Bicycle Mechanics - Ultegra 6700 FD with 46T Chainring too close to chainstay?
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12-23-10, 10:04 PM
I'm helping a friend build up an S-Works Crux (essentially the old S-Works TriCross frame) from the module (frame/crank) up. We're using Ultegra 6700 with the chainrings that came on the frame from Specialized -- 46T/34T.
When I adjust the position of the front derailleur so it's close enough to the chainring the back end of the derailleur cage is very close to the chainstay (see pictures -- the clearance is less than 1 mm). He and I have discussed it and think we have two options -- move the FD up a little bit and maybe compromise shift performance, or replace the chainrings with a 50/34 set, which would require moving the derailleur up a bit.
I'm trying to decide if this is going to be a problem that will need to be addressed before the bike is ever ridden. Obviously it'll be far enough away when he's riding on the 46T ring but I'm concerned if the clearance is enough for any movement/deflection of the chainstay or derailleur.
either solution would work.
seems like, true to the frame's purpose, the designers never thought about a CX type chainring setup.
12-24-10, 12:45 AM
I have a somewhat different bike (steel frame, triple crank) that presented me with a similar situation. Due to the low bottom bracket, the FD cage comes very close to the chainstay when on the smallest ring. There's enough space for a credit card to fit...and just a smidge more. If it's true that a credit card measures 0.7mm (based on another member's statement), then I might have 1.0 - 1.25mm clearance. That's just a reasonable guess, though. The cage has never marked the frame, and I have no reason to believe it has touched it.
Looking at your pictures, it appears that (a) you have wiggle room to raise the dérailleur a tad, and (b) the edge of the cage just overlaps the edge of the chainstay. Given that the rings only have a 12t difference, am I wrong in thinking that shifting will not be noticeably affected if you raise the dérailleur 1 - 2mm? And if this still failed to provide adequate clearance, perhaps a spacer under the drive side bb cup would help, though then chainline may not be as precise.
The first thing I'd want to do is be certain the cage is angled properly. This requires the chain on the bike, in addition to the rest of the drivetrain.
I suspect that the area near the bb is stiff enough that it would be rideable with the minimal clearance you state, but the cautious side of me would still want to bump the FD up as long as shifting remained good. Then again, if the owner would really rather be happier with a 50/34, then none of this is to bother with.
IMO that's almost certainly enough of a gap.
If the derailleur is adjusted to shift optimally in that position, I'd just stick a little blob of plasticine in the chainstay and take it for a hard ride, then inspect the blob to see how thin it got squashed. I'm betting the gap wouldn't even be halved.
12-24-10, 01:43 AM
I would go with the option of moving the FD up by 1 mm, the effect on shifting may not even be noticeable.
12-24-10, 07:57 AM
Thanks all -- I'll give moving the FD up a smidge a go.
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