BOTTOM BRACKET BEARINGS TUTORIAL
This basic tutorial will demonstrate the workings of a typical 3 piece bottom bracket, showing the removal and installation of the various components that make up the bearing system. Although there are slight size differences between different manufacturers, the basic principles of operation and assembly are virtually the same. In this tutorial, the bottom bracket shell has been cut from the frame for later use in a custom bicycle project.
The crank axle is a hardened steel rod that includes a pair of machined bearing races, tapered square ends, and either a bolt or internally threaded section at each end to affix the crank arms. There is some debate between "axle" and "spindle" as being the correct name, but most seem to use axle now, so I will as well. Why make things more complicated, right? Anyhow, you will need to remove the bolts or nuts from the end of your axle before disassembling it, so turn them in the counter clockwise rotation to remove them from the ends of the axle.
On this crank axle, bolts were included at the ends, so nuts are used. The bolts or nuts can be removed using the appropriate sized socket wrench or in some rare cases, a hex key. The axle bolts drive the crank arm firmly along the tapered section until it is pressed extremely tight to the axle. Crank arms and removal will be discussed in another tutorial.
The components on the right and left side of a bottom bracket differ in size as well as thread direction, so take note of all parts that are removed, especially on a bottom bracket that has been removed from a frame. It is a good idea to mark the right and left side so you don't accidentally cross-thread the bearing cups or end up installing the bottom bracket in reverse in a custom bike project. To begin disassembling the bearing components, start on the left side of the bottom bracket, removing the large locking nut (aka lock ring). To loosen the lock nut, tap it in the counter clockwise direction with a chisel or use a pipe wrench to loosen it half a turn. The lock nut can be easily removed by hand once it is released.
The purpose of the left side locking nut is to secure the bearing cup once it has been tightened to the desirable tension. Without the locking ring, the bearing cup would eventually loosen, creating slop in the bearings as the gap between the cup and the axle race became larger. Plat in the axle will cause the bearings and cups to wear prematurely, and could cause misalignments in your chain due to the chain ring moving slightly from side-to-side.
The left side threaded bearing cup (aka "threaded bearing race") is the only one you should remove. Since the right side threaded bearing cup has reversed threads, it is easy to accidentally cross thread either cup into the wrong side of the bottom bracket shell if the bottom bracket has been removed from a frame and not marked. Bearing cups are rarely bad, so unless you can see obvious damage, only remove the left side cup and leave the right side cup installed with the bottom bracket. To remove the left side bearing cup, find an adjustable wrench that will fit onto the flat section.
The left side bearing cup has a standard thread, so it is loosened by turning it in the counter clockwise rotation. If your parts are new and clean, you can probably turn the cup around a few times with the wrench, and then remove it the rest of the way by hand. This will not be the case on an older bicycle that has lived outdoors through rain or snow.
Once the bearing cup has been removed, the bearings will be exposed. Be careful when you first pull the cup from the bottom bracket as some bearings are just loosely packed in grease and they may all fall out if the grease has dried up. Typically, the bottom bracket bearings have a retaining ring, but this is not always the case. Also, take note of the orientation of the bearings, which in most cases is balls into cups.
The bearing system is completed by the race on the crank axle, the ball bearing set, and the race on the inside of the bearing cup. Both sides of the bottom bracket work the same way, but there are differences in the hardware such as thread direction and the length of the axle at each end.
When you are salvaging bicycle components that include ball bearings, it is advisable to completely clean and de-grease the bearings and wear parts so that you can determine if any of them need to be replaced. It's often difficult to identify a bad bearing when it is packed in grease. Many times it may only be the grease that is rusty looking, not the bearing.
A rag dipped in Varsol or paint thinner will easily degrease and clean a bicycle ball bearing. Be careful not to put too much pressure on the retaining ring or a ball may fall out of the slot. If a ball has come loose, just press it back into the retaining ring slot.
A bearing that is obviously rusted or balls that have lost their shine need to be replaced. If you run a rusty bearing, it will eventually destroy the bearing race, wearing a groove into the hard surface. If only one or two balls are rusted, you could replace them, but chances are the damage will be even throughout the entire bearing. Also, check bearing cups for rust or cracks that can develop over time from heavy use. Cracked bearing cups need to be replaced, but some amount of rust may be removed using steel wool inside the bearing cups. Each year of service, you should check your bicycle's bearings for wear and give them a fresh coat of grease.
With the left side bearing cup removed, all of the parts can be removed from the bottom bracket shell except for the right side cup, which is normally left installed. The crank axle, two ball bearings, and the left side cup are now freed from the bottom bracket shell.
Because the crank arm on the right side of the bicycle has to carry one or more chain rings, it needs to have a little more space between the frame, so for this reason the right side of the crank axle is slightly longer than the left. If you inspect the crank axle closely, one side will have more distance between the end and the built in bearing race. This is always the right side.
There are several reasons for not removing the right side threaded bearing cup: avoiding an accidental cross thread, helping keep the bottom bracket round during welding, and because this cup can be extremely difficult to remove when on an older bicycle. If you do have to remove the right side bearing cup for some reason, then remember that it has a reversed thread, so you will turn it clockwise in order to remove it. A large adjustable wrench or pipe wrench is used to grips the small flat side on the bearing cup for removal.
The right side bearing cup looks the same on the inside as the left side, having a smooth bearing race surface where the balls are to ride. Because clearance is an issue on the right crank arm, there is no locking nut and the flat area is quite small, making it difficult to get a wrench to hold when the part is stiff. If you don't have to remove this cup for replacement, then let it stay where it is on the bottom bracket shell.
When completely disassembled, a 3 piece bottom bracket bearing set includes: the bottom bracket shell, the crank axle, two threaded bearing cups, two ball bearings, two axle nuts, and a right side locking nut. Most of these parts are interchangeable on a bicycle, but in some cases, the crank axle length may differ depending on the number of chain rings on the crank.
Remember that the ride side bearing race has a reversed thread, so it will be reinstalled by turning it in the counter clockwise rotation. When reinstalling either threaded bearing cup, be careful not to cross thread the part, as it can be tricky to get the fine threads started. If you can't seem to get the part to begin threading, ensure that you do not have the right side cup on the left side of the bottom bracket or vice versa.
You should be able to turn the crank axle easily, but it will have some resistance. If you push hard on the axle and the friction seems to increase, then there is a possibility that the bearings are installed with the retaining ring into the cups, rather than the balls into the cups. A reversed bearing will be stiff and could damage the race if you tried to use it that way.
This completes the reassembly of the bottom bracket hardware. Every one of our Downloadable DIY bike or trike plans will require one or more bottom bracket assemblies, so never toss an old cycle frame that includes a salvageable bottom bracket. If you clean and lubricate your bicycle bearing components once per year, they give you many trouble free years of service.
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