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Short Cage on a Triple - Check My Derailleur Math

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Short Cage on a Triple - Check My Derailleur Math

Old 07-26-21, 01:40 PM
  #1  
Harold74
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Short Cage on a Triple - Check My Derailleur Math

The details...

Cassette: 7 speed, 12-28
Chainring: 9 speed triple, 26,36,48
Desired derailleur: 10 speed 105 short cage, 33T capacity
Bike: 1992 Miyata 1000. 19 rides out of 20 I ride it as a double. That said, I'd like to preserve the granny gear option for when I tow the dog trailer and/or attempt to climb walls.

This short cage derailleur won't have enough capacity for me to run the full cassette on all three chainrings. However, my understanding is that, if I avoid cross chaining, I can still make it work. If I wanted to know how many of my cassette sprockets would be usable on the granny gear, would I look for the sprocket having no less than:

28 - (33 - (48 - 26)) = 17 Teeth.

Is that right? If it is, that means that I could still access the largest four cogs of my cassette from the granny gear (the fourth is 18T). And I don't see attempting to access anything smaller than the second largest cog.

As a secondary question, if I were to, say, lend the bike to my elderly father and he tried to use the small ring and small cog, what's the worst case scenario there? The chain would slip the granny gear chainring? Is there any chance the loose chain would hop the cassette and fly into the rear wheel spokes somehow?
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Old 07-26-21, 02:17 PM
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You are at 38 capacity... 5 over. Won't work. Look for a long gage DR.
You are in danger of riping the rear DR off the bike.
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Old 07-26-21, 02:25 PM
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you really need to get the GS (longer cage)

your math is incorrect chain wrap (capacity) is Diffference of smallest largest rear + difference high low front

in this case capacity needed = (28-12) + (48-26) or 16 + 22 or 38

I think your spec on the short cage are wrong also, what I see for the GS (medium cage) is 40 warp capacity and 32 max rear cog and 22 difference up front the short cage seems to be 34 wrap
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Old 07-26-21, 03:21 PM
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9 speed shimano mt RD are readily available that there is no reason to run something else
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Old 07-26-21, 03:42 PM
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Shimo mountain rear derailer. short cage road won't work. not even close.
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Old 07-26-21, 03:45 PM
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Exceeding the chainwrap capacity of a rear derailleur can be done safely and effectively, with a couple of caveats:

A couple of teeth over is all you're really gonna get, and size your chain so that the big/big works. It doesn't have to work all that well, but you need to able to shift the bike into it and not have the drivetrain lock up. Cuz that's a really nasty crash and a broken bike if it happens. I've seen it. 'Twas ugly.

That said, you're on the right track with your math.

(12-28) + (48-26) = 16 + 22 = 38, so 4 teeth more than the 34T chainwrap of your derailleur. 4 teeth is a lot when pushing specs like we're doing, I like to limit it to 2 so that everything just works. In that case, you either need to lose 2 teeth on the top or gain 2 at the bottom. 48x12 is a 105.83" gear, 26x28 is 24.60"; 46x12 gives 101.33" and 28x28 is 26.46", so your choice is lower high or higher low. Me, I'd go with the lower high, but I'm old and slow. YVMV. (Your Velocity May Vary.)

Your short-cage rear derailleur also has a 16T difference-of-chainring spec, which determines if the chain will clear the tail of the front derailleur in small/small. Yours won't. With a 46/26 or 48/28, you're at 20, so we need to get 4 teeth back somehow. 12 + 4 = 16, so you won't be able to ride the granny in any cog smaller than a 16. If you do, you'll hear it, and if the chain jams on the FD, bad things may happen. 48-26 is 22 teeth, which is too much, IMNSHO.

"Short cage on a triple" is a venerable and worthy setup, but there's compromises. (There are always compromises. TANSTAAFL.) If you haven't already bought the rear derailleur, just buy the medium or long cage one and don't sweat any of this. The Miyata 1000 is a touring bike, after all, and would have come with an MTB rear derailleur. (If it were my bike and my money, I'd put a 9-speed XTR on it... plentiful, cheap, and fantastic!)

--Shannon
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Old 07-26-21, 04:11 PM
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If your plan is to only use the larger cogs in the rear with the granny, then with proper chain length, it should work. Shifting to smaller cogs with the granny will cause chain slack but that's not a serious problem, big/big is where the major problems happen.

Assume that you're looking a newer 105 RD. The RD-5700-A has 34T total capacity. The older RD-5700 has just 33T. Any older short cage RD have even less total capacity.

As for front derailleur clearance, it depends on what you have, since you didn't mention it.
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Old 07-26-21, 04:57 PM
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Originally Posted by KCT1986 View Post
If your plan is to only use the larger cogs in the rear with the granny, then with proper chain length, it should work. Shifting to smaller cogs with the granny will cause chain slack but that's not a serious problem, big/big is where the major problems happen.
Actually chain wrap on a correctly sized chain only effects the small-small. Maybe the 1st and 2nd position depending on how far over capacity someone goes.

On the large large the wrap is “closer” to zero, depending on the extension of the cage, since the chain is “almost” going straight from the cog to the chainring.

On the small-small the derailleur cage folds back on the derailleur until the chain sags.

John
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Old 07-26-21, 05:36 PM
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Originally Posted by 70sSanO View Post
Actually chain wrap on a correctly sized chain only effects the small-small. Maybe the 1st and 2nd position depending on how far over capacity someone goes.

On the large large the wrap is “closer” to zero, depending on the extension of the cage, since the chain is “almost” going straight from the cog to the chainring.

On the small-small the derailleur cage folds back on the derailleur until the chain sags.

John
True... although if your chain goes slack enough to drag on the front derailleur cage, I could see bad things happening if it was slack enough to wrap itself up in the derailleur or seat tube under load. I wouldn't have a problem doing this on my own bike, because I'd know that it was there and would never ride in those gears, but I hesitate to suggest it to anyone else. Especially if someone other than the builder is going to be riding the bike.

A little bit beyond specs with a proven combo, no problem, as the specs are very conservative. Once you get past a couple of teeth beyond spec, I start getting nervous. (Especially because tolerances stack. 2 extra teeth of big cog, a couple more teeth of chainwrap, and then a few extra teeth of chainring difference, and you can end up with combos that are 6-8 teeth too much, and then it can suddenly become a Very Bad Thing. ) Locking up your drivetrain at speed tends to really, really suck.

--Shannon
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Old 07-26-21, 05:48 PM
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If you size the chain for Big-Big, you will be fine, and the worst case scenario is that your chain goes slack in the couple smallest cogs if you accidentally cross chain. And that will make enough racket that you will know it is happening.

But really, the only reason to do this is if you already have the RD. If you need to buy one, there is no reason not to get an RD with the proper capacity. This is especially true if you want the bike truly idiot-proof. Because idiots WILL cross chain, and a bunch of noise from the RD is not going to stop them.
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Old 07-26-21, 06:11 PM
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Originally Posted by 70sSanO View Post
Actually chain wrap on a correctly sized chain only effects the small-small. Maybe the 1st and 2nd position depending on how far over capacity someone goes.

On the large large the wrap is “closer” to zero, depending on the extension of the cage, since the chain is “almost” going straight from the cog to the chainring.

On the small-small the derailleur cage folds back on the derailleur until the chain sags.

John
benefit to small/small sizing is you get max amount of chain, so fewer issues if the big/big is a bit tight......I know from personal experience (fall and go boom....but at least is was slow and only hurt my ego)
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Old 07-26-21, 06:12 PM
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Originally Posted by Kapusta View Post
But really, the only reason to do this is if you already have the RD. If you need to buy one, there is no reason not to get an RD with the proper capacity. This is especially true if you want the bike truly idiot-proof. Because idiots WILL cross chain, and a bunch of noise from the RD is not going to stop them.
Yep.

For example, I'm putting a 45/42/30 x 14-16-18-20-23-26 half-step + granny on my '85 League Fuji. Why? Because of the NIB 1980 Superbe rear derailleur I put on the bike when I got it. (Was a single-speed and I hate 'em.) That derailleur is a thing of joy and beauty, and I already had it so of course I used it. But it imposes limits. I'm already breaking one of them, the 23T max cog. With Suntour dropouts, which have a longer hanger, it's fine, and I could almost certainly get a 28 in there, but still... TANSTAAFL. With 45/30 and a 14-26, I'm at 27T of chainwrap, which is only 1 more than the 26T rating. This I'm confident will work fine, and just be a bit slack in the 30x14, which I'm unlikely to shift into even by accident.

But that's a odd case, and I'm an odd dude.

--Shannon
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Old 07-26-21, 06:22 PM
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1) Thank you all for your gracious responses.

2) Many responders seem to have not picked up on the fact that I intend to NOT use my entire gear range when on the small ring. So the 33T capacity shouldn't rigidly apply as it normally would as I won't be using the full 38T demand. Rather, I'll be restricting my gear usage to suit the 33T capacity.

3) I plan to size my chain for big-big and, as such, I see the only difficulty as being small-small chain slack. So my derailleur math was basically small ring compared to as small a cog as the short cage DR would allow me to go at the back without the chain going slack (17 teeth). That calculation seems to be correct. Or, at the least, I've seen nothing here so far to indicated that it's incorrect.

4) The front derailleur will remain the Deore triple that came stock on the bike. So long as that's the case, and I'll not have chain slack, that should be fine, right? This is strictly rear derailleur fiddling.

5) I don't expect that anyone will ride this bike other than me. That was just a hypothetical and was me trying to solicit the "doomsday scenario" of what could possibly go wrong.

6) The Miyata was indeed intended to be a touring bike. That's not how I'm using it however. For me it's a cool, super comfy, fitness rider. There will be no touring. Like I said though, I want the granny ring option occasionally for the dog trailer. I have a separate bike that is currently the full time dog trailer hauler but I foresee a future in which I'm forced to divest myself of a bike or two and the Miyata is pressed into that role.

7) I don't want ANY long cage derailleur on the bike, no matter how cheap or how high a quality of a component it is. My reasons are unique to me and this project and are not altogether rational:

a) I'm in love with the frame of this bike and the comfort of the ride. I'm taking the bike in a non-vintage direction, modernizing everything that I can figure out how to upgrade for sport.

b) It may just be my imagination but I feel as though my short cage road setups shift more crisply than the long cage setup on the Miyata. And I LOVE crisp shifting.

c) As it stands, with the original long cage Deore derailleur, small-small is already almost unusable. The chain is loose and, on rough terrain, it bounces on (and scratches) a little bar on the chain stay meant to protect the chain stay in just such situations. Cross chaining isn't currently dangerous for me but, at the same time, it's hardly functional.

d) This more than anything else. I'm a structural engineer and, as a result, I view certain thing in certain, unorthodox ways. I see a long cage derailleur as a fragile, upside down cantilever. When I look down at it, it feels like it's the length of a baby's forearm and flopping around all ungainly like. I'm simply repulsed by it. And it's my bike so, if I say it goes, it goes.

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Old 07-26-21, 07:22 PM
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I’m running 38t capacity with a medium cage rated at 33t capacity. Because it is medium, I can go small-small and get a slight chain-to-chain rub without chain sag on the stand. I say on the stand because I have never come close to running a small-small in real life.

You’re smart enough to understand the situation. Just ride it.

I won’t go as far as agreeing that your SS 105 is crisper than a GS Dura Ace or even an XTR RD.

John
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Old 07-26-21, 07:22 PM
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Wanting a short-cage derailleur on the bike just because that's what you want is all good. They do look better and shift better, no doubt. And, yes, what you're planning will work, you're just pushing harder on the limits than many would do. It's just that there's tradeoffs. (Your're an engineer... you grok that there are always tradeoffs.) Here, the tradeoff you're making is that you get what you want, at the cost of a bigger failure band, with the possibility of a catastrophic faliure mode if you screw up badly enough. The odds of that happening are probably pretty slim... but sometimes Slim shows up and wrecks the party. (Slim is Murphy's crazy cousin.)

It'll work, for sure. You just have to respect the limitations that you're building into it, which it seems that you do.

--Shannon
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Old 07-26-21, 07:52 PM
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If you have your reasons for using a short cage, go for it. You seem to know what the deal is. Just don't lone it to your elderly father.

As far as your math as to what the smallest cog is you can use in the small ring... makes sense to me, though the chain wrap capacities Shimano states are usually conservative, so you will probably be able to use even smaller rings.

FWIW, nobody on the cares that you - or anyone else who says they are an engineer on the internet - are an engineer.

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Old 07-26-21, 07:58 PM
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Originally Posted by ShannonM View Post
with the possibility of a catastrophic faliure mode if you screw up badly enough.
Yes... tell me as much as you can about that, in graphic detail. The chain might come off at the chainwheel which is scary but, I think, manageable at granny gear speed if one doesn't panic. Somebody mentioned the derailleur ripping off?? That seems unlikely unless the chain flies off the cassette and into the spokes. I do plan to remove the dork disk with this overhaul.

One thing that I've considered doing for safety is this:

1) Sizing and running a chain to be used on only the two, larger chainrings. And setting the limit screws accordingly.

2) Sizing and running another chain to be used on only the two, smaller chainrings. And setting the limit screws accordingly.

When/if I do the dog trailer thing with the Miyata, it will be a bike then dedicated to that purpose for entire seasons at a time. And when I ride with the dog trailer, I rarely get out of the granny gear, yet alone the middle ring. My middle weight poodle weights a solid 50lbs. In this sense, the bike would cease to really be a triple.

Another thing that I've considered, although it may not be possible, is setting the front derailleur limit such that I can't shift to the granny gear but I could move the chain down to it with my hand. This way, whenever I went granny, at least there would be a natural moment of pause and reflection. And I wouldn't be shifting into it at 40km/h or anything.
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Old 07-26-21, 08:07 PM
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Originally Posted by Kapusta View Post
You seem to know what the deal is.
If I've created that impression, that's wonderful. I've really just done barely enough research that I finally felt competent enough to ask the question intelligently.

Originally Posted by Kapusta View Post
FWIW, nobody on the cares that you - or anyone else who says they are an engineer on the internet - are an engineer.
It wasn't my intent that anyone should care that I'm an engineer. Rather, it was my hope that they might use that piece of information to better understand my particular view of the world. Some kids see dead people. I see fragile things buckling. Without my structural engineering concerns regarding long cage derailleurs, my preference would come down to nothing more than "I don't like how it looks". I wanted to make it clear that it is about more than that, for me.
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Old 07-26-21, 08:15 PM
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Originally Posted by 70sSanO View Post
I won’t go as far as agreeing that your SS 105 is crisper than a GS Dura Ace or even an XTR RD.
You sound like a man speaking from experience. For what it's worth, I won't be divesting myself of any of the original componentry. If one of my "improvements" turns our not to be, I'll just take my time polishing up the original kit before I put it back on.

The shifting thing is interesting to me. I went to Park's website to research how to tell if an RD is worn. I was expecting it to be mostly about front to back spring stiffness. Instead, they mostly focused on lateral play. And a lot of authors seem to claim that short cage RD's shift more crisply which sounds related (longer cage = less lateral stiffness all other things equal). That said, I would have thought that quality shifting would be mostly about the upper pulley being positioned well and the lower pulley would mostly just be along for the ride. If the lower pully has some play in it, so much the better. Viewed from the top, most gears are cross chaining to some degree which means that there's always going to be some torque on the pulley cage, short, long, new, or worn..
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Old 07-26-21, 08:19 PM
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Originally Posted by Harold74 View Post
Without my structural engineering concerns regarding long cage derailleurs, my preference would come down to nothing more than "I don't like how it looks". I wanted to make it clear that it is about more than that, for me.
And yet that's exactly what it is, no more. There are no "structural engineering concerns" with long cage derailleurs. It's fine to prefer one look over another, but you'd think an engineer would prefer a properly designed system over some kludge with a sagging chain. Anyway, maybe a compact crank would give acceptable range while keeping the look you like.
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Old 07-26-21, 08:22 PM
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Also FWIW, it's the upper pulley that has play, by design.
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Old 07-26-21, 08:23 PM
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Is this the thread to share one of my pet peeves? The usual maxing out the capacity of rear ders, even on new bikes? The Op talks about of his OCD (my term) of failures in structures but seemingly no same concerns about exceeding a design capacity. I often use automotive analogies at work. This is like buying the smallest vehicle then pulling a trailer way past the car's load limit. Sure many do this but your heirs won't see that as a reasonable excuse when your turn comes up. Andy (who will crawl back into the cave now)
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Old 07-26-21, 08:43 PM
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You'll probably be able to use the low limit screw on the FD to 'lock-out' the smallest ring if you desire. Set it up as usual, shift to the middle ring and tighten the low screw. Even if it not long enough to totally keep the cage in the middle ring position, it will probably prevent the shift to the small ring if you try to shift to the low position. You'll may get chain rub, but that will let you know that your shifter is in the low position. When you know you will need the granny, just loosen the screw like you're setting up the FD again.

I wouldn't bother with the 2 chains. Size for the big/big and you're fine. Try it out and see. Since you understand the issues and what to avoid, it won't be a problem.
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Old 07-26-21, 08:49 PM
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Maybe use a Suntour Triple Pulley rear derailleur? Uses the two rearmost pulleys in the bigger gears and then the third forward pulley takes up the slack in the smaller gears.



How it works. Skip to the 1:05 point in this video.


Cheers
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Old 07-26-21, 08:50 PM
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If I understand this, you love the frame, you don't need all the gears, and you hate using anything but a short cage derailleur. So why not just get rid of the triple and use the short cage without concern?
jon c. is offline  

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