Bottom Bracket axle length...... check my thinking.

*AJ*
*AJ* Posts: 1,080
edited August 2010 in MTB workshop & tech
Right, my old frame had a shimano ocatlink BB which measures 68 x 121mm and the crank arms were very close to the BB, but did rotate freely.

My new frame is a 73mm BB shell.

Now am i right in thinking that if I buy a 73 x 121mm BB the axle is going to be too short, because the shell being 5mm bigger, reduces the length of the clamping area on the axle by 5mm.

I therefore need to order a 73 x 126mm bb to enable me to get the same fitment as previously got by the 68 x 121mm.

Does this seem correct?

Comments

  • Chunkers1980
    Chunkers1980 Posts: 8,035
    Yep that's correct.

    BUT - ditch the crank set and get an HTII set instead - problems then go away.

    Cartridge BB ~20 quid.

    New crankset and BB ~50/60 quid... for new deore.

    Your choice, but HTII is MUCH better.
  • *AJ*
    *AJ* Posts: 1,080
    Chunkers1980 wrote:
    Yep that's correct.

    BUT - ditch the crank set and get an HTII set instead - problems then go away.

    Cartridge BB ~20 quid.

    New crankset and BB ~50/60 quid... for new deore.

    Your choice, but HTII is MUCH better.

    TBH, I probably will do this, but im gonna save a bit and get myself some Saint cranks.

    73 x 126 bb is £11 from CRC, so that'll do for now.
  • nicklouse
    nicklouse Posts: 50,675
    nope, you need the same axle length to give the same chain line.

    shell width has nothing to do with it.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • *AJ*
    *AJ* Posts: 1,080
    nicklouse wrote:
    nope, you need the same axle length to give the same chain line.

    shell width has nothing to do with it.

    How can shell width have nothing to do with it?

    I thought this at first but the more i think about it the more i realise 5mm must be taken from the amount of spindle sticking out the end. (2.5mm from each side)
  • *AJ*
    *AJ* Posts: 1,080
    on a 121mm bb, a 68mm shell will leave 26.5mm sticking out each end.

    a 73mm shell will only leave 24mm??

    Doesnt that make more sense?
  • nicklouse
    nicklouse Posts: 50,675
    nope. the chain line is the important bit.

    the axle, if the correct one, is the same length for 68 or 73mm. the chain line does not change.

    longer axle and you will have an incorrect chain line and may have front mech issues.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • supersonic
    supersonic Posts: 82,708
    Nick is right. The crankset will still fully engage the splines, which isa lot shorter than 25mm. The distance from the centre of the bb shell to the end of the spindle is unchanged, hence chainline is exactly the same.

    I also disagree that HT2 is much better. Both are good systems.
    GT ZASKAR CARBON TEAM

    VOODOO CANZO

    Come and see me at https://www.facebook.com/biketyke/
  • *AJ*
    *AJ* Posts: 1,080
    OK Guys, a friend of mine had a knackered BB which was 73 x 121mm and although there was play in it ive put it in to check fitment, and sure enough its tightened up against the BB shell so the cranks wont rotate.

    So 68 x 121mm works fine, if a little close

    73 x 121mm tightens up against the frame.

    I then tried the 68 x 121mm BB in the frame and the drive side crank is fine, but obviously the non drive side one wont fit. (because the shell is to short) pulling the axle in by 5mm.

    Surely this means my theory was correct?

    Ive ordered a 73 x 126mm one now anyway, I dont run a front mech so if the chainline is out is shouldnt be miles out?
  • nicklouse
    nicklouse Posts: 50,675
    nicklouse wrote:
    nope. the chain line is the important bit.

    the axle, if the correct one, is the same length for 68 or 73mm. the chain line does not change.

    longer axle and you will have an incorrect chain line and may have front mech issues.

    Ok then what is the part number for the cranks.

    on the back of the arm.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • nicklouse
    nicklouse Posts: 50,675
    lets say the cranks are M442-8s

    with a 121 axle you get a 47,5mm chain line. with a 126mm axle you get a 50mm chain line.
    the width of the shell (BB) does not matter.

    So i would guess your old frame was for a different chain line to the new.

    Like both Sonic and I have said if all things were equal then the axle length stays constant.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • *AJ*
    *AJ* Posts: 1,080
    FC-M540
  • nicklouse
    nicklouse Posts: 50,675
    not listed for compatibility but

    this might help in the info

    http://techdocs.shimano.com/media/techd ... 657937.pdf

    just for info what was the old and new frame?
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • *AJ*
    *AJ* Posts: 1,080
    Old frame was a GT LTS.

    New Frame is Orange Patriot.
  • *AJ*
    *AJ* Posts: 1,080
    Cheers for the link Nick,

    Noted it stated 121mm axle length for both 68mm and 73mm shells.
  • supersonic
    supersonic Posts: 82,708
    What chainline does your new frame need?
    GT ZASKAR CARBON TEAM

    VOODOO CANZO

    Come and see me at https://www.facebook.com/biketyke/
  • *AJ*
    *AJ* Posts: 1,080
    Right....

    The 73 x 126mm came today (thank you CRC very fast postage).

    The amount of axle sticking out from the shell measured the same on the 73x126 as it did on the 68x121.

    The cranks are now fitted and run very nice. (I wouldnt have got away with any shorter axle though).

    Next problem, forks - but thats another topic. :lol:
  • supersonic
    supersonic Posts: 82,708
    I run a 73x118 Octalink with my LX M572 chainset, and works fine, except the chainring fouled the frame. I don't use the small ring on this bike so removed it.

    Yours is a very odd problem if the actual crank was hitting the frame. It should not do that on any model crank.
    GT ZASKAR CARBON TEAM

    VOODOO CANZO

    Come and see me at https://www.facebook.com/biketyke/
  • *AJ*
    *AJ* Posts: 1,080
    The axle was just too short and therefore the cranks were tightening right up against the BB shell thats all.

    The 126mm BB solved this and now everythings fine :)
  • supersonic
    supersonic Posts: 82,708
    My BB wouldn't have been much good then! All I can think is your crank is built to a different standard than normal V2 octalink crank, some strange non series stuff. They are supposed to work with all axles to vary chainline.

    All I want to make clear to other readers that this is not the case with other octalink cranks, you don't have to get a longer axle just because you have a wider shell.
    GT ZASKAR CARBON TEAM

    VOODOO CANZO

    Come and see me at https://www.facebook.com/biketyke/
  • *AJ*
    *AJ* Posts: 1,080
    Yeah sorry, thus is for my case only.

    I have however found out what the problem was.... When I've changed frames, I also picked up some polished crank arms, with the same part number as the originals and therefore should be I direct replacement, however someone had modified the crank arms by cutting them down so the clamping area was thinner than the originals. Proper bodge, but it means that they should have had a 126mm axel all along but they had been bodged to fit the 121mm bb
  • nicklouse
    nicklouse Posts: 50,675
    some people.

    :wink:
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • *AJ*
    *AJ* Posts: 1,080
    Might explain why the originals came loose every few weeks, they'd been hacked away! God knows how I'd missed it before, but i'l get pics up just for entertainment.
  • *AJ*
    *AJ* Posts: 1,080
    Heres the pics, just for closure on the thread really, and general amusement.

    New crank arm in situ (nice and snug)
    Iphone4094.jpg

    Original crank arm (bodged/modified)
    Iphone4097.jpg

    Comparison
    Iphone4098.jpg

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