Gotta get this front mech away from the frame somehow

Manc33 Posts: 2,157
edited October 2014 in Workshop
I have a triple with a R443 front mech. On the inner limit screw, I have to have the left outer part of the cage literally touching the chainstay, because in the lowest gear it rubs, I mean it rubs even with the limiter screw set so the mech cage is half a millimeter from touching the chainstay.

For the same reason, I have to have the front mech a little higher than the recommended 2mm above the large ring, lowering it would again mean the mech cage hitting the chainstay (since it curves out there).

My chainset is meant to be used with a 113mm bottom bracket and I am using a 115mm bottom bracket and still finding my front mech to be too close.

Its almost as if... if I just put on a 127mm BB on it would solve everything. That would add 6mm to each side though! It would solve why the cable touches the mech when on the granny ring + why the lever is stiff there. I am sure it is all too close in.

What happens if you put on a BB with an axle too wide? :lol: Does it mean the front mech might have trouble shifting to the biggest chainring?

Maybe its because my seat tube is a 34.9mm. If that means the FD body itself (the band mount) is say 3mm too far out, it would explain why it needs to be set so far in (the cage).

Not sure what the margin for error is on BB's. :oops: I am past caring about the chainline that much. :P The chain will just have to cope.


  • Manc33
    Manc33 Posts: 2,157
    I think because I have a 34.9mm seat tube on a road bike, it is causing the problems.

    I am going to bring the chainline out by 3.75mm (swapping a 115mm axle for a 122.5mm axle), wish me luck. :lol:

    I worked out if I do that, it will "fool" the FD into thinking it is on a 27.5mm seat tube, which is dang close to the most common size of 27.2mm.

    A road frame with a 35mm seat tube. :roll: That is the maximum size across road and MTB. The FD is road specific... as far as the frame of the bike is concerned, where shifters are concerned it is a MTB mech then. Don't even ask, its the R443.
  • cycleclinic
    cycleclinic Posts: 6,865
    I doubt that frame was made with long cage triple mech in mind that is your problem. -wheel building and other stuff.
  • Manc33
    Manc33 Posts: 2,157
    I doubt that frame was made with long cage triple mech in mind that is your problem.

    The outer plate of the FD cage is about 1mm from touching the crank arm, while the inner plate of the cage nearly touches the chainstay on the inner chainring.

    Its usable as it is, but dangerous since the FD could get ripped off by the crank arm pretty easily when on the big ring. All you'd have to do is have the limit go out slightly. The cable has to be really slack on the granny ring and needs a big shove to get it to middle (it always did with a triple, but this is an exaggerated version).

    What I wanna know is how far out can chainrings go before the shifting to the big ring gets sloppy? I don't know what the margin for error is with it.
  • Fit a spacer between the frame and the driveside cup.Cassette spacers fit, this will move your chainrings about 2.5-3 mm further away from the frame.
  • Manc33
    Manc33 Posts: 2,157
    freezing77 wrote:
    Fit a spacer between the frame and the driveside cup.Cassette spacers fit, this will move your chainrings about 2.5-3 mm further away from the frame.

    Too late, already ordered a 122.5mm BB to replace the 115mm one. :oops: Its only £8.99. What worries me now though is the chain line, on middle>middle its already nothing like parallel to the frame, but I'm not sure if its meant to be anyway. It only looks parallel when in the highest gear.

    Last night I measured from the center of the seat tube to the middle ring and it was 45mm exactly as its meant to be for a triple, but that measurement is not taking the tubes thickness into account and probably assumes a 27.2mm seat tube, not 34.9mm.

    Thats just where the mech goes, the seat tube clamp is 36.6mm! I have tried and tried to find a QR version of that collar/clamp but they seem to not exist. You can get 34.9mm ones but my fat seat tube tapers up to be even fatter at the top. This, on a road bike! :lol: A 34.9mm seat tube is fat even if it was a MTB.

    Looking on Wikipedia, 27.2mm is right in the middle of all the known sizes, they also list 34.9mm as the fattest you can get. I think this is what's pushing the FD out on its band meaning the FD has to be all "tucked in" more underneath and be pivoted inwards more to accommodate for the fat seat tube.

    By using a BB thats going to be 3.75mm oversized on each side will accommodate for the difference between a 27.2 and a 34.9 (half that difference is 3.85mm). So the FD will be "fooled" into thinking it is on a 27.2mm seat tube and the chainrings will be the right distance.

    I probably should have just got a hardtail MTB, just kidding. :mrgreen:
  • Ben6899
    Ben6899 Posts: 9,686
    How much have you spent on this bike now? I only ask because you seem to be chasing your tail and throwing money at new parts, trying to make it work.

    Bikes: Donhou DSS4 Custom | Condor Italia RC | Gios Megalite | Dolan Preffisio | Giant Bowery '76
  • Manc33
    Manc33 Posts: 2,157
    The new 122.5mm bottom bracket has fixed it.

    The triple FD has to have the cage nearly touching the chainstay to work, but it all works properly now.

    Middle chainring > 32T sprocket = very slight rubbing.
    Middle chainring > 11T sprocket = slight rubbing but I would be on the big chainring.
    Granny chainring > 32T sprocket aka lowest gear = no rubbing.
    Top gear = mech cage about 2mm-3mm away from crank arm.

    Low limit screw - like above, FD cage just about not touching chainstay. :oops:
    High limit screw - screwed so the cage meets the shifter's last click.

    On the granny ring the front mech cable now is not all loose like it had to be before widening the bottom bracket spindle.

    My sig has never applied more, it took me about 4 hours getting the FD on straight (stupid carbon band mount).

    Ben6899 I paid £9 for that 115mm bottom bracket delivered and will just sell it on eBay for about £12-£15 since new ones are selling on there in their hundreds for £19. I will make the £9 back, don't worry. :P It has been used for about 100 miles if that.

    I never really was throwing money at it, just building a bike.

    Not sure what the total outlay is up to by now, well over £1000 but probably under £1500.

    All that remains of the original Triban 3 now is the brake calipers. :lol: It used to be the brake calipers and chainset, but I just put a new chainset on it.

    Its closer to a MTB now than a road bike... just with thin 700c wheels.

    "So you've managed to botch together a bike that is neither fast on the road, or suitable off road?"

    Essentially, yes. :mrgreen:

    But thats exactly what I wanted, a fast commuter bike. To me, road bikes are too racy and MTB's are too off road, I mean its two extremes.

    Don't tell me you can get carbon fiber commuter bikes with custom chainrings for under £1500? With a 1400g wheelset? With XTR shifters? Show me. :twisted:

    Once the ground dries up, gonna give it a try up that notorious hill near me.