Square taper BB question

Total Newb when dealing with Square BB…

I have a Northrock XCF Fat bike that comes with a Square taper Crank (155mm with 34T) and I would like to switch to 170mm with 30t.

Can something like this be installed without any issues?

Can I assume that Bottom bracket is the factor that controls the chainl ine/frame compatibility? and any type of crank arm can be installed?

The crank length and chainring size really don’t impact the bottom bracket compatibility. You should be fine. I gotta say, going from a 155 crank to 170 is a large jump. Why? Did you buy the bike used?

Yes, bought it used and I use 170mm on all other bikes.

So I guess square taper cranks don’t have a “spindle length” that will be determined by the BB?

There are different spindle lengths for square taper BBs. You need to get the correct length to put the arms in the right spot. Phil Wood, as an example, offers a very wide set of spindle lengths from 103 up to 145. White Industries offers fewer options at 108, 113 121.

Also check your BB Shell width (most bikes will be either 68 or 73mm; your fat bike will likely have a wider shell than either of those), and if the crank has JIS or ISO taper.

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Got it. So if the bike already has a Square BB installed, spindle length/chain line already has been determined?
It doesn’t matter what type Crank arm I install?

Could be small variance on how a different crank fits. Just match JIS/ISO taper and give it a shot. Probably be OK and with fat bikes “precise” probably isn’t a big deal.

Have fun

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The crank will have a stated BB length to give a standard chain line, ie the crank is what makes the choice. If you want to change chain line for some reason you can change BB length but otherwise refer to the crank manufacturer for BB length. If you’re planning to fit on a current BB and it’s similar (not double instead of triple or anything) it’ll probably be fine though.

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