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Chain length and rear derailleur

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Chain length and rear derailleur

Old 05-12-19, 01:17 PM
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_ForceD_
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Chain length and rear derailleur

On one of my road bikes I had to replace the chainrings (chain and cassette too), and for the smaller ring went with a 36. The old one was a 42 tooth. I have a short cage derailleur. But I can't get the chain length right. If I use the big ring-big cog method to determine chain length...it's too long even with the B adjustment screw to the extreme. Shifted to the small-small the chain rubs on the bottom of the derailleur cage. If I take a link out of the chain...then it's too short on the large-large combination...the chain almost binds in the derailleur. Is the 36 small ring really too small? Should I just leave the link in and let the chain rub on the derailleur? (although it rarely would because I wouldn't normally be on the small-small combination)
Hope that's not too confusing.Dan
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Old 05-12-19, 01:37 PM
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I would replace the derailleur with a medium or long cage if you want to keep the chainrings.
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Old 05-12-19, 01:44 PM
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Assuming you are running 7/8 speed, I would just spend $20 and buy a new RD with the capacity you need.

I just bought one of these, it runs great, with take a 34 t sprocket in the back and has a chain wrap capacity of 43 teeth. Has nice tight springs and the shifting is excellent, a little heavy though. That big jockey wheel takes up a lot of chain, and it looks OK on a road bike.

https://www.ebay.com/itm/Shimano-ALT...gAAOSwZW5aKOed

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Old 05-12-19, 01:52 PM
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Three choices:

1- Get a larger “small” ring.

2- Get an RD with a longer cage.

3- Avoid the small-small combo.

Personally, I would do #3. It gives you the low gear you want and is cheaper than buying a new RD.
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Old 05-12-19, 02:08 PM
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What size is the big ring? What's the cassette/freewheel range?

Regardless, your description sounds like a classic case of insufficient chain wrap, although as others have mentioned you could also just avoid the small/small combination.
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Old 05-12-19, 02:18 PM
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It's rubbing on the front derailleur, right? That's what I'm envisioning with this setup. If you want to get rid of that, get a front derailleur made for a triple ring mountain bike.
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Old 05-12-19, 04:42 PM
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Originally Posted by rccardr View Post
What size is the big ring? What's the cassette/freewheel range?

Regardless, your description sounds like a classic case of insufficient chain wrap, although as others have mentioned you could also just avoid the small/small combination.
+1.
You likely have a short cage "road" RDER that has insufficient take up capacity for the 6T additionally required.
Increasing the size of the largest cog in back will also exacerbate this situation.
If you avoid the smallest 2-3 cogs on the small ring and it shifts fine otherwise, live with it.
If you plan on going larger in back, a different RDER would be needed.
providing details re: how many speeds, what DER's & shifters would give one a much better idea of what you have and what recommendations to make/avoid.
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Old 05-12-19, 07:56 PM
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Originally Posted by Kapusta View Post

Personally, I would do #3 . It gives you the low gear you want and is cheaper than buying a new RD.
I think this is what I'll do for now. I won't use the small-small combo so no big loss.

Honestly...I don't even know why I went with the 36 small ring!?!?!? I guess I made a mistake when I ordered. I wanted to go with a 38. I probably wouldn't have this issue with that size. But then when it came...instead of returning it I just thought...what the he11...I'll put it on. Too late to return it.

Dan
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Old 05-12-19, 08:14 PM
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There is no reason to ever use small-small. You can get the same gear using the large chainring and one of the intermediate cogs.
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Old 05-24-19, 07:22 AM
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I decided to reconfigure yet again. Originally the rings were 52/42. I went to 52/36 and got the chain rub. But I changed again to a 53/38 and all is well.

Dan
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Old 05-24-19, 08:18 AM
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Oversize bottom pulley does have an effect like a bit longer cage , for chain slack take up..
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Old 05-24-19, 02:14 PM
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Originally Posted by HillRider View Post
There is no reason to ever use small-small. You can get the same gear using the large chainring and one of the intermediate cogs.
What rear cogs are you able to use with the small chainring? If it's just on 1 or 2 cogs, I wouldn't do anything. I'd just use the derailleur cage rubbing as an audible warning to shift into some big ring combination.

Whatever you do, don't shorten your chain. If you ever accidentally shift into the wrong gear combination with a too short chain, you can do a TON of damage to your bike.
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Old 05-24-19, 02:29 PM
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I've been know to use small-small (usually on triples) where the chain rubs. I only use it for short stretches of relatively level on climbs so I don't have to double shift twice. Aside from the noise, never been an issue.

Ben
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Old 05-24-19, 05:01 PM
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fwiw I have 53/39 and 13/28 combo working with old Dura Ace short cage derailler. I don't think I could get any more out of it

Most important (IMHO) is that Big/Big works.... it is not uncommon to hit that combo if you are a) not paying attention or b) not planning..... and the crash can be no fun

also check out small/small sizing...here is a cut and paste from a post in C&V

I have always done big/big and add 2 links chainsizing.

with my recent build with 5800 105 with a compact crank (50/34) and 13/32 cluster, the result was not as good as I wanted in terms of shifting and rear derailler position

My mechanic suggested trying low/low saying that it is shimano's current technical recommendation

it worked great for me, not sure how it would work with vintage deraillers and crank/cluster sizing, but will try soon

this method if you are not familiar with it (and I wasn't) is:

put deraillers in low/low combination
thread the chain thru the deraillers and have on the low rings and teeth
size the chain so that it does not rub on rear derailer

here is a video link

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Old 05-31-19, 02:43 PM
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I never ride small to small or large to large, old timers told me this year's ago
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