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)

Popping back over to this thread to wonder about pushing the chain wrap on Shimano 8/9/10-speed road rear derailers. Rated 29t wrap.
My cousin has 53/39 chainrings and 12-28 cassette (30t total wrap, although I don't know whether he ever cross-chains). But he wants a higher max gear. Easy way to do that is 11-28 cassette (nobody makes 11-25, 26, or 27 in 8-speed)
But that would put his total wrap at 31. I've never tried to push this, as I've stuck with triples and later with "mtb" derailers for larger max cog, plus I spin more and don't need a big top gear. My cousin lives in a really flat area and finds a 53-12 not-big-enough.

Just to be clear, we were talking about large cog capacity, not total "wrap capacity" that you're bringing up. They are somewhat unrelated - you could have a derailleur that could take a really large cog but had very little capacity - or vice versa.

In the case of using a short cage derailleur over capacity, the only thing you can do is set up the chain on the big-big method, and accept that the chain will be loose in the small-small or the smallest two cogs. This is really not a big deal - you are completely unlikely to ride in small-small, and even if you do shift into that gear nothing bad will happen. I've had multiple MTBs set up like that.

Just don't go the other way and cut the chain based on small-small, or you'll snap the hanger if you shift into big-big.

two factors not mentioned... roller Size and upper roller position in relation to the cage pivot/spring.

see: Shimano MegaRange Designs.
they are compact, offer superior ground clearance for 650b conversions, or low profile tires on 26" rims, and work brilliantly, for 1/3 the cost, or less.
https://www.jensonusa.com/Shimano-Al...n=&pt_keyword=

Shimano is known to be conservative, so 2 teeth over is likely to not be a problem.

I differ in approach to Sir Kontact in that when pushing limits I like to do small/small sizing such the the chain is very close to or slightly rubbing on the derailer. This gives the max possible chain length

Of course then you have to make sure big/big works and if it doesn't then my personal approach is to say I am pushing the limit too much.... not right or wrong, but how I approach it.

repeating big/big has to work as it is pretty easy to get into that situation

OP: For short-cage RD, you may need to remove two chain links for an 11T on the small chainring, if the chain is rubbing against itself at the RD or not taking up slack. Just make sure first that when on the big/big combo, the RD is not wound forward beyond its ability. If that doesn't work, go for a medium (GS) length RD (and a longer chain). My cheap Shimano one is 43 capacity and 34T max cog size, works like a champ on 50/34 x 11-30, so 35 wrap, easy.

Here's a chart listing chain wrap and capacity for many different derailleurs. Bicycle Cassettes & Drivetrains - Google Sheets

Pushing the wrap capacity by two teeth is fine if you know what you're doing, I've done it a bunch of times. Shimano specs are conservative, as mentioned.

Just for clarity, TallRider is talking about exceeding the wrap capacity of the derailleur, which means that if you start with some tension at on small/small you will break the bike when you shift into big/big. Now, if the capacity is 34t and you very carefully try 36t - it might work. But if you know you are at or just beyond capacity, big/big is going to be exactly the same chain length as small/small.

If you are under capacity, I use small/small rather than the big/big recommended in current manuals - for the same reason you probably do: Long chains last longer, and long chains are less likely to break anything if you slip the wrong wheel in there with a bigger cassette.


When you are way past capacity, big/big is just going to minimize the combinations that leave the chain dangling.

I like to use the longest chain I can on small/small, as then the cage is not wound forward to near its limits on big/big, I think it's easier on the cage spring. Unless, I need the cage swung more forward for adequate ground clearance with small wheels; A blogger online had to do that with a long cage on a Bike Friday with a wide triple.

when considering removing slack from a chain in small/small, i always shift to big/big, then see if i can fold a link back over a link, without binding... it's an easy way to check for too-tight chain in big/big.

and i always set up chains with the Small/Small method... single ring setups then allow some tightening/link removal, but a loose chain is a lower drag, smoother running chain.

A 53/12 at 80 cadence is almost 30 miles an hour. On the flat? Is your cousin’s first name Tadej?

Arguably, at least from my vantage point, the finest RD of the era; road or mtb.

RD-7700 GS users may differ, but the M900’s ability to push the specs with DA performance tilts the scale for me.

John