Rear Derailleur "Total Capacity"

[Louis]

Are the statements below accurate? Please correct or comment as required.

1) "Total capacity" is primarily related to the ability of the R-der to take up the chain slack when in "small in front" & "small in back" gear positions.

2) Shimano designs for the least common denominator. If I am a semi-competent rider and avoid those combinations I can exceed the stated limits by several T and not risk destroying anything.

TIA
Louis

[Kontact]

Total capacity is the difference between the big/big combination and the small/small combo. It is the amount of chain slack the derailleur can take up without being yanked apart or allowing the chain to droop.

You can exceed max capacity, and many people do. But you probably need to set it up so the chain is drooping in certain combinations, because this is less disasterous than hitting the other limit with a mis-shift.

[christian]

Yup. Nothing to add here.

[RPS]

Depending on how many you mean by “several” it MAY not even require avoiding certain combinations. I believe Shimano rates capacity based on worst case scenario for chainstay length. If you get lucky with your particular bike, you may have a few extra teeth of capacity with full range of gearing.

[Kontact]

Depending on how many you mean by “several” it MAY not even require avoiding certain combinations. I believe Shimano rates capacity based on worst case scenario for chainstay length. If you get lucky with your particular bike, you may have a few extra teeth of capacity with full range of gearing.
Sure, but it is really difficult to plan for, since actual capacity is affected by CS length, hanger length and actual cog choice's affect on B screw adjustment.

Not arguing against your point, but I consider that situation good fortune, rather than something that you can bank on, due to all the variables. If you're going to push capacity, you set the chain up in the Big/Big, then see if you get lucky.

[BobbyJones]

Ok guys, explain to me how CS length comes into play..percentage based?


Cause I'm thinking that if all of sudden the "slack" now increases by, lets say 4 inches, it doesn't matter how long the CS is- 4 inches is 4 inches. Then again, I never really thought about this at all before now.

[palincss]

Total capacity is the difference between the big/big combination and the small/small combo. It is the amount of chain slack the derailleur can take up without being yanked apart or allowing the chain to droop.

You can exceed max capacity, and many people do. But you probably need to set it up so the chain is drooping in certain combinations, because this is less disasterous than hitting the other limit with a mis-shift.

On a wide-range triple with a small inner ring, it makes no sense to use the inner ring with sprockets smaller than the 3rd or 4th largest, so the "small-small" combination can droop all it wants, since it's never encountered.

[Dave]

Look at the rigorous chain length formula at the Park Tool website. Note the effect of the chainstay length. For any big/big combination, there is a chainstay length that would require a chain length that is exactly a whole number of inches (53, 54, etc.). If you run the calc and get something just over 53 inches, then you'll need a 54 inch chain. All RDs have to be designed with 3T of extra wrap capacity to cover the worst case. That's why certain gear combinations, exceeding the advertised wrap capacity, will work on some bikes and not on others. Some bikes will have a chainstay length that provides up to 3T more wrap capacity than others.

The formula gives the shortest possible chain length. I much prefer to use the little/little method of setting the chain length, so I know that I'm getting the maximum wrap capacity from the RD. With Campy 11 speed, that insures that I can use any 11 speed cassette that Campy offers and never have to change the chain length.

http://www.parktool.com/blog/repair-help/chain-length-sizing

Also note that hanger length and cage length are not part of the equation. You can swap from a short cage RD to a long cage and not add chain length without fear of damaging something. Of course if the idea is to use larger cogs, then you must add 1 inch of chain to use a cog with up to four more teeth.

I've changed from a 53/39 double to a 53/39/28 triple and used the same chain length with the same 12-25 cassette. Of course a medium cage Campy RD was necessary to take up all of the slack, when in the little ring.

[Ralph]

Like Dave above said. I've been making rear derailleurs work beyond their rated capacity for about 40 years. Generally I start out with the chain on the long side. Figuring it's easier and safer to remove links than add them, if I start with wrong length. I'm not too concerned with how little to little shifts, but do want enough chain tension so I don't throw off chain with FD change. And then sometomes you have to run chain pretty tight, or on the barelly long enough side, to get upper pulley wheel out of the way, to clear a rear cog bigger than what RD is rated for. "B" screw doesn't always do it, especially on older RD's....or on bikes with a short RD hanger.

I've even run a triple front crank set (30-40-50) with short cage rear, and only 3-4 rear cogs working on small front ring (a one ride set up with minimal change to normal double set up). Sometimes I have run chain a couple links short so big to big wouldn't work.... when operating beyond RD capacity.....but don't recommend that, and for sure don't loan that bike to anyone.

[Kontact]

On a wide-range triple with a small inner ring, it makes no sense to use the inner ring with sprockets smaller than the 3rd or 4th largest, so the "small-small" combination can droop all it wants, since it's never encountered.
Yup. Yet that is what "capacity" is defined as, and long cage derailleurs are sold because they absorb droop in all combinations.

I've been running a MTB with a compact triple and an 11-28 with an Ultegra short cage for the past 15 years. Clearly, that works. But MTBs normally have long cages for those "never encountered" gear combos.


Dave,

The lack of hanger or cage length in Park's formula just is another reason to avoid using it. It will produce a chain length, but not a good one.

For Shimano (and Campy), the vertical pulley method works really well to produce the best shifting with pantograph derailleurs.

[BobbyJones]

Ah hah- So its just for rounding then and won't ever exceed an extra 3T. Best to think of capacity as more of a range then.

I knew they always built in a little extra- now I know exactly why.

Thanks

Look at the rigorous chain length formula at the Park Tool website. Note the effect of the chainstay length. For any big/big combination, there is a chainstay length that would require a chain length that is exactly a whole number of inches (53, 54, etc.). If you run the calc and get something just over 53 inches, then you'll need a 54 inch chain. All RDs have to be designed with 3T of extra wrap capacity to cover the worst case. That's why certain gear combinations, exceeding the advertised wrap capacity, will work on some bikes and not on others. Some bikes will have a chainstay length that provides up to 3T more wrap capacity than others.

[cnighbor1]

Has you can get to 24T smallest on front and 34T biggest on front why the need to push capacity on a rear derailleur?
Just buy the right components
Charles

[RPS]

If I am a semi-competent rider and avoid those combinations I can exceed the stated limits by several T and not risk destroying anything.

TIA
Louis
P.S. – One other thought on intentionally exceeding derailleur capacity on basis that certain gear combinations can be avoided. In practice I think it’s much easier to “avoid” some combinations in order to reduce RD required capacity when riding with a triple. For whatever reason, I think most riders tend to use the granny gear for steep climbing only, and as soon as the grade levels off slightly, they will change to the middle ring. I have never noticed any triple rider using the granny ring in combination with the right side of the cassette.

With standard double rings it’s too easy to cross the chain all the way to big-big or small-small. I notice other riders doing it all the time. I have to admit I sometimes go to the big-big for a few seconds if required to crest a fast roller.

[palincss]

[A]s you can get to 24T smallest on front and 34T biggest on front why the need to push capacity on a rear derailleur? Just buy the right components
Charles

In fact, on ebay XT long cage rear derailleurs are usually much less expensive than 105 short cage derailleurs.

[Louis]

In fact, on ebay XT long cage rear derailleurs are usually much less expensive than 105 short cage derailleurs.

The primary motivation for the question was whether or not I have to use an XT long cage (SGS) or if I can get away with using a medium cage (GS). Because I'm only a few T above the GS limit I'm going to try it.

As pointed out above, it's super-easy to avoid small-small, and I never do big-big either.

[RPS]

The primary motivation for the question was whether or not I have to use an XT long cage (SGS) or if I can get away with using a medium cage (GS). Because I'm only a few T above the GS limit I'm going to try it.

As pointed out above, it's super-easy to avoid small-small, and I never do big-big either.
I'm curious, what are you building? MTB or road?

[Louis]

I'm curious, what are you building? MTB or road?

MTB cassette and R-der with road triple crank on a road bike.

[BobbyJones]

Louis, hope this can help in some way-

I'm running a 53 /39 double with an XT GS RD (NOT SGS) and two different cassettes: a 12-27 6500 and a 11-32 HG-50. My Soma Smoothie ES has the chainstays listed at 420mm.

I used the shimano method of chain length (perpendicular pulleys) with the 11-32 on.

If did my math right when I set this up, I think I was two teeth over rated capacity. I haven't had an issue and I've been cross-chaining like crazy, athough now that I look the small / small combo is a little close for comfort but it works fine.


MTB cassette and R-der with road triple crank on a road bike.

[RPS]

Ah hah- So its just for rounding then and won't ever exceed an extra 3T. Best to think of capacity as more of a range then.

I knew they always built in a little extra- now I know exactly why.

Thanks
Actually, the “theory” on which this idea is based doesn’t suggest that they “ALWAYS” include a little extra; but instead that they always include at least the minimum rating. Although maybe not entirely correct, I prefer to look at it in a slightly different way involving probability. If chainstay lengths were truly random (which they are not) and everything else like RDs were equal, then 100 percent of the time you’d be able to get at least the full capacity rating, 75 percent of time you’d be able to exceed by at least 1 tooth, 50 percent of time you’d be able to exceed by at least 2 teeth, and 25 percent of time you’d be able to exceed by 3 teeth. Maybe this isn’t an exact math representation but you get the gist.

Sometimes we hear of riders who are running 4 or more extra teeth but I have to guess it’s mostly because they are operating outside Shimano’s recommended functional parameters.

As an actual example of how variations in chainstay lengths can affect outcome, my Co-Motion is presently at the maximum rear derailleur rating because I’m using a 53/39 with 12-27; which at 29T is the capacity rating for Ultegra 9-speed doubles. However, with the same length chain I can also fully run an 11-21, which means I could also run an 11-27, exceeding “minimum rated capacity” by 1 tooth. Based on observation, I’m also thinking that with the same length chain I can also cover a 53/28 combination so I’ve ordered an 11-28 cassette to replace the 12-27. If it works out OK I’ll be exceeding the rated capacity by 2 teeth.