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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, 08:51 PM
  #26  
Harold74
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Originally Posted by shelbyfv View Post
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.
I disagree. The cage is a column and a column's propensity to buckle varies with the square of its length. So a 50% longer cage is only about 45% as strong as the shorter cage. Originally I thought that longer cage derailleurs would come with laterally stiffer construction but, from what I can tell, that's not the case. Sure, long cage derailleurs don't fail this way and are, in that sense, well designed. That said, It would not surprise me at all to find that long cage derailleurs tend to age less well that short cage derailleurs for this very reason. The two vintage, long cage derailleurs that I own do seem quite "tired" relative to the vintage short cage derailleurs that I have. Maybe they always were softer, who knows.

Additionally, if you read my proposal in detail, you would know that there will be no sagging chain. I intend to specifically use my gearing in a way that precludes that.
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Old 07-26-21, 08:54 PM
  #27  
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Originally Posted by shelbyfv View Post
Also FWIW, it's the upper pulley that has play, by design.
They both have play, with the lower pully having more of it. The lower pully basically has the upper's play plus whatever flexibility accrues over the length of the cage. I understand that the play in the upper pulley is a somewhat modern design feature meant to make shifting a bit more forgiving.
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Old 07-26-21, 08:58 PM
  #28  
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Originally Posted by Andrew R Stewart View Post
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)
If you read my original proposal carefully, you'll see that I have no intention of exceeding the derailleurs design capacity. Rather, I intend to limit my gearing choices to remain within that capacity. I'll only use my two biggest cassette cogs with the granny ring. I could use the four biggest cogs and still have one tooth to spare without even encroaching into the two extra teeth's worth of capacity that most folks seem to think are characteristic of Shimano derailleurs.
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Old 07-26-21, 09:02 PM
  #29  
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Originally Posted by jon c. View Post
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?
Quoting myself from the OP: "That said, I'd like to preserve the granny gear option for when I tow the dog trailer and/or attempt to climb walls."

Also, I dropped good money on a new Sugino triple this time last year. At that time, I was about two weeks into trying to be a DIY bike mechanic and I just assumed that a triple should be replaced with a triple. I could probably just pull the little ring off of the crankset if I cared to. Frankly, I'm now starting to wonder if it was even a good decision to replace the crankset. The only thing wrong with the original was it being a Biopace which I'm starting to think might actually be an asset for me.
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Old 07-26-21, 09:14 PM
  #30  
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Originally Posted by Miele Man View Post
Maybe use a Suntour Triple Pulley rear derailleur?
Wild! I might feel differently about the long cages if the modern builds were more robust. I've got the derailleur shown below on another bike that I'm working on and I LOVE it. At 40 yrs old or whatever, it's straight as an arrow, has negligible lateral play, and is much stiffer than the Deore that came with my Miyata. Even the jockey wheels are awesomely fat and robust feeling. I'm half tempted to go friction on the Miyata and see if I could make it work.

I'd be happy to double the weight of a modern derailleur in exchange for the added robustness but, again, that surely reflects my particular perspective on things. I kind of just really like derailleurs in general. Last fall I read an entire book on them called The Dancing Chain. Pretty cool stuff.


Last edited by Harold74; 07-26-21 at 09:19 PM.
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Old 07-26-21, 09:31 PM
  #31  
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Originally Posted by Harold74 View Post
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.



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.
Sorry, the engineer crack was intended to be a playful dig. Re-reading it though, I see I came off being a real d!ck there. My bad.
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Old 07-26-21, 09:49 PM
  #32  
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Originally Posted by Harold74 View Post
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.
The catastrophic failure mode is this:

For whatever random reason, you shift into, say, the 26x12 or 14. At speed. Your chain goes totally slack, and drags on the tail end of the front derailleur. But you're pedaling furiously, because you forgot to shift the front and upshifted the rear as fast as you could... so the chain wraps itself around the front derailleur, or folds in half and jams in the cage of the rear, and instantly locks your drivetrain... at 30 mph and 110 rpm. Picture the resulting crash in all its gory detail.

Don't do that.

The drivetrain you're designing allows that to happen. And Mr. Murphy is always waiting,

As I said, dropping your big chainring to a 46 gives you a safety margin that's worth having, at the cost of a few top-end gear inches that you're unlikely to miss. 46/36/26 was very common on road touring bikes, for good reasons.

Since you've got an engineer brain, go here, and move the sliders back and forth and watch the numbers change. Enlightenment will likely follow. (For fun, plug in the gearing that I'm putting together: 45/42/30 and 14-16-18-20-23-26, which is a 7% half-step + granny 6 speed. Then try your 7-speed 12-28 with a 45/42/26... you might like what you see.)

--Shannon
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Old 07-27-21, 06:58 AM
  #33  
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24/34/46, 12-28 7sp, and short cage XT was very common in the early 90s. I ran it with no issues. I think only 3-4 gears were available in the granny but thats all we needed. Short cage for snappy shifting is right.
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Old 07-27-21, 07:10 AM
  #34  
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Originally Posted by Kapusta View Post
Sorry, the engineer crack was intended to be a playful dig. Re-reading it though, I see I came off being a real d!ck there. My bad.

I, too, was a bit dickish with my reply. I had a stressful day and had a couple of customer interactions involving a lack of understanding as to the current market and what a shop's obligation is. One was about this very issue, how to change their gearing without spending much $ (and thus retaining their der). Andy
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Old 07-27-21, 08:31 AM
  #35  
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Originally Posted by Harold74 View Post
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.
What you have is fine because you really are riding it as a double, with an occasional granny.

John

Edit added: And your RD-5700-SS may be called a short cage, but it really is a medium cage. A traditional 7 speed short cage will only give you a 28t wrap. I believe Shimano started to lengthen their cages in the 90’s while retaining their nomenclature.

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Old 07-27-21, 09:13 AM
  #36  
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Originally Posted by Kapusta View Post
Sorry, the engineer crack was intended to be a playful dig. Re-reading it though, I see I came off being a real d!ck there. My bad.
Thanks for that Kapusta, no harm done. Cyber humor is much like under capacity derailleurs... still fun but fraught with limitations potential danger.
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Old 07-27-21, 09:16 AM
  #37  
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Originally Posted by Andrew R Stewart View Post
I, too, was a bit dickish with my reply. I had a stressful day and had a couple of customer interactions involving a lack of understanding as to the current market and what a shop's obligation is. One was about this very issue, how to change their gearing without spending much $ (and thus retaining their der).
Again, thanks for the note and no sweat. It's cool that there are real, bonafide bike mechanics here. If you all ever need any structural engineering done on your homes, do let me know.

I'm almost in the reverse camp where I'll happily spend a ridiculous amount of money for great shifting. I might get an XT derailleur too and just see which one suits me best.
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Old 07-27-21, 09:33 AM
  #38  
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Originally Posted by ShannonM View Post
The catastrophic failure mode is this:

For whatever random reason, you shift into, say, the 26x12 or 14. At speed. Your chain goes totally slack, and drags on the tail end of the front derailleur. But you're pedaling furiously, because you forgot to shift the front and upshifted the rear as fast as you could... so the chain wraps itself around the front derailleur, or folds in half and jams in the cage of the rear, and instantly locks your drivetrain... at 30 mph and 110 rpm. Picture the resulting crash in all its gory detail.--Shannon
I can picture that as I had the misfortune of having a front cantilever brake saddle wire hopping off and catching my tire as a teenager. I've got a fake front tooth to help me remember.

To be clear:

1) Locking the drive train in many instances does not mean that the bike comes to a complete stop, right?

2) Is it my own, high RPM pedaling coming to an instant stop that will really cause the accident?

And thanks for the online gearing resource, that's awesome! The original chainrings on my bike have the middle ring not so much smaller than the larger ring, at least compared to modern bikes. That probably means that it's a half step situation right? I'm new to that terminology but I think I get it: finest gradation shifting would be done by messing with the FD a lot. I tend to change chainrings infrequently but I'm not opposed to altering that if there are benefits. I think that the sugino has a larger step from big to middle but I'll have to verify. Really, I need to figure out all of my gear ratios someday so that I can truly understand what I'm riding.
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Old 07-27-21, 09:50 AM
  #39  
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Originally Posted by Harold74 View Post
I can picture that as I had the misfortune of having a front cantilever brake saddle wire hopping off and catching my tire as a teenager. I've got a fake front tooth to help me remember.

To be clear:

1) Locking the drive train in many instances does not mean that the bike comes to a complete stop, right?

2) Is it my own, high RPM pedaling coming to an instant stop that will really cause the accident?

And thanks for the online gearing resource, that's awesome! The original chainrings on my bike have the middle ring not so much smaller than the larger ring, at least compared to modern bikes. That probably means that it's a half step situation right? I'm new to that terminology but I think I get it: finest gradation shifting would be done by messing with the FD a lot. I tend to change chainrings infrequently but I'm not opposed to altering that if there are benefits. I think that the sugino has a larger step from big to middle but I'll have to verify. Really, I need to figure out all of my gear ratios someday so that I can truly understand what I'm riding.
IME when I hit a big/big combo with too short a chain (which I new was too short....famous "I'll be careful until if fix it" I stopped pretty much dead in my tracks. I fortunately was going very slow behind a stroller, on a trail, on a small uphill so my fall was a monty python slow tip. Had I been going at any speed at all it would have been a really bad crash.

I think it is ill advised to set up a bike that will not use all the gearing just because of "look" at best the solutions like 2 chains etc are fiddly and at worst you can get into a crash or destroy gear situation.

and at at least to my eyes there is not that much look difference

there is a lot of experience here, hope you take advantage, but enjoy the ride no matter what





also from a looks point of view
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Old 07-27-21, 10:39 AM
  #40  
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Originally Posted by squirtdad View Post
there is a lot of experience here, hope you take advantage, but enjoy the ride no matter what.
1) I value that experience and I am listening, whether I chose to to go my own way with it or not. If I'm still capable of typing after the big crash, I'll report back here to close the loop.

2) See, when I look at those two derailleur pics, all I can think of is man does that short cage look sexy next to that ungainly long cage! To each his own I guess.

3) I still don't see the problem with big-big. In my mind, I will set the chain up for big-big just as I would on a true double and, as such, using an RD meant for a double should be fine. I'll do whatever needs doing with respect to the B-screw etc. What am I missing?
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Old 07-27-21, 12:03 PM
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Originally Posted by Harold74 View Post
1)
3) I still don't see the problem with big-big. In my mind, I will set the chain up for big-big just as I would on a true double and, as such, using an RD meant for a double should be fine. I'll do whatever needs doing with respect to the B-screw etc. What am I missing?
You are not missing anything.... other than the fact that some commenters donít understand what you are doing before commenting (like the big-big comment).

And you are also correct that even if the chain got jammed in the RD or FD (which IMO is unlikely), it is not going to stop the bike.

FWIW, I ran a 2x9 setup on an MTB for over a decade with a short cage RD which could not support a small-small combo. Total non-issue.

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Old 07-27-21, 01:59 PM
  #42  
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I forgot that I still have one rig with a short cage and triple. All XT except the cranks
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Old 07-27-21, 02:04 PM
  #43  
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I was thinking about this on my commute this morning: the very fact that short cage derailleurs exist surely suggests that there is some performance advantage, right? Otherwise, why not just use long cage for everything and produce fewer models? It's hard to imagine that the weight difference of 9g of non rotating mass would matter to anybody. For reference, I believe that I myself weigh about 86,000 grams.
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Old 07-27-21, 08:02 PM
  #44  
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Originally Posted by Harold74 View Post
I was thinking about this on my commute this morning: the very fact that short cage derailleurs exist surely suggests that there is some performance advantage, right? Otherwise, why not just use long cage for everything and produce fewer models? It's hard to imagine that the weight difference of 9g of non rotating mass would matter to anybody. For reference, I believe that I myself weigh about 86,000 grams.
The reasons why are varied but one is that the short cage ders are what the pros are paid to ride (and train really hard to be able to not need lower gearing). Too many average riders lemming whatever the cycling media pushes WRT what the racers are using. This is the same with using attractive (sexy) people in ads for products that have little to do with human relationships. Hope, ego and dreaming sells. Practical and effective are far down that list. Andy
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Old 07-27-21, 09:26 PM
  #45  
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Originally Posted by Harold74 View Post
See, when I look at those two derailleur pics, all I can think of is man does that short cage look sexy next to that ungainly long cage! To each his own I guess.
If you want sexy for your 7 speed 1992 Miyata...



Unfortunately you won't get any more wrap. But you won't be riding some generic 105 either.

John
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Old 07-27-21, 10:11 PM
  #46  
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Originally Posted by 70sSanO View Post
If you want sexy for your 7 speed 1992 Miyata...



Unfortunately you won't get any more wrap. But you won't be riding some generic 105 either.

John
That may very well be the single best-looking component that Shimano has ever made.

--Shannon
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Old 07-28-21, 08:25 AM
  #47  
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Originally Posted by 70sSanO View Post
If you want sexy for your 7 speed 1992 Miyata... Unfortunately you won't get any more wrap. But you won't be riding some generic 105 either.
Is that an M900? The best I could find on eBay was a long cage version at $425 USD.

Generic 105?? Man, you bike-o-philes have high standards. So far, I've been riding Acera's, Deore's, and Suntours from 20yrs/30yrs ago. A new105 is nothing short of aspirational for me! I guess I have an Ultegra on a 2000 Airborne but I haven't been on that since 2006...
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Old 07-28-21, 08:34 AM
  #48  
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Just funning you.

Old parts cost are ridiculous now. 10 years ago it was $50. Nothing wrong with 105’s. Good stuff. And it looks good.

While I imagine the newer stuff is better, that XTR is at the top of my list of best Shimano RD’s.

John
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Old 07-28-21, 09:52 AM
  #49  
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Originally Posted by 70sSanO View Post
Old parts cost are ridiculous now.
I hear it. Just last week I nabbed the last two, decent, reasonably priced 7 speed cassettes that I could find on eBay for this bike. I did that in the hope of being able to keep riding the Miyata for another 5-10 years, hopefully, and had to make the following compromises:

1) HG50 which is a downgrade from the HG70 that I think is on there now.
2) Black instead of silver colored. Ick... although it actually matches the original color scheme of the bike.
3) They take up space and there's a 50/50 chance that I'll wind up forgetting where they are before I install them

The XTR derailleur is a beauty, no doubt.

I was riding the Miya yesterday and trying to be thoughtful in studying what I do with the FD which is friction shift. I feel like the greatest safety concern for me would be just getting tired / heat stroke and accidentally shifting down to far when shifting from the big ring to the middle ring. While that likely wouldn't stop the bike, I can see how the abrupt stop to my pedaling could trigger a crash. Part of being the zone, which is usually a good thing, is being in the zone. It would indeed be a sad state of affairs if my lust for structurally sound, crisp shifting led to me not riding for a spell.
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Old 07-28-21, 11:48 AM
  #50  
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Bikes: 1986 Cannondale SR400 (Flat bar commuter), 1988 Cannondale Criterium XTR, 1992 Serotta T-Max, 1995 Trek 970

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I'm not so sure about upgrade/downgrade with 7 speed cassettes. I do agree that the nickel ones seem to stay cleaner. I typically use Sunrace and Shimano cassettes with KMC chains.

As for heat stroke and downtube shifters. You can always install STI shifters.

With 7 speed cassettes, I would take them apart and space them like 8 speed. It doesn't impact the front derailleur, but it lets you run 8 speed shifters. It also helps, a tiny bit with front derailleur trim. The reality is that I'm not running many stock cassettes. Most of them are a mix of cogs to give me what I want. Obviously they are not spidered cogs with a single carrier. But with fewer cogs the few grams do not concern me. I'm old so I use lower gearing. As an example, on my road bike I am running a 14-34 (14-16-18-20-23-26-30-34) which. as far as I know, doesn't exist. I might dump the 34t for a 36t except the gap is pretty huge.

My wife has Claris ST-2400 3x8 (Tiagra FD/XT RD) that seems to work well for her. It has enough trim to work with 8 cogs and it will have more than enough for 7 cogs spaced as 8. She used to have ST-2300 with a double, (flat chainrings). Those are on our son's bike with a 2x7. I have recently gone to Kelly TakeOffs for my 3x8; you can Google them but probably can't find them anywhere. The jury is out on them, as they definitely require some hand gyrations, but I like not having to remove my hand at speed as I did with downtube shifters for decades.

John
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