# How to calculate what BB spindle length I need for a mystery crankset?

I'm building up a bike frame with spare/junk components to be my beater bike for commuting. It will be a single speed, chainline on my rear wheel is 42mm. I have a crank set to use, but I'm not sure how to calculate the correct bottom bracket spindle length since I don't have manufacturer's specs on the cranks or any way to look them up since they're not clearly branded. Is there a way to calculate spindle length using measurements from the crank?

I'm using JIS square taper with an english-threaded bottom bracket. I don't expect to get the chainline exactly perfect, I figure I may need to use a couple mm of spacer washers between the chain ring and crank for small adjustments. But I'd like to get as close as possible.

ojs' answer is the the more practical one most of the time, but if it's a used square taper crank and you absolutely have to land on the right chainline without a dummy bottom bracket to use, there is a way that doesn't require special tools.

Used square taper cranks have witness marks from the spindle. You can usually tell or get to a good inference where the tip of the spindle was. Hold the crank down flat against a flat surface. With a caliper or small ruler, measure from the surface to this witness mark. Then measure from the surface to the teeth centerline on the middle ring on a triple or the only chainring on a single, or to each chainring centerline on a double and then average them. Subtract the measurement to the spindle mark from the measurement to the front "chainline measurement plane," and you will usually get a negative number. That number plus half the length of whatever spindle you plug in will give you the resultant chainline. From there you can calculate what you need for your target rear chainline.

The biggest reason to not do it this way is many times you also need to worry about chainring and crank clearances in the bottom bracket area. Modeling that without having any kind of dummy BB to work with is time consuming and tricky, and very easy if you do have a dummy BB. But, there are also times when you have a pretty good idea that it's not going to be an issue.

Note that if you're singlespeeding an old double or triple crank and you want the ring mounted on the outside position to look cool and minimize your Q, that is an application where it's easy to have the crank hit the chainstay unless you model things carefully first and/or have a dummy BB to work with. If you're willing to sacrifice the Q and use the inside/middle position, usually you're then safe to not worry about clearances, but that's also not the "good" way of doing it for most people.

Another good reason to do it ojs' way is with a dummy BB, you'll know up front whether the cranks you have want an asymmetric spindle, which many old random cranks do. The counterpoint here is if you want to go really nuts, you can do the same witness mark trick but measured to the pedal mounting surface. (If it does need an asymmetric spindle, usually the way to solve it on a 68mm shell is use a 73mm BB and add spacers under the drive side cup equal to half the offset).

• Thank you for the detailed description... while I happen to have a dummy bb in this case, it's good to know how to do it without one. What does "Q" stand for? Feb 19, 2022 at 21:03
• @Loralrose Q-factor, or the distance of the pedal mounting surface to the frame centerline. Feb 20, 2022 at 6:12

Taking the required measurements from the crank itself would require precision that is not practical at home, so the easiest way is to borrow or buy a temporary bottom bracket and install the crank on it. After installing, measure the chainline, and subtract the target 42mm. The bottom bracket length you need is the length of the temporary bottom bracket, adjusted by 2 times the difference.

For example calculation, suppose that you have a 122mm bottom bracket and the chainline is 45mm. The difference to target is 45mm - 42mm = 3mm. The correct bottom bracket is then 122mm - 2 * 3mm = 116mm.

• I'm glad to have finally found something useful on this subject. I've been wanting to change my BMX with a 68mm "American" pressed cup bottom bracket shell to a 3 piece crank. I have an adapter that presses into where the cups go and will accept a sealed bearing bb cartridge and I have a set of square taper 170mm crank arms and have been trying to find out what spindle length I need on the cartridge. I do have 3 different length 73mm bb cartridges from my mountain bike mishaps so hopefully one of them will get me in the ballpark. Sep 24 at 0:10
• FYI for anyone looking for info on a BMX conversion, there are plenty of kits available and it seems that they will all do the trick, you only need to pick a spindle diameter, 19, 22, or 24mm. You can get just the interface, bearings and spindle or you can get it with crank arms and even pedals depending on your needs They run from \$40 on up to a few hundred. But if you're trying to do it with recycled used parts like me, they sell a \$25 part that installs where the cups go into the shell and accepts a standard sealed cartridge mtb bb, 68x113, and all you then need are arms & pedals. Sep 26 at 1:45