#1
|
||||
|
||||
Chain skipping - too long?
Just took a test ride on a bike I've been overhauling.
Chain is brand new SRAM PC-1091. 114 links and I've taken out 4. Cassette is like new 11-32 SRAM PG-1070. Chainrings are FSA 52/36. 6700 short cage rear derailleur. Seems to only skip when on the 36 chain ring and on the 11 and 12 cogs under load. On the repair stand, it's fine. I see no movement of the RD with a video I took on the phone. Everything seems to stay aligned and in place. I thought I cut the chain to the proper length. Pics below when in the 36 chainring and 11 and 32 cogs. Should I actually go shorter? B screw is about 2/3 the way in. |
#2
|
||||
|
||||
Short cage Ultegra capacity 35t, you are at 37t. Looks like you will be to tight if you take a link out.
How to calculate tooth capacity (largest cog – smallest cog) + (largest chainring – smallest chainring) = required capacity For a modern, double chainring road bike drivetrain, we would have something like: (32 – 11) + (52 – 36) = 37t capacity For the sake of example, let’s assume you are looking at a Shimano 11-speed road derailleur here. The total capacity of an SS (short cage) Shimano derailleur is 35t and 39t for a GS (medium cage) derailleur. As such, in this circumstance, you would require a medium cage derailleur. I would try it with an old chain first, it looks like you may have contact between the bottom run of chain and the derailleur cage wrapped chain going over the pulley on small/small. The only in-between might be to wind the B screw in or a hanger extender.
__________________
Marc Sasso A part of the resin revolution! Last edited by m_sasso; 06-05-2023 at 06:43 PM. |
#3
|
|||
|
|||
Re: the skipping. Put the chain on that 36 x 11 or 12.
Turn the cranks backwards slowly and observe the chain as it exits the bottom derailleur pulley. If you see a link that is not straightened out, you then have a tight link in the chain. Grab the chain on each side of the stiff link and work the chain laterally with your hands until it rotates freely.
__________________
http://hubbardpark.blogspot.com/ |
#4
|
|||
|
|||
It's unlikely a chain skip will be caused by a chain that is too long, so you should look for another cause. One possibility is what Peter P. suggests, a stiff link.
But I'm wondering if the skip is really at the cassette. You say it only happens when in the 36 (inner ring) and on the 11 and 12 sprockets (outer sprockets). It is not unusual on modern drivetrains that the chain will rub against the big chainring when the chain is on the small ring and on the outer 1 or 2 sprockets (extreme cross-chainging angle). And since the big ring has pick-up pins to help with shifting, the chain will often get caught on the pins, which can cause the chain to lift up and then drop again, sometimes skipping on the chainring. The propensity for this to happen increases with the degree of cross-chaining, and with the differential in chainring sizes (you've got a 16 tooth differential). If the problem is the chain getting caught on the outer ring's shift pins, then the solution is simply not to use the smallest sprockets when in the small chainring. There's several other reasons to avoid these gear combinations, even if the chain isn't getting caught on the shift pins. |
#5
|
|||
|
|||
what crankset?
Wondering if Mark is on the mark and this is a chainline issue? Could a spacer installed under the bottom bracket cup move the rings out an mm or two and prevent the chain from catching on the chainring?
UOTE=Mark McM;3254925]It's unlikely a chain skip will be caused by a chain that is too long, so you should look for another cause. One possibility is what Peter P. suggests, a stiff link. But I'm wondering if the skip is really at the cassette. You say it only happens when in the 36 (inner ring) and on the 11 and 12 sprockets (outer sprockets). It is not unusual on modern drivetrains that the chain will rub against the big chainring when the chain is on the small ring and on the outer 1 or 2 sprockets (extreme cross-chainging angle). And since the big ring has pick-up pins to help with shifting, the chain will often get caught on the pins, which can cause the chain to lift up and then drop again, sometimes skipping on the chainring. The propensity for this to happen increases with the degree of cross-chaining, and with the differential in chainring sizes (you've got a 16 tooth differential). If the problem is the chain getting caught on the outer ring's shift pins, then the solution is simply not to use the smallest sprockets when in the small chainring. There's several other reasons to avoid these gear combinations, even if the chain isn't getting caught on the shift pins.[/QUOTE]
__________________
Crust Malocchio, Turbo Creo |
#6
|
||||
|
||||
Quote:
BTW-DON'T do any spacer type gig to make this not happen,,Just don't ride there. small ring, small cogs . Spacing out the big ring will have an effect on FD shifting and on some systems, just result in chain rub on FD when in big ring.
__________________
Chisholm's Custom Wheels Qui Si Parla Campagnolo |
#7
|
|||
|
|||
What I've seen happen on bikes ridden much in the small-small sprockets is that eventually the lower pully teeth wear sideways, and then the chain is found to have worn a deep pathway into the inner cage plate, at which point the chain can derail the cog and get jammed between the pulley and cage.
Very not good. If caught in time, it's an inexpensive repair on say an Ultegra derailer (with $17 replacement cage plates being widely available). The other causes of this skipping could be worn smallest cogs or pulleys having too much rotational drag. Recent compact doubles have wider chainring spacing with a sloping inner face applied to the big ring. This was to help prevent chain drop when downshifting, but also enables quiet operation while cross-chaining. If we take a cue from Shimano's Di2 algorithm, we find that the shifting system will not allow such cross-chaining, because Shimano is well aware of the issue having to do with the lower pulley and inner cage plate wearing into potentially-dangerous destruction. |
#8
|
||||
|
||||
I was rushing the inspection yesterday.
Slowed down and looked from the other side. In 36-12, there is chain-chain contact. Thought I had read that the 6700 RD could handle a 32 cog when I was overhauling this thing but I guess not. Will have to go back to the 11-25 that this bike came with but I'll keep the chain long enough for a 28. I wanted to keep the 11-32 cassette for myself anyway. |
#9
|
|||
|
|||
If you use wax lube, sometimes it builds up in the valley between the small cogs. That will prevent the chain from fully seating on the cog and it will skip.
|
#10
|
|||
|
|||
Quote:
|
#11
|
||||
|
||||
See post #2
__________________
Marc Sasso A part of the resin revolution! Last edited by m_sasso; 06-06-2023 at 01:22 PM. |
#12
|
|||
|
|||
Quote:
I find myself using Shimano's 12-30t cassette for mainly 126mm-width 10s builds using short-cage 6600 or 7800 rear derailers. No problems thus far, and the 12-xx 10s cassettes can be fitted to unmodified Shimano 7s, 64xx Hyperglide freehubs using a longer-threaded SRAM alloy 11t lockring: |
#13
|
|||
|
|||
get a mid cage derailleur
and do what you want.
__________________
Crust Malocchio, Turbo Creo |
Thread Tools | |
Display Modes | |
|
|