alexk_il

A noob here, please be gentle

Somehow installing a properly sized chain didn't work well on my first bike project - MTB frame, R7000 shifters, M8000 SGS derailleur with Jteck Shiftmate 8, 11-42t 11 speed on the rear and single 38t on the front, Red 22 chain.

I followed the official Shimano/Park tool instructions (chain wrapped around largest cog/chainring plus adding two rivets). For some reason the chain didn't want to go all the way to the largest ring, maybe because of the incorrect position of the B screw. Or maybe I didn't install Shiftmate 8 adapter properly, hard to tell now. The chain was too tight, the derailleur cage was pointing to 7 o'clock while on 11t, and the chain quickly flatten out the cage before reaching the last two largest cogs.

Well, without even trying to think I simply added 6-8 more rivets to the chain, added cable tensioner, reinstalled the Shiftmate adapter and it all tuned up nicely to shift accurately both with the clutch on and off. The bike survived it's first 20miles ride in the rain and mild mud, the shifting is still crisp and accurate.

Anyway as much as I feel proud of my otherwise incredible achievement, I wonder if I should go back to sizing the chain properly:

• What happens if I don't do that?
• The chain goes three cogs down from 42t if I rotate backwards the pedals by hand. Do I care if I never backpedal in real life?
• Can I simply leave the clutch on all the time to avoid risks of chain skipping when bikes hits road pots and cracks?

Thanks

FBinNY 

First of all the chain may not be too long.

Minimum chain length allows use of the big/big combo, ie. 48/32.

Maximum length allows small/small combo.

Anything between is acceptable, though fine tuning within that range can improve shift performance.

Personally, I a proponent of running chains at or near maximum length for a number of reasons, but to each his or her own.

easyupbug 

I would go back and check it as it was so far out I suspect you had the Shiftmate upside down; upside down you would pull a third less cable than what is needed and you do need to check your B screw position.

Iride01 

Well what didn't work correctly that you attribute to chain sizing?

What was the front difference between your small and large ring on the cranks? Did you exceed the max front diff of 18 teeth that the RD-M8000-sgs is spec'd for?

For me, too long a chain seems to result in poor shifting into the higher ratio cogs on the rear and possibly gives me chain drops. However if you aren't having any issues then don't worry whether it's the "right" length.

easyupbug 

OP has a 1x11, he installed the shiftmate upside down.

oldbobcat

To keep it from flopping around. Also, a chain that's too long makes for more difficult rear shifting because of added chain span from the jockey pulley to the smaller cogs.

And here's a hint for testing chain length after you've cut it. Start on the big ring and work your way up the cassette to make sure it's long enough. That prevents an unpleasant surprise in case you cut it too short. Then shift to the small ring and work your way back down to make sure it's short enough.

Far Side of 50

Too long a chain decreases spring tension on the derailleur cage. Hit a bump, jump a curb and your chain may fall off the chainrings. If it wraps up you may crash.

FBinNY 

I hate to seem picky, but this is an all too popular misconception. RD cage springs typically have well over 5 turns, so even 120 degrees of arc are not going to make much difference.

That said, there are two factors that drive chain tension; leverage and geometry. Longer cages will always produce lower tension simply because of the leverage factor. The math is simple --- Chain tension = spring torque/lower pulley radius (measured from chain to cage pivot). The geometry is trickier, but simply put, the chain tension will decline as the lower pulley sweeps up from 7 to 9 o'clock.

So, it's not necessarily a problem with longer chains, but more of something to think about when trying to optimize chain length. Keep in mind that the RD is only operating in the "slack chain" condition when using smaller sprockets. At the same time, the upper chord tension which is in play when winding the chain onto the chainrings, is only set by the RD when coasting. Once you're turning the pedals that tension is whatever you're making to to propel the bike.

holytrousers

Minimum length also allows small-small combo. So the small-small combo cannot be enough to define the maximum.

Bill Kapaun

Sure it does, but you need to read what FB means.
Maximum that doesn't allow chain sag on Small:Small is what the vast majority of wrenchers would read into that.

holytrousers

"chain sag" is what was missing. That why i said "is what allows small-small combo" is not enough to define the maximum limit.

Bill Kapaun

Than say so instead of acting like an ankle biting Pekingese.
Ignore list because I don't like ankle biting Pekingese.

FBinNY 

You don't know me, but I tend to be fairly precise in my use of the language. In this case, I wanted to avoid the obvious, figuring it was implied.

Moreover, I intentionally didn't mention chain sag, because while the minimum is a hard number because of the potential consequences, there's fudge room regarding the small/small considerations.

It's not rare for folks with triples to use a chain too long for use with the granny and the smaller sprockets. Even if they end up in those combinations, it's a "no harm, no foul" situation.

holytrousers

You are overreacting.
I couldn't say so because it didn't occur to me at that moment, i just saw the beauty of those formulas and automatically saw the flaw in the second condition.

holytrousers

The verb allows, while appropriate for the minimum length definition, is wrong with the maximum definition if you do not precise the sag, allows means a hard limit as you have mentioned. I liked your formula and it struck me with it's simplicity but i felt there was something wrong with it so i thought you would appreciate my contribution. Some people are very fast at judging others and that's sad.

FBinNY 

Yes and no. "Allows" is one of those "big" words with plenty of subtle shades of meaning, from permitting to enabling, and beyond. Ie. I don't allow my teenage daughter to stay out past 11PM, or by cutting them some slack I allow my children to grow and mature.

Here (as I choose to use it) it implies a functional consideration. I give readers credit for realizing (even if by trial) what isn't allowed for the big/big combo. At the same time I'll allowing for their discretion at the opposite end.

Sometimes, depending on the context, I clarify the rule by referring to a hard or absolute limit vs, a soft, discretionary limit. However when posting to forums, I allow the context and circumstances to determine how I balance perfect clarity against brevity.

Please note that Allow has plenty of shades of meaning in the above passage.

BTW- thank you for liking my "rule" and feel free to use or post it any time, adding more info at YOUR descretion.

holytrousers

Humm, i'm not convinced because you used the same verb in a quasi mathematical set of rules : The first one describing the impossibility to go shorter, it implied that the second also described such an impossibility. Even your emphasis on the importance of the second rule implied it wasn't soft.
But it doesn't matter and i'm glad you appreciate my appreciation

hokiefyd 

I like the way you wrote this, and I think it makes good intuitive sense. Minimum length describes the shortest possible chain that physically permits big/big. Maximum length describes the longest possible that permits the small/small without dragging the chain on itself due to the cage angle. Anything between these two lengths will work well in most cases, though certain optimizations might be possible in certain situations.

Like you, I prefer longer chains to shorter ones.

alexk_il

• It didn't want to go up. The chain was stretched and it felt that if I press the lever harder something would snap
• Single speed on the front, the difference is zero teeth
• The only issue is the chain dropping to smaller cogs while backpedalling

alexk_il

The derailleur has a clutch to prevent exactly this from happenning. Do I still need to worry?

tFUnK

Not an answer to OP's question, but an anecdote that may be of relevance:
I was setting up a road bike, 34/50 up front, 11/32 in the rear, with an appropriate rear derailleur. I had a few used chains with acceptable stretch in the bin, and I brought them out, cleaned them up, etc, and hoped there would be a properly sized one in the bunch.

I took one chain and sized using small/small and experienced just what the OP did - the last cog (32t) simply won't clear and if I forced it I felt like it could have broken something. I added links but it made the small/small feel unacceptable. I went back and forth a couple times, including making b-screw adjustments, and was stumped.

Then, I just so happened to grab another one of my used chains, and found that it matched the correctly-sized length. I put it on, made no additional adjustments to the RD, and everything just worked as one would expect. I then switched back to the first chain and again was having the same problem of not being able to get on the 32t.

Needless to say, I went back to the chain that worked and called it a day. Not sure if the links were too stiff, or the plates shaped slightly differently, or what. If anything, the problematic chain had the less visible wear of the two. Both were Shimano, one was an Ultegra, the other was either an older Ultegra or Dura Ace.

hokiefyd 

I've experienced what tFUnK has as well. Sometimes, maximum small/small chain length will be too long for the specific derailer-and-cassette combination, especially if the large sprocket is at or even just near the derailer's largest allowable. This is not a problem on most of my bikes, but is a problem on a few.

As an extreme example, I'm using an old 105 RD-5600 on a '70 Peugeot, but the derailer's springs appear to be fairly loose and the B-spring won't keep it out of the 28-tooth large sprocket with the chain length that I prefer. I could have significantly shortened the chain to get more cage movement in the larger sprocket, but that brought other side effects. I ended up using a road link to lower the derailer down so that I can run a longer chain and still have the derailer clear the large sprocket.

bikebikebike

Where this may be a real non Pekinese issue is on boom adjustable tadpole trikes.
Boom adjustment is like seat height there, and may produce a problem and cause distressing failure, when on a steep hill going to Big-big cogs.
Boom adjustment changes chain tension (duh) , but may not get noticed , as the adjustments may be incremental
and attention focused elsewhere. Easy to overlook, as Big-big combo is not used as often there, or when tweaking adjustment.

alexk_il

Accidently found one reason to fix it. I have to manually roll my bike backwards from my garage. I just discovered that rolling the bike backwards is somehow equivalent to backpedalling; the chain makes clicks till it eventually drops down by three cogs. This is not a real issue and other than that all is good. Nevertheless I find this to be annoying and will eventually fix it.

FBinNY 

Sorry to be the messenger of bad news, but "auto-shifting" when backpedaling isn't fixable. It's inherent in the system.

Understand that the chain is precisely fed onto the sprockets by the derailleur, and in order for things to work nicely it has to be easy to move the chain from sprocket to sprocket. After all you wouldn't want to struggle shifting because of a willful chain that doesn't feel like moving over.

OTOH- there is no guide on the top of the cassette, so the chain is free to do whatever it pleases. The same "gates" (cut down or specially sculpted teeth) that improve shifting now make it easier for the chain to do it's thing when unguided. As a rule, the chain will shift towards better chain line, ie, towards the center of the cassette, or down to smaller sprockets because of gravity.

Your basic options are to either shift toward the middle of the cassette before having to walk the bike backward, or shift the RD to match where the chain has moved to so it spools smoothly and is ready for you to start riding.

It's not a big deal, and nothing to fret over, it's the nature of the beast, and for you to accept and adapt.