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Derailleur wont go to smallest cog

Old 12-10-15, 04:51 PM
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vintagerando 
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Derailleur wont go to smallest cog

I have a road bike, double rings in the front. When I have the chain on the large ring, I cannot shift to the smallest rear cog, even with the shifter push into the furthest position. This is a 5 speed friction, Campy SR deraillues from the early 80s.
Here is the weird thing: it I shift the front derailleur to the smaller front ring; the chain hops on the smallest cog.
What is causing this?
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Old 12-10-15, 04:57 PM
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Could the chain be too short?
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Old 12-10-15, 05:08 PM
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I doubt it would be the chain being too short, unless you were also having issues shifting into the largest sprocket while in the largest chain ring.

Sight down the rear of the bike, do the pulleys on the rear derailleur look like they are parallel with the freewheel?
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Old 12-10-15, 05:43 PM
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return spring pulls to high , its default when no cable pull .. so things may be dragging in the cable/housing ..

un hook the cable & see if it drops on, then..
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Old 12-10-15, 05:54 PM
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Originally Posted by vintagerando View Post
I have a road bike, double rings in the front. When I have the chain on the large ring, I cannot shift to the smallest rear cog, even with the shifter push into the furthest position. This is a 5 speed friction, Campy SR deraillues from the early 80s.
Here is the weird thing: it I shift the front derailleur to the smaller front ring; the chain hops on the smallest cog.
What is causing this?
Let's be clear, the bike will go into small - small with no problem, and will not go into the smallest rear cog when the front is on the big ring.

1st thing. When the shifter is all the way to high (smallest cog in the back); is the the RD against it's limit stop screw.

If yes - the limit screw needs a tweak (1/4 turn or so) to allow the RD to travel a fraction more.

If no - the cable is too short or stuck somewhere. Check by disconnecting the cable from the RD, and move the RD by hand (easiest on a stand, or hanging off a rack).
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Old 12-10-15, 06:17 PM
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the only thing, given a slack enough shifter cable, that forces the parallelogram on a RD to collapse, which is what is necessary in order to change to a smaller cog, is the internal spring in the parallelogram. not to be confused with the chain tensioner spring in the cage. when the derailleur body becomes worn, or dirty, it can become impossible for it to collapse completely. and the tighter the chain the worse it gets.

i would imagine a couple more links of chain could do it, but maybe not. old derailleur's parallelograms pivot points can and do wear after being cycled for a few decades, causing the two parts of the parallelogram to rub against one another, eventually causing so much friction that they cannot be collapsed into one another far enough to reach the smallest cog. a good cleaning may help. and i would take the chain off and examine how well the RD's parallelogram is functioning in general without it. IME, sometimes a little filing of the wear spots can help.

Last edited by hueyhoolihan; 12-13-15 at 03:23 PM.
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Old 12-11-15, 12:48 AM
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ok....some great approaches here. I messed with it a bit in the garage. Now the chain is going to the smallest rear while on the large front ring. So, maybe there is some combination of shifting that I am doing while riding that is causing this issue?
I am thinking the cable is dragging somewhere, hanging up somewhere. FYI The cable and housing are brand new. Maybe I have ridden only a couple hundred miles in 3 months.
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Old 12-11-15, 08:50 AM
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As the chain is played out to mount the large ring the rear der cage rotates and it's upper (guide) pulley drops down and away from the bottom of the cogs (on a campy NR with the cage pivot between the two pulleys). Then when the chain shifts back to the small ring the cage/pulley rotates back up and closer to the cogs. This changing gap between the cogs underside and the top of the guide pulley is some of the mechanism involved with how responsive the shifting on the rear cogs is. The more links of chain that span that gap the more flex and slop that has to be compensated for by allowing the guide pulley to move further then being directing below the intended cog. When shifting between the cogs NOT at either end of the freewheel/cassette this over shifting (and subsequent trimming back for smooth chain run) is done by how one moves the control lever. When shifting onto the last cog (and the smallest cog will have the greatest gap to the guide pulley) the over shift ability is controlled to a degree by how the limit screw has been set.

needing to adjust the limit screw some to let the guide pulley move further outward to effect a shift to the small cog is a very common need and is done on many bikes. Andy.
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Old 12-11-15, 10:38 AM
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Originally Posted by Andrew R Stewart View Post
As the chain is played out to mount the large ring the rear der cage rotates and it's upper (guide) pulley drops down and away from the bottom of the cogs (on a campy NR with the cage pivot between the two pulleys). Then when the chain shifts back to the small ring the cage/pulley rotates back up and closer to the cogs. This changing gap between the cogs underside and the top of the guide pulley is some of the mechanism involved with how responsive the shifting on the rear cogs is. The more links of chain that span that gap the more flex and slop that has to be compensated for by allowing the guide pulley to move further then being directing below the intended cog. When shifting between the cogs NOT at either end of the freewheel/cassette this over shifting (and subsequent trimming back for smooth chain run) is done by how one moves the control lever. When shifting onto the last cog (and the smallest cog will have the greatest gap to the guide pulley) the over shift ability is controlled to a degree by how the limit screw has been set.

needing to adjust the limit screw some to let the guide pulley move further outward to effect a shift to the small cog is a very common need and is done on many bikes. Andy.
i don't envy you, trying to explain that in a post. i would have made a bodge of it.
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Old 12-11-15, 11:30 AM
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As a side note. I acquired a Derailleur Hanger Alignment Tool. This is a great addition to your tool box if you do a lot of refurbishing like I do. It puts the derailleur hangar in the right position before you do the adjustments and saves a lot of time. I notice when the derailleur hangar is straight and the derailleur is in the correct position, many derailleurs need little or no adjustment at all..
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Old 12-11-15, 01:43 PM
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Originally Posted by elmore leonard View Post
As a side note. I acquired a Derailleur Hanger Alignment Tool. This is a great addition to your tool box if you do a lot of refurbishing like I do. It puts the derailleur hangar in the right position before you do the adjustments and saves a lot of time. I notice when the derailleur hangar is straight and the derailleur is in the correct position, many derailleurs need little or no adjustment at all..
EL- Certainly as a starting point using a DAG or N tool is a very good step. But as mentioned by me, in another current thread, this alignment is no guaranty of having the cage (which is what the chain is concerned about) be as straight as it might otherwise be. http://www.bikeforums.net/bicycle-me...ex-normal.html

Huey- Thanks. Sometimes the thousand monkeys in my mind actually type out sense. Andy.
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Old 12-11-15, 07:10 PM
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FYI This is a 30yr old Campy SR derailleur, if that matters. The cable runs under the BB. There are brazed on cable guides under the BB. When I bought the frame/fork used there was the remnants of a plastic tube in the cable guides. I could not find a good replacement for these tiny tubes. I ended up using small tubes from the V brakes (??) of an old mountain bike. I wonder if the cable is hanging on that tiny tube.
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Old 12-12-15, 10:34 AM
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Wonder as you want, did you buy new cables? the die slicked ones are made smoother..
(NB Campag compatible gear cable heads are smaller than the others)

There needs to be a little grease on the back side of the outer plate, of Trad record/SR RD

since the return spring on the back side slides a little bit as RD goes in and out.
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Old 12-12-15, 11:00 AM
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There are multiple possibilities, and a key question is whether this is a sudden thing, or was like this from the time you set it up.

If this is sudden, it's an indicator that the RD hanger was bent inward since the last time the limit screw was adjusted. If so, the inner limit is likewise misadjusted, and you run the risk of overshifting into the wheel.

Otherwise, this is often caused by a bent RD cage or hanger, so the cage is not parallel to the central plane. When the cage rotates through an arc to take up chain slack, the upper pulley does also. If it's arc of rotation isn't in a parallel plane it will move closer or farther from the wheel, so all trim will change when you shift the front.

This issue of non-parallel arc of travel is why hanger alignment is so critical, especially with the narrower spacing of 9,10 and 11s systems.

So regardless of the specifics, the repair starts with checking hanger alignment.
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Old 12-13-15, 02:35 PM
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Originally Posted by vintagerando View Post
FYI This is a 30yr old Campy SR derailleur, if that matters. The cable runs under the BB. There are brazed on cable guides under the BB. When I bought the frame/fork used there was the remnants of a plastic tube in the cable guides. I could not find a good replacement for these tiny tubes. I ended up using small tubes from the V brakes (??) of an old mountain bike. I wonder if the cable is hanging on that tiny tube.
I had this exact same issue withe the same derailleur and cable routing. I could adjust the derailleur stops so that it would shift all the way when on the big ring but then it would tend to over shift when on the small ring.
Apparently these derailleurs were kind of known for this.
After making sure all of the alignments were correct (to no avail);
Here's what fixed it:
I added 2 links to the chain.

Why it worked:
Before, when on the big ring, the derailleur's jockey wheels were pulled away from the rear cogs so far by the tight chain that they couldn't command the chain to move all the way down.
Adding 2 links allowed the jockey wheels to stay close to the rear cogs and get the chain to move all the way down.
The chain will seem to have slightly too much slack now when on the small front chainring and the smallest rear cog but you really wouldn't ride this combination anyway and you can take up some of the excess by ensuring that the rear wheel is positioned as far back in the dropouts as possible.

This worked for me and I bet it will work for you too.

Last edited by KJK; 12-13-15 at 02:39 PM.
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Old 12-13-15, 03:11 PM
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And this exactly relates to what I was saying. Andy.
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Old 12-13-15, 07:28 PM
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Originally Posted by Andrew R Stewart View Post
And this exactly relates to what I was saying. Andy.
Somehow I totally missed that post or I would have quoted it also.

So I'm just reiterating (and not as well I might add) what Andrew already explained!
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Old 12-17-15, 10:07 PM
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Originally Posted by KJK View Post
I had this exact same issue withe the same derailleur and cable routing. I could adjust the derailleur stops so that it would shift all the way when on the big ring but then it would tend to over shift when on the small ring.
Apparently these derailleurs were kind of known for this.
After making sure all of the alignments were correct (to no avail);
Here's what fixed it:
I added 2 links to the chain.

Why it worked:
Before, when on the big ring, the derailleur's jockey wheels were pulled away from the rear cogs so far by the tight chain that they couldn't command the chain to move all the way down.
Adding 2 links allowed the jockey wheels to stay close to the rear cogs and get the chain to move all the way down.
The chain will seem to have slightly too much slack now when on the small front chainring and the smallest rear cog but you really wouldn't ride this combination anyway and you can take up some of the excess by ensuring that the rear wheel is positioned as far back in the dropouts as possible.

This worked for me and I bet it will work for you too.

Thank you for the response
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