I have been on the PCs almost exclusively this year.  Minus a few groups rides that I rode my non-PC bike, I have built up to about a 5 hour PC ride without much difficulty.  That somewhat changed recently.  I have had a few harder rides in preparation for IM Louisville in a few weeks.  I find that above 90rpm, I really hurt.  Specifically, my anterior tibialis muscle gets fatigued and will not cooperate by recovering.  My solution to continue the ride is to chase back my rpm to the 60-70 range, stand on the hills at 50-60rpm, and give-up pedaling above 90rpm.  As long I stay below my rpm-threshold of ~90, I can ride 'normally' at about 80-85rpm.  I am not a big power producer, so I feel my 54-11 is enough... until I can build my high cadence stamina.  With IM about 3 weeks away, I will work on my higher cadence with respect.  The raw-meat feeling of my a-t muscle is quite interesting. 

I hope to get a lab VO2 test with PCs and one without in the next couple weeks.  Until then, keep on Powercranking!

Matt