Have a Trek 100 with 3X7 drivetrain. Chain sags when it is on the small-small sprockets. I measured the chain with a chain checker and it is worn but not at the change out point. The bike shifts well. Is the chain stretched or too long(too many links)?

Top picture seems to be big-big. Bottom picture appears to be small-small. Top picture looks like there is some way to go before the rear dérailleur is fully extended. Guess your chain is too long. Rumour has it you ignore the dérailleur, wrap the chain big-big and add two links in order to get the right length. I may be wrong about how many links you add... I may be thinking too simplistically but I would be surprised if any stretch you measure would result in the the rear derailleur being under extended to the extent that it currently appears to be.

Yes, your chain is probably longer than need be to allow big-big so you may be able to shorten it a bit.

BTW it's not rumor, it's proper technique. Loop the chain over big-big without going through the rear derailleur. Overlap the ends and add 1" (two half links) and make the chain that length. Be sure the ends are a proper match (inner/ outer if using a joining pin, two inner links if using a master link). If you come up between links, go to the next longer pin. Then route the chain properly and join the ends.

Slack in small-small isn't a major problem as you should never use that combination and it's just a nuisance if you do, not damaging despite warnings to the contrary.

^ What he says.

I disagree, as you well know. Yes, you should never use the small/small combination but you shouldn't use the big/big either. But with the chain as slack as El Gato27's, you risk chain bounce that can trap the chain in a chain stay or the chain can end up wrapped around the derailer and will end up tearing the derailer off the mount. In my experience, this kind of damage is far more likely and far more common than damaged caused by having the chain too short.

^ What he says.

Well, the possibility of damage is there but it gives you ample warning. As soon as you shift into the small-small combination under these circumstances the noise and rattling you will know it doesn't feel or sound right and you will shift out of it promptly. Having the chain too short for big-big gives you no warning at all. As soon as you shift into it, the damage is done immediately.

FWIW I will set things up so I can use big-big and small-small. Saying 'never' would mean I have to give up on using the big/small cog on the back. Others may be situationally aware but I'll stick with the limit screws and thread locking compound along with a Goldilocks, just right, chain. I guess Murphy will still apply.

Speaking of Murphy I would like to claim the Godwin's Law version of Chain Lubrication and call it Chorlton's Law. I do realise there is prior art but I am just going to plagiarise and name it.

So.... about that chain lubrication thing?

I shortened the chain, no more sag. I did have to adjust the b-screw. New pictures attached.

el gato27, If you aren't changing the chain, at least remove links two at a time until it isn't drooping. The chain is, IMHO, loose enough to cause damage.

The chain still has to fit big-big in the end. I can't tell by the photo if the RD can cope. On big-big will the cage still move forward?

Brad

Big-big picture

I suspect you could have just turned the "B" screw in a turn or two and removed say half the slack and everything would have been find.

Observe if the drivetrain feels quieter now or before you removed the links when you are in your normal gears. If it was better before, add those links back (maybe with your next chain) and tweak the "B" screw to pull the derailleur back. See if that seems better. I have run many triples with real slack in the small-small and had no issues at all since it only gets used going uphill then the hill has leveled out for short stretches. If after my suggested tweak, the small front-next to small rear is solid, I'd just call it good. In fact, I'd call it just about perfect.

Ben

That's the correct way assuming your rear derailleur has sufficient "wrap" capacity to handle the total teeth involved. I tend to exceed the rear derailleur's rating on my bikes by changing the standard 30T granny chainring on my road triples for a 26T. Even with a GS road rear derailleur, it's a couple of teeth over Shimano's rating. That said, the slack in small-small is minimal and certainly not enough to cause damage from sag.

El Gato27 I am not yet there myself. Your latest small-small shows the chain is no longer dragging on the top Jockey Wheel. Your latest big-big suggests you still have some leeway to go shorter but..

Dérailleurs suffer from 'degrees of freedom'. Reducing the length of the chain will wind up the dérailleurs springs and pull the top jockey wheel closer to the big cog. Then you adjust the B thing.

Looks like you are already on the case.

Now that everybody has weighed in, let me toss a con wrench into the mix.

I suggest folks look at the photo, and read the question more carefully.

First of all the, at the risk of being called Goldilocks, the chain is neither too long, nor too short, and is just right as it is. Note that the RD has moor travel room in either direction, so there's no reason to make any change.

Next, the chain sag isn't the result of chain length but is a normal artifact of changing spring tension as the cage rotated through it's arc. As the chain takes up slack with smaller sprockets, the spring gets more relaxed, and the chain tension. ALL derailleurs do this, and it's a progressive effect, with more tension with bigger sprockets and lower tension with smaller.

The amount of tension and how much it changes is a function of the derailleur's design, relating to the spring used, and the number of turns it has. Odds are that the chain always sagged this way, but the OP never noticed it because he never had a reason to. (and still doesn't).

As for any change being related to chain stretch (wear), consider that we consider chains toast long before they "stretch" 1/2" overall, no exactly an earth shattering factor.

So, to the OP, everything is OK, continue checking your chain for wear, keep it lubed and ride your bike instead of obsessing over every observed anomally.

Or just turn the "B" screw in one turn and call it good!

BEn

Original photos suggested it was wrong. New photos suggested it was better.

Chorlton's law says we have to get into a discussion about Chain Lubrication. I see you are already there. Perhaps your product is in the Goldilocks Zone.

Actually, I was referring to the original photo, which was also OK.

There's a range of OK chain lengths, and both the original and shorter are within the RD's working range.

I guess we will have to agree to disagree which basically means you can have an opinion but you are wrong. The original, small-small, in conjunction with the chain length was outside of the working range. Slack chain and small-small it was running against the upper jockey wheel.

No problem, I've been told I was wrong before, and am sure I will be later.

I'll try that next time. Timing is everything.

I've ridden many triples set up so that was the case. Hardly an issue unless for you it is an issue. It it works, and I usually find that it does, for me it is simply not an issue. I just tune out the noise. If I hear it then, good. Reminder that I am in the small-small and I don't want to spend a lot of time there. I really don't like tight big-big combos and will take a loose small-small instead any day. (The big-big can make rear derailleurs old very fast. Never killed one on the small-small.)

Ben

The chain catching on the derailer when the chain bounces which results in the chain tangling and twisting off the derailer doesn't give much warning at all. It most often happens when you hit a pothole or some other object that causes the drivetrain to bounce.

On the other hand, when shifting into a big/big with too short a chain, you feel it which should be ample warning that you are doing something wrong. If you have to apply enough force to bend axles as others have claimed, you are applying way to much force.

And, since this is the Mechanics forum, I would hope that someone putting on a new chain would run through the gears to see how everything works before taking it out on the road. A chain that is too short for the big/big combination is immediately apparent and anyone who has a bike set up that way...there are reasons to do it...should be well aware of the problem and have no one but themselves to blame if they force it and damage the bike. I almost guarantee that they won't make the same mistake.

If the chain running from the top of the cassette to the chainwheels didn't have slack in it, you might have a valid point. However, there is no more room for the rear derailer to travel in back to take up the chain slack because the chain is slack.

Yes, all springs loose tension as they get near the end of their arc. But that's something that you should adjust chain length to avoid. Once the chain is as slack as El Gato27 is in his original photo, there chances of the chain bouncing and catching on some part of the frame, derailer, tire, etc. is greatly increased. Entangled chains result in far more damage in my experience than a chain that is too short. The lower chain at the derailer is slack enough that it could bounce up, catch either the inside or outside of the derailer cage and rotate the derailer right off the hanger.

We can debate this until the cows come home, and nothing will change. If the OP hadn't shortened the chain I would have bet 3 beers to one that he could rotate the cage back (help the spring) and the chain would come taut (top and bottom) before he hit the stop or the chain doubled over itself under the jockey wheel.

But the original question posed was whether this was a problem, and we need to accept that he's been riding it this way since he got the bike and no problem has manifested, which I feel answers his question persuasively.