Bicycle cranks and pedals are pretty straight forward once you know which threads are reversed and the basic differences between one piece cranks and three piece cranks. Tools needed: a 15mm box wrench for the pedals, a pipe wrench if you plan on removing the crank set or axle from the bottom bracket, and optionally, a crank puller to easily remove crank arms from a three piece bottom bracket axle.
Figure 1 - The two standard crank types
There are two types of crank sets and bottom brackets as shown in Figure 1. On the left is the one piece crank set, which is often found on single speed BMX bikes and lower quality cycles. Single piece crank sets are heavy because they are made of steel, but this also makes them practically indestructible, which is why they are chosen for use on BMX cycles. On the right of Figure 1 is a three piece bottom bracket and crank set. This type of crank set uses steel or aluminum crank arms that are bolted to an axle. They are the most common type found on road bikes and mountain bikes.
If you are planning to build your own cycle, always choose a three piece crank set for any speed bike or vehicle that is intended for serious riding, and use single piece crank sets for experimental bikes and fun rides like choppers and cruisers.
Figure 2 - Single piece and three piece cranks
Both types of crank sets are available with varying lengths of crank arms, which typically vary in length from 165mm to 175mm. The length of a crank arm is measured from its center of rotation to the center of the pedal axle. Crank length preferences vary, and many schools of thought exist to what the best crank length might be for a certain leg length or even bicycle type. Shorter cranks are becoming a trend on recumbent bicycles, even to the point where builders are cutting the crank arm shorter than the standard 165mm to redrill and thread a new pedal hole. The best way to determine which crank length suits your body and riding style is experimentation, but I have not noticed any difference with varying crank lengths myself.
Oddly enough, pedals are not interchangeable between single piece crank sets and three piece cranks sets. The threads on a single piece crank pedal are 1/2" by 20 threads per inch, whereas threads on a three piece crank set are 9/16" at 20 threads per inch. It may seem odd that the pedal axle on a BMX crank set would have a smaller diameter than a road bike, but since the BMX crank arm is made of steel, it is more robust and will bend far before a pedal will strip from the threaded hole. I have seen many stripped pedal threads on an aluminum crank arm, though.
Figure 3 - The right pedal has right hand threads
One thing all bicycle pedals have in common is that on the right side crank arm (chain rind side), pedal threads are standard right hand threads, and on the left side crank arm they are reversed left hand threads. Also, most pedals can be removed using a 15mm wrench, although some may differ or even require a special thin profile pedal wrench. Figure 3 shows how to remove the right side pedal; using a counter clockwise rotation to unscrew the threads.
Figure 4 - The left pedal has reversed threads
Figure 4 shows the clockwise rotation used to remove the left side pedal, which has reversed threads. Often, recycled bicycle parts have been well used and may be dirty or rusted, so don't be afraid to use a few dirty tricks to free those stuck pedals. I have a few 15mm wrenches that I use specifically for bicycle pedals, as there are times where a hammer is necessary to "help" things along. Another trick you can use to gain leverage over a stuck pedal is to place a 2 foot long pipe over the wrench for mechanical advantage, but be aware that you may snap the end from your wrench. A blow torch can also be used to heat up the area around the pedal axle in order to expand the crank arm hole a bit, but be careful not to melt the pedal bodies, which are often made of plastic.
Figure 5 - Right pedal on a three piece crank arm
By now, many astute garage hackers may be thinking, "Wait! The threads should be the other way around on the pedals!" Really, the pedals rotate in the same direction that would actually unscrew their threads, right? It does seem strange at first, but due to the fact that the bearings actually transfer force at 180 degrees to the other side of the race, the opposite is true. When a pedal is rotating in the clockwise direction, forces are transferred around the bearings back to the pedal axle with counter-clockwise force. Confusing, isn't it? But 120 years of engineering can't be wrong!
Figure 6 - Most pedals have their axles stamped
Rather than attempting to cross-thread the wrong pedal in a crank arm, you can usually just look at the threaded end of the axle and see a stamp indication either "L" or "R". The pedals for three piece cranks will not fit into a single piece crank set and vice versa, so that is not really much of a concern. The only thing you need to be mindful of is the first few turns of a pedal into an aluminum crank arm. If you can't get the first few turns in by hand, then either you have the pedal on the wrong side, or it is not going in straight. Aluminum is lightweight, but it is very easy to damage, and a cross-threaded aluminum crank arm is not repairable, even with a pedal threading device, since there will not be enough material to cut new threads. Start a pedal by hand and then switch to your wrench after a few rotations.
Figure 7 - Two sizes of pedal threads shown
The larger 9/16 inch threads for three piece cranks are shown at the top of Figure 7, and the smaller 1/2 inch threads used on single piece crank sets are shown in the lower part of Figure 7. Pedals are also available in a wide variety of styles and types, some of which hold the pilot's shoes in place using some type of clip or strap system. Clipped pedals require a special cycling shoe and allow the rider to push and pull on the pedal, allowing the legs to deliver force to the drive system at all times during a pedaling rotation.
Clipped pedals are generally used by more serious long distance cyclists as they require special shoes, as well as the confidence to ride with their feet somewhat bonded to the pedals. Although the rider's feet can be removed from both clipped and strapped pedals without much difficulty, it could still become a problem in an emergency situation. The typical and most common type of pedals are platform pedals (shown on this page), and have no constraints since the rider simply pushes down on them.
Platform pedals only allow force in the downward direction, but for most cycling, this is more than enough efficiency. In defense of clipped pedals, a foot slipping from a platform pedal on a very low recumbent can cause the rider's foot to catch the ground and be pulled under the cycle, so consider your choice of pedals based on our riding style and bicycle type.
More of this and other free DIY tutorials at AtomicZombie.com.