Chain wrap theory
 

So in updating Mrs. Trotsky's Super Sport I goofed up by not considering chain wrap. It has a short cage Shimano 600 SIS rear derailleur (6208), which has a stated chain wrap of 28. Without thinking, I installed a compact crankset (50-34) and and 11-28t cassette. Oops. That works out to 33. What to do? I have not yet installed the chain.

Option 1: Replace the RD with a longer cage model. Boring.

Option 2: Get creative with chain length.

After all, Shimano probably fudges the number a little on the stated specs, right? And chain wrap presumes that you're using the full range from the big/big combo to the small/small combo, when in fact you shouldn't be using either of those, yeah? If I eliminate both of those combos, will that solve my woes? How should I calculate the proper chain length?

set the chain length with Big-big so that it won't explode if she makes a mistake and then see if there is an issue with small-small (might be fine) but it won't seize up even if she accidentally goes there. another thing you can do is just set the RD not to use the 11t depending upon how fast she likes to ride.

not theory, Math ! its (Big + big) minus (small + small) , that is how many links of slack in tooth count numbers
your RD has to take Up.

Greek

You could try riding it, see if it works ok that way. Shift it through all the gears while you have it on the stand, first, of course.

And yes, your chain length will be important here. But 5t beyond stated capacity is the absolute minimum I'd expect from any derailleur I've messed with. I've pushed some a lot farther than that.

What jet boy said. I am major violator of chain capacity numbers. They are generally very conservative, especially if you avoid cross-chaining and the only thing you seriously have to watch out for is a derailleur or dropout destroying big-big situation. Sometimes adjusting the position of the rear axle in the dropouts can also have an effect on the capacity.

You can sometimes push the spec a bit, but one thing you cannot do is put too short a chain on with the idea you'll never use big-big, because eventually the rider will forget and rip the derailleur off the bike. You have to make all your unusable gears on the small-small side where the only consequence will be a slack chain.

- Mark

As others have said, you can cheat a the small-small end with little or no risk, but make sure you have enough chain to handle the big-big, or disaster will befall you sooner or later.

Many of my bikes exceed the theoretical derailleur capacity because I like to use enormous 34-tooth cogs in back. Most of you are made of sterner stuff, I know.

i would start with the small-small combo, one i would never use while riding except in a stupor.

so ... make the chain as long as possible while still allowing the derailleur to keep the chain tight in the small-small combo ... and still able to shift to larger rear cogs without the upper pulley hitting the bottom of the next larger cog (as they sometimes do with a too-long chain).

and that's absolutely as long as you can make the chain .... unless you do the above but instead start with the small-second smallest cog combo and completely forgo the use of the small-small (because the chain will be too long).

When the RD lacks the capacity to deal with enough chain, you either get sag on small/small, or too short on big/big. Too short on big/big, and you risk significant damage to the RD, wheel, and more. Sag on small/small has no such risk.

OF course, if you are close, you may not get sag at all. I am running several RDs past their "rated" capacity without sag. I also have others with noticeable small/small sag.


I use Sheldon't guide on chain length, which is measured using big/big.

No, always start with the big-big. It's not a disaster if can't you use one or more of the smaller cogs with the small chainring because of slack in the chain. It is a disaster if the chain is too short to go onto the big-big. Like some others around here, I have had that experience, and it's not one I'd be eager to repeat.

I'm with jetboy and janwvara.

I don't know about Mrs. Trotsky, but Mrs. Tunes spends time shifting among 4-5 rear cogs and leaves the front alone. Now, our terrain doesn't require it, but if I said "go the the smaller front ring" I'd get a bewildered look.

A big/big fail generally involves an RD, the chain, the RD hangar, and quite likely spokes. Add in the fact that it'd be on an uphill grade, under load, and you add injury factor, worse if clipped in. I saw it once on another's bike, staples in the back of his head were required (no helmet). Better a bike that won't go fast than a large metal road hazard with adjacent soft tissue and bone....

I had a similar fail on a rebuilt 6400 RD (shop built) which mimicked a big/big. The RD mounting bolt snapped, it unwound into the spokes, which of course stopped the bike very quickly, bent the RD hanger, bent the spokes, chain twisted beyond repair, and RT was very much a road hazard. Car behind him swerved to avoid, ran oncoming car off the road. Both stopped, and drivers began to yell and then shove. RT got up and continued up the hill in his cleats, carrying his bike. A former highway patrolman lived nearby, and he gave RT a ride home and said "those two idiots can call a tow truck." The shop repaired my bike free, which included straightening the RD hanger, replacing 3 spokes, replacing the RD, chain, and the labor. I touched up the gouges on the frame.

Still don't know what happened, all was fine and I was not exceeding chain wrap.

This is irrelevant to the main purpose of this thread, but I can't resist:

I was leading a commercial bike tour here in Vermont a few years back, and one of the guests, a guy from Florida, seemed pretty confused when I discussed how to work the shifting on the hybrid bikes the company provides. (You always have to go through this in a fair amount of detail and demonstrate with a bike on a stand, because you can't assume that everyone is familiar with derailleur gearing.)

He asked me what gear we should be in on that day's ride. I told him that we'd probably use most of the gears over the course of the day. He had told me that he rode his own hybrid bike at home, so I asked him how he managed shifting there. He said "Well, the bike was in a good gear when I brought it home from the bike shop, so I've never had to shift."

Somewhat to my surprise, though, he did get the hang of shifting pretty quickly.

In most of Florida, the only reason to shift is for the winds, and it's not even essential if you don't mind slogging into a headwind in a high-ish gear.

Simply remove the cable to the FD

i suggested a method for realizing the longest chain you could install and still use a popular (and inefficient) gear, the small-small. after sizing it, you still need to determine if it is long enough to shift through all gears. if it's not long enough for the big/big, you either need to refrain from using that gear or make changes to the components. i understand some cannot refrain from using certain gears, so the freewheel, derailleur, or both may need to be replaced or modified.

the way i normally size a chain is with the sheldon method, where you put the chain over the biggest cog and big ring, detour the rd, and add two links to where the chain meets itself. if the rd has a long cage, i've found an even longer chain both possible and helpful.

i usually use a longer chain than absolutely necessary, because i highly value my old rear derailleurs.

i don't usually have any issues because i always match the componentry properly.

the problem i have had on my own bikes (when building them up for the first time) with a too-long chain is the upper pulley of campy record derailleurs hitting the rear cog. the chain needs to be short enough to pull the derailleur cage far enough away from the freewheel to work smoothly.

on only one occasion could i not use the small/small because the chain was too long for its granny gear. i made sure the new owner knew this. i see it as a safety issue.

No offense meant! I also like the wrap-the-chain-around-the-big-big-and-add-two-links method. That's the approved bike-school method.

Still, I have to ask: When you wrote "i understand some cannot refrain from using certain gears," did you mean that some people can, with 100% certainty and 100% of the time, avoid certain gears (presumably the big-big)? If you believe that, you have a lot more faith in human perfectibility than I do. But quite possibly I misunderstood you. I think it's nuts to ever ride a bike with a too-short chain.

And you're right, of course--to really be correct, a bike should be able to handle the small-small as well as the big-big. I suppose there is a potentially minor safety issue there, since a slack chain could derail unexpectedly, though the risk is trivial compared to that of a too-short chain. I build up some of my own bikes that way, since I'm fond of 1970s Suntour derailleurs like the VGT Luxe, Cyclone GT, and Vx-GT, all of which have maximum capacities of 34 or 36 teeth. In practice, I often run with a 40-tooth difference, which means I cheat a little sometimes. But I would never build up a bike that way for someone else.

yes, i never use the small/small nor big/big combo on any of my bikes. ever.

but, unfortunately, i can only speak for myself.

i used to tell buyers not to use these two gears as well, but that just confuses them into assuming there's something wrong with the bike. if i send a follow-up email about a purchase, i may send them a link to sheldon's page about efficient gear selection.

on a few occasions during a test ride, i've witnessed potential buyers really struggling with crazy, random shifting. it can be sickening. but it comes with the territory. sometimes, a sale requires quite a bit of hand holding.

I'll take your word for it that you have never accidentally shifted onto the big-big or small-small. But it may happen someday. I had never shifted onto the big-big either, until that one time I did.

In other words, "never have" and "never will" are two different things

But I apologize about going on and on about this--everyone has their own way of doing things. I'll shut up now. And I do have to respect your can-do attitude. Carry on!

I'm with [MENTION=52458]jonwvara[/MENTION] in that I'd rather be safe then sorry. While I try to never go big-big, sometimes at the end of a long hard ride, when weariness has peaked, making a shifting mistake on the final hill can be either a disaster or an inconsequential mistake. I'd prefer the later over the former.

http://i125.photobucket.com/albums/p...psb3lorzty.jpg

http://i125.photobucket.com/albums/p...psn8zyw7lx.jpg

Lets see: :innocent: (50+38)-(31+16)=41 Now that's some chain wrap! :p

The fact that you would only use the big-big in a stupor won't help you at all when your wife does. And suggesting her she was in a stupor won't impove the situation.

Do the big-big first. Too much at stake. Her complaining about the chain dragging in the small-small will be a far smaller negative consequence.

I'm guessing you do not thrive on climbing hills and doing so fast and hard that you hit the top close to seeing stars, deep in anaerobic debt. Most of us who climb with that kind of passion (or used too) have more than once or twice hauled that left lever back as we rolled over the top to keep the speed up.

Ben

I've done the big-big thing many times, always unintentionally. I don't like disasters.

you have me confused with someone else.

How about this one: I have a chain sized to big/big + 2 links but am exceeding the rd capacity by 2-3 teeth. What's the risk here? I assume it's just possible sag in small/small combos but no catastrophic possibilities in big/big, right?

EDIT: I guess I'm just re-framing the op. If I'm correct then big/big + 2 links seems to be the safest way to test/treat exceeding rd capacity. But do correct me if I'm wrong.

EDIT 2: The real problem/risk here is *too short a chain*. As long as your chain is long enough to handle big/big then the primary risk when exceeding rd capacity is chain sag/rubbing in small/small combos. This is not catastrophic so test/ride this way.

EDIT 3: Is it obvious I'm trapped in a snow storm?

I size the chain in big-big, through the derailleur, as it will actually be used. No need to guess how many extra links, no need to worry if cage length is a factor , and 100% sure it's long enough.
Seems the most straight forward method to me.