What limits rear derailleur front capacity?

[Tobias]

What actually limits a rear derailleur’s front capacity? Other than its rating by the manufacturer, what affects function that limits its front capacity?

The new Shimano RDs have more capacity than before; almost half way between the previous double and triple RDs (reminds me of the Campy short/medium/long offers). However, the front capacity is the same – listed at 16T (presumably to cover the compact 50 to 34 jump). However, in previous model years the same exact derailleur may have been rated at 14 T (presumably for the standard 53 to 39 jump). Without apparent modifications did it go from 14T to 16T? Or did Shimano actually make minor changes to Ultegra RDs without changing the model number (maybe springs, etc…)?

I’m asking only because I’m curious what would happen if a standard double RD with 33T capacity was used with a custom triple as long as the total capacity did not exceed 33T. Would it be able to handle 18T at the front? How about 20T?

Inquiring minds want to know. And thanks for sharing your insight.

[Ti Designs]

There are two things to think about in terms of capacity of a derailleur, the total wrap and the largest cog. The largest cog thing is a little fuzzy 'cause they don't know which dropout the derailleur is going to hang from, which can lower or raise that number. Come to think of it, the total wrap isn't a solid number because nobody sets up chain length so the small/small is right where the spring tension starts... Anyway, it all comes down to the legal department. If you put a derailleur on your bike, then shift into the big ring and rip the dropout off your bike, they're going to say "it's in the fine print".

I go by more of a working definition. Largest cog is the one that the derailleur can run in without the upper pulley running on the teeth (about 1/2 chain link clearance is a good rule). Subbing out that 4mm B-tension screw for something longer can get you that little extra there. As for total wrap, how well do you know how to shift? What are your expectations for use of stupid cross-over gears? If you're looking at the total wrap being the difference in teeth between small/small and big/big you're looking at a few gears I would never use. Big/big is the dangerous one, if you don't have the capacity for that it's going to do some damage. And which of us hasn't forgotten we were in the big ring on a climb? I would allow the small/small to run slack if you need the extra capacity, assume that if you need the climbing gear you're not going to be in the smallest 4 gears in back.

[RPS]

There are two things to think about in terms of capacity of a derailleur, the total wrap and the largest cog.You cover the rear capacity well, but I think his question is about front capacity.

[Tobias]

Yes, my question is about the front capacity listed by Shimano. As an example to clarify my question (not that it's what I want to do), what keeps a Dura Ace RD with 33 tooth capacity from working on a triple (52/39/30) if used with a 12-23 cassette? The RD can handle the capacity (33T rating) and the cog size (28T max rating), but according to Shimano the 22-tooth front capacity is more than the rated 16T for the rear derailleur. :confused:

[dave thompson]

Yes, my question is about the front capacity listed by Shimano. As an example to clarify my question (not that it's what I want to do), what keeps a Dura Ace RD with 33 tooth capacity from working on a triple (52/39/30) if used with a 12-23 cassette? The RD can handle the capacity (33T rating) and the cog size (28T max rating), but according to Shimano the 22-tooth front capacity is more than the rated 16T for the rear derailleur. :confused:
As I understand it, the 22 tooth front derailleur capacity has to do with how much the derailleur can lift the chain, from the small ring to the big ring. In the case of a triple, it's 22 total teeth; 30 small to the 52 tooth big. Has nothing to do with the rear at all.

[Ti Designs]

Yes, my question is about the front capacity listed by Shimano. As an example to clarify my question (not that it's what I want to do), what keeps a Dura Ace RD with 33 tooth capacity from working on a triple (52/39/30) if used with a 12-23 cassette? The RD can handle the capacity (33T rating) and the cog size (28T max rating), but according to Shimano the 22-tooth front capacity is more than the rated 16T for the rear derailleur. :confused:


OK, I get the question now. It's that number that says the maximum front capacity is 16T for the rear derailleur...


To get the whole answer you must first understand that Shimano engineers were born to parents who were Shimano engineers. They have lived their whole lives within the Shimano factory, they have no windows and have never seen the outside world. Yeh, it's a lot like MicroSoft's inner ranks... They design certain components to work with certain other components. They then make large and confusing compatibility charts which state that what is compatible with what has more to do with what it was designed to work with than reality. Take for instance their mountain bike cassettes and 9-speed road shifters - there's no little dot there in their compatibility charts, yet who hasn't seen that combination on a touring bike?

So here's their thinking: There are two chain tension springs in a derailleur, the lower chain tension spring which wraps the pulley cage, known as the A tension spring, and the upper chain tension spring which wraps the body of the derailleur, known as the B tension spring. Here's where it gets a little tricky. The design goal is to make a derailleur which keeps a half chain link between the upper pulley and the cassette cog, which means the paralellagram must track the shape of the cassette, the pulley cage must rotate back to adjust for the larger cogs as it moves in, and the B tension spring must adjust for the difference in front chainrings. It's called a linear aproximation of a non-linear system, but it kinda works...

Outside of the Shimano factory/test grounds/amusement park there's another way of knowing. It's called try it and see. Oddly enough, they're mostly right, it doesn't shift well. In the small ring the pulley cage is almost fully wrapped, so there's too chain wrapped around the cog.

On a side note, if you use the wireless FlightDeck, this gearing info has been sent to Shimano, and you can expect a late night visit from two short guys with pocket protectors. They will take your bike for violations of the user agreement you signed, and they'll probably take your toaster too, 'cause Shimano doesn't make those so it's damn hard to get a good slice of toast in Japan.

[Johnnyg]

Just for the record I run a 30 tooth cog when I go to the Mts. I have done this with 7800 and the new 7900. As Ti said, screw the derailur screw all the way in. JohnnyG

[Ray]

When I bought my first nice bike back in '97, I had it built up with Campy triple, but I used a regular Chorus double rear derailure (why? - beats the crap out of me - no memory of my rationale). As long as I didn't use the two largest cogs on the big ring or the two smallest on the granny, I was OK. I rode it like that for a few years and never had any problems. But my largest rear cog was 26, which I think was the largest Campy was making in those days and any rear derailure could handle that cog. These days I run a 12-34 in back and use a XT rear derailure because none of the road derailures will shift up to a 34 (30-32 seems to be the real world limit). I don't need all of the chain wrap of the long cage, but I need to be able to shift to the 34 (rarely, but no use having it if you CAN'T use it).

-Ray