How to choose the right cranks

Choosing the right size of crank of your bike is crucial. And yet, if we think about it, how much time do we spend on this decision? Not even 2 seconds, for sure. The most common is to use the cranks that come standard with the bike itself, the one proposed by the manufacturer, which normally defines the length of the crank simply according to the size of the bike.

But the choice of cranks is a very important issue, which affects the health of the muscles and the physical performance of the cyclist.

Let’s see, then, some advice.

WHAT PHYSICS SAYS

What does physics tell us about cranks? Let’s keep it simple: a larger crank draws a wider circle and therefore takes longer to complete a full revolution. A shorter crank, on the other hand, will do exactly the opposite.

What does it all mean?

  • By using longer cranks, we can push harder gears, but the pedalling frequency will be lower;
  • Using shorter cranks will make it more difficult to push the long gears but at the same time will allow us to pedal more agile and at higher rpm.

In practice, the use of larger cranks will increase the force used, allowing us to “push” more, or be able to perform better uphill. Shorter cranks increase your agility, allow you to pedal at higher cadences and have a beneficial impact in sprints and races.

WHAT BIOMECHANICS SAYS

Bike physics is useful, yes, but biomechanics is necessary, too. So there are other considerations to take into account when choosing cranks. Increase the length of the crank will obviously bring the knee closer to the torso when the crank itself is vertical and with the pedal at the top ready to push (at the so-called top dead centre). The knee angle created between the thigh and calf will be smaller and this can lead to technopathies in the joint.

The same is true when the pedal is at bottom dead centre: the leg is more extended, compared to using a shorter crank.

This is why a range of cranks is always defined for each athlete, who can use them according to his needs (pushing longer gears or pedalling more agile).

Currently on the market the different lengths of connecting rods available are:

  • For road bikes: the most common are 170mm, 172.5mm, 175mm. There are also shorter cranks (167.5 mm and 165 mm) that are reserved for the youth categories or the track sector, while some companies offer 180 mm cranks only for certain groups;
  • For MTB: here the crank length has increments of 5mm at a time, so we find 165mm, 170mm, 175mm, 180mm.

HOW TO CHOOSE THE RIGHT SIZE

If you want to try to find the right size on your own, here’s a tip.

It all depends on the length of your femur: sit with your shoulders against the wall and your legs at 90°. Then measure from hip to kneecap. Compare this measurement with the table below.

Femur length [mm] Connecting rod dimensions [mm]

300                                          162

320                                          164,8

330                                          166,6

360                                          167,4

380                                          169,1

400                                          170

420                                          172,2

440                                          176

460                                          177,1

480                                          177,6

500                                          180

LADIES, WATCH OUT!

Women should pay close attention to the choice of the connecting rod.

In fact, their build normally has longer femurs but with lower overall heights. Therefore, using the table above can be misleading.

For them, sports medicine recommends not to use cranks shorter than 165-170 mm in length.

All the more reason why, as you can see, the choice of a good biomechanic is fundamental.

Find all cycling products at Deporvillage

Stefano Francescutti

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