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Sora RD bumps against big cogs, b-screw maxed out

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Sora RD bumps against big cogs, b-screw maxed out

Old 06-09-21, 06:58 PM
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pstock
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Sora RD bumps against big cogs, b-screw maxed out

moving to the rear end of my Rivendell Sam Hilborne, I am also having trouble with my RD setup.
since I was initially just throwing this bike together quickly to see how it rode, how it felt and if it even fit me, I just grabbed a simple 8 speed Sora RD-3300.
I have higher end RDs I could use instead.
but this Sora bumps up against the #1 and #2 biggest cogs.
I believe I would normally adjust the b-screw to push the derailleur back off the cogs a bit. BUT I have maxed out the b-screw.
I believe I have the correct chain length - on Big Front, Big Rear, bypassing RD allow 1 link overlap.
so, what's going on here?
should I:
- find a longer b-screw?
- swap on another RD?

interesting, just now reading the label on the RH barend shifter I see it says "compatible RD-7400 8Speed".
I have an RD-7400 I think but I would ideally not use one for a bike I plan to use as a commuter and for bashing around the country side.....



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Old 06-09-21, 07:04 PM
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Not going to try to count the teeth from your pic, but are you exceeding the MAX cog size for that RDER? Typically, "road" RDER's max out around 28T, although you "MIGHT" be able to get a couple more depending on hangar geometry etc.
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Old 06-09-21, 08:54 PM
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We have tried both 1 and 2 a number of times and generally find a different der that is made to handle the gear range at play works better and for more miles then cobbing up a der that is maxed out WRT it's gear range. We've installed many Shimano ATB rear ders on 8 and 9 speed road bikes to help contend with the search for lower gearing. Andy
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Old 06-09-21, 09:16 PM
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Turn the b-screw around. Or try longer. Take a link out of your chain.... but I don't think you can.
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Old 06-09-21, 09:25 PM
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the max tooth capacity is 27 teeth (per shimano) on that derailleur. it has a greater chainwrap than the short cage, but still the same cog clearance

https://www.birota.ru/manuals/shiman...e/5TT0B_EN.PDF
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Old 06-09-21, 09:33 PM
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According to Shimano Docs, the maximum cog is 27t for both the SS and GS cages. The capacity is 29 for the SS and 37 for the longer GS.

As Andy noted, a ATB/MTB rear derailleur is probably what you need. I would opt for the SGS long cage.

Just getting a longer b screw may not solve your issues if you are too far beyond the rated capacity.

John
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Old 06-10-21, 04:51 AM
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You can also try a derailleur hanger extender, like a Wolf Tooth RoadLink. I have one of these on an older Peugeot with a 5500 series 105 rear derailleur. I have a 14-32 7-speed cassette on it and like the look of the derailleur on the bike, so I dropped it down with a RoadLink and it works pretty nicely. You can really relax the B-screw in this case and the derailleur still tucks up pretty nicely to the smaller sprockets, despite being lowered by about an inch. I have this bike setup 1x, so it was pretty easy to tune chain length to what I needed for this configuration. Chain length will of course determine the guide pulley's vertical position and you can use chain length to help get a marginal setup to work. But this is harder to do if you have a 2x or 3x crank set and need to get multiple chain ring/sprocket combinations to play together nicely.
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Old 06-10-21, 08:27 AM
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My biggest cog is in fact 32t.
I'd always understood that a regular cage length RDER would only handle max 27t (or 28t) BUT somewhere recently I read that cage length had more to do with chain length than it did with max rear cog size.
I guess that was wrong intell.

Although this Sora RDER is long cage (well, longish) 7.5cm between jockey wheel centers. shouldn't that be enough to handle more than 27t? or is it not about cage length? (I'm confused about what dictates Tooth Range)

In any event, I will swap on anATB/MTB RDer.

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Old 06-10-21, 10:37 AM
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Cage length has absolutely nothing to do w/ how large of a big cog the derailleur can handle. Zero. Nada. A medium or long cage only has one job...wrap more chain. The location of the upper pulley and the angle at which it moves determines the max cog size. And no...a long B screw or turning it around is NOT the right way to do it.
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Old 06-10-21, 11:44 AM
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Originally Posted by pstock View Post
BUT somewhere recently I read that cage length had more to do with chain length than it did with max rear cog size.
I guess that was wrong intell.
No, that's right...a longer cage can wrap more chain (it can store more chain links). It does not influence the largest sprocket the derailleur can support.

Mountain derailleurs usually have longer cages AND they support larger sprockets, but those two facts are not directly related (the first does not cause the second).
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Old 06-10-21, 01:56 PM
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and.... I swapped on a Deore XT RD-M739 RD and Voila! problem solved. (not suprisingly. the spec sheet says it'll handle 32t and Lo and Behold, it does!)
she's shifting very nicely.

I will now read spec sheets more carefully.

many thanks for your patience and advise.

it feels great to have this bike more dialed in.

Peter
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