I have a DiamondBack Apex Mountain Bike with a 3×7 drivetrain that I picked up off facebook marketplace for 20 bucks. It's a 29er and I'm having a lot of fun with but the last 2 times I've ridden it the cranks have fallen off on the non drive side. I'm looking to upgrade the bottom bracket and crankset because I believe it to be an issue with the spindle. I have a digital caliper and after taking the measurements of the spindle, and bottom bracket housing im a little stuck on what new bottom bracket to purchase because I'm in between sizes. Looking for any advice or recommendations.
by Signal-Run6295
5 Comments
I’m not an expert but only responding because I recently did something similar. However, after I took the cranks off, I just measured from one end of the square taper to the other, you seem to have a threaded metal portion sticking out. Is that part of the spindle or just got stuck in the square taper? As you can see in the pics for the new ones, the screw will go inside the square taper so the length will be shorter than what you are measuring. In my case, I ordered an almost 1:1 replacement after measuring and found that it was still too long so it was pushing my crankset so far out that I couldn’t adjust into the big chainring even with the limit screws at max. I ended up having to get a size smaller. With Amazon free returns, maybe you can order two sizes and return one that doesn’t work. The closest per your measurements is the one you showed in the pic so just go with that if you buy one to try.
The smaller measure is “bottom bracket shell width”. You should measure it without the black cover from the bottom bracket, only the width of the frame. I guess this will be 68 or 73mm (mtb standard).
The wider measure, you should measure without the thread, only from 4square to 4square. With these measures you should find it.
Check the chainline after changing.
The length of the spindle you need is going to be dependent on the new crankset. From the looks of things you are most likely going to need a 73×123 bb if you’re sticking with 3 gears on the front. 80mm bottom brackets are usually found on e-bikes or fat bikes so very unlikely to be what you need.
You’re measuring everything incorrectly.
Measure the white BB shell. Don’t include the black BB cups already installed.
The axle width doesn’t include the threaded parts. Most, but not all bottom brackets come with bolts, rather than nuts to affix the crank arms.
Your bottom bracket shell is 68 or 73mm wide. The spindle is probably 118 or 122.5 mm long.
You are measuring incorrectly, old bottom bracket have different design. You should measure square spindle without threaded round part on both sides.