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Crankset FC781 48-36-26 : possible to replace 48T with 38T

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Crankset FC781 48-36-26 : possible to replace 48T with 38T

Old 11-17-21, 02:21 PM
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gauvins
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Crankset FC781 48-36-26 : possible to replace 48T with 38T

My touring bike is equipped with Shimano's trekking group. The crankset is nominally a triple 48-36-26 (104/64 BCD) and the cassette an 11-34 XT. As I rarely push gear-inches above 90 and definitely do not like the 13T-11T step, I've replaced the smaller 11-19 XT sprockets with Miche's 15T-19T, which gives a reasonable range (495%) and narrow gaps (5%) for gear-inches above 50 or so (see here).

Finding replacement parts is becoming somewhat challenging, so I am considering reverting to a stock XT 11-34 cassette, which could work great in half-step mode, if I were to swap the 48T chainring for a 38T.

Is this possible? Other thoughts?

[EDIT - initial post didn't link correctly to the proper page. My current gear range is [url=https://www.ritzelrechner.de/?GR=DERS&KB=22,36,48&RZ=15,16,17,18,19,21,23,26,30,34&UF=2075&TF=90&SL=2.6&UN=KMH)]this. Replacing the larger chainring and cassette would give this)

Last edited by gauvins; 11-17-21 at 03:55 PM.
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Old 11-17-21, 03:21 PM
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I do not understand why you would want to drop to a 38 when you have a 36 middle. I understand you are not pushing big gear inches, but the 38 makes the gearing awfully low. I also am not familiar with 104/64 front, ergo, I do not know what rings might be available. Maybe look for a 44 tooth big ring. Sometimes you just have to play around to find what is going to work best for you.
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Old 11-17-21, 03:28 PM
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Is this your front crank, an FC-T781 ?


https://bike.shimano.com/en-EU/produ...0/FC-T781.html

If so you can change to 2x easily enough. Though I'd expect you to get a new 2x crank, 2x front DR and 2x front shifter or STI.

Or did I read your title wrong and you are just wanting to replace the big ring for smaller?
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Old 11-17-21, 03:36 PM
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I agree that a 38-36-26 doesn’t make much sense. If a 38t is the correct large ring, drop the 36t and replace it with a 38t. Run it as a double.

John
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Old 11-17-21, 03:50 PM
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It will REALLY SUCK as far as functionality.
The FDER will be "looking" for a big ring that isn't there.

I converted my bikes to a 22-32-44 w/ a more roadish cassette.
I changed the large ring to 36T on one bike and 38T on the other.
The 38T ring shifts reasonably well, but does have a little "lag".
The 36T ring results in MUCH crappier shifting than the 38T.
My advice is to stay at LEAST 6T larger than the middle.
You can get an idea how ugly....

On my hybrid, I'd run 12-23 or 13-25's and my smaller wheeled 86/7 Rockhopper/grocery getter a 12-27 cassettes. I'm pretty flat terrain wise though.


Last edited by Bill Kapaun; 11-17-21 at 03:53 PM.
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Old 11-17-21, 03:50 PM
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Originally Posted by delbiker1
I do not understand why you would want to drop to a 38 when you have a 36 middle.
Please look at the linked pages (current and planned). Should be obvious.
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Old 11-17-21, 03:54 PM
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Originally Posted by Iride01
Is this your front crank, an FC-T781 ?


https://bike.shimano.com/en-EU/produ...0/FC-T781.html
Yes, this is the one. My reason to want to ditch the 48 for a 38 is to be able to reduce the gaps between gear-inches. a 2x would leave rather large gaps (10% or so) whereas being to go up to the 38 would more or less split the gaps in half.
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Old 11-17-21, 03:58 PM
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The 38/36 shift is going to be a problem. Your front derailleur's inner cage plate will hiit the 36T if the cage is set properly above the 38T chainring and if the derailleur is set to clear the 36T, it will be way to far above the 38T to shift at all well. Unfortunately, half-step gearing pretty much went away as cassette cogs exceeded 6 or 7-speeds
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Old 11-17-21, 04:57 PM
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HillRider thanks for your input. This being said, I don't understand why that is going to be a problem. I mean, the 26T (I've actually swapped a 22T) is much lower than the FD's cage, yet it shifts very well. I (naively) thought that lowering the FD, perhaps with a wee bit of excess clearance WRT the larger chainring, would work well... funny as something that looks simple on the surface turns out to be difficult to implement
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Old 11-17-21, 05:58 PM
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Originally Posted by gauvins
HillRider thanks for your input. This being said, I don't understand why that is going to be a problem. I mean, the 26T (I've actually swapped a 22T) is much lower than the FD's cage, yet it shifts very well. I (naively) thought that lowering the FD, perhaps with a wee bit of excess clearance WRT the larger chainring, would work well... funny as something that looks simple on the surface turns out to be difficult to implement
The FDER will jam against the middle ring if you lower it more than just a "bit". That's WHY the HUGE gap between the largest ring and cage.
The smallest ring is moot since you won't get past the middle.
The "slope" the FDER travels is going to remain the same no matter what you do to ring sizes.
IF it currently just clears the middle ring, how can it clear the middle ring if you lower it 4-6T more?
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Old 11-17-21, 08:15 PM
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Bill Kapaun Yes, I see what you mean. What isn't really obvious to me is why exactly would I want to lower the front derailleur??
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Old 11-17-21, 08:36 PM
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Originally Posted by gauvins
Bill Kapaun Yes, I see what you mean. What isn't really obvious to me is why exactly would I want to lower the front derailleur??
Most FDs are designed to have the outside of the cage located just a millimeter or 2 above the chainring. Maybe it will still work if it's way above the chainring, but I bet it won't work as well.

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Old 11-17-21, 09:14 PM
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I'm running my mtb's with 34-24 chainrings and the FD's are more than 25mm above the middle ring. It is basically set where it would be if I had an outer ring.

It is a bit of a catch 22. It shifts fine as the inner derailleur cage is aligned at the correct place to move the chain from inner to middle; the FD doesn't know there isn't an outer ring. I did install a bash guard in the outer position as a precaution against throwing the chain off the middle ring. That is the catch 22 part. I'm not convinced I wouldn't drop a chain over rough terrain with the gap between ring and FD cage so great.

I would guess a 36 to 38 outer would require a wide gap for the middle to outer. Chainrings with ramps and pins would help, but I would think the inner cage needs to be set correctly for upshifting from the the inner and middle chainrings.

John
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Old 11-17-21, 11:12 PM
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Originally Posted by gauvins
Bill Kapaun Yes, I see what you mean. What isn't really obvious to me is why exactly would I want to lower the front derailleur??
You were the one that mentioned it.
I responded why you can't.
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Old 11-18-21, 08:53 AM
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Originally Posted by Bill Kapaun
It will REALLY SUCK as far as functionality.
The FDER will be "looking" for a big ring that isn't there.
Originally Posted by HillRider
The 38/36 shift is going to be a problem. Your front derailleur's inner cage plate will hiit the 36T if the cage is set properly above the 38T chainring and if the derailleur is set to clear the 36T, it will be way to far above the 38T to shift at all well.
You could get it to work by replacing the front derailleur with a road model, that doesn't have the dropped inner plate. It might not play well with an indexed front shifter, though.

Seems like a lot of work for not much gain.
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Old 11-18-21, 09:02 AM
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Originally Posted by gauvins
What isn't really obvious to me is why exactly would I want to lower the front derailleur??
Performance can suffer if the front derailleur is positioned too high. It can either have a hard time lifting the chain to the outer ring or throw the chain completely past the outer ring and into the void, or both. This is particularly an issue with indexed front shifting. Best performance is when the outer plate of the derailleur sits only a few millimeters above the teeth of the outer ring.
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Old 11-18-21, 09:16 AM
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I see -- thanks for the information. (back to square one
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Old 11-18-21, 03:12 PM
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All my touring has been with 52/46 in the front and 5 cogs at 14-28 in the rear. If the 46/28 would have been to high a gear then I would have replaced the 28 with a 30 tooth cog. All one really needs is a granny gear that is low enough that you can go up any grade you will encounter. Unlike racing there is no need for even intervals with the rear cassette or freewheel. More than 80% of the time I am using the 52T chainring. With more speeds available at the rear I would first go to 11T and have a taller gear for the downgrades. I pick up a lot of time by pedaling down the grades and it provides more speed for pushing up small hills on rolling terrain.

My maximum tour load is 25 lbs and when I started credit card touring and staying at hotels and inns and eating at restaurants my load dropped below 15 lbs on the rear. Even easier to carry fewer tools and replacement parts with so many excellent bike shops all over the place.
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