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Shimano Crankarms and Bottom Bracket Spindle

ShotsawayShotsaway Posts: 175
edited October 2010 in MTB workshop & tech
The Shimano M440 crankarm and spider on my Rockhopper sheared apart. The M440 square tapered crankset doesn't appear to be available anymore, so I ordered the M442 to replace it.

Anyway I fitted the new crankarms but they don't sit as close to frame as the original crankarms. Therefore when the front mech was in the original large/outside ring position the chain would be on the middle ring of the new crankset. The original middle ring position had the chain on the granny ring of the new crankset. etc.

To rectify the problem I have had to adjust the front mech, so that it moves as far as the outside ring, however the front mech only just gets there. I have riden and tested the bike for 16 miles this afternoon, without any shifting problems. So good news there.

See the picture below for the spindle gap.

1) So why is there such a big gap between the crankset and the frame, in comparison to the previous M440 crankset?
2) And will the positioning of the crank arms cause addition wear problems on the bottom bracket?

P1090463.jpg

Posts

  • Chunkers1980Chunkers1980 Posts: 8,035
    1. It's tapered, meaning very very small variation in either the cranks or the BB or both will make an exagerated difference.
    2 Cant see it making too much difference to wear one side may possiblyt have slightly more leverage.
    3.You should have just gone HTII
  • nicklousenicklouse Posts: 50,675 Lives Here
    you need a new BB with a shorter axle to give the correct chain line.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • Chunkers1980Chunkers1980 Posts: 8,035
    Is it not the same BB?

    Should be very similar in distance it takes to lock on between the same fitting cranks.
  • ShotsawayShotsaway Posts: 175
    The square tapered fitting on the crankarm/spider of the original came out as far the middle ring. On the new crankarm/spider the tapered fitting comes out as far as the granny ring.

    The granny ring is approximately 6mm further away from the frame over the original crankset.

    So do I replace the 113mm bottom bracket with a 107mm (say a Shimano UN54) and will that move the crankset back towards the frame by 3mm (as the spindle is 6mm shorter)?
  • nicklousenicklouse Posts: 50,675 Lives Here
    odd, the M442 should use a 117.5 or a 123mm axle.

    see
    http://techdocs.shimano.com/media/techd ... 657937.pdf
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • Chunkers1980Chunkers1980 Posts: 8,035
    That will make it exactly the same the other way.

    If it works as is - leave it.

    I
  • ShotsawayShotsaway Posts: 175
    I've got to the bottom of it.....I think?

    The Specialized website lists the bottom bracket as 113mm, but I've had a look at the one fitted and it is actually 118mm.

    http://specialized.com/gb/en/bc/SBCBkMo ... nuItemId=0

    The chain line is currently 55mm and it needs to be 50mm ish. So if I buy the 113mm BB does the spindle reduce by 5mm on the driveside or 2.5mm on each side?
  • TuckerUKTuckerUK Posts: 369
    Bear in mind that chainline MIGHT be specced differently for large diameter downtubes (at least for Shimano).

    http://www.sheldonbrown.com/chainline.html

    Hope that helps.

    I guess that one of the benefits of integral external bearing cranksets anyway (althouhg still using my old indestructible XT square taper BB myself).
    "Coming through..."
  • ShotsawayShotsaway Posts: 175
    I bought a new Shimano bottom bracket (UN54) this morning and in response to my own question the spindle/axel on drive side reduces with each smaller bottom bracket.

    For example the spindle/axel on the drive side of a 113mm bottom bracket is 5mm's shorter than the 118mm spindle/axel. There is no difference in the length of the spindle/axel between the bottom brackets on the none drive side.
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