ONE PIECE BOTTOM BRACKET BASICS

Figure 1 - Single 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 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.

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 three piece aluminum cranksets for speed bikes, lowracers and any ride designed for speed and low weight.

Figure 2 - Cantilever brakes on a front fork

A much better (and more modern) variant of the pad brake is the cantilever or “linear pull” brake shown on the front fork in Figure 2. Because of the way the cable force is applied to the arms, the leverage is so much better than the side pull brake, making these brakes highly effective. This style of bicycle brake is just as good as a disc brake, and is very easy to install and setup.

The cantilever brake does require a pair of cantilever “studs” be mounted to the frame in the proper position, but this is not difficult to do, as the studs can be cut from one frame and welded to another frame. These brakes suffer a bit of wet rim weakness as well, but will still have enough power to lock up a wheel even in the rain.

Figure 3 - A tabbed lock washer

Under the locking nut will be a tabbed lock 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.

Figure 4 - Removing the bearing race

The bearing race is basically a nut that has a slot on the top and hardened cone shaped surface on the bottom so it can become a bearing surface. To remove the bearing race, just tap it in the clockwise rotation as shown in Figure 4. This part will not be very tight, so unless the threads are rusted, it should be easy to remove it by hand or by light tapping in the slotted section with a flat screwdriver.

Figure 5 - Removing the bearing

Once you remove the threaded bearing race, the actual ball bearing will be exposed, as well as a huge blob of grease, which has been removed to make Figure 5 look nicer. Take note of the orientation of the bearing, which will have the balls facing the bottom bracket cup and the small retaining ring body facing upwards. The bearing will not work properly if reversed as the retaining ring will rub against the bearing cup, causing massive friction.

Figure 6 - Move the bearing to remove the crank arm

To remove the S-shaped crank arm from the bottom bracket, you must first slide the right side bearing out of the way so that the arm can be pulled though both cups as shown in Figure 6. Once the bearing is pulled away from the right side cup a bit, the crank arm can be completely removed from the bottom bracket. If your chain ring is large, you may need to find the best path around the fame tubing when you remove the crank, but one way or the other, it will come out.

Figure 7 - Removing the right side race

If you need to remove the chain ring, then there will be another threaded bearing race and a washer to remove. As shown in Figure 7, this will be a standard right hand thread, so to remove the race, turn it in the counter clockwise rotation. Single piece crankset chain rings will almost always be interchangeable due to the standard size of the crank arm and drive pin, but be aware that single speed cycles often use a wider chain than multi-speed cycles.

Figure 8 - A complete disassembly

All of the parts that make up a single piece crankset are shown in Figure 8 after a few minutes of cleaning the grease. If you see any rust on the ball bearings or crack in the cups or races, then replace them. Almost all single piece crank set parts are interchangeable, so they are very easy to repair. Crank arms are also available in various lengths, so you can swap them as well. If you plan to heavily modify the crank arm, then be aware that the part is forged, so it is slightly hardened. Although you can cut and weld a crank arm, it should not be trusted because as a load bearing part it may become brittle.

Figure 9 - Proper bearing orientation

Figure 9 shows the proper way to insert a single piece crankset bearing into the cup. If you place the bearing in the other way around, then the steel retaining ring will rub on the cup and the cranks will not spin properly due to excessive friction.

Figure 10 - Testing the bearing friction

To check the bearing, and see that it is installed properly, place the race over the bearing, then push as hard as you can while spinning the part. No matter how much pressure you put down on the bearing race, there should be practically no friction at all.

Figure 11 - Removing the bearing cups

The bottom bracket cups can also be removed as shown in Figure 11 by tapping them out using a long bolt or rod. Tap them out by placing a rod into the bottom bracket shell so that you can hammer them out from the rear side. It does not take much force to push out the cups, and it can usually be done just by ramming a solid rod or tube into the bottom bracket by hand. The cups are reinstalled by placing them flat over the bottom bracket shell to drive them back in. Use a piece of scrap tube or angle iron over the cup so you can drive them in parallel to the bottom bracket shell using a hammer.

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