Road fd cable pull ratio, shimano vs sram.

Discuss light weight issues concerning road bikes & parts.
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alcatraz
Posts: 1295
Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

Hi wws...

I'm trying to set up a dura-ace 7900 front derailleur on a sram red 10s groupset. I'm getting rub in either smallest or highest gear.

Limit screws can be completely loose so they aren't interfering. Fd is mounted perfectly straight.

I'm left to conclude that the cable pull ratio must be higher with shimano shifters as the cage doesn't move far enough.

Do you have any information or experience with this? Thanks....

I'll try to clamp the wire as far down on the fd arm as possible to kind of increase the movement. What else can I try?

by Weenie


joejack951
Posts: 451
Joined: Fri Feb 18, 2005 6:50 pm
Location: Wilmington, DE

by joejack951

Shimano and SRAM have rarely agreed on cable pull requirements for derailleurs. I'm sure there is some hack you can do to make it work reasonably well (like you've noted about adjusting where the cable is clamped) but why not just use the proper FD?

DJT21
Posts: 223
Joined: Sun Mar 27, 2011 7:35 pm

by DJT21

The orevious generation Shimano front mechs (i.e. 7800 for Dura Ace) used to work ok with SRAM 10 speed. Not sure whether the later generation worked as well.

Hexsense
Posts: 556
Joined: Wed Dec 30, 2015 12:41 am

by Hexsense

I'm using Sram Red 22 left shifter to command Shimano 9100 FD.
It works very well. There are two settings.
Low trim and High.
Low trim: small ring and 28-25-23-21-19-17-15-14 in the back without rubbing. small ring and 13-12-11 rub FD cage.
High: big ring and full frange of cassette 28-11 are usable without rubbing.

alcatraz
Posts: 1295
Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

I'm getting rub without trim, letting the derailleur go as far as it'll go.

I just let it rub a little in the smallest granny gear.

I will try to lower the cable mounting point if I can.

Maybe my cables are a bit old too and not as smooth as they should be. If I push on the fd cage on the low ring I can get it to be silent barely. I means it has a little bit left to move but doesn't from the spring tension alone.

Marin
Posts: 3062
Joined: Wed Jan 22, 2014 11:48 am
Location: Vienna Austria

by Marin

Widen the cage a bit?

wrcompositi
Posts: 125
Joined: Tue Dec 04, 2012 3:57 am

by wrcompositi

FD-7900 has a longer linkage than other Shimano FDs(FD-6700,5700,7800… basically all FD before the 11s groups, which have long upper arms like FD-9000), therefore it must be used with ST-7900 to operate properly.

There's a solution though. If you put a tab washer(the type of washer on rear derailleur's cable fixing bolt) under the FD-7900's bolt, than route the cable below the tab and bolt, the effective lever arm length is reduced to be compatible with Sram shifter. Here's the example I used on one of my bike, the red arrow points to the tab of added washer:
Image

After this mod, FD-7900 wil index correctly with Sram shifters, as other short-armed Shimano FDs do.

I don't use this setup anymore, IIRC, the shiftng feels similar to FD-5600(normal short-arm). I'm curious whether FD-9000/6800/5800 will work with Sram shifter, but I'm more inclined to use Sram 11s FD with yaw features, which should give the best results.Hope this helps.

alcatraz
Posts: 1295
Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

wrcompositi wrote:
Fri Nov 09, 2018 9:15 am
FD-7900 has a longer linkage than other Shimano FDs(FD-6700,5700,7800… basically all FD before the 11s groups, which have long upper arms like FD-9000), therefore it must be used with ST-7900 to operate properly.

There's a solution though. If you put a tab washer(the type of washer on rear derailleur's cable fixing bolt) under the FD-7900's bolt, than route the cable below the tab and bolt, the effective lever arm length is reduced to be compatible with Sram shifter. Here's the example I used on one of my bike, the red arrow points to the tab of added washer:
Image

After this mod, FD-7900 wil index correctly with Sram shifters, as other short-armed Shimano FDs do.

I don't use this setup anymore, IIRC, the shiftng feels similar to FD-5600(normal short-arm). I'm curious whether FD-9000/6800/5800 will work with Sram shifter, but I'm more inclined to use Sram 11s FD with yaw features, which should give the best results.Hope this helps.
Thank you for that detailed post. I'll certainly try something similar to get the cable clamped further down.

I'm not sure how the extra washer helps to achieve that unless it's simply not possible to clamp the cable further down against the arm itself.

/a

wrcompositi
Posts: 125
Joined: Tue Dec 04, 2012 3:57 am

by wrcompositi

alcatraz wrote:
Fri Nov 09, 2018 4:06 pm

Thank you for that detailed post. I'll certainly try something similar to get the cable clamped further down.

I'm not sure how the extra washer helps to achieve that unless it's simply not possible to clamp the cable further down against the arm itself.
At first I clamped the cable to the right of fixing bolt(opposite to standard routing path), but the derailleur still can't move far enough. The tab washer I used is from RD-5501, the tab is about 2mm wide, so it can force the cable just a little bit closer to FD's pivot. The tab washer solution is like a crude version of Shimano's "converter", as found on FD-9000/6800/5800 etc.

alcatraz
Posts: 1295
Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

Aha so is it a small diameter washer that has the cable resting on its edge?

I needs to be thin right so it doesn't take all the clamping force.

/a

by Weenie


wrcompositi
Posts: 125
Joined: Tue Dec 04, 2012 3:57 am

by wrcompositi

alcatraz wrote:
Sat Nov 10, 2018 5:20 am
Aha so is it a small diameter washer that has the cable resting on its edge?

I needs to be thin right so it doesn't take all the clamping force.

/a
No, it's not what I did. :D The cable must rest against the right edge of tab for this trick to work, and the cable is clamped between the tab washer and original washer(the thick silver washer). Since the tab washer lacks the groove for optimal clamping, I superglued a short piece of shift cable on the left half of tab washer, to prevent the top washer from tilting and make it press evenly (not really necessary, just OCD… :lol: ). Lastly, a longer bolt is required.

I think your idea of using a small-diameter washer to alter cable path may leave too little space to clamp the cable. Worth a try though.

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