bfloyd

I just aquired a new road bike from a friend which has a triple chain ring front. I am wanting to change it to a double that I have laying around and need to know if I should change to a shorter BB spindle length. I currently have a 118 with the triple. What length would be suitable for the double (older Shimano 105)? Thanks.

acape

I don't think you can use that bottom bracket with a double. I just purchased a 105 double crank and bottom bracket, and the spindle length for the double is 109.5 mm.

operator

You most likely will have to change the bb to something like a 107. The specific lenght depends on which crankset you buy.

DMF 

Is it a square taper BB? Have you measured the front chain line? Nominal on a triple is 45mm. Nominal on a double is about 42mm. But if it's a square taper, it's hard to predict how a different crank set will fit.

capwater

If the cranks are square taper, pick up a Shimano UN-53 107mm spindle length which is proper for a double crankset.

bfloyd

Yes, it is a square taper. I'll look into the UN-53 107, thanks.

jnkbl

hi, i'm still trying to find an answer to this question in the forum but can't find it, maybe someone can help me here...

i have an old bottom bracket with axis/spindle length: 111mm and wanted to install an old crankset, but its recommended bb length is 113.5mm...

would it be possible to mount this crankset in that bb? what difference will those 2,5mm make? will the cranks occasionally loosen, damage... explode?

thanks a lot in advance!

andrewclaus

The issues are whether the the front derailleur will shift correctly, clearance to the chain stay, and chain alignment for smooth, even wear.

jnkbl

thanks a lot!

1) front derailleur will not shift at all, cause it will be a singlespeed bike
2) chainstay clearance checked, and -so far i can say now- ok, and
3) the chain alignment was a problem from the very beginning because, again, singlespeed etc.

ok, if thatīs all, then i think it will work!

thanks again for the info!

Russ Roth

The idea is to pick a bb that lets the chain run in line with the cog from the crank. Going to SS you might want to actually go shorter. Install the crank though don't wrench it down really hard just most of the way and see how far the chainline is off of center. Easiest is to measure the distance of the center of the cog from the outer nut of the hub. I usually do this but resting the outer nut on a workbench and then measuring to the cog. So assuming a 130mm hub, half is 65mm, if table to cog is 20mm then the cog is 40mm out from center. You then want to measure from the center of the seat tube to the center of the chainring. A mm or 2 out from center isn't too bad, I prefer to stay within a mm to cut down wear and noise.

jnkbl

thanks! will keep that in mind, let's see if it works with the bb and cranks i have!

phughes

Here are some articles that explain chainline. https://www.velodrome.shop/chainline https://www.parktool.com/blog/repair...nline-concepts https://sheldonbrown.com/chainline.html

Remember that many BB spindles are not symmetrical, they are longer on one side than the other.

jnkbl

thanks

Moe Zhoost

To determine what spindle length you need, mount the new crank on your current spindle then measure the chain line. Add or subtract 2x the difference between this measurement and your desired chain line to the length of your current spindle and you have it. This assumes that the replacement BB has the same taper end width as your old one. You can sometimes find your target chain line in the specification for your rear hub - for single speed it is typically 41-42 mm.

jnkbl

thanks!