rear derailleur to cassette - huge gap
10-14-14, 10:16 PM
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rear derailleur to cassette - huge gap
here's my situation:
- first bike build
- chinese carbon frame, brand new
- sram pc-1070 11-28, 10 sp
- sram force rear derailleur, 10 sp, medium cage, brand new
- chain not yet installed
- cable routed, but not yet connected to derailleur
- derailleur hanger looks like this one
- derailleur hanger alignment checked with with park tool DAG-2 alignment guage
- sram recommended derailleur hanger length (L) is 26 - 28 mm
- derailleur hanger length as measured on my frame is 27 mm
- confirmed the b-adjust washer tab is clear of the rear derailleur dropout tab
- the closest i can get the top pulley to the cassette cogs is 9-10 mm, this is with the b-adjust screw COMPLETELY REMOVED
- inserting the b-adjust screw just makes the gap even larger
- gap remains as i push the derailleur in with my thumb along the full range of cogs
- reference picture with a 6 mm hex for scale
at first i thought the hanger was too long, but it measures within recommended range. checked the hanger alignment, can't imagine RD is bent since it's new out of the box, some googling indicates medium cage should work with 11-28t cassette... i'm not sure what else to check. any suggestions?
- first bike build
- chinese carbon frame, brand new
- sram pc-1070 11-28, 10 sp
- sram force rear derailleur, 10 sp, medium cage, brand new
- chain not yet installed
- cable routed, but not yet connected to derailleur
- derailleur hanger looks like this one
- derailleur hanger alignment checked with with park tool DAG-2 alignment guage
- sram recommended derailleur hanger length (L) is 26 - 28 mm
- derailleur hanger length as measured on my frame is 27 mm
- confirmed the b-adjust washer tab is clear of the rear derailleur dropout tab
- the closest i can get the top pulley to the cassette cogs is 9-10 mm, this is with the b-adjust screw COMPLETELY REMOVED
- inserting the b-adjust screw just makes the gap even larger
- gap remains as i push the derailleur in with my thumb along the full range of cogs
- reference picture with a 6 mm hex for scale
at first i thought the hanger was too long, but it measures within recommended range. checked the hanger alignment, can't imagine RD is bent since it's new out of the box, some googling indicates medium cage should work with 11-28t cassette... i'm not sure what else to check. any suggestions?
10-14-14, 10:34 PM
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First install the chain, then make the adjustments.
10-14-14, 10:35 PM
#3
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The B-screw is set for the largest cog (28T) not the smallest. That gap is pretty standard for upper pulley wheel to to smallest cog. Connect the cable, then adjust the B-screw.
10-15-14, 04:45 AM
#4
Mechanic/Tourist
+1 - Thank you for all the info - a welcome change from many posts - but it's far more than we need to determine your problem. Did it not occur to you that rotation of the pulley cage from the chain being installed would take it closer, that the chain would be closer than the pulley to the cassette, or that the chance of standard frame, derailleur and hanger causing a fit problem would be almost non-existent? Install the chain (correct length) adjust the derailleur and worry only if things don't work properly. There's such a thing as being overly meticulous.
10-15-14, 09:10 AM
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Use all the above advice it's all correct.
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It may not be fancy but it gets me were I need to go.
https://www.jtgraphics.net/cyclist_bicycles.htm
10-21-14, 05:45 PM
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Yeah... I don't know about the recommendations so far, fellas. SRAM's own video (2:55) shows them adjusting the chain gap BEFORE installing the chain.
Anyone experience this?
Anyone experience this?
10-21-14, 05:49 PM
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10-21-14, 06:25 PM
#10
Mechanic/Tourist
You asked for advice. If you're not going to trust it why ask? In 20 years of working professionally and quite a few since I've never come across that procedure, nor have I needed to do it to get good operation. Why don't you just hook up the chain and see if things work?
Last edited by cny-bikeman; 10-21-14 at 06:42 PM.
10-21-14, 09:31 PM
#11
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Keep in mind that what you're really doing is a coarse initial alignment. You'll want to fine tune all of the adjustments fully assembled for optimum shifting.
For the record, Shimano's instructions call for connecting the chain and cables before adjusting the B-screw.
Last edited by gsa103; 10-21-14 at 09:36 PM.
10-21-14, 09:42 PM
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That's fine. If you notice, he manually moves the RD to adjust the gap with respect to the largest cog. In general, that's the cog with the most clearance issues. How's the clearance if you shove the RD under the 28T cog? If the clearance looks about right, go ahead and finish assembly.
10-22-14, 06:22 AM
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Curious how this turned out once you got the chain on.
10-22-14, 08:18 AM
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well, there were no suggestions for further tuning, so i slapped the chain on.
the gap didn't change. b-screw still removed.
i'll just join the club and convince myself it's normal.
gotta give props to gsa103 for the calm responses, though.
the gap didn't change. b-screw still removed.
i'll just join the club and convince myself it's normal.
gotta give props to gsa103 for the calm responses, though.
10-22-14, 08:26 AM
#16
Mechanic/Tourist
...yeah, because the rest of us were abusive and impatient, whereas you were open to trying our suggestion to mount the chain and try it.
The medium cage SRAM has a 32 tooth capacity. That is of necessity achieved in part by a longer mounting bolt to top pulley distance. Because it was only medium cage I did not suspect the capacity was that much over your 28 tooth setup.
The medium cage SRAM has a 32 tooth capacity. That is of necessity achieved in part by a longer mounting bolt to top pulley distance. Because it was only medium cage I did not suspect the capacity was that much over your 28 tooth setup.
Last edited by cny-bikeman; 10-22-14 at 08:33 AM.
10-23-14, 10:24 AM
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