How to match bottom bracket and crankset sizes?

Question

I'm trying to buy a new bottom bracket. But I am not sure if sizes fits together.
VGEBY1 Bottom bracket with CYSKY Single Speed Crankset 48tT.
The bottom bracket has the following sizes;

68X118mm
68X122.5mm
68X124.5mm
68X127.5L

I can't find the match for the crankset.. It has the following infromation;
48T 170mm Crankarms 130 BCD.
How to pick the correct size?

juhist · Accepted Answer

If the crankset manufacturer has not specified the type of taper (ISO or JIS) and the required length of the bottom bracket, I think your only option is trial and error. The best way forward would be to pick a defective bottom bracket of known length and taper type -- perhaps if there's a bike shop nearby that might have some defective bottom brackets that haven't yet been thrown out -- and test what the chainline, chainstay-to-crank clearance and Q-factor are. Then you will have an idea of what bottom bracket to buy next. So this trial and error approach requires two bottom brackets: first a trial bottom bracket, then since you can calculate the correct length, the one that fits best.
If you can't find a defective bottom bracket of known length and taper type, your only option is to buy two bottom brackets. The first one, the trial bottom bracket, should be the cheapest you can find.
Note also there's a difference between ISO and JIS tapers. Ideally you would use the correct taper type required by the crank, but since you can't know if the crank is intended for ISO or JIS taper, you can mix and match ISO and JIS tapers, but selecting the wrong taper type will cause 4.5mm alignment error so you should compensate this in choosing the spindle length.

Nathan Knutson · Answer

There is a nerdy way if you have to choose a starting spindle length completely in the dark. It's not perfect but usually gets close and may nail it.
Arrive at a premise for JIS vs ISO. Almost all Asian cranks are JIS with the exception of a few higher end track cranks.
Measure the width of the square taper spindle at its tip. I don't have those numbers in front of me. Mark the taper bore about 2mm out from where it hits that inner measurement. The 2mm is to account for the press fit. It's a fudge but is about right.
With a vernier caliper and squinting, get a measurement for the lateral distance your mark falls past the centerline of the chainring(s), ie the tooth centerline for 1x or 3x or the midpoint for 2x. Call that distance X. If somehow it wasn't past that centerline, it would become a negative number.
Arrive at a number for target chainline. Call that C.
You now have everything you need for taking a guess at the spindle length math. Call spindle length S. You're now solving S= 2(C + X). (This assume a modern crank that wants symmetrical spindle.) Again it's not perfect, but it's a way to get to a starting point if you must order a BB with no other information.

How to match bottom bracket and crankset sizes?

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