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Diagnose my rear derailleur problem and I'll buy you a beer

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Diagnose my rear derailleur problem and I'll buy you a beer

Old 08-22-10, 03:42 PM
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Diagnose my rear derailleur problem and I'll buy you a beer

Yep, if you can correctly tell me what is going on here and how to fix it I'll PayPal you $10 for a nice 6 pack or soda if you're a youngster.

Background - I built up my Lynskey Cooper with 2010 SRAM Force. 50/34 up front and 11-28 in the rear. The force derailleur is supposed to be able to handle a 28t max cassette, however my 28t grinds on my rear derailleur cage.

I took it to two LBS' today and both said the same thing "We've never seen this before, just stay out of that gear combo". I'm not really satisfied with that solution (and yes i know i shouldnt be cross-chaining to begin with). The B screw is all the way in and it's still grinding.

This is the only gear combo (50x25) I have any issues with, ive ran the exact same gearing with SRAM Rival and had no issues. You will notice that when I use this combo it's the 28t cog that is grinding the derailleur cage.

Here is a video and a pic




any ideas?

thanks
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Old 08-22-10, 03:45 PM
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Do not know about Scram but on Shimano you can remove the B-screw and put it in from the other side to get more range.
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Old 08-22-10, 03:45 PM
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swap out your current b-tension screw for a longer one. Most derailers use a standard metric threading. Then crank up the b-tension more.

Can I get my beer now?
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Old 08-22-10, 03:49 PM
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When you are in the big big combination is the cage almost straight?
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Old 08-22-10, 03:53 PM
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How did you size the chain?
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Old 08-22-10, 03:53 PM
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2 fixes to attempt .. A + 1 on two mentioned..

B screw further in preloads the spring around the mounting bolt to turn the RD further to the rear opening a gap

May need a longer B screw, it screws against a stop on the frame, if it wont hit that tab right, turn the screw over so the head faces and hits the frame tab..

and / or shorten a link from the chain the cage getting pulled by the shorter chain may pull the cage around too for more clearance,
but then you really have to watch that Big - Big combo since the chain will be too short for that.

Avoid it like the Plague .. may break stuff, in that case.. if you go to that combo.
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Old 08-22-10, 03:54 PM
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LBS said that reversing the screw wouldnt be enough and a longer B screw would disrupt the angle on the derailleur and it wouldnt shift properly
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Old 08-22-10, 04:07 PM
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So far the plan is to get a longer B screw this afternoon. If this resolves the grinding issue and doesnt effect the shifting, Fuzz2050 is the grand prize winner.

It may not work so feel free to keep the suggestions coming

thanks guys!
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Old 08-22-10, 04:44 PM
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Add two links, try it, rinse and repeat till it's sussed.
 
Old 08-22-10, 04:48 PM
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Originally Posted by thompsonpost
Add two links, try it, rinse and repeat till it's sussed.
I added one link and it solved this problem but resulted in sloppy shifting and too much chain slack. went back to the proper length chain and started from square one.
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Old 08-22-10, 04:55 PM
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Before fooling with the "B" screw see if it'll make a difference. Manually pull the RD back (as if the B screw were longer) a few degrees and see if that solves the problem.

From the photo, the pulley/sprocket seems close but looks to be adequate, so something else might be going on. Also double check that you have enough chain length by seeing if you can push the lower pulley forward (as if the chain were shorter yet).

A few seconds determining what is and what isn't the problem will save you time compared to making changes willy-nilly.

BTW- it's hard to know from the video because you're backpedaling and the freewheel ratchet noise is covering up the grinding. Post a new video pedaling forward, and pull the camera back a bit so we can see the big picture.
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Old 08-22-10, 05:05 PM
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The video shows forward pedaling. Look closer.
 
Old 08-22-10, 05:15 PM
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It must be an optical illusion, but I see the chain running from the cassette to the RD, and hear what really sounds like a freewheel ratchet.
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Old 08-22-10, 05:22 PM
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I also see it in reverse. You know that it could grind in reverse and be fine when pedaling, right? Don't adjust anything based on what it does when pedaling backwards the derailleur tensioning does not work correctly that way.
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Old 08-22-10, 06:12 PM
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Originally Posted by FBinNY
It must be an optical illusion, but I see the chain running from the cassette to the RD, and hear what really sounds like a freewheel ratchet.
Yeah, weird. To me it looks forward but the spokes are still. I jes doh no.
 
Old 08-22-10, 06:17 PM
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I've never had this issue, but if all else fails....

...can somebody tell me what harm there would be in filing the slight bit of cage plate that's contacting the big cog?
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Old 08-22-10, 06:21 PM
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Originally Posted by desertdork
I've never had this issue, but if all else fails....

...can somebody tell me what harm there would be in filing the slight bit of cage plate that's contacting the big cog?
There's no need to do that. If the bike shop can't solve this get a better bike shop.
Post number three is the answer. It's not complicated.
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Old 08-22-10, 06:24 PM
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Stupid question. The photo is too closeup to see the big picture, but the cage surrounding the pulley looks more like that on the lower, rather than upper pulley. The cage is usually cut away on the inside of the upper pulley, so now not only is your video running backward, but the cage seems to be upside down.

My question is, are you trolling for tuna, or do you really have a problem?
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Old 08-22-10, 06:36 PM
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To the OP. Have you disassembled the derailleur cage recently? If so, you may have put it back together incorrectly, which is nearly impossible to do on most mechs.
 
Old 08-22-10, 06:40 PM
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Originally Posted by FBinNY
Stupid question. The photo is too closeup to see the big picture, but the cage surrounding the pulley looks more like that on the lower, rather than upper pulley. The cage is usually cut away on the inside of the upper pulley, so now not only is your video running backward, but the cage seems to be upside down.

My question is, are you trolling for tuna, or do you really have a problem?
Eye think you are right! Turn that sucker around to the right attitude...........
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Old 08-22-10, 06:58 PM
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Originally Posted by thompsonpost
To the OP. Have you disassembled the derailleur cage recently? If so, you may have put it back together incorrectly, which is nearly impossible to do on most mechs.
Hey!, you're not trying to hijack my beer are you?
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Old 08-22-10, 07:28 PM
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Pulley cage is bolted on correctly. The problem is that you are at the extreme range of operation for this derailleur. You are asking it to operate in a 33t capacity scenario when that is the maximum rating.

It works, just not very well at all. Increase B tension and/or file the pulley cage appropriately.

The other option is to ditch the granny gearing and go for 11-25 or 53x39.
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Old 08-22-10, 07:37 PM
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Originally Posted by FBinNY
Hey!, you're not trying to hijack my beer are you?
Busted. Man.

I'm gone. Hope you get it sussed.
 
Old 08-22-10, 08:26 PM
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I went to the Sram site, and blew up the photo of the RD, and your cage isn't upside down, or threaded or assembled wrong. So I went back to the close up still photo, and realized that it doesn't grind while on the 28t sprocket, but grinds against the 28t while on the adjacent smaller sprocket.

It's because of the unique shape of the Force's inner cage plate which comes up beyond the pulley about 1/3 of the way around. That means that when the lower pulley is pulled forward, this high section of inner plate is at the 12 o'clock position.

The fix is probably to lengthen the chain to the point that the cage is more vertical in this gear. Before cutting the chain experiment by shifting to the 2nd largest sprocket, then removing the chain from the chainring and feeding it back slowly and watching the angle of the cage and inner plate/28t sprocket clearance. If it looks like adding chain will help, check that it won't be too long in the small/small combination.

You might also manually pull the RD back while testing chain length, to see if there's a best balance between chain length and RD hang angle. If necessary, go with a longer screw for the B adjustment and lengthen the chain both.

Lastly, if you can't find a point of best balance between chain length and B screw adjustment, consider filing back the offending part on the inner cage so it doesn't touch the teeth of the next sprocket.

BTW- when you're all finished fixing it, consider finding a new LBS. This was hard to diagnose from the description and overly close up photos, but any decent mechanic who had it in his hands should have seen the problem within minutes and been able to fix it.
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Old 08-22-10, 08:32 PM
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swap in a MTB RD? FWIW, SF alone, has a dozen Bikeshops..
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