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Chain sag on small cogs!

Old 08-10-19, 05:45 AM
  #1  
Dafydd_Cymru
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Chain sag on small cogs!

Hi! I have a Voodoo Bazango. Never noticed before but on the small front cog and 2nd smallest and smallest rear cog, the chain sags at the bottom (photo coming). It sags so much in rear2nd smallest that it's just touching the frame, same with the smallest rear. Gears change quick and smooth.

Questions:
Is this normal?
I read that the low rear cogs on the low front cogs are never used?

Many thanks I'll take some pics now.
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Old 08-10-19, 05:47 AM
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Ah sorry, I need 10 posts before I can upload images.
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Old 08-10-19, 05:59 AM
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If the chain has been replaced it may have been sized too long. Also possible there is a problem with the pulley tensioning spring. "Never" is an exaggeration, but especially the small-small combo is generally avoided. On almost all bikes the next smaller chainring with one of the larger cogs is close to the same gear ratio. Running the small-small is much harder on your chain and cogs - fewer teeth engaged and a large chain angle.
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Old 08-10-19, 06:08 AM
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Sounds like a chain wrap issue. A longer-arm rear derailleur would likely fix this. Sometimes, all that is needed is a little adjustment of the "B-screw" on the derailleur.


https://www.parktool.com/blog/repair...eur-adjustment
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Old 08-10-19, 06:17 AM
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Ah noticed that when I adjust, is it the B screw, to widen the gap it corrects itself. But is the gap between the rear large cog and inner derailer cog supposed to be about 6mm? It was sold with a wide gap but I watched a video by park tool about gap being small? Is the Idea to adjust the B screw to get the smallest gap without sag? Reading your link now.

Last edited by Dafydd_Cymru; 08-10-19 at 06:28 AM.
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Old 08-10-19, 06:31 AM
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Thatsthe video I got the 6mm from...maybe mine is a lot bigger.
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Old 08-10-19, 08:52 AM
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As long as you don't change the size of the largest cog on your cassette and the largest chainring on your crankset during the life of the chain, here is a summary:

The chain should be sized by running it around the cassette's largest cog and the crankset's largest chainring, without running the chain through the rear derailleur, and finding the shortest point on the chain where the chain could be connected together, and then add one inch (from one pin to the other is one half-inch, so skip one pin and go to the next to get the inch measurement).

If, in doing this, you are "between links" in finding the nearest point where the chain can be connected, size up rather than down; better to have a chain that is a half inch longer than it needs to be than a chain that is a half inch too short. This chain sizing method is often referred to as the "big-big plus one method," and will give you the shortest chain possible to work safely with your drivetrain. Keep in mind that the most important rule in chain length sizing is that the chain absolutely must be at least long enough to safely shift to the largest chainring/largest cog combination. If not, even if you are determined to never shift to that combination, all it takes is one brain lapse and you will likely do major damage to your bike.

If you use the "big-big plus one" chain sizing method, and find that your chain is slack when in the smallest chainring and smallest cog(s), then your rear derailleur doesn't have enough chain wrap capacity for your drivetrain. As you have found, the result of this is usually not a big deal. Those gears are rarely used anyway, and the bike will usually still work okay unless it is an extreme case of too much chain for the derailleur to handle.
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Old 08-10-19, 01:06 PM
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Here's the way I check chain length on the bike.
With the chain on BIG:BIG, see how much slack you can "pull". Pull it snug, but don't force it.
Anything over 2 link SETS is excess.
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Old 08-10-19, 01:25 PM
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Originally Posted by Dafydd_Cymru View Post
Gears change quick and smooth.
Never Fix A Running Piece.
You shouldn't be using the small cogs when in the small chain ring anyway, it is called cross-chaining and it wears the drivetrain more quickly and is less efficient than using the equivalent ratios on the larger chain ring.
If you decide to mess with it anyway and want to shorten the chain, you MUST always have the chain long enough to go onto the big cog and big chain ring without strain, which you shouldn't do either (cross-chaining again) but you might forget.
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Old 08-10-19, 01:31 PM
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@Dafydd_Cymru's photos:

https://www.bikeforums.net/g/user/503957
https://www.bikeforums.net/g/album/14804753


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Old 08-10-19, 01:41 PM
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Chain too damn long or no tension on the DR

Last edited by trailangel; 08-10-19 at 01:46 PM.
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Old 08-10-19, 01:48 PM
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Pictures show a chain that is a bit too long. @Dafydd_Cymru: Could you post a similar shot of the chain on the largest rings and cogs?
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Old 08-10-19, 02:00 PM
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I think you have the chain too long. See @Bill Kapaun's notes about chain sizing, big/big rings. But, removing a single double link pair may be enough, although I like to size my chain so it is in the middle, not over-streched on the big/big, but not too lax on the small/small.

As @dsbrantjr mentions, the problem occurs with cross-chaining, small/small.

Your derailleur looks like it should be able to take up a fair amount of chain. I can't see exactly your cassette/chainring details. The modern systems aren't designed for 40+ teeth on the rear + a triple on the front. But, I don't think that is the case on your bike.

Here is the "B-Screw" adjustment. It might help a little bit, but I'd check the chain length first.

Derailleur_Adjustment.jpg
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Old 08-10-19, 02:33 PM
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Originally Posted by Moe Zhoost View Post
Pictures show a chain that is a bit too long. @Dafydd_Cymru: Could you post a similar shot of the chain on the largest rings and cogs?
Here is @Dafydd_Cymru's Big/Big photo.



It looks to me like the chain could be shortened at least one double-link, if not a pair of double-links (2 or 4 single links).
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Old 08-10-19, 06:22 PM
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Originally Posted by CliffordK View Post
Here is [MENTION=503957]
It looks to me like the chain could be shortened at least one double-link, if not a pair of double-links (2 or 4 single links).
Agree. Remove one double-link and if still not satisfactory do another.
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Old 08-12-19, 05:09 PM
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hey there Welshman, or woman,

check out this reliable, well made, how to vid.
My guess also is chain is a bit too long, its easy to find out, if you can figure out how to undo your chain, hopefully it has a quick link.


easiest way is once chain is apart, big big , not through derailleur, and add two links.

watch vid, and read up on it to be sure, its not a hard thing to do, and if you've never taken apart a quick link , (if your chain has one) its a good exercise to do. Can be a bit fiddly, but good to know how to fight with one and remove it and put it back on.
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Old 08-12-19, 05:35 PM
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My question - do the slack chained small-small combinations work? Would you use them if they did?

On most of my bikes, those combinations run funky. On some, the chain fully doubles back on itself and rubs. But on all the bikes where the next chainring doesn't pick up the chain (cursed pick-up pins), these combinations are ride-able and I use them, although not for long periods of time. (Say when a tough climb levels out for a short stretch and I really do not want to do two double shifts.) That they are noisy is a good reminder not to stay there.

Edit: looking at CliffordK's first photo of the OP's bike, small-small - looks to me like the spring for the derailleur cage isn't tight enough. The cage is not pulled all the way back/up and not fully tightening the chain. (It won't take out all that slack but it should be taking out more.) I do not know the Deore derailleurs but on the old SunTours, we would just take the cage off, open the spring cylinder and wind the spring a a notch or more tighter and reassemble. I'm guessing someone here can say how that is done with Deores.

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Old 08-12-19, 07:43 PM
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Mr mooney, I dunno, I cant imagine that a fairly new deore rd is slack. Heck, I have a nearly 30 yr old one on my old touring bike that is still good and snappy, and have a 9 or 10 season old one on one of my bikes, same thing.
I also dont know if you can change this spring tension, like on older v brakes with changing the hole the spring goes into.

it seems to me that in the big big photo, the rd isnt as far forward as it is when I do this funky test on my bikes, which I really dont like doing....which would point to the chain being a bit too long.

if his chain is a shimano, it prob wont have a quick link, so probably not encouraging to do a chain length test.

who knows, maybe this was a replacement chain put on without measuring, and put on with all the links on it in box.
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Old 08-13-19, 05:30 PM
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Yup chain too long.
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Old 08-14-19, 01:09 AM
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Did the bike come with these cassette chain rings, and rear derailler hanger?

From the Shimano web site:

Total capacity 41T

Do the math. Big chain ring number of teeth + biggest rear cog # of teeth. Then get the number of teeth on the small front + small rear. Is the difference greater than 41?
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Old 08-14-19, 06:02 AM
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hey there Dafydd,
did you put this chain on your bike?
If you did, we always have to take a few links off a new chain, and we have to do that chain length test, "big-big not thru derailleur" to see how many links to take off.
Did you buy it new, used?

get back to let us know.

Had family in Aber for years btw
cheers
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Old 07-26-20, 09:52 AM
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Solved

First time running serious hills on my new bike. Small chainring, and largest rear cog for the main climb. Once that had levelled out and before the next climb I went through the gears to a smaller rear cog and found a load of slack (probably more than what the OP has shown).

Solved by tightening the B screw, which took up all of the slack. As an added bonus, turns out my pulley and cog were too close together and my chain was getting pinched in the large chainring and largest rear cog. Now this is also solved.

Not planning on cross chaining for long periods but in amongst climbs it's better than double shifting.
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Old 07-26-20, 10:15 AM
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Originally Posted by Bill Kapaun View Post

Here's the way I check chain length on the bike.
With the chain on BIG:BIG, see how much slack you can "pull". Pull it snug, but don't force it.
Anything over 2 link SETS is excess.
Thatís a great idea for a chain thatís already on the bike. The one refinement I would suggest is to use hemostats instead of two screwdrivers
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Old 07-26-20, 10:27 AM
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the welsh dude came, got a few answers and then buggered off.
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Old 07-26-20, 10:56 AM
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Originally Posted by cyccommute View Post
Thatís a great idea for a chain thatís already on the bike. The one refinement I would suggest is to use hemostats instead of two screwdrivers
Don't know why?
2 screw drivers work fine, especially if you aren't trying to hold a camera in the other hand.
A person could use a couple nails as far as that goes.
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