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Old 11-10-10, 09:50 AM   #1
danlea
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Front derailleur / shifter - insufficient index spacing?

I've just purchased a Viking Virago Front Suspension Gents 19" town bike and have adjusted the front derailleur (fully) as best I can. I seem to have the peculiar problem of not having enough of a difference between the second and third indexes on the shifter: when the derailleur is set to allow riding in the top gear (both shifters) without the outside of the cage touching the chain, the chain touches the inside of the cage when on the middle sprocket with lower (rear) gears.

Is there any way of changing the index settings on the Shimano shifters (say by looping the cable at a larger radius)? If not, the problem would seem to be that the chainwheels are too far from the frame, or the whole front derailleur is positioned to close to the frame, since the rotation of the mechanism results in a smaller lateral shift at higher tensions.

That's a pretty long-winded explanation I know. Basically I'm not getting enough lateral shift between the second and third front gear settings, and it's not due to a prohibitive high limit screw. Here's an ASCII art representation, just in case it's still still not clear:

What I want:
Code:
                                          |   |  <- derailleur
|   |                |   |                |   |
|   |                |   |                |    
|         |          |     |                |    <- chainwheels
      |   |            |   |            |   |
  |   |   |        |   |   |        |   |   |
  |   |   |        |   |   |        |   |   |
What I can currently get:
Code:
                                         |   |
|   |                |   | <-too small-> |   |
|   |                |   |               |    
|         |          |     |                |
      |   |            |   |            |   |
  |   |   |        |   |   |        |   |   |
  |   |   |        |   |   |        |   |   |


                                          |   |
|   |                 |   | <-too small-> |   |
|   |                 |   |               |    
|         |           |    |                |
      |   |            |   |            |   |
  |   |   |        |   |   |        |   |   |
  |   |   |        |   |   |        |   |   |

Any bright ideas? Thanks,

Dan.
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Old 11-10-10, 10:10 AM   #2
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You didn't say exactly what components you're using, but I believe that Shimano road and mtn front derailleurs have different cable response ratios, so mixing might be the cause of your problem. It's also possible (but you'd need to check with Shimano) that as they went from 8s & 9s to 10s systems they narrowed the spacing of companion left shifters to match the narrower chainring spacing.

If you don't want to change anything, but want to increase the FD travel, try sliding a bit of plastic tubing up the bare wire to where it winds on the shift lever cam. That will increase the effective diameter so it'll pull more wire. If the shift lever isn't grooved, you can do a better job by super-gluing a piece of steel shim stock to the cam once you figure out how much you want to increase the diameter. (This is how I modify Campy ergolever cams)
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Old 11-10-10, 10:52 AM   #3
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It is likely it is a derailleur setup issue. If the Front derailleur is a SHimano model and the shifter is a shimano model then they will work together when properly set up (provided you don't have a road derailleur and mtb shifters or vice versa).

Go to parktool.com and follow their derailleur setup instructions to the letter, and don't skip any steps. The alighnment and height of the derailleur is just as important as cable tension so pay particular attention to those aspects.
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Old 11-10-10, 12:02 PM   #4
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In top gear (large chainring & smallest cog), you want the outer plate of the fd cage to just clear the chain. Perhaps you have it adjusted so there is excessive and unnecessary clearance to the outer plate, thereby robbing you of sufficient movement on the middle ring.
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Old 11-10-10, 08:00 PM   #5
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Thanks everyone.

I believe the derailleur is set up correctly, including height and angle (I did in fact spend many hours on this, so even though I am new to it, I'm happy with the setup). In top gear there is indeed minimal clearance from the chain - I'm pretty certain it's the amount of travel between the second and third index points that's the problem. Given that the components come as standard on the bike (it's certainly not a custom job), they should be entirely compatible (the shifter is a Shimano Rapidfire Plus ST-EF50-L, which with the SIS 7-gear right shifter should apparently have a FD-C051 front derailleur and FC-TX71 front chainwheel - I'm taking for granted that this is the case as I can't see model numbers on the actual components aside from the shifter).

FBinNY, thanks for the advice regarding increasing the effective lever radius - I think that's about the only option I have if I really care about using the lower four gears on the middle front chainwheel. Cheers!
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Old 11-10-10, 11:04 PM   #6
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Same thing happened to me when I purchased a new bike. It was the limit screws on the derailleur that improperly limited the range of cage movement.
Once I increased the range of motion of the cage, the indexing could be adjusted to work fine.
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Old 11-10-10, 11:26 PM   #7
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Loosen the cable so that the FD is more inwards when shifted to the middle-ring click.

Also, in 90% of the cases, you still have to do a half-click trim shift on the middle-ring. Let's say you're in top gear, big-ring/small-cog and you shift 2-3 gears lower in back. Then you swap to the middle-ring. Then you start shifting into lower and lower gears. In the vast majority of cases, once you're in the last 2-3 gears you have to trim the FD inwards by 1/2 click so that the chain doesn't rub on the inner-cage in the lowest 3 gears.

To combat this, I've actually unbolted the rear of the FD cage and inserted a 2-3mm spacer between the ends and used a longer bolt to put it back together. The shape is more trapezoidal than rectangular (wider at the back, narrow at the front).

Last edited by DannoXYZ; 11-10-10 at 11:29 PM.
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Old 11-10-10, 11:33 PM   #8
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to the OP, I don't know offhand which side of the FD pinch bolt arm the cable is supposed to be, but if you run the cable to the inside of the bolt, thereby shortening the effective length of the arm it'll increase the cable response, possible enough to solve your problem. (conversely if it's supposed to go to the inside and you bolted to the outside, that might be the problem)
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