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Shimano 7800 won't shift into smallest cog

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Shimano 7800 won't shift into smallest cog

Old 07-25-21, 06:23 PM
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Nessism
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Shimano 7800 won't shift into smallest cog

Brand new SRAM 1070 cassette won't shift into the smallest cog with my Shimano 7800 shifters and derailleur. Chain checks fine and system worked okay with my old wheel/cassette. Cable is clean and recently greased. Limit screws properly set.

I'm wondering if the cogs themselves are in need of deburring or something like that.

Or bite the bullet and buy a Shimano cassette.

Open to suggestions. I searched for answers but didn't find anything appropriate.
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Old 07-25-21, 06:35 PM
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Cassette should be compatible. Cable tension? Derailleur alignment?
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Old 07-25-21, 08:09 PM
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During the 10-speed era, weren't there different specs for different hub/cassette combos that required different thickness spacers to be used? Maybe you’re suffering from this. Is there a spacer, or two, behind your cassette?
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Old 07-25-21, 09:13 PM
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Some of the cogs have spacers between them, but the last 2 or 3 gears on on a spider which goes on first w/o a spacer.

Since posting this I ordered a Shimano cassette too see if that's any different. I also verified none of the gears are flipped backwards.
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Old 07-26-21, 06:00 AM
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Originally Posted by Nessism
Limit screws properly set.
I am a bit confused here, because if the limit screws are properly set then it should go into all the gears otherwise how can you set them properly?

So does it not go in only while while riding? Were the limit screws set with the bike upright? Are they the settings for the previous hub? With the limit screw fully backed out, can you see the derailleur not moving out far enough, and what is stopping it?
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Old 07-26-21, 07:44 AM
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Originally Posted by Geepig
I am a bit confused here, because if the limit screws are properly set then it should go into all the gears otherwise how can you set them properly?

So does it not go in only while while riding? Were the limit screws set with the bike upright? Are they the settings for the previous hub? With the limit screw fully backed out, can you see the derailleur not moving out far enough, and what is stopping it?
So again, the derailleur doesn't like to move the chain to the smallest sprocket. The chain will drop sometimes, but it's hit or miss and requires pedaling very slowly. I've backed out the rear derailleur limit screw all the way, such that it is not restricting derailleur movement at all. The spring inside the derailleur pulls the derailleur to a fully retracted position so to speak but even that position doesn't move the chain over far enough to nudge the chain over onto the smallest cog. When in this position the cable tension is low, with even some slack. I checked the derailleur hanger alignment and tweaked it a smig; previously it was slightly open in front but now it's pretty much square/parallel to the rear wheel. I've also tried fussing with the B tension adjustment, but that doesn't seem to have any affect either.

I'm wondering if I should remove the rear derailleur completely and soak it in mineral spirts to clean it? Maybe there is some grime keeping it from fully retracting like it should? This bike is not a high mileage ride, in fact, it's been babied for most of it's life until recently. Maybe I'll try swapping gears from an old cluster onto this new one? With the old rear wheel the bike didn't have this issue. It showed up with the new wheel/cog set.
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Old 07-26-21, 08:05 AM
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Originally Posted by Nessism
I'm wondering if I should remove the rear derailleur completely and soak it in mineral spirts to clean it?
Try shooting some WD-40 or solvent of your choice on the pivots with the wheel removed so you can manually push the derailleur checking for stickiness. A longshot but possibly some crud is stuck between the return spring coils which could cause derailleur to hesitate sometimes trying to move all the way to the small cog and some solvent might help remove as well. Also, how many miles do you have on this derailleur? Could be just wearing out too.
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Old 07-27-21, 09:29 AM
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Cleaned the derailleur in mineral spirits and it's movement is smooth and snappy. I started to reinstall the rear cassette but stopped after noticing that the smallest cog sits quite a bit proud of the freehub body. It seems that the stack is too tall. I've used SRAM 1070 cassettes before with good results so this is confusing me. I've got a Shimano cassette scheduled for delivery today so I'll compare the two and then go from there.
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Old 07-27-21, 09:45 AM
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Originally Posted by Nessism
Some of the cogs have spacers between them, but the last 2 or 3 gears on on a spider which goes on first w/o a spacer.

Since posting this I ordered a Shimano cassette too see if that's any different. I also verified none of the gears are flipped backwards.
No need to check that, it's not possible.
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Old 07-27-21, 09:57 AM
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Different size cassette? 11-nn instead of a 12-nn or something like that? Maybe your chain is now too long.

As always, one little strand of frayed cable in the STI can cause this issue too.
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Old 07-27-21, 10:03 AM
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Shimano 10 speed cassettes require a 1 mm spacer be installed before you install the cogs. SRAM 10 speed cassettes do not require this spacer. Is it possible that you have one of these thin 1 mm spacers installed on your wheel? You will have to remove the cassette to find out. If there is one, remove it
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Old 07-27-21, 12:43 PM
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Originally Posted by alcjphil
Shimano 10 speed cassettes require a 1 mm spacer be installed before you install the cogs. SRAM 10 speed cassettes do not require this spacer. Is it possible that you have one of these thin 1 mm spacers installed on your wheel? You will have to remove the cassette to find out. If there is one, remove it
Hum, I didn't know that.
I'm using a Shimano 7700 hub. No spacer.
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Old 07-27-21, 02:01 PM
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I have something for you to look at. There is a washer w/ a couple of tabs on it under the cable anchor bolt. If the washer rotates to the wrong position the tab will not let the derailleur get all the way into the small cog position. Take a look and see if that's it.

Last edited by cxwrench; 07-27-21 at 08:04 PM.
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Old 07-27-21, 07:53 PM
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Few questions.

What kind of dropout on the frame, vertical or slanted?
Smallest cog, 11 or 12?
Is the DA hub stock? Could the freehub body possible have been changed?
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Old 07-27-21, 08:04 PM
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Originally Posted by KCT1986
Few questions.

What kind of dropout on the frame, vertical or slanted?
Smallest cog, 11 or 12?
Is the DA hub stock? Could the freehub body possible have been changed?
I don't think you can swap the freehub from any one D/A hub to any other.
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Old 07-27-21, 08:32 PM
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Installed the newly purchased CS-6700 11-28 (ultrega) cassette without the spacer and shifting is back to normal.
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Old 07-27-21, 09:30 PM
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Originally Posted by cxwrench
I don't think you can swap the freehub from any one D/A hub to any other.
Yes, I know that the Fh-7700 has a unique freehub body, ones that inserts deeper into the hub shell. There are other non-DA that could possible work with this deeper insertion requirement. See the FH-6500 and FH-6700. Those are standard steel bodies.

OP never gave a history of the wheel/hub.

OP seems to have come to the conclusion that the cassette is the issue. Maybe he will figure out why....
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Old 07-28-21, 09:45 AM
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The 7700 hub is NOS. Wheel is newly built. Old wheel also used a stock 7700 hub, with a SRAM cassette.

The problem may have been related to a grungy rear derailleur. I'm not positive but I do know it's operating smoother since cleaning..

The SRAM cassette, and the new Ultegra cassette w/spacer, both extend well proud of the freehub body. The Ultegra cassette w/o spacer still sits proud but less so, so I decided to go that way.
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