What is BCD? Focus on cranksets

We have a preliminary understanding of the transmission system through previous articles( https://www.pi-feel.com/a-brief-discussion-on-the-transmission-system/ ). In this article, we will focus on its important parts. Crankset and crank. The number of crankset teeth, crank length, different materials, and whether the crank is hollow or not will directly affect riding experiences.

Bike crankset

1. Detailed explanation of crankset specifications and materials 

The main differences between cranksets are the number of chainrings, number of teeth, and material.

Mountain bike cranksets are divided into single-piece, double-piece, and triple-pieces. The common teeth of single-piece are 34T, 36T, 38T, 40T and 42T. The common teeth of double-piece are 42-24T, 42-28T and 38-26T. The common teeth of triple-piece are 42-32-24T, 44-42-22T and 48-36-26T. 

Road bikes usually use double-piece cranksets. The common teeth are 53-42T, 52-39T, 53-39T, 54-42T, 55-42T, 56-44T and 50-34T.

T (Teeth) is a unit which is the number of gears in the chainring. If you often need to climb hills or ride long distances and want to save effort, you can choose a chainring with fewer teeth. 

The main materials of the crankset are steel, titanium alloy, aluminum alloy and composite materials. Steel crankset is heavier but cheap and has good wear resistance. In contrast, the cranksets made of other materials are lighter and more expensive. Nowadays, aluminum alloy cranksets are more popular. It has good comprehensive performance.

Cranks are usually divided into two types. The first type is the ‘three-piece crank’ which has two cranks and a bottom bracket. 
The second type is ‘Two-piece crank’ which the bottom bracket is fixed on one crank.

The main difference between cranks is length and material. Common lengths are 165mm, 167.5mm, 170mm, 172.5mm, 175mm, 177.5mm, 180mm, etc.; and common materials are steel, aluminum alloy, titanium alloy, Carbon fiber, etc. The great crank generally is hollow, which can minimize weight while ensuring strength. 

2. How to choose crank length

Personal physical conditions and riding habits need to be considered when choosing the crank length. Riders with a higher pedalling frequency can choose a shorter crank, and sprint riders can choose a longer crank. For the selection of length, you can refer to the following two methods. 

The first method is to calculate the crank length based on the leg length (the deviation range does not exceed 5mm). Table 1 shows the relationship between crank length and leg length.

Table 1: The relationship between crank length and leg length

Calculate crank length based on leg length

leg length/cm

76.2

78.74

81.28

83.82

86.36

88.9

91.44

93.98

96.52

Minimum crank length/mm

160.2

165.4

170.7

176

181.4

186.7

192

197.4

202.7

Maximum crank length/mm

164.59

170.1

175.6

181.1

186.5

192

197.5

203

208.5

Leg length, thigh bone length, and height

The second method is to calculate the crank length according to the length of the thigh bone (the deviation range does not exceed 5 mm). Table 2 illustrates the relationship between crank length and thigh bone length.

Table 2: the relationship between crank length and thigh bone length.

Calculate crank length based on thigh bone length

Thigh bone length/cm

39.48

42.41

43.38

45.34

48.27

49.25

50.22

51.20

Minimum crank length/mm

150.01

161.15

164.86

172.29

183.42

187.14

190.85

194.5

Recommended crank length/mm

155.93

167.51

171.37

179.09

190.66

194.52

198.38

202.24

Maximum crank length/mm

161.86

173.87

177.88

185.89

197.90

201.91

205.91

209.92

Crank length

3. Routine maintenance of crankset and crank

The crank needs to be cleaned in time, especially near the bottom bracket; Periodically check whether the mounting screws of the crank are loosened. After riding, the impurities on the crankset should be wiped in time to extend the service life of the crankset and chain. 

4. Common crankset problems

(1) What is BCD?

The holes on the spider arms used for attaching a chainring can have a variety of spacings, referred to as the bolt circle diameter, commonly abbreviated as BCD. 

The replacement crankset must have the same BCD as the crankset and have the same bolt locations. The BCD of a given crankset generally sets the number of compatible teeth for the crankset. Generally, a 130 mm BCD crank is at least compatible with a crankset of 38T, and a 110 mm BCD crank is at least compatible with a crankset of 33T. However, the Shimano Road crankset has changed this situation. All crankset sizes use 110mm BCD, giving users the option to increase or decrease the crankset size without changing the crankset. 

(2) The impact of the crankset on riding

The effect of the gear ratio is that the more teeth (T), the faster the speed. For example, the standards for racing bikes are 54T and 53T. Climbing and endurance road bikes are 50T. Gravel road bikes focus on the ability to pass through bumpy terrain, so the teeth number is smaller, with 42T or 46T.

The hollow crank and the carbon crank are lighter and have higher rigidity. They can generate force more directly and improve the pedalling efficiency. 

(3). What is a hollow crank? What are the advantages?

The hollow in the hollow crank refers not to the bottom bracket but to the crank legs.

Hollow crank

The hollow crank is lighter. The main advantage is that the inner and outer sides of the crank have a similar structure, which means it is less likely to deform when pedalling. It has better rigidity and force transmission, which not only reduces the rotational weight but also helps accelerate. 

Thanks for watching. 

Don’t hesitate to leave a message if you have anything to share. We are looking forward to your idea and story.

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References:

[1] ‘Cycling Bible’ Editorial Board. Cycling Bible [M]. 1st Edition. Beijing: Machinery Industry Press, 2013.

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