variability of RD chain wrap capacity ratings
 

In the process of setting up my wife's random bike, I noticed that Shimano short cage rear derailers have different chain wrap ratings, despite appearing to be exactly the same dimensions. 6401 (8-speed 600 Ultegra tricolor) is rated with 26 or 28t chain wrap, while 6600 (10-speed Ultegra) is rated with 37t capacity. (I also have a Microshift 10-speed-era Shimano-compatiblee short-cage road rear derailers, officially rated 33-tooth wrap capacity. Appears to have same dimensions as both Shimano derailers listed above.)

The main thing I'm paying attention to with comparing these derailers is the cage length, which is exactly the same. But the angle of the slant parallelogram appears to be the same as well.

Is there some other difference between these derailers, or was Shimano quite conservative in rating the 8-speed and then acknowledged over time that effective capacity was greater?

Note: I have plenty of options for this bike (33t total difference combining front and rear differences) but I'm interested to work out in advance what is likely to work, and then choose based on aesthetics. Here I'm interested in learning the general differences in how chain wrap capacity is calculated or estimated, not in asking what would work with my specific setup.

Largest cog compatibilty vs chain wrap capacity?

Mmmm…my memory may be failing me at age 72, but seems to me the wrap on a 6600 is 37 only on the long cage.
Pretty sure the short cage wrap is the same as the 640x- somewhere in the high 20’s.
Most of these originally came with a 53/39 (14) and maybe a 12/26 (14) = 28. Although I can testify that either will happily shift a 48/34 (14) and 12/30 (18) =32.

I'm specifically curious here about chain wrap capacity, not max cog.

The wrap capacity of 6600 short cage is 29:

https://si.shimano.com/en/pdfs/si/5V...0C-001-ENG.pdf

Thanks! (and to Kontact too) If I accidentally looked up the wrap of the 6600 GS (not SS) that explains my confusion here. High 20s official would make more sense for both, as you say, given the typical setups.

​​​​​​My wife's bike has 42/28 double crank with 11-30 cassette (the Shimano "road" rear derailers of 8/9/10 era usually worked with 30t large cog in practice).

I'll go with a 9-speed-era 105 medium-cage model, which will have more than enough wrap for this setup, unless it doesn't handle the large cog in which case I'll use a medium cage mtb derailer.

The variables with getting extra large cog capacity comes down to the length of the hanger (it varies between frame models), and the strength of the B spring. Often tired Shimano derailleurs would not crank back far enough even with the B screw buried.

Medium cage derailleurs look good and work well. If not the 105, find an XTR.

You have to look at Shimano's ratings with this in mind-
IF for example, a group set's largest cog is 28T, that's what the RDER will be rated at even though it's "probably" good for a couple more.
Another example is largest chain ring. If they were to offer both a 52 & 53T chain ring, the capacity would magically contain that extra Tooth.
Part of it is to make sure part is so parts will work well together.
I think another part is don't buy another cassette in a size we don't sell.

The M900 medium cage is going on MY random bike (with 12-32 cassette, 46/30 crank)