kenshireen

earn.performancebike.com/bikes/advice/buyers-guides/bike-parts-and-components/understanding-rear-bike-derailleurs

Based upon this article you need to be concerned with 2 factors when acquiring a RDR.
I have a Shimano 6400 and from what I have read the max cog that it is rated for is a 28.
I have never come across anything dealing with "Capacity" on this RDR.

I have 53/39 and 12/28... according to this article I would be outside the limits since I believe the 6400 is a short RDR.

I am somewhat confused....

79pmooney

If it works and you can get onto the big cog - big chainring combo without straining the derailleur, you're good. The manufacturer's ratings are suggested limits. Riders have been pushing them as long as derailleurs have been around. That said, I used to push the Simplex derailleurs on my Peugeot UO-8, boosting the 26 tooth FW to a 28 when even the 14-26 was a pushlot with the 52-36 in front. Being not very careful about setup and shifting, I dumped the chain into the spokes a few times and mangled the dropout.

Understanding Rear Bike Derailleurs - Performance Bike Learning Center

Ben

Andrew R Stewart 

The wrap capacity id assuming that all cog combos are in use and no chain droop is allowed. But people have been setting up drivetrains with chain droop when in the small ring and small cog combo for decades and just avoiding this combo when riding. It's best to slowly hand shift the chain the first time you run through the gears in case you messed up something and haven't realized it yet Andy.

kenshireen

I am still not clear what is meant by capacity. Sheldon has indicated that 28 is the largest cog that a shimano 6400 RD can handle.... He didn't mention capacity....
Also, what is chain droop. I use the small/small and other than the cross chaining it works fine for me.

Bill Kapaun

VeloBase.com - Component: Shimano RD-6400, 600 Ultegra

Your wrap capacity is listed as 26T.
(Largest Ring- smallest ring) + (largest cog - smallest cog)
In your case-
(53-39) + (28-12)
14 + 16 = 30T wrap.

Andrew R Stewart 

Sorry but I thought that yo did have an understanding of the term. Capacity has two meanings WRT rear ders.

First is the cog diameter (tooth count) which the der pulley wheels can get under. So a 28T max capacity, in this case, means that with the der hanger of the spec that the der brand chooses
the upper cage pulley wheel will clear the underside of that 28T cog.

The wrap capacity is the amount of chain length (in teeth) that the der/cage design (independent on the hanger) can play out and take in so that when the chain is in the big/big combo there is enough chain to fit but not so much so when in the small/small there is chain droop. So an 11-28T cassette has 17 tooth of range and a 34/50 crank ring pair has a 16 tooth for a total chain wrap capacity of 33 teeth.

Chain droop is when the der cage can't fully take in the chain so the chain hangs down, or droops. A der cage tension spring that is broken/weak could do this with an otherwise properly speced out system. Or a chain that is too long could do this. Usually seen when in the small/small combo. Andy.

kenshireen

Andy, thanks for the explanation...But my 6400 rd is rated at 28T max. This is the largest cog that it can handle on the rear... Is there another RD rating for capacity....

I found the link that shows that my max wrap should be 26T.....but I have 30T right now based upon 39/53 and 12/28... Shouldn't I see an issue somewhere

kenshireen

I follow the link and see the number... but I currently have a 39/53 and 12/28. That is 30T so I am beyond the 26T... But how come everything shifts fine.... Shouldn't this be causing me an issue somewhere

Bill Kapaun

The chain will sag when on the smallest ring and 2-3 smallest cogs.

Bike tinker man

Shimano sizes quoted are conservative, if they say 28 you can normally get 30 etc, I've two bikes using beyond Shimano limits no problem, typically I've a 36 large cog cassete with a Deore LX RD-M581 which Shimano states 34 largest spocket. Longer arm RD are a better choice, if you use enough chain to run big x big cogs then on small x small the RD may not have enough tension & be wrapped up behind the cassette.

Kopsis

The wrap calculation assumes worst-case cross-chaining. You can re-write it as (big ring + big cog) - (small ring + small cog) and it's a little more obvious. In your case, if you don't run the two extreme gears, you end up with (53+26)-(39+13)=27T. If you set chain length to allow the big-big, then as was already mentioned, you'll have some sag running the small ring out at the small end of the cassette. If you rarely use those gear combinations, you won't see an issue.

Bike Gremlin

Der. capacity explained:

Rear derailleur


Chain sizing and going around exceeded chain wrap capacity:

Chain length sizing for bicycles with derailleurs

Bill Kapaun

You're using a 13-26 cassette, whereas the OP has a 12-28.
Thus you come up 3T short.

Kopsis

Yes, I was trying to illustrate that if you don't cross-chain you can exclude the 53/28 and 39/12 ratios and the effective wrap is what you would get from running a narrower range cassette. That's why, people can run 28T capacity RDs on a setup that should need 30T capacity without any shifting problems. It would probably be even more realistic (since most people will size the chain to handle big-big even if they don't use it) to knock off two ratios from the small-small end. That would give effective wrap of (53+28)-(39+14)=28T -- exactly what the RD can handle.