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Front derailleur chainline for FD-3503 with FC-T8000

Hi - I'm looking for best-practice tips for resolving chainline issues using a 3x9 Sora front derailleur with an MTB triple chainset (for lower gearing on a touring bike). The bike manufacturer did it by leaving out a drive side spacer on the BB, crushing its plastic inner shell, and ramming the NDS crank arm further onto the spindle to take up the slack, but before I commit to repeating that setup I'm curious if anyone has any better ideas. Details below.

Cheers!



So I recently changed chainset on my touring bike (needed shorter cranks, chose to upgrade as well). Decided to swap the Bottom Bracket at the same time since it was cheap and easy to do at the same time.

When I pulled out the old BB (Alivio I think) the plastic inner shell had been crushed. I noticed that it had been fitted without its 2.5mm drive side spacer. Presumably this is why the shell came out crushed, so I put all the appropriate spacers in during install of the new BB-M8000

Then I couldn't get the front derailleur to swing out far enough to put the chain in the big ring. No amount of limit screw or cable tension adjustment would get it out there. It was close but never going to happen. I had a look at some specs online and saw that my FD-3503 has a 45mm chainline when all the MTB triple derailleurs have a 50mm chainline. I figured this was why the spacer had been left out, but it seemed odd for a manufacturer to spec and ship a bike this way (2017 Dawes Galaxy Plus).

First I looked at moving that spacer over to the NDS, but there are less threads on that BB cup so I was worried whether it would get enough purchase to hang in there safely. I decided the best course of action was to swap to an appropriate front derailleur instead and do it all 'the right way.' So I found a NOS 9 speed triple Deore front mech FD-M591 on closeout and bought it for peanuts.

I've just fitted the new derailleur and after much cursing and forum reading I've discovered (or re-discovered) that while Shimano 9 speed REAR derailleurs are cross compatible between Road and MTB, the FRONT derailleurs have different cable pull and won't work together. So if I want to keep my STI levers then the Deore mech is out.

So I'm trying to work out my best solution - is it to crush my new BB-M8000 the same way Dawes had done so that the original Sora front derailleur works again? Seemed to be fine before but feels an odd mis-use of parts. I also wonder how much it matters that leaving out a spacer requires you to take up the slack by pushing the NDS crank arm onto the spindle an extra 2.5mm - again it worked fine before but definitely doesn't seem like best practice.

Here's the info on my chainset swap - I went for FC-T8000 in the end, after finding it listed in various places as being both 9 and 10 speed compatible: https://forum.bikeradar.com/discussion/13106587/10-speed-triple-chainset-to-replace-9-speed-triple
Custom Albannach Torragar [BUILD IN PROGRESS]
2020 Ribble CGR SL
2019 Vitus Vitesse Evo CRS Disc
2014 Boardman MTB Pro 29er HT

Posts

  • Update: I decided best to do a 'proof of concept' install before crushing a brand new BB (on which I got the model number wrong BTW, it's BB-MT800 not 8000). So I took out the MT800 and re-installed the old crushed SM-BB52, omitting a spacer as originally built. DS bearing is on its way out but still has some life left. Probably another tour at least. NDS spins as new.


    [Crushed BB]

    I can just about take up all the slack on the T8000 chainset if I leave out the safety clip. I found a washer that fit over the nylon pre-tension bolt which allowed me to snug that up pretty well, but it's not ideal. I can't feel any lateral play in the cranks but I can kind of hear something, so I think there's just a hair's width of movement. Long term I think I probably need to find a washer with 24mm inner diameter to sit between the NDS crank arm and BB, which should allow me to fit the safety clip and preload bolt as Shimano intended.

    It will result in an off-centre Q-Factor with the left pedal being 1-2mm wider and the right pedal being 2.5mm narrower. How big an issue is that from an ergonomics standpoint? Hopefully helped by the fact that the old chainset had a Q Factor of 181.7mm and the new one is 176mm (my main reason for upgrade). I'd rather not shuffle my cleats every time I go touring...

    Once I did all this I was able to index the Sora front mech perfectly. It required incredibly precise setup, but if everything is 100% perfect then the shifting is clean across all three chainrings and the entire spread of the rear block. Took me 3 tries though as it's only possible to avoid chain rub in 48-11 if the (band-on) mech is mounted at the perfect height, perfectly parallel, and with the high limit screw backed all the way out. But I guess that's to be expected when you're making a 5mm chainline adjustment using only a single 2.5mm spacer...

    Why would Dawes spec a bike like this in the first place?!

    (And also, does anyone have any better ideas that I'm overlooking?)
    Custom Albannach Torragar [BUILD IN PROGRESS]
    2020 Ribble CGR SL
    2019 Vitus Vitesse Evo CRS Disc
    2014 Boardman MTB Pro 29er HT
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