Hi,

as noted in here -
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http://www.bikerumor.com/2012/11/28/hac ... -cassette/" onclick="window.open(this.href);return false;
"stuck a 9-speed cassette on there and was able to use their micro adjust feature on the Di2 to get it shifting perfectly"
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i wonder how that can be achieved? As indicated -

http://sheldonbrown.com/cribsheet-spacing.html" onclick="window.open(this.href);return false;

, the center-to-center spacing is 4.34mm and 3.95mm for 9-speed and 10-speed cassettes respectively. Assuming the

derailleur is perfectly adjusted at the 5th cog on a 9-speed cassette, it would at least be mis-aligned 0.39mm at the closest cogs and 1.56mm at the furthest cogs.

This would be a cool thing to see in person. I'm a little skeptical how "perfectly" this really was shifting.

Using micro adjust doesn't change the fact that the derailleur will only move in 10 speed spacing increments. I would think that at some point on the 9 speed cassette the adjustment would have to be slightly off, even if adjusted to the best possible position.

If his hanger was bent in slightly (unbeknownst to him), it would increase the horizontal movement of the derailleur per cog. That, and the floating upper pulley taking up a little slack, and the 10speed chain having more room between itself and the adjacent cog before making noise...