Cranks And Pedals - Page 1 of 3
Figure 1 - The two standard crank types
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.
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 re-drill 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 ring 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.
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