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Front derailleur tab position

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Front derailleur tab position

Old 09-10-14, 03:35 AM
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JonnyHK 
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Front derailleur tab position

I'm riding my bamboo bike around and it is ok, but the front derailleur is not quite right. The chain rubs slightly on the cage in my lowest gear combos (34t at front, 21-23-25 at back).

See photos:
http://www.bikeforums.net/framebuild...l#post17098392

Two issues:
1 - I mounted the tab too high (dumb error)
2 - the tube is too large, so the derailleur is too far over

I've just fixed number 1, so now the cage is lower and can sit the usual 2-3mm above the large chainring.

However, even with the limit screw as far as it will go, the cage just won't get close enough to the frame to stop that slight rub.

Am I correct to assume that the wider-than-normal tubing (seat tube approx. 40mm diameter here) is placing the derailleur mount too far out from the centreline of the bottom bracket?

My budget solution for now is to see what a thin spacer moving the crank outwards a little will do. Hopefully it solves this problem without creating any chain vs seat stay issues at the back.

For my Mk2 frame I think it would be easy enough to carve out a small recess for the tab so that it can sit 2-3mm further in.

What dimension (centreline of frame to the centre of the derailleur mounting bolt?), or range of measurement, would you consider to be OK?
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Old 09-10-14, 09:06 AM
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Canaboo
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Why not just recess this one? It would be a good idea to internally reinforce the area if you are going to be cutting into the fibres on the outside. The best idea is to just use a smaller diameter of tubing which is your main problem.
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Old 09-10-14, 09:07 AM
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reddog3
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Here's all the info for frame dimensions for builders- http://www.peterverdone.com/wiki/ind...for_Post_Mount
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Old 09-10-14, 08:15 PM
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Originally Posted by Canaboo View Post
Why not just recess this one? It would be a good idea to internally reinforce the area if you are going to be cutting into the fibres on the outside. The best idea is to just use a smaller diameter of tubing which is your main problem.
I may yet attack this frame again!

Just wanted to be sure before I started to chisel things out!



Originally Posted by reddog3 View Post
Here's all the info for frame dimensions for builders- Shimano Framebuilder Info - Pvdwiki
Brilliant.

Everything I needed. Thank you.
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Old 09-10-14, 11:40 PM
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We've just put a 2mm spacer in to offset the crank and things are much better. Still a very slight rub when in the 25t even after angling the cage a little.

I don't think this frame can be modified further. The tubing is too thick, then the carbon wrap adds to that.

Even if I recessed the tab it would cause the lower arm of the derailleur would hit the carbon lug before the limit screw.

So, on Mk2:
- thinner tubing
- recessed tab mount
- small area notched out of bamboo tubing below tab for derailleur arms to swing (done before carbon wrap), but would need to be done anyway as I would need to counter the depth of the recessed tab.

Also:
- thinner joins on the chain/seat stays (or further from axle) at the rear dropouts
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