Is my BB too long?

Kiblams
Kiblams Posts: 2,423
edited August 2009 in MTB workshop & tech
I have recently fitted a 3 chainring chainset (SR Suntour XCR) and a Simano Deore front mech, had only granny and middle ring previously and I am having an issue.

I have managed to get the shifting between the middle and granny rings perfect, but changing into the big ring is causing me a headache.

I have noticed that the mech stops moving half way through pushing the shifter into the biggest ring, which leads me to thinking that the mech is at its full extension (no, it isn't touching the limit screw)

Does this sound about right? Do I need a shorter BB?

Comments

  • stevebrock
    stevebrock Posts: 239
    no your limit screws need setting!

    take a look on Park Tools!
  • Kiblams
    Kiblams Posts: 2,423
    stevebrock wrote:
    no your limit screws need setting!

    take a look on Park Tools!

    The limit screw isn't touching the metal! there is a visible gap of more than 2mm below the bottom of the screw.

    Anyone else got any ideas?
  • Zeroman_IR
    Zeroman_IR Posts: 290
    How many driveside spacers are there on the BB?
  • Kiblams
    Kiblams Posts: 2,423
    I am afraid the BB is the one thing on the bike I haven't changed so 'driveside spacers' makes little sense to me I'm afraid. Can you enlighten me at all? It is a sealed cartridge square taper BB.
  • Kiblams
    Kiblams Posts: 2,423
    Not sure if this information will help anyone, but I have measured the distance from my seat tube to the centre of my big ring; and it is 45mm.
  • stu8975
    stu8975 Posts: 1,334
    Make sure have correctly tensioned and routed the cable through the pinch bolt on the front mech, it may sound daft but it can affect it. ie mine is correctly routed on the left of the pinch bolt, if its routed on the right side it doesnt change correctly. Post some pics aswell. Can you pull the mech into position over the big ring by hand? to measure your chainline, measure from centre of seatube to middle of the middle chainring.
    It should be around 50mm chainline and 113mm bb axle length.
  • Kiblams
    Kiblams Posts: 2,423
    stu8975 wrote:
    Can you pull the mech into position over the big ring by hand? to measure your chainline, measure from centre of seatube to middle of the middle chainring.
    It should be around 50mm chainline and 113mm bb axle length.

    No, the shifter is pulling it to full extension. The BB length is 121mm so that has to be the issue :(

    Here are some images (please excuse the focus - phone camera in low light)

    Not sure you can see it here, but I assure you that there is a gap:
    limiter%20screw.jpg

    Not too clear here either, the chain is being pressed against the big ring but not on it - but this is the max extension I can get with the shifter or by hand:
    max%20extension.jpg

    This is the routing of my cable (ignore the hoop, this will be cut afterwards)
    pinch%20bolt.jpg

    Hope this helps guys.
  • Kiblams
    Kiblams Posts: 2,423
    Damn, this is the one thing I was dreading, Looks like more tools and a new bottom bracket are in order. I really need the bike in working order for a charity ride next weekend, but I work the hours my LBS are open so this is the worst news! :cry:

    Is it wrong for me to even be considering using spacers to move the chainrings in a bit? :?

    Thanks for your help though.
  • supersonic
    supersonic Posts: 82,708
    Measure your front chainline.
  • Kiblams
    Kiblams Posts: 2,423
    Front chainline is between 53 and 56mm (only have a ruler to hand; which is why I hadn't tried to measure it until you asked)
  • supersonic
    supersonic Posts: 82,708
    Bb is too long most probably.
  • Kiblams
    Kiblams Posts: 2,423
    :lol: we already established that! you got my hopes up there for a moment SS :P

    Is there nothing I can do just to get through next weekend that doesn't involve a new BB? Don't like the idea of road riding (with slicks on) and no big chainring...
  • stu8975
    stu8975 Posts: 1,334
    You should be able to pick up a new bb/crank extractor/bb tool for about £20-£25 from chainrectioncycles.com
  • Kiblams
    Kiblams Posts: 2,423
    Crisis overted! :D I had a sit down to think about what I could do... and it hit me!

    If you can't move the chainrings towards the mech (before next friday)... why not move the mech to the chainrings! A little tape later and I have it shifting like a dream! :D

    Just a temporary measure for next weekend, but a big weight lifted off my shoulders :D