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What you're looking at is a Non-drive side crank making contact with the chainstay. Drive-side has plenty of clearance.

enter image description here

Components involved:

  • Cranks: LDC 155mm square taper
  • BB: 108mm Box One Ti.
  • Frame: DK Swift size expert.

I've seen a few people talk about filing down the inside of the arm(s) in situations where the cranks make contact with the chainstay.

Not an ideal solution IMO but probably a lot cheaper than buying a 113mm ti spindle (I didn't see one sold separately on Box's website) or a new BB...

Safe to file a tiny bit off the end of the crank arm to avoid rubbing the frame?

Additional Info

No, I don't really want to file the crank arms!

I measured the exposed length of spindle on DS and NDS, approximately 18mm of exposure each.

I figured maybe crank arm on NDS is bent since my kids do not take the greatest care of their bikes despite my constant admonitions to do so :) I checked both crank arms for any obvious bends or warps but they pass the eyeball test and mate back-to-back without any bows/etc.

Regarding fit/measurements, and whether I should buy Ti stuff until I know how to measure things out in advance ;) the shape and offset/angle of the crank arms (not just their length) might mean that certain brands would work with this BB but other brands would not. I have a set of Sinz 150mm 2-piece cranks that have pronounced bow-shaped arm, they're nominally shorter but both sets of cranks are ~168mm from spindle center to end of arm. These would probably fit without contacting the chainstay, but I didn't try that because I already swapped the BB for a longer spindle.

Finally:

I went with a longer spindle BB, because I happened to have a 113 or 115 mm sealed BB laying around the parts bin. Both DS and NDS have about the same frame clearance now.

enter image description here enter image description here

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    Safe to. Really in my opinion that is completely up to you. What you consider safe, others might not and vice versa. But it looks to me, there is not a lot in it and taking a little of the the inside of the crank "should" be ok. I'm assuming this is a new crank set, so I would be more inclined in the first instance to see if I could exchange it for something that fits the bike better
    – Hursey
    Commented Oct 4, 2022 at 3:38
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    Sounds like you’ve already decided to file it, but I’d recommend buying a steel axle replacement and spending money on titanium only after you’ve learned to look up the specs before buying. The weight difference will be unnoticeable.
    – ojs
    Commented Oct 4, 2022 at 3:42
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    If the DS has lots of clearance, perhaps that points to an unbalanced left-right centering issue. I would recommend against any significant filing.
    – MaplePanda
    Commented Oct 4, 2022 at 5:18
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    I’m wondering the same as MaplePanda, is it really symmetrical? Is the pedal-to-center distance the same for both sides?
    – Michael
    Commented Oct 4, 2022 at 11:24
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    Rubbing? from the image I would say striking, but it can be only from perspective. I would suggest you against filing. It looks like you are working on a fixie. Due to the strong loads cause dby riding style, you may discover that the frame flexes more than the few mms you plan to file, with all the risks involved in the wheel locking because of pedal strike/rub instead of spinnning with your pedaling.
    – EarlGrey
    Commented Oct 4, 2022 at 12:29

2 Answers 2

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You want about 3mm of clearance at the closest point with the frame to account for flex. That it's Expert doesn't necessarily make that rule any different, since the parts and frame you're working with are also built to a strength/weight target with that size rider in mind.

Filing a crank is not off the table in every situation, but the problem is going from hitting the frame to 3mm clearance everywhere is often more than just a little bit. Sometimes when people start down this path, more needs to come off than anticipated. Also, you would need to assess whether getting to that much clearance will wind up needing some spindle material to go too.

Before doing anything you should assess why it's like this. This includes looking at:

  • Confirming the 108 is giving you the chainline you want in the first place.
  • Is the clearance on one side and the interference on the other due to a frame alignment problem?
  • Is the Q-factor symmetrical even as the crank clearance is asymmetrical? Why is the frame clearance asymmetrical if so, or why is the q-factor if not? It's possible the crank Q is imperfect by design, but that's not super common.

If the above yields nothing you're probably left with the conclusion that it's happening because they're Mini cranks that were made to achieve a very low Q, and they're going on an Expert frame. If that is the only problem here and you're doing this to address fit considerations or some other good reason, here are some things you could think about:

  • Find a precision washer that's the right dimensions to fit between the DS cup bore and the cartridge, not interfere with the bearing, and give you the NDS clearance needed without taking away too much on the DS, then use chainring spacers to get the chainline dialed back.
  • Determine how much gap you can afford to lose on the DS and how much clearance you need to add on the NDS, then math out what length 73mm JIS BB would allow you to use spacers under the DS cup shoulder to wind up with functional chainline and clearance, then get a comparably fancy version of it and ebay the box.
2
  • I've got 3 or 4 cheap BBs I could put back on the bike all with longer spindles that's probably the way to go for now. Also a possibility that the NDS crank is bent slightly my kids don't take the greatest care of their bikes despite my reminders to do so lol. I don't think a spacer or washer of any sort will solve it, my first thought was a lock ring on the NDS -- that could adjust the "depth" of the cup insertion but it won't change the length of the spindle itself and that seems to be the root cause when combined with cranks this long. Commented Oct 4, 2022 at 13:13
  • @DavidZemens The length of the spindle itself is the problem if it would be impossible even without the DS cup shoulder in the way to give you the clearance needed on both sides at once. If that's true then you have to go longer. If it's not true then the trick above of going to a 73mm BB and tuning the position with spacers will probably work. Commented Oct 4, 2022 at 17:10
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Personally I would be happy to file some small amount off the rear corner of the crank arm.

I'd aim to have the file parallel to the chainstay, and I wouldn't file any closer than the pedal's hole.

Plan view (ie, from above) exaggerated for image:

    |      |        /
    |      |       /
    |Crank |      /
    |      |     /
----|------|    /
Axle|      |   /Chainstay
----|------|  /
    |     /| /
    |    /X|/
    |___/XX/ 
          /
         /

Less is most definitely better than more. I wouldn't worry about any weight imbalance - the difference won't be noticeable while riding.

I'd also suggest adding some paper tape to the chain stay and see if flex is ever allowing your filed crank to hit the chainstay still.


However if you're uncomfortable with this idea, then simply don't. The alternative is a longer BB axle/cartridge to push the left crank further out.

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    From the image posted, the amount of material that needs to be filed off could very well include part of the pedal axle. Time for a slightly wider $14 BB-UN300... Commented Oct 4, 2022 at 11:30
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    That diagram is basically the approach I had in mind (as a last resort). At this point I'd prefer not to mar the otherwise new cranks and probably just put a different BB on there since I've got 3 or 4 of cheaper ones in a few different sizes Commented Oct 4, 2022 at 13:15

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