What do you need to know before buying your MTB cranks?

autorRotor Bike Components
fecha26th June 2019

Before buying a new mountain bike crankset, there are some things you should take into account in order to make the right decision.

What is a crank?

In case you’re new to this, let’s start with the basics: cranks are the components that transmit power from the pedal to the chainring. The two cranks, left and right, are connected to the pedals and to each other through the bottom bracket axle.

Crank sizes usually range from 165 mm to 180 mm, which is the result of measuring the distance from the centre of the bottom bracket axle to the centre of the pedal axle.

Choosing the right MTB crank

What is a crank?

When choosing cranks for mountain bikes, there are three fundamental factors we must take into account: type, model and length. So, our choice will depend on which mountain biking discipline we practice, our height and the so-called Q Factor.


Choosing the length of the crank based on the height of the cyclist has been the standard in cycling for a long time. However, there are those who choose the length based on crotch height.

These two approaches give us a standard measurements table that must always be corrected for each individual cyclist.

Height Crotch height Crank Length
Under 152 cm Under 70 cm 165
Between 152 and 158.5 cm Between 70 and 74 cm 167.5
Between 158.5 and 174 cm Between 74 and 80 cm 170
Between 172 and 183 cm Between 80 and 86 cm 172.5
Between 183 and 198 cm Between 86 and 93 cm 175
Over 198 cm Over 99 cm 177.5 – 180.0

Q Factor

The well-known Q Factor is the distance between the cyclist’s legs when pedalling.

Maximum Q Factor in cranks is the distance between the outer ends of the cranks at the level of the pedals. We can use it to calculate Minimum Q Factor, which is the distance between the outer end of the crank plus the outside edge of the pedal. Total or functional Q factor is the distance between the centre and middle of the pedals.

To calculate our Q Factor we must measure the distance between the iliac crests of the pelvis and check if it matches the alignment of our knees and the centre of our feet.

Enduro and All-Mountain cranks

Calculating these two factors will give us our ideal crank size, and we can then select the model according to our MTB discipline.

At ROTOR we have developed enduro and all-mountain cranks, which we have sought to make as lightweight and rigid as possible thanks to the combination of aluminium alloys, making them well-suited for these disciplines.

The two models, R-HAWK and R-RAPTOR, come in 3 sizes (165, 170 and 175 mm) and are compatible with the most common bottom bracket measurements and 30 mm axles.

They have protective rubber bands that come in different colours to safeguard the crank from possible impacts and scuffing. When used in combination with the oval Direct Mount Q Rings and ROTOR’s OCP system, you can fully optimise your pedalling.

XC–Marathon cranks

Kapic Carbon MTB cranks

ROTOR’s MTB, XC–Marathon cranks are grouped into three categories: the lightest and stiffest are the new KAPIC CARBON cranks designed for the most demanding cyclists and professionals. This carbon fibre crankset only weighs 429 g and is our response to newer mountain bike designs, prepared for the most demanding conditions. Thanks to ROTOR’s modular system you can combine it with different individual components, such as the standard, Boost or SuperBoost axle, according to how you’ve configured your bike and your own physical traits. You can also adjust the accuracy of your oval Q Rings to suit your pedalling thanks to OCP mount technology.

What’s more, ROTOR offers KAPIC cranks that, like KAPIC CARBON, are inspired by the 7 victories of ROTOR athletes in the Absa Cape Epic race in South Africa. These are also designed to get the most out of your pedalling along with oval Q RINGS® and OCP Mount technology, not to mention the REX2 crank, designed to handle any off-road challenge.

Choosing the wrong cranks

Crank size directly affects your pedalling cadence. We often think that longer cranks are the way to go. However, we have to be careful.

The longer the cranks, the more difficult it will be to maintain a high cadence. Changing the crank length also means that we will have to adjust the height of the saddle, keeping in mind that your leg must be completely straight when the pedal is in the lowest position.

Not choosing the correct cranks can have negative effects on your health. If the distance to Bottom Dead Centre (when the pedal is in the lowest position) increases after raising the saddle we can end up with tendinitis.

If we increase the length of the crank, decreasing the distance to the Top Dead Centre, our knees will bend further, with all the health problems this can entail. It will also cause our quads to lengthen, diminishing our power and physical performance.

Remember, if you have any questions when looking for your new crankset, contact us and we will be delighted to give you a hand.

And please click here if you want to buy ROTOR MTB cranks.

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