Fatbike Crankset and BB widths WTH???

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Fatbike Crankset and BB widths WTH???

Postby flamingo » September 29th, 2018, 7:47 pm

So I started ordering components for my fatbike frame. Looks like I am totally confuesed by now. The BB width is 100mm. It is a PF30.
However the genius who sold me the frame put a Race Face external bearing bottom bracket in it, and now the width of it is 120mm.
I have been looking for cranksets and they come in 100, 170 and 190mm width flavors. There is nothing in 120mm width even from Race Face.
Is the width of a cranset equivalent to the length of the spindle? Why would Race Face make a bottom bracket with no equivalent crank for it?
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Re: Fatbike Crankset and BB widths WTH???

Postby c0nsumer » September 29th, 2018, 9:19 pm

RF makes a bunch of different spindles to go on their Turbine cranks.

Consult this to figure out which spindle you need: https://www.raceface.com/media/Crank_Q- ... nlines.pdf

Which frame is it?
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Re: Fatbike Crankset and BB widths WTH???

Postby Rickwetherald » September 29th, 2018, 9:51 pm

It sounds like you're looking at a few different sources with measurements for different items. 100mm (aka BSA100, or threaded 100mm) refers to the width of the bottom bracket shell of the frame. If there are threads in this (sounds like there are), then the previous owner has the only bottom bracket that will fit, which will pull the width out to ~120mm. 170 and 190 refer to the crank's ability to interface and maintain chainline with two different rear hub width standards (170/177 and 190/197). If you have the larger 190 or 197 variety, you'll need a crank that brings the chainring(s) out wider. Sometimes companies get tricky with that stuff. Trek, for example, uses a 197 rear hub, but intends Farley frames to be run with a crank for 170mm frames but with the direct mount chainring flipped backwards, thus pushing it out wider. Confused yet? ;)

If you post your frame model and intended crank, I'm sure we can crowd source an answer for you.
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Re: Fatbike Crankset and BB widths WTH???

Postby Rickwetherald » September 29th, 2018, 9:56 pm

Oops, just re-read your post where you said it's PF30 (I'm assuming with a threaded adapter). If that's the case, a Race Face crank/spindle designed for 100mm threaded BBs should work fine (I think). Keep in mind that Race Face makes a BSA30 bottom bracket, where bearings designed to fit BB30 cranks are in cups that thread into the normal threaded frame where a normal 24mm (Shimano, etc) would go.

I think I'm now 98% as confused as you are :)
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Re: Fatbike Crankset and BB widths WTH???

Postby flamingo » September 30th, 2018, 5:05 am

Rickwetherald wrote:Oops, just re-read your post where you said it's PF30 (I'm assuming with a threaded adapter). If that's the case, a Race Face crank/spindle designed for 100mm threaded BBs should work fine (I think). Keep in mind that Race Face makes a BSA30 bottom bracket, where bearings designed to fit BB30 cranks are in cups that thread into the normal threaded frame where a normal 24mm (Shimano, etc) would go.

I think I'm now 98% as confused as you are :)


Thanks.

The frame in question is Specialized Fatboy Carbon. The BB shell is 100mm wide and it is PF30. My confiusion is that there are PF30 bottom brackets that
push into the shell maintaining the width of 100mm by keeping the bearings inside. Thus the required spindle length of the cranks keeps being 100mm. In my case the bearings are outside of BB shell adding 20mm width so I need a crankset with a spindle length of 120mm. When cranksets are specified they only say 170 or 190 meaning the width of the rear hub. There is my confusion.
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Re: Fatbike Crankset and BB widths WTH???

Postby flamingo » September 30th, 2018, 5:35 am

c0nsumer wrote:RF makes a bunch of different spindles to go on their Turbine cranks.

Consult this to figure out which spindle you need: https://www.raceface.com/media/Crank_Q- ... nlines.pdf

Which frame is it?


Specialized Fatboy Carbon.

Went through the charts and got even more confused. Looks like for a my bike with 190 rear spacing accordingly to table 1 the nominal chainline is 76.5mm.

Now looking at table #3 since I have BSA 120 or PF30 120 the required spindle is RF189.

HOWEVER looking at table #4 column 2 the spindle with closest chainline of 75mm is RF169 with flipped chainring. I would much more prefer the 169 because of its lower Q-factor. I feel like I need to go back to school and get a PHD to decipher all this info.

What is spindle length? Is it the width from inside of the arm to the inside of the arm or is it the outside to the outside. Or is it neither.

Thanks.
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Re: Fatbike Crankset and BB widths WTH???

Postby c0nsumer » September 30th, 2018, 8:54 am

It may do you well to visit a Specialized shop and find out what shipped stock on that frame. They were typically Race Face cranks, so you can then buy just that. Be careful with cutting Q factor down too much as you can risk crank arm strikes.
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