The theme of the correct length of the cranks is surrounded by various myths. There are cranks with a length of 130 to 220mm. We are done a small research of different opinions for this theme. Personal experience in the article is missing, only the analysis of the opinions of "experts" and manufacturers. I will try to be brief.
The myth says than longer the crank, the greater the lever which you push and the “more powerful” you push. This is not entirely true. It is necessary to consider the total lever of the system, and since we have a chain transmission, the change in the length of the crank can be compensated by changing the speed. This will change the cadence. For example, an increase the length of the crank from 170mm to 175mm is equivalent to a decrease in the leading chainring from 34z to 33z.
Primarily the length of the crank affects the angles of flexion of the joints. And here opinions divided. Everyone says that the length of the crank is very important that it should be chosen as the size of the frame and the height of the saddle. But changing crank is expensive.
Points of view
Below is a graph of crank length versus height for the following variants:
1) Many refer on information 9.5% of height (there is also an variant of 9.7%)
2) 1.25 * inner seam (in cm) + 65 - sounds more logical because with one height the length of the legs may be different. The graph is called Machine head (because came up with seemingly www.machinehead-software.co.uk), for the average man
3) Specialites TA have their own point of view.
4) Common standard.
As you can see, opinions differ and do not coincide with manufacturers installation cranks on bikes.
The guys from bikedynamics.co.uk, whom we took diagram, by their experience bike fitting done the formula
Crank length (mm) = -0.0123 * x ^ 2 + 4.8121 * x - 293.8
( where x = internal seam + leg length to the hip joint in cm).
As we can see, the recommended length is almost always less than the standard. At the same time, the opinions Specialites TA, Machine head & BikeDynamics do not differ so much.
Signs of wrong cranks length
Signs of too long cranks:
1) you prefer low cadence
2) usually you are first who gets up on the pedals on the rise
3) pain in the knees, back and lower back (of course there may be other reasons)
Signs of too short cranks:
1) you have very big cadence on flat areas or downhill
Really in the general case it is worth choosing cranks shorter than usually. However, there are authors who give examples of successful athletes who are out of these recommendations. With a height of 180cm, all my life I traveled on 175mm cranks, and I had no problems, but now I want to try 170mm.
We made a new batch of our cranksets. Let me remind you about their review:
While standard lengths are 165, 170 & 175mm, but we can do others if will be more requests.