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Thread: BB question

  1. #1
    my legs are carbon thebeatcatcher's Avatar
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    BB question

    I am replacing an old bb with a new cartridge one. I got the old one out and it is 68x116mm (printed on top). I happen to have on hand a 68x113mm new cartridge bb. Is it too small?

    If I need a 68x116, most other replacements I saw are 68x115 or 68x117, will one of these work?

    The bike is an old Schwinn with a double chainring and 6spd cassette.

    Thanks.

  2. #2
    Elitist Troglodyte DMF's Avatar
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    Same crankset? (important to the following advice)

    115 or 117 will work. 115 is slightly preferable. 113 is really too small.

    "Printed on top"?? Is the old BB a cartridge too?
    Stupidity got us into this mess - why can't it get us out?

    - Will Rogers

  3. #3
    my legs are carbon thebeatcatcher's Avatar
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    Quote Originally Posted by DMF View Post
    Same crankset? (important to the following advice)

    115 or 117 will work. 115 is slightly preferable. 113 is really too small.

    "Printed on top"?? Is the old BB a cartridge too?
    Same crankset. The old BB is not a cartridge, it has the serviceable bearings. 68=W=116 is stamped into the axle part of the old BB and it does measure ~116mm.

    The crankset is Shimano 600 and when I looked that up on Sheldon Brown's BB Size Database it had "113 (115)" as the appropriate sizes.

    Brown also says "Shimano sometimes specs two different lengths for the same crankset. This usually relates to the diameter of the seat tube. The shorter dimension is normally preferred, but bikes with fat seat tubes may require the longer size to accommodate the swing of the front derailer."

    The seat tube on this Schwinn 6spd is not fat, just the "regular" size.

    So it seems a 113 could work, but 116 is what came out of the bike, so I'm not sure.

  4. #4
    Elitist Troglodyte DMF's Avatar
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    "116" may be the overall length of the old axle, but unlike cartridges bare axles are often asymmetrical. Measure the distance from each race to the end.

    Two notes on 113 v. 116:

    1) Quoted (symmetrical) axle length is to achieve a standard chainline. The actual front chainline needed is determined primarily by the rear chainline. If that's non-standard then the crank OEM's recommendation needs adjustment.

    2) Chainwheel position can be limited by the chainstay, as you noted. But a fat stay is far from the only reason for it to run into the (optimally positioned) chainwheel. Some bikes have flared stays to clear fat tires. That's why mountain bikes (fat tires and maybe suspension) now have a different "standard" chainline than road bikes (skinny tires and human suspension).
    Stupidity got us into this mess - why can't it get us out?

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  5. #5
    Sometimes knows stuff. rmfnla's Avatar
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    I haven't had the chance to use my favorite phrase in quite a while, but since you do have the parts on hand... Why not try it and see if it works?!

    Ahh, that felt good; thanks!
    Today, I believe my jurisdiction ends here...

  6. #6
    my legs are carbon thebeatcatcher's Avatar
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    Quote Originally Posted by rmfnla View Post
    I haven't had the chance to use my favorite phrase in quite a while, but since you do have the parts on hand... Why not try it and see if it works?!

    Ahh, that felt good; thanks!
    B/c I might possibly be able to return the new cartridge BB and I didn't want to funk it up.

    But I think I just might try it anyway.

  7. #7
    JRA...
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    assuming the old spindle was symmetrical (which it probably was on 600) you're talking a 1.5 mm difference per side.

  8. #8
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    Install the new bottom bracket dry and only hand tight. Then install the drive side crank arm as a hand fit and see how much clearance between the chainstays and the chainrings you have. If there is plenty and fully seating the crank won't cause the rings to hit the stays, you are good. If it seems the chainrings will hit the chain stays when full seated, return the bb and get a longer one. That way you won't have marked up the bb and can return it as unused.

  9. #9
    my legs are carbon thebeatcatcher's Avatar
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    Thanks for all the advice guys! I think I'm going to return the bb, just to be on the safe side...

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