THREE PIECE BOTTOM BRACKET BASICS

Figure 1 - A three piece bottom bracket and crankset

Unless you are building a motorized vehicle, you will probably need to salvage the bottom bracket and cranks from a bicycle frame in order to give your creation a human power transmission. A bottom bracket is the small tube that supports the bearings and crank hardware, creating 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, as well as the transmission system. A bottom bracket and the included bearing hardware are easy to maintain, repair and modify once you know the basics.

A 3 piece bottom bracket gets its name from the fact that there are two crank arms and an axle that make up the crankset. A 3 piece crankset is easy to identify because each crank arm will have a center nut or bolt that fastens it to the axle, whereas a single piece crankset has a solid S-shaped crank arm. Figure 1 shows a typical 3 piece crankset and bottom bracket with a triple chain ring. Higher quality crank arms are made of aluminum, whereas inexpensive cranks are made of steel.

Figure 2 - Removing the plastic dust cap

To remove the crank arms from the bottom bracket axle, the two bolts (or nuts) must be removed. There might be a plastic or rubber dust cap over the crank center as shown in Figure 2. This can be removed by unscrewing it using a penny or screwdriver, or by popping it out with a blade (if it is just a rubber cap). On bikes that have had a lot of sun exposure, the plastic cap may break due to being brittle, so take care.

Figure 3 - Removing the crank axle nuts

The axle nuts are both standard right hand threads, so they are loosened using a counter-clockwise rotation as shown in Figure 3. Most of the department store variety cycles will have bolts holding the crank arms to the square tapered axle, so they can be removed using the appropriate sized socket wrench. Some bolt heads may require a hex key as well. The crank axle may have a bolt or bolt hole, but both types work essentially the same way, working the crank arm onto the tapered axle where it is held securely by friction.

Figure 4 - A crank puller is a useful tool The easiest way to remove the crank arms from the square tapered axle is by using a crank puller, like the one shown in Figure 4. This device can be purchased at most cycle shops and will make your job much easier, and ensure you do not damage the chain ring. The crank puller screws into the threaded inside of the crank arm (where the dust cap was) and then forces a pin down onto the crank axle, removing the crank from the axle. The other method of crank arm removal involves hammering a long bolt or rod against the backside of the crank arm until it is forced off the axle. The hammering method has to be done carefully, as you can easily bend a chain ring if you smash it with a hammer. I must admit that I do not own a crank puller, and use the hammer method as well as the blowtorch to heat up stuck crank arms that have rusted in place over many years of service.

Figure 5 - Both sides of a bottom bracket

Once the two crank arms have been removed, you will be left with the bottom bracket shell and all of the bearing hardware that allows the square tapered axle to spin freely. The threaded bearing cups on each side of the bottom bracket have alternating threads, so take note of which parts are on the right (chain ring side) and which parts belong on the left side.

Figure 5 shows what can be found on a typical 3 piece bottom bracket once the crank arms have been removed. On the left side of the bottom bracket (left image), the bottom bracket hardware consists of a threaded bearing cup and a locking nut which will usually have several notches around the edge. On the right side of the bottom bracket (right image), the hardware consists of only a single threaded bearing cup.

The right side cup (chain ring side) has reversed threads, so it must be unscrewed in the clock wise rotation. I actually recommend that you never remove this cup since it makes it easier to identify the right and left side of the bottom bracket after it has been ground clean. Also, having very fine threads makes it easy to accidentally cross thread the wrong cup into the bottom bracket, so by only having the standard threaded cups removed (left side cups), you never run the risk of cross threading a bottom bracket or welding on in backwards on your homebuilt creations. Having one cup installed when you weld to the bottom bracket shell also helps to minimize distortion from the welding heat.

Figure 6 - Removing the lock nut

To remove the left side bottom bracket hardware, start by unscrewing the lock nut by turning it in the counter-clockwise rotation as shown in Figure 6. This thin nut can be removed using a pipe wrench or by simply tapping into one of the slots using a chisel and hammer. The lock nut locks against the bottom bracket shell, keeping the bearing cup from unthreading.

Figure 7 - Lock nut and axle nuts removed

The locking nut is shown removed in Figure 7 along with the two axle nuts. Sometimes the axle will be threaded and bolts will be used to hold the crank arms onto the tapered axle.

Figure 8 - Unscrewing the bearing cup

The left side bearing cup is also removed using a counter-clockwise rotation since it is a standard thread. There will either be a square top for a wrench as shown in Figure 8, or several holes that can be tapped by placing a punch into them. If a bottom bracket is badly rusted inside, it may take a lot of force to unscrew the cups, so you may have to decide to waste the cup and use a pipe wrench on it to get it out.

Using a pipe wrench will likely damage the threads, but at least you will have a bottom bracket that can be salvaged with new bearing hardware. A little heat from a blowtorch around the bottom bracket shell can do wonders to free a stuck bearing cup as well.

Figure 9 - Take note of the bearing orientation

Once the bearing cup has been fully unscrewed from the bottom bracket shell, the bearings and axle will be freed. Take note of the orientation of the bearings, as they have to be put back into the cups in the same way they came up (with the top of the retainer ring facing inward). Some manufacturers want to save two pennies and make your quality cycle weight .2 ounces less, so they will “feature” loose ball bearings that will fall all over your workshop floor! Watch out for this.

Figure 10 - The axle usually has a long side

Another thing to take note of is that the bottom bracket axle is usually longer on the right side so the triple chain ring has more clearance between the inner ring and the frame tubing. Not all axles have a longer side, and often the longer side is only a fraction of an inch longer, so look carefully. Figure 10 shows the bearings, cup, and a typical axle with one side slightly longer than the other. Side (A) is about 1/4 of an inch longer than side (B).

Figure 11 - Both bearing cups and bearings

Balls into cups – that is a general rule for ball bearings that have a retaining ring such as the ones shown in Figure 11. If you put them in backwards, there will be massive friction.

Figure 12 - The right side bearing cup

It is best to just leave the right side bearing cup installed as shown in Figure 12. Since it locks flat against the bottom bracket shell, you can weld and grind without risk of damaging the part, and it will help keep the bottom bracket shell from distorting when welding something to it.

Figure 13 - The right cup has reversed threads

If you do have to remove the right side bearing cup, remember that it has a reversed thread, so it has to unscrew in the clockwise rotation as shown in Figure 13. Often, these cups will be wedged in tightly due to years of service, rust and dirt, so it may take a great deal of force to get them out. Some right side cups have a ridge for a wrench, but many only have a few small notches that can easily by chipped using a pipe wrench, so take care when trying to get one free.

Figure 14 - Bottom bracket and bearing hardware

Figure 14 shows all nine parts of a typical bottom bracket assembly. There is a threaded bottom bracket shell, a tapered square axle, two bearing cups and bearings, two axle nuts and a lock nut. All parts should be cleaned and inspected for excessive wear, cracks or rust. Toss any cracked bearing cups or rusted bearings.

Figure 15 - A cartridge bottom bracket tool

A cartridge style bottom bracket assembly will include the axle and bearings as a sealed single unit, and although it will fit into a standard threaded bottom bracket shell, it will require a special splined tool in order to remove or install. Figure 15 shows a cartridge bottom bracket assembly and the small splined adapter tool that will fit into the part and allow you to remove it with a standard wrench.

Figure 16 - Removing the cartridge bottom bracket

The splined tool fits into the cartridge as shown in Figure 16 so you can use a wrench to turn the threaded assembly. The same rule applies to thread direction - right hand threads on the left side and reversed threads on the right side (chain ring side).

Figure 17 - Cartridge, end cap, and axle bolts

A cartridge style bottom bracket is shown in Figure 17 after removing it with the adapter tool. The main body will include a sealed tube that contains the axle and bearings and the other side will have a plastic retainer that wedges against the body and keeps out dirt. The square tapered axle will also have either nuts or bolts to secure the crank arms.

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