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GRX front derailleur hack

Old 09-05-21, 06:37 AM
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GRX front derailleur hack

I know FDs are relatively inexpensive but sometimes you want to try what's in the parts bin. I recently treated an old steel frame to a new GRX crank. First I tried a derailleur with the proper 28.6 band clamp. I couldn't get the trim set to suit me. Next I tried one with a 31.8 clamp plus the shim. This worked well. Apparently the shim brought the FD out far enough to satisfy the GRX crank. Maybe someone else will find this helpful.
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Old 09-05-21, 07:49 AM
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We have considered 3D-printing a shim for this purpose where the inner hole is offset by 2.5MM to compensate for th GRX wider spacing. We have not yet had to do it, but glad to hear thsi wirked as well. We have 3D-printed parts for other purposes as well and it is at times a handy solution o a strange problem.
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Old 09-05-21, 08:44 AM
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I don't quite understand how the standard shim helps, since they are symmetric and should put the FD in the same place.

I like the idea of an eccentric shim, that's a good use of a 3d printer.
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Old 09-05-21, 09:52 AM
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JTEK makes an eccentric shim that lets you use a 34.9 der on a 28.6 seat tube and sells it through SJS Cycles in England. They are fairly painless to order. Great for making the wrong derailleur work better!

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Old 09-05-21, 02:54 PM
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Originally Posted by unterhausen
I don't quite understand how the standard shim helps, since they are symmetric and should put the FD in the same place.

I like the idea of an eccentric shim, that's a good use of a 3d printer.
It seemed to me that the FD with 31.8 clamp (shimmed to fit the 28.6 seat tube) sat a little more to the drive side than the FD with native 28.6 clamp. I could be wrong....
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Old 09-05-21, 06:59 PM
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Originally Posted by JacobLee
JTEK makes an eccentric shim that lets you use a 34.9 der on a 28.6 seat tube and sells it through SJS Cycles in England. They are fairly painless to order. Great for making the wrong derailleur work better!

Nice!
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Old 09-06-21, 04:05 AM
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SRAM makes a "wide spacing" front derailleur clamp that moves the derailleur over 2.5mm. It's a little pricey, but it was designed to work with their "wide spacing" cranksets, which was their version of GRX for road cranksets. The cranksets were pretty much an OEM only thing for gravel bike manufacturers, but the clamps are available. I have used the clamps to adapt standard road derailleurs to mountain cranksets.
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