How to Adjust the Euro Bottom Bracket on a BMX Bike
A Euro bottom bracket for a BMX bike consists of two threaded bearing cups, a spacing sleeve that goes between the bearings, two bearing dust caps, a spindle, spacing washers that ride on the spindle, and two crank arms that attach to the outside of the whole assembly. Adjustment of the system requires that it be disassembled and reassembled. All modern setups use cartridge bearings built into the cups, but some older types have loose ball bearings on races. If your bike has this older bottom bracket, it's best to switch to cartridge bearings while everything is apart.
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Things You'll Need
- Pin spanner wrench
- Crank arm puller tool or mallet
- Socket wrench
- Grease
- Rags
Instructions
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1
Remove the crank arm bolts with the socket wrench. Pull the crank arms from the spindle with the crank arm puller. Some systems will use a proprietary puller and some will even require a mallet, so check the specifics for your crank.
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2
Slide the spacers and the bearing caps off of either end of the spindle. Make note of how many spacers were in place on each side of the spindle.
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3
Use the pin spanner wrench to remove the bearing cups from the frame. Remember that the cup on the left side of the bike will thread out counterclockwise and the cup on the right side, or drive side, of the frame will thread out clockwise. The spindle and the spacing sleeve will both come out with one of the cups.
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4
With everything apart, inspect the pieces for damage or wear. Don't reuse anything that looks suspect. Get everything nice and clean with a rag so you can get a clear view. If you have a handful of loose ball bearings, consult your bike shop for a switch to cartridge bearings.
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5
Clean and grease the threads of the frame. Thread one cup into place by hand, making sure that it goes into place smoothly.
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6
Place the spindle inside the correct size spacing sleeve and feed the pieces into the bottom bracket shell of the frame. Thread the other cup into place by hand to capture the spindle and sleeve.
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7
Use the pin spanner to tighten both cups until they are firmly in the frame. The cups are designed to fit flush with the frame and the spacing sleeve without binding. Use the right sized sleeve for your frame's bottom bracket shell.
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8
Slide the bearing caps and then the spacers over either end of the spindle. The spacers will determine the chain alignment, and there should be the same number of spacers on both sides. Check the chain alignment by sliding the drive side crank arm onto the spindle and putting the chain onto the gears. The goal is to create a straight chain without any lateral twisting.
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9
Grease the splines of the two spindle ends. Attach the crank arms and tighten the crank bolts with your socket wrench. The spindle ends should fill at least half of the space in each crank arm and both the arms should be flush with the spacers.
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10
Check everything and make changes as needed. If the cranks do not spin freely, the sleeve between the bearings is too narrow. If the arms wobble when you pull on them, more spacers are needed between the bearing caps and the crank arms. When everything spins freely and there is no play in the cranks, you have adjusted the bottom bracket properly.
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Tips & Warnings
Double-check your work before you ride the bike, and if you have any doubt let a bike shop check your repair.
References
- Photo Credit bmx image by Claudio Calcagno from Fotolia.com
