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I have a seasoned 26" Dirt Jump bike where the time has come to replace the bottom bracket. The bike currently has Profile BMX cranks, with a single speed rear wheel (not an MTB freehub). Current BB uses external cups which means that the standard 5.5" axle is too short to tighten up correctly and occasionally have the cranks work it's way loose. With the external cups the chainline also isn't great.

Now the simple option is to go back to an internal Euro BB, but was surprised to see how expensive these are now, so was looking at my options given that 1x drivetrains have become popular. Would also give me the opportunity to go up a couple of teeth on the sprocket to 32.

Am I likely to run into any issues converting to a modern MTB crank. It looks like I can find a 73mm BSA threaded DUB BB/Hollowtech. My two main concerns are:

  1. What chain compatibility do I have we a 1x chainring, can I use a single speed chain?
  2. 1x drivetrains are common on bikes with 142 mm rear spacing compared to my bikes 135mmm rear spacing. Will my chainline be ok on a crank designed for a wider rear spacing?

update: I've found the driver on from the manufacturer. Doesn't specifically say what chains are compatible unfortunately, but I'm guessing I could measure the width of the teeth to see if a narrower multi-speed chain would work or am I oversimplifying this? link

  • What bottom bracket shall does the frame have? You imply BSA/ISO threaded, what width? – Argenti Apparatus Apr 22 at 10:28
  • correct, 73mm shell width – el_oso Apr 22 at 13:54
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At $40US for a Profile internal Euro BB I'm struggling to see what you're hoping to gain here. One major thing you'll lose is the narrow Q of your current cranks, which most would consider negative.

But,

  • Normal BMX 1/8" chains like a 410 or 410H will work on a narrow-wide ring. I haven't put a halflink chain on one but I don't imagine there being a problem. Note that the NW profile might as well not be there in a singlespeed drivetrain and is basically money wasted, but they do work.
  • 142x12 is essentially the thru-axle adaptation of a 135 QR rear end. The bigger number is due to how the ends of a 142 hub recess into the frame. So for the most part, 142 and 135 bikes have the same chainline considerations and use the same front end configurations. Presuming you have some kind of SS freehub, you should be able to use spacers to get your chainline good, but it will be different from where you've had it. Most 135 SS freehubs can go to about a 55mm chainline at the outside. Since Superboost cnailine is 56 that probably means you have impunity between getting a 142, Boost, or Superboost crank were you to do this.
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  • pricing up the new BB + a 32tooth BMX ring came up a similar price as a new MTB crank, and MTB consumables generally being cheaper. Also found internal Euro BB's to wear much quicker than outboard types. That being said they took quite a lot of abuse from general dirt jump riding in very sandy conditions. Since the land where my local jumps has been bought the jumps have been levelled and this bike is now only used for street riding. I think for now i'll stick with what I know until the n+1 bike comes along and try a trial swap – el_oso Apr 23 at 8:54
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What chain compatibility do I have we a 1x chainring, can I use a single speed chain?

You'll have to use a 3⁄32" / 2.4 mm derailleur chain with an MTB crank I think, not a 1⁄8" / 3.2 mm single speed chain.

1x drivetrains are common on bikes with 142 mm rear spacing compared to my bikes 135mmm rear spacing. Will my chainline be ok on a crank designed for a wider rear spacing?

A crank for 142mm rear will probably be OK. All the current Shimano cranks for 142mm spacing run 48.8mm which is not much more than the classic 47.5mm MTB chainline. Beware than cranks for 148 and 157mm rear spacings also exist and these obviously run wider chainlines.

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