9 speed shifters - 10 speed cassette - HELP!

Please forgive me if this topic has been answered many times before. I figured it would have been when the shift (pun intended ) came from 9 speed to ten speed. I tried searching but couldn't find the exact answer to my questions.

My situation:
two bikes:
1) dura-ace 10 speed (ultegra 10 cassette) - wipperman connex chain
2) full dura-ace 9 speed - 9 spd shimano chain? (came with the bike)

I have built a Powertap (PT) hub into bike #1 which I would like to be able to swap between both bikes. I can afford a 2nd bike harness for the PT but I can't afford a 2nd PT hub and I can't afford to upgrade the shifters and deraileurs on bike #2 to 10 speed. So my questions are...

a) can I just flip the wheel over without changing the cassette - in other words, what happens if I put a 10 spd cassette in with 9 spd deraileurs and shifters (can I use 9 of the 10 gears or are they completely incompatible)?
b) if that won't work, can I use the 9 spd cassette with the 10 spd shifters/deraileurs? I'd lose two gears but what happens if I shift too far into that 10th gear that isn't there?
c) is the 9 spd cassette even compatible with the free hub of the powertap wheel? I'd rather not have to switch the casette every time I swap the wheel between bikes but is that my only option? Is that even a viable option?

Any advice would be much appreciated!

Pad

 

Neither will work.

In terms of travel from one end of the cassette to the other, both 9 and 10 speed systems move the same distance. It's the cog spacing that is different.

You might find one gear that just happens to work without making a hell of a racket, but even then, none of the others will line up.

Yes, your 9 speed cassette will/should fit. The only exception is if it has a Shimano Dura-Ace freehub body, which has deep aluminium splines. But this is only an issue on Shimano-brand Dura Ace hubs and wheelsets.

You may also run into issues with the 10-speed chain being too thin for 9-speed cogs/9-speed chain being too wide for 10-speed cogs. Never tried doing either, but technically it may be a problem.

 

Would you consider getting a chainwhip and lockring tool? You could switch the cassettes over yourself. Or are you looking for something more convenient. If you are, shimano have done the inconveniece thing again. They tend to make all their high end stuff completely incompatible with anything else. Their centrelock hubs only work with centrelock rotors. Their 10 speed chains only work with 10 speed cassettes. Their 10 speed shifters only work with 10 speed systems. Pure shimano. Sorry.

 

Take the suggestion to get a chainwhip and cassette lockring tool--less than $25 total. Switch cassettes as needed, as this takes about 3-4 minutes or less, then there will be not any issues with component incompatibility.