Wednesday, March 31, 2010

One piece bottom bracket - a free DIY tutorial from

Unless you are building a motorized vehicle, you will probably need to salvage the bottom bracket and cranks from a bicycle frame to give your creation a human power transmission. A bottom bracket is the small tube that supports the bearings and crank hardware, as well as creates a junction for the seat stays, chain stays, seat tube, and down tube on a standard bicycle frame.

The bottom bracket is kind of like the kingpin in a bicycle frame, holding the main tubing together in addition to the transmission system. A bottom bracket and the included bearing hardware are easy to maintain, repair and modify once you know the basics.

Single piece bottom bracket and crankset
Figure 1 - Single piece bottom bracket and crankset

A single piece bottom bracket and crankset are easy to identify since they will have a single forged steel crank arm that forms an S-shape as shown in Figure 1. Often, a single piece crankset will also only have one chain ring because it’s used on BMX type cycles due to its durability. Multi-speed single piece cranksets are also available, but usually found on cheaply made mountain bikes. As a general rule, I use only single speed cranksets for fun bikes like tall bikes and choppers and only use the higher quality 3 piece aluminum cranksets for speed bikes, lowracers and any ride designed for speed and low weight.

The left side has reversed threads
Figure 2 - The left side has reversed threads

A single piece crank arm is like a large S shaped bolt with threads in the center that allow bearing races to be threaded in place to hold all of the internals together. Like many threaded parts on a bicycle, there are right hand threads and left hand threads due to the way bearing forces act upon the parts. On a single piece bottom bracket, things are simple; everything on the left side has reversed (left hand) threads and everything on the right side (chain ring side) has right hand (standard) threads.

To begin disassembling a single piece crankset, remove the locking nut shown in Figure 2 with an adjustable wrench or pipe wrench. Being on the left side of the bottom bracket, this will be a reversed thread so it must be loosened by turning the nut in the clockwise rotation as indicated by the arrow in Figure 2.

A tabbed lock washer
Figure 3 - A tabbed lock washer

Under the locking nut will be a tabbed washer as shown in Figure 3. This washer allows you to tighten the lock nut without it forcing the bearing race to tighten. Since the tab prevents the washer from turning, the rotation of the locking nut will not affect the rotation of the bearing race. Sometimes the locking washer might become jammed if the top nut is cranked too tight, so you may need to pry the tab a bit to push it back into the slot if it was jammed into the threaded area on the crank arm.

Removing the bearing race
Figure 4 - Removing the bearing race

More of this free and other DIY plans,