Originally Posted by
zzyzx_xyzzy
There is zero give, therefore the location of the top pulley does not change with chain tension. Therefore it shifts just as well with a minimal length chain as is does with a longer chain. Hence, it is insensitive to chain length.
You may be thinking of what happens if you put on a chain shorter than minimal length, that doesn't fit around big/big. Only an idiot would do that; when considering the impact of chain length on shifting performance it is completely outside consideration.
Also, the cage pivot IS sprung (you know, "single sprung pivot", as opposed to other derailleurs have two sprung pivots). If the pivot were not sprung, it would not be able to take up slack, obviously.
No.
I know exactly what i'm talking about. The rear derailleur has no tolerance for even the slightest incorrect chain length that a shimano derailleurs would've taken up no problem. This is precisely why we didn't warranty a bunch of sram road derailleurs when they were sent back - the derailleur has no give and is thus it is *extremely* important to get the chain length correct -user error. Something that the shimano derailleurs give you more leeway on. It's unfortunate, but too bad.
Go look at the road derailleurs and compare to the shimano ones, where the tab stop is for b-tension.