed to balance out the center force than the outside one. So it would be more difficult for a good inside edge in second half spiral when your speed is reduced.
What's do you think?
BTW, I think judge a spiral edge position and quality, can't take it from the photo. You have to watch it alive or on the motion. I've seen ppl post the inside edge spiral from MK, SH, IS, all of them got very deep inside edge from the photo. But when I rewatch some of my old tape (2001 worlds) Sarah though has deep inside edge but its edge running was not as smooth as MK's. SH's had some glitch/wobbling in the middle. Not sure how IS's have to go back take a look.