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-   -   Front derailler throw (http://www.bikeforums.net/recumbent/559234-front-derailler-throw.html)

oldacura 07-06-09 03:44 PM

Front derailler throw
 
I've got an older Vision SWB USS bike that I inherited from my brother in law.

It has a Shimano triple crank and Shimano thumb / forefinger indexed mountain bike shifters. I replaced the shifters after the ones it came with got damaged. It seems like the front shifter has too much throw for the front der. If I adjust the front der so the cage is centered on the middle ring, the der tries to throw the chain off the high side when I shift to the big ring. I have the upper stop screw adjusted to prevent this but I really have to push hard on the shifter to get the indexer to hold the der against the stop.

I have the cable attached to the der arm as far from the pivot as possible. I could move the cable to the other side of the clamp screw but this would move it closer to the pivot and should make the der move farther.

I know that some shifters have 5 indexed positions. This one has only 3. If it had 5 positions, I could half-step the der to trim it. What am I doing wrong?

Bobsk8 07-06-09 04:13 PM

http://www.youtube.com/watch?v=ngm6dr-1na0


http://www.youtube.com/watch?v=ngm6dr-1na0

cranky old dude 07-06-09 04:16 PM

I had a similar problem last Spring with a front Deraileur. My shifter and DR were a proper match but my skill level was sub-par. Releasing and re-affixing the cable with the proper tension resolved my issues. It turned out that I had a minor amount of slack in the cable...could too much tension also cause this type of problem?

Like I said, my DR skills are limited...what worked for me may not be at all related to your issues.

JanMM 07-06-09 04:23 PM

It can get confusing when you post the same thing in two forums.
I replied in Mechanics.

Jeff Wills 07-06-09 05:43 PM

Quote:

Originally Posted by JanMM (Post 9229485)
It can get confusing when you post the same thing in two forums.
I replied in Mechanics.

Me, too! :D

BlazingPedals 07-06-09 07:11 PM

Just a guess, but you might not have gotten the cable seated in the 'slot' in the clamp mechanism. Depending on which side of the slot it got clamped, the derailleur would move either too much or not enough. If that's not it, I got nuttin'.

iconoclast 07-07-09 07:35 PM

I've read and re-read your post several times and don't quite understand what you're saying when you adjust the derailleur so the cage is centered on the middle ring.
My understanding and the way I adjust the front derailleur is not for the center chainring.
The derailleur is adjusted for the small chainring and the large chainring.
The cable is adjusted for the center chainring.
This is the procedure I use.
1. Low adjustment
I adjust the low to bring the inner derailleur surface to almost touch the chain when it's on the small chainring and on the largest rear sprocket.
2. High adjustment
I adjust the high to bring the inner derailleur surface to almost touch the chain when it's on the large chainring and on the smallest rear sprocket.
These two setting should be independent of the cable. In other words, if the cable was not connected and you moved the derailleur by hand to the two extremes, the low and high adjustments would be as described above.
3. Intermediate chain ring adjustment.
This is where the cable is adjusted using the barrel adjustment usually located at the shifter.
With the chain on the intermediate chainring and the largest sprocket, adjust the cable so the derailleur inner surface almost touches the chain.
The cable adjustment barrel can be further used to fine tune the derailleur position and the shifter indexing.

BlazingPedals 07-07-09 07:54 PM

I've got 'micro adjust' SRAM stuff, and it has 7 or 8 clicks available. I start with the shifter indicating "2" then downshift 2 clicks. Then I pull all of the slack out of the cable and tighten it. The cable *must* be in the slot or the pull radius will be wrong. Then I shift up two clicks and use the fine-adjust to center the derailleur over the middle chainring. Now downshift _3_ clicks and use the low-limit to center the derailleur on the granny ring. Finally, shift 2 clicks *above* the "2" indicator and make sure it's centered. (It should be.) Adjust the top limit screw to to about a half turn above the current location of the derailleur. This allows for overshifting to get the chain up on the big ring. I have 12-14 difference from the middle ring; if you have less you may not need the overshift function. If your derailleur is riding within a mm or two of the chainring, that shouldn't be enough to let it shift off the outside of the big ring.

Yeah, someone over in mechanics will probably say this is all effed up, but it works for me.

oldacura 07-16-09 09:25 AM

Yes - I did double post on the Mechanics section as well but I do appreciate any input.

What I understand now is that the FD is a Tiagra (road?) and the shifters are Alivio (mountain?) and that these are incompatible.

I've determined that both the shifter I removed (Deore) and the one I replaced it with (Alivio) pull the same amount of cable: ~11mm from low to mid and ~8mm mid to high. I don't know why these are different or if they should be. The problem seems to be the mid - to - high with too much throw.

The easiest / cheapest way to go would be to replace the FD with a mountain version but I don't know if this would solve the problem. I've checked and re-checked the cable attachment to the FD and it seems right: pivot/pinch bolt/cable. The cable is in a groove adjacent to the pinch bolt.

Do mountain FDs have less throw for a given cable pull than road FDs?

Jeff Wills 07-16-09 12:38 PM

Quote:

Originally Posted by oldacura (Post 9291299)

Do mountain FDs have less throw for a given cable pull than road FDs?

Yes. The lever arm (the one the cable bolts to) is longer from the pivot to the bolt. This gives it more leverage on the cage (for shifts under pressure) but moves the cage less for each millimeter of pull on the cable.

I've run into this problem dozens of times- three times on my own bikes. The only good solution is getting the correct front derailleur.

oldacura 07-16-09 12:57 PM

Jeff - thanks. What is on there is a bottom pull, top swing (?) clamp-on FD with about a 29mm (28.6mm) tube.

Should any Shimano bottom pull, mountain triple FD with the correct clamp diameter work?

iconoclast 07-17-09 05:11 AM

Quote:

Originally Posted by Jeff Wills (Post 9292686)
Yes. The lever arm (the one the cable bolts to) is longer from the pivot to the bolt. This gives it more leverage on the cage (for shifts under pressure) but moves the cage less for each millimeter of pull on the cable.

I've run into this problem dozens of times- three times on my own bikes. The only good solution is getting the correct front derailleur.

Really?
When I switched from a Shimano FD-R443 road front derailleur to a Shimano Deore FD-M531 mountain front derailleur, I used the same shifters and experienced no problems.
When I read your reply, I got out my calipers and measured my two derailleurs (road vs mountain) from the pivot point to where the cable pulls on the lever and found them to be exactly the same.
Perhaps, there are derailleurs with different lever arm lengths.
However, from my limited experience with road and mountain derailleurs, I see no difference in the lever arm length.
Also, think about all the recumbent riders who have gotten rid of their road cranksets and replaced them with mountain cranksets to provide lower gearing.
Most of them have also replaced their front derailleurs from road to mountain.
I don't remember any posts discussing problems when switching to mountain derailleurs and using their existing shifters.

BlazingPedals 07-17-09 06:01 AM

Claiming that mountain and road stuff are incompatible is a cop-out. I have a 105 front derailleurs and use SRAM Attack on one bike and SRAM X-7 on another. No problems. Almost all recumbents use mountain shifters, and quite a few use road derailleurs. Mountain or road, the distance between the chainrings is the same.

oldacura 07-17-09 07:39 AM

I ordered a FD-M531. Hopefully this will do the trick.

oldacura 07-23-09 08:10 AM

Jeff - you were correct. I got an FD-M531, installed it and it works great. Thanks for the help.


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