dandyMECHANICS: Cranks

dandyMECHANICS: Cranks

By Brent Robinson

Illustration by Chris Simonen

The first documented type of cycle was a French machine that had two in-line wheels connected by a wooden frame. The rider sat on top of this machine and pushed it along with their feet. This type of vehicle with no pedals was in fact called a dandyhorse. This machine had hubs that allowed the wheels to roll however; walking still had its mechanical advantages. The French played around with crank drives on the front wheel (also known as a penny-farthing) and enlarged the front wheel to increase efficiency, but these were difficult and dangerous to ride.

The British inventor J.K. Starley created the Rover, which looks more like something we would recognize today. One of the biggest innovations of the Rover was the crank, a device that introduced a chain drive to the rear wheel. This allowed the pedals to be mounted much lower and further back than before, allowing the rider to sit within reasonable distance to the ground. It was the invention of this crank that paved the way for modern bicycles. Almost all bikes are equipped with cranks so let’s take a closer look at them.

Cranks have several parts: crank arms which the pedals thread into, chain ring(s) that carry the chain and a bottom bracket which allows the crank assembly to spin within the stationary frame. Crank arms themselves are relatively simple. They are usually made from aluminum, although some older bikes may have steel cranks and very high-end bicycles may have ones made of carbon fiber. Crank arms come in different lengths, to accommodate leg length and fit of a bicycle. Usually, a smaller bicycle will come with shorter crank arms and a bigger bicycle will come with longer ones.

A chain ring carries the chain, allowing the rider's power to be transferred to the rear wheel. Single speed and internal hub bicycles will always have only one chain ring on the crank, most road bikes will have two (double) and most hybrid and mountain bicycles will have three (triple). A larger chain ring dictates a harder or “faster” gear ratio, whereas a smaller chain ring gives an easier gear ratio. For this reason, most road bicycles will have larger chain rings and mountain bikes smaller ones, with hybrid bicycles falling somewhere in the middle. Geared chain rings (doubles and triples) have ramps and pins, which help the derailleur move the chain from one ring to the next when switching gears. Chain rings come in different bolt circle diameters (BCD). This is measured as the diameter of a circle going through the middle of the chain ring bolts. The BCD of the chain ring(s) must be the same as the BCD and as the crank it is going on.

The bottom bracket is the final part of the crank assembly and is usually sold separately from a crank. There are many different types of bottom brackets, but the most common is called a square taper. The spindle of the bottom bracket is in a square shape and tapers away from the frame. The crank is pressed onto the spindle with crank bolts, which thread into the spindle. The spindle has bearings between it and the cups, which thread into the frame. An older type of bottom bracket is called a cotter pin, where the crank is held onto the spindle with a tapered pin. Most new and high-end cranks integrate the spindle into the crank itself, which allows for a much larger but lighter spindle and an outboard (bearings outside the frame) bottom bracket that results in less flex therefore, reducing wear. Many creaks and clicks can come from the bottom bracket area as all of the force a rider puts into the crank goes through the bearings in the bottom bracket. They can be simple to adjust however, because they thread into the frame, misadjusting them can damage it. If you suspect that your bottom bracket is showing signs of wear and needs replacing your best bet is to check into your local bike shop and have them take a look at it.

Cranks and bottom brackets are relatively simple devices, but technological advances and innovations in cycling have resulted in many different types and sizing options. If you're unsure about what sort of crank you have or about your options for replacing them, check into your local bike shop and discuss what choices may be available for you.

Until next time, keep the rubber side down.

Brent Robinson is a veteran bicycle salesman and manager at Sweet Pete’s Bike Shop. He enjoys long bike rides to the beach and a cold one when he gets there. Apart from bikes he is an enthusiast of fast cars, David Attenborough documentaries, and tongue-in-cheek humour. One day Brent hopes to work as a journalist in either the bicycling or automotive field. www.sweetpetes.com

Information on early bicycles and dandyhorses taken from the Canada Science and Technology Museum: http://www.sciencetech.technomuses.ca/english/collection/cycles.cfm

 Related on the dandyBLOG:

dandyMECHANICS: Hubs 

dandyMECHANICS: Tires

dandyMECHANICS: Handlebars 


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