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Front mech affect rear mech???

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Front mech affect rear mech???

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Old 02-12-19, 12:20 PM
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pixelstick
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Front mech affect rear mech???

I enjoy working on my own bikes, but am still building confidence and running into an issue. A couple months ago I had the bike shop replace my front derailleur with a SRAM Red on my road bike, great and it worked just fine. At that point I had a 2x10-spd groupset. After looking for quite a while I found an awesome deal on a pair of new SRAM Red 11-spd shifters. I changed out the right shifter, cassette (Ultegra lightly used) and added a new SRAM PC1170 chain and new 11-spd Rival rear derailleur myself. And yes, my wheels are 11-spd. I had a spacer for the 10-spd setup. Shifting with the new right shifter was amazing compared to my Apex shifter! Shifts were clean and crisp every time up and down the cassette. Fast-forward a week later and I changed out my left shifter but it wasn't engaging, no cable pull at all, so I took it to the bike shop for them to finish it up. I then get a call from the shop... the left shifter is working correctly and shifting perfectly, but the rear isn't shifting properly. They said that if the rear is adjusted to shift properly while the front is in the big ring then the rear won't shift correctly when the fd is switched to the little ring, and vice versa. The limit screws and indexing have to be adjusted based on what front chainring the chain is on. This is the exact same rear hardware, chain, crankset/chainrings and front mech that was on the bike before. The only difference is the new left shifter and the front mech had to be adjusted. Is it possible that a clean-shifting rear mech is affected by the front mech somehow? They are thinking it is the cassette, but it doesn't appear to be worn at all (it came with my used-but-in-pristine-condition wheels I got last year).

Last edited by pixelstick; 02-12-19 at 01:22 PM.
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Old 02-12-19, 01:03 PM
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Just to get it straight after the shop looked at it the left shifter that wasn't working now is but the rear was and now isn't.
Perhaps they can't eliminate chain rub?
But it should be possible to adjust that out when you are not cross chained.
The Sram fd has a yaw function which is maybe causing the problem.
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Old 02-12-19, 01:06 PM
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Friction shifting ..

When you shift the rear , then your chain scrapes on the front cage , so you recenter the front derailleur to stop that..


I can say that , from my experience..





drop by and show me what your issue is ... loss of the fine tuning friction shifting offered.. perhaps ?




...

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Old 02-12-19, 01:19 PM
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The Answer!

Ok, just as I finished up writing my post I got a message from the shop saying my bike was finished. And yes, blamester, that is correct. It was a really strange situation. So this is an Allez Sprint with internally routed cables. Both shifter housings stop at the down tube and the cable continues thru the tube and out again by the bottom bracket back into more housing. Turns out that the cables were crossed inside the tube so when tension was applied to the front cable it was affecting tension on the rear cable, which explains the effect on rear shifting depending on which chainring was being used at the time. Big ring = more cable tension = adjust rear mech and things work fine but only while in the big chainring. Once back in the little chainring the extra tension on the rear cable from the front cable disappears and shifting is off again in the rear. Doubtful I would have ever figured this one out!
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Old 02-12-19, 01:24 PM
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Originally Posted by pixelstick View Post
Ok, just as I finished up writing my post I got a message from the shop saying my bike was finished. And yes, blamester, that is correct. It was a really strange situation. So this is an Allez Sprint with internally routed cables. Both shifter housings stop at the down tube and the cable continues thru the tube and out again by the bottom bracket back into more housing. Turns out that the cables were crossed inside the tube so when tension was applied to the front cable it was affecting tension on the rear cable, which explains the effect on rear shifting depending on which chainring was being used at the time. Big ring = more cable tension = adjust rear mech and things work fine but only while in the big chainring. Once back in the little chainring the extra tension on the rear cable from the front cable disappears and shifting is off again in the rear. Doubtful I would have ever figured this one out!
Kudos to your shop for figuring it out. I have heard of this before but it can't be common
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Old 02-12-19, 06:33 PM
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This can be detected as you thread a cable all the way through the frame you should hold it in your fingers and then move/shift the other shift cable. If the cables are crossed/tangles/twisted on each other you'll feel the tug on that first cable. Andy
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Old 02-13-19, 09:59 AM
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Nice one.
The mechanic did a good job. Just took him a bit of time to figure it out which is understandable.
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Old 02-14-19, 01:55 PM
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I agree

Those guys at the shop are awesome! I learn something new every time I have to take my bikes to them. They know I like doing things myself so they give me the 411 on how they did whatever.
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Old 02-14-19, 02:40 PM
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Wait, the cable inside the frame is without housing?
I have 1 internally routed bike and it has full housing inside the frame.

Without housing inside the frame I see many potential problems. Like cable cutting into the frame, noise.... Why would they not use full housing?

I guess same question on why anyone still uses cable stops externally. But externally you can at least control the cable and verify.
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Old 02-15-19, 07:52 AM
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Mountain and gravel bikes with internal routing generally have full runs of housing, where road bikes usually have a cable stop with just the inner cables running through the frame. After you've replaced a few of them, it's really not any harder than replacing external cables, just a different process. I've seen a handful of the crossed over cable issue. It's fairly easy to recognize once you know what to look for. Another issue that's closely related and common with Specialized is the small teflon tubes used in the cable stops. I've seen many instances where the cables have been replaced and the cable stop or guide tubes were left out. What happens then is that the inner cable will actually "saw through" the aluminum cable stop causing damage and poor shifting. I made a video some time back with some demonstrations of both potential problems as well as general internal cable routing tips.

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Old 02-15-19, 11:40 AM
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Originally Posted by pixelstick View Post
They said that if the rear is adjusted to shift properly while the front is in the big ring then the rear won't shift correctly when the fd is switched to the little ring, and vice versa. The limit screws and indexing have to be adjusted based on what front chainring the chain is on.
Um... I don't understand how this beaten-to-death, FAQed and overFAQed at every web site, kindergarten-level error with obvious and instantly recognizable symptoms puzzled your bike shop mechanic for more than a split second. This is like... fundamental folklore of internal cable routing. Bicycle mechanics begin their day (every day!) by telling each other jokes about double-crossing shifter cables inside the downtube.

And your bicycle mechanic had to give you a call, saying that "they are thinking it is the cassette"???! Lemme just sit still for five minutes, speechless and with my mouth agape...

(Five minutes later.)
It is not clear from your original post who made the routing mistake: you or the bike shop. Who, really?

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Old 02-18-19, 12:18 PM
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Originally Posted by Le Mechanic View Post
Mountain and gravel bikes with internal routing generally have full runs of housing, where road bikes usually have a cable stop with just the inner cables running through the frame. After you've replaced a few of them, it's really not any harder than replacing external cables, just a different process. I've seen a handful of the crossed over cable issue. It's fairly easy to recognize once you know what to look for. Another issue that's closely related and common with Specialized is the small teflon tubes used in the cable stops. I've seen many instances where the cables have been replaced and the cable stop or guide tubes were left out. What happens then is that the inner cable will actually "saw through" the aluminum cable stop causing damage and poor shifting. I made a video some time back with some demonstrations of both potential problems as well as general internal cable routing tips.
Yep, this Specialized has what I would call cable guides going into the frame, basically plastic cable stops that screw into the frame. I couldn't get the one for the front shifter off because the screw was so tight and I was stripping it so I had to resort to the string-and-vacuum method of pulling the cable thru. At that point I realized that the shifter wasn't pulling the cable and after spending so much time routing the cable I just took it to the LBS, which is where they saw the problem. This being my first time messing with internally routed cables (well second if you count the right shifter) I doubt I would've figured this one out quickly so I'm thankful that those guys had it back same day... because at some point I did actually want to be able to ride again.
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Old 02-18-19, 02:22 PM
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Originally Posted by pixelstick View Post
At that point I realized that the shifter wasn't pulling the cable and after spending so much time routing the cable I just took it to the LBS, which is where they saw the problem.
In your original description (and here as well) you state that "shifter wasn't pulling the cable". This is, of course, a completely different problem, which has nothing to do with double-crossed cables in the downtube. At the same time, you said that bicycle shop later had to deal with double-crossed cables in the downtube. So, we are observing two different unrelated problems here:
1) "shifter not pulling the cable" (apparently, your original mistake),
2) "double-crossed cables in the downtube" (apparently introduced by an incompetent mechanic while trying to fix problem 1)

So, what what the reason for the original problem?
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Old 02-18-19, 03:11 PM
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Yeah I tried to be as complete as I could, but I can see where you're coming from. The series of events from beginning to end:

1) I changed my right shifter, cassette, chain, rd from 10-spd to 11-spd and things were awesome. Left shifter was still my old/existing shifter and hadn't been touched.
2) About a week later I changed my left shifter and routed the cable thru the down tube myself, but didn't hook it up to the fd because I noticed there was no cable pull from the left shifter (I put tension on the cable by holding it and switched big-little-big). Not something I wanted to or had the time to deal with.
3) Took the bike to the LBS
4) LBS called me and said front shifting was awesome but changing rings affected rear shifting
5) I posted on this forum because at the time I didn't see how that could even be possible
6) LBS found the issue, rerouted the left shifter cable and problem went away

So as it turns out I am the incompetent mechanic, although I don't know how in the world anyone (even a competent mechanic) could possibly route a cable thru a tube and 100% know that it isn't going to affect the other cable. Bueller? Bueller?
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