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What happens if I exceed the front difference on a rear derailleur?

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What happens if I exceed the front difference on a rear derailleur?

Old 01-10-17, 09:21 PM
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What happens if I exceed the front difference on a rear derailleur?

According to this site: Shimano Rear Derailleur Specification - Bicycle Parts at discount prices / the Buyer's Guide / Bicycle Parts at their finest! / Professional Bicycle Source / Bike Pro, my Acera X derailleur should only be able to handle a front ring difference of 18T. Currently, it is equipped with a 24/34/42, right at 18T. I'd like to take that to a 22/32/42, simply because they are far more available without buying absolute bottom of the barrel or digging around for NOS (plus, I want the slightly lower gearing).

What are the exact consequences of exceeding the limit? I will still be in the total capacity limit, keeping the stock 13-28 rear, albeit right at the edge (42-22=20 + 28-13=15 for 35T). Is there any concerns, or are the concerns something you can deal with (i.e., something like you can't be in 22/13 at the same time)? I could go out and buy a new derailleur, or something used like a Deore XT, but I'd rather not if possible.
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Old 01-10-17, 09:35 PM
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As long as the chain wrap capacity is sufficient, you are good.

There's a bit of confusion on this-
If you looked at the spec's across the model line, it'll show a certain group of cassettes that "fit" in the "gruppo".
They use the required chain wrap of the worst case cassette (largest-smallest) and subtract that from the total wrap of the RDER.
In your case, that leaves 18T for the front difference.

I do find 35T wrap for an Acera RDER to be suspiciously small.

Last edited by Bill Kapaun; 01-10-17 at 09:44 PM.
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Old 01-10-17, 09:41 PM
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A couple of teeth more of wrap need is not too big a deal if everything else is kosher. But if some other aspects are a bit off (like the axle to der mounting bolt dimension or the B spring strength or stop tab location) then the "flexibility" may be less. Worst case is a tad of chain sag in the small/small. (Which we know is a bad combo to be in anyway).


But, perhaps, the other issue of front shifting will be the challenge. The front ders of Shimano have specifically sculpted cages and match up with intended rings. With only a two tooth difference this is not too big a deal but... Andy.
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Old 01-10-17, 10:21 PM
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Just be sure you can shift into your big/big combo and then shift out of it under load. In the effort to get slack out of your small/small the temptation is to remove enough chain links to prevent sag in that "combo you'll never use," but be sure the other "combo you'll never use" is possible in order to prevent an unpleasant surprise out on the road. Don't ask me how I know this.
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Old 01-10-17, 10:29 PM
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Originally Posted by Andrew R Stewart
But, perhaps, the other issue of front shifting will be the challenge. The front ders of Shimano have specifically sculpted cages and match up with intended rings. With only a two tooth difference this is not too big a deal but... Andy.
It's a cheap FD, a FD-C500. I can't find much info on it, but this incredibly helpful tech manual in Italian seems to indicate it should handle 20T, and an outer of 48T. I'm less attached to the FD, I can buy a decent new one of those cheap enough if this doesn't wind up working:

https://books.google.com/books?id=z2...d-c500&f=false
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Old 01-10-17, 10:39 PM
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(not an expert -- largely speculating)

1. I've swapped a 22T on a 48-36-26 (XT), i.e. outside of the nominal range. No problem whatsoever.
2. Derailleurs' capacity is a way to express the amount of slack that the cage can take. If the chain is long enough to wrap large-large, and you exceed derailleur specs by a wide margin, then you may expect that the derailleur will not be able to take all the slack when you shift to small-small and something will rub. If you shorten the chain such that everything runs OK on small-small, then you may be unable to have enough length to wrap large-large. Probably not that big of a deal. I've inadvertently shortened a chain to the point where it becomes next to impossible to wrap large-large (48x34 in my case). Which is bad practice anyway and would occur only if I were distracted.

Bottom line - it is certainly worth a try. Worst case you'll reinstall the 24T. Second worst is that you'll lose large-large. If you are lucky, 22T will work without a problem
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Old 01-10-17, 10:48 PM
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Since the big/big combination is the same, the VERY WORST that might happen with the change to 22-32 chainrings is that the RD may not take up the slack when you use the 22 with the smallest rear sprocket.

That my not happen, but so what if it does? You wouldn't have a reason to ride the crossed 22/13 for anything anyway. So no loss.
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Old 01-11-17, 03:58 AM
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@jefnvk, You have to have enough chain to prevent pulling the RD into the rear spokes on the big-big combo. If then the RD can't wrap enough chain for the small-small combo all that will happen is some noise and you'll soon enough up shift.

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Old 01-11-17, 09:44 AM
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Originally Posted by jefnvk
It's a cheap FD, a FD-C500. I can't find much info on it, but this incredibly helpful tech manual in Italian seems to indicate it should handle 20T, and an outer of 48T. I'm less attached to the FD, I can buy a decent new one of those cheap enough if this doesn't wind up working:

https://books.google.com/books?id=z2...d-c500&f=false
When it comes to Shimano, don't be fooled into thinking that the more expensive the front derailer, the better it works. This is true for rear derailers but not for the front ones. Shimano monkeys around with their more expensive derailers and gives them more ramps and sculpts them a whole bunch more. In my experience, this results in derailers that are less forgiving and more difficult to set up for a wide range cassette than the cheaper offerings. This holds for both road and mountain front derailers.

For example, the Ultegra triple derailer is a gorgeous piece of useless junk while the Tiagra is an ugly gem. The distance between the plates for the Tiagra is significantly wider than those of the Ultegra. This results in being able to shift across a wider range on the rear with the Tiagra without having to trim.

Same goes for the XTR triple mountain derailer and the Deore triple. The XTR is gorgeous but useless. It's a royal pain to set up so that it works properly. The Deore shifts like a dream.

But, if you really want the bees knees in front derailers...and if you aren't trying to mate the derailer to an STI shifter...go with Sram's front derailers (X5, X7 or X9). Dead easy to set up, works for either bottom pull or top pull and has a much better range than any of Shimano's offerings. They aren't too expensive either.

Oh, and ignore those "capacities". The derailer won't explode if you exceed them and they are way too conservative anyway. I have a 26 tooth difference on my touring crank (46/34/20) and derailers with a capacity of 20T work just fine.
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Old 01-11-17, 10:00 AM
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what happens up front is you potentially drag the chain in the bottom of the cage when you're on the smallest rear cog and smallest front ring. Shouldn't be a reason to be there.

Exceeding the chain wrap capacity of the RD gives you slack chain slap.

My cyclotouriste triple ranges from 26T to 46T up front, 12-30t teeth in the rear. I'm exceeding the spec'd capacity of both front and rear derailleurs, Shimano 600EX and SunTour Cyclone GT. It works in all combinations except the 26T-12t combination, which there is never any reason to engage, except maybe for removing the wheel.


Last edited by bulldog1935; 01-11-17 at 10:03 AM.
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Old 01-11-17, 11:02 AM
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Something to watch out for is the shape of the cage. If the curve of the cage doesn't follow the shape of the chainring reasonably close what can happen is that shifting from the smallest ring to the middle ring can result in the cage jamming on the chain instead of moving the chain to the next ring. I've replaced a number of deraillers on bikes that had that problem when the owner put on a smaller ring.

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Old 01-11-17, 11:10 AM
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The FDER, in this case, is pretty much a non issue.
You're only changing some of the rings by 2T.
You shouldn't even need to change the height, since you still have to clear the 42T ring.

The only adjustments to the FDER will likely be a small adjustment on the limit screws & barrel adjuster.
With a NEW crank and likely a new BB to fit the crank, the rings may be a couple MM in/out more than the old rings.
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Old 01-11-17, 02:31 PM
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Originally Posted by cyccommute
When it comes to Shimano, don't be fooled into thinking that the more expensive the front derailer, the better it works.
Yeah, that is why I'm not too concerned if I have to swap it out, although it seemingly won't be an issue.

Oh, and ignore those "capacities". The derailer won't explode if you exceed them and they are way too conservative anyway. I have a 26 tooth difference on my touring crank (46/34/20) and derailers with a capacity of 20T work just fine.
Cool, good to know. Didn't think there would me much of an issue, just verifying.

Originally Posted by Bill Kapaun
With a NEW crank and likely a new BB to fit the crank, the rings may be a couple MM in/out more than the old rings.
It's a JIS square taper, new as of the end of summer. As far as I am aware, any newly manufactured square taper should fit, correct? Or do I need to worry about BB/crank matching as well?
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Old 01-11-17, 02:37 PM
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Originally Posted by jefnvk
What are the exact consequences of exceeding the limit?
What everyone who posted here seemed to miss is the height differential between FD's tail and the lower edge of FD's outer plate.

I have never used Acera, but if Shimano has noted a range limit, their concern may be of either:

A. having the chain rub onto FD's tail in combinations of too small a granny + too small a rear cog (while the FD is properly positioned for a given large ring);
B. having the FD's tail hitting the chainstay in case of too small granny ring at a given large ring.

Case A. would prevent you from using the granny ring with certain cogs, which means you would miss gears you might really need.
Case B. would prevent you from a proper downshifting.

Since you are only changing to a smaller granny while keeping the large ring the same size, you are safe from running into Case B, and are at risk only with Case A.

While we all know that Shimano is always way too conservative in their specifications, the above cases are still something to consider.

Somewhat related, I have experienced the above Case B while trying to combine a Campy Centaur FD with an odd aluminum frame with too much of a BB drop (read: chainstay angle) plus a 46/30 crankset. In order to make the combo work, I had to jeopardize the FD height setup, adjusting its outer plate at a bit over 2 mm higher than the big ring's teeth (to ensure the tail cleared the chainstay). While many would consider this being "in spec", I have been setting up my bikes with ~1 mm clearance for decades, and was disgusted with this setup to the point that I abandoned it after less than 2k miles.
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Old 01-11-17, 03:05 PM
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Originally Posted by jefnvk
Yeah, that is why I'm not too concerned if I have to swap it out, although it seemingly won't be an issue.



Cool, good to know. Didn't think there would me much of an issue, just verifying.



It's a JIS square taper, new as of the end of summer. As far as I am aware, any newly manufactured square taper should fit, correct? Or do I need to worry about BB/crank matching as well?
Yes-
Do your research. You may find a crank that matches, or it may require a new BB in order to get a crank that meets your goals. Just don't assume it fits. Hopefully, it isn't hard to do. I haven't bought cranks in about 6 years, so I'm not totally aware of how much compatibility in sq. taper is the current norm.

As far as the FDER is concerned- I doubt you'll notice ny difference. As I mentioned earlier, you might have to adjust the limit screws and/or barrel adjuster, but that "should" be relatively simple.
I DON'T recommend repositioning the FDER on the tube. The "A" series of Shimano FDER's are very forgiving on mismatches of a couple teeth. You have a very minor "mismatch".
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Old 01-11-17, 03:26 PM
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Originally Posted by Bill Kapaun
Yes-
Do your research. You may find a crank that matches, or it may require a new BB in order to get a crank that meets your goals. Just don't assume it fits. Hopefully, it isn't hard to do. I haven't bought cranks in about 6 years, so I'm not totally aware of how much compatibility in sq. taper is the current norm
Yeah, after the last comment I went back and checked. The one I am currently eyeing up (SRAM S600: SRAM S600 8-Speed Square Taper Crankset - 22-32-42t in Tree Fort Bikes Cranks) matches the 113mm spindle that is on the bike, with a 48.5 driveline (RD should be able to handle 47.5-50, according to Sheldon Brown: Sheldon Brown's Bottom Bracket Size Database)
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Old 01-11-17, 03:30 PM
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Originally Posted by jefnvk
...It's a JIS square taper, new as of the end of summer. As far as I am aware, any newly manufactured square taper should fit, correct? Or do I need to worry about BB/crank matching as well?
Crank sets don't all have the same "back spacing". The crank set manufacturer often will have recommended BB spindle lengths compatible with a particular crank set. Like a lot of bicycle things, there is a little bit of fudge factor. Hollowtech II crank sets sometimes come with the external BB cups.

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