What Are Cranks On A Bike? (Does Size Matter?)

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A buddy of mine (we’ll call him, Gus) asked me earlier today what bike cranks are.

He’d heard the term being discussed but, like so much bike jargon, didn’t know what it meant…so just had to nod his head and appear to be knowledgeable about what this bit of bike tech actually was.

It got me thinking that there are so many technical terms involved in cycling and it was high time that I did my bit to try and explain what some of it meant.

Today, then, we’re going to look at cranks: where they are and what they are, whether longer or shorter cranks are best, and how to diagnose faults with your crank.

Let’s dive in before you get too cranky.

Where and what is the crank on a bike?

The crank (sometimes also referred to as the ‘crankset’) is the collective term for the components at the center of a bike that sit between the pedals.

Depending on who you’re talking to, there are roughly four components in the ‘crank’, these being the crank arms, crank spider, chain ring(s) and bottom bracket.

Though this can get a little confusing because sometimes the crank arms are called simply the ‘crank’.

And also because the crank spider and chain rings (or crank arms) can be one component.

Let’s look at each of these four and see what their job is.

Crank arms

Crank arms are the pair of levers that the pedals are attached to.

Push the pedals around and the crank arms will turn like hands on a clock.

The job of the crank arms is to turn the up and down motion of your legs into rotational motion of the back wheel, that in turn propels you and your bike forwards.

Crank arms come in different lengths, which are measured from the center of the hole where the pedal attaches to the center of the hollow tube that goes through the axle of the bike.

Crank spider

The crank spider is a star-shaped component that joins the crank arms to the chain rings.

Sometimes this is a separate unit, but often it is part of either the right crank arm, or the chain rings.

Crank spiders can come with various numbers of ‘star’ points, so it’s important to check how many star points your existing chain rings have before purchasing a new crank spider.

Chain rings

Chain rings are the toothed cogs that carry the forward part of the bike chain.

Bikes will usually have between one and three chain rings, depending on the number of gears a bike has.

For example, a bike with 11 gears will only have one chain ring (with 11 cogs at the back wheel hub). A bike with 27 gears will have three chain rings (with 9 cogs at the back wheel i.e. 3 x 9 = 27).

Bottom bracket

The bottom bracket joins the two crank arms together and it sits inside a hollow tube in the bike frame.

Unlike the crank arms, chain spider and chain rings, the bottom bracket (or BB) is the one part of the crank that you can’t see on a bike.

It allows the crank arms to turn freely as you pedal.

Without the BB, you wouldn’t be able to pedal with your left leg as it would only be the right pedal that was attached to the chain rings – so it’s quite a critical component!

READ THIS NEXT BB90 Bottom Brackets Explained

Do bike cranks make a difference?

Clearly, bike cranks make a difference. Without them you wouldn’t be able to move the chain round, and therefore the rear wheel.

But, how important is the length of the crank arms themselves?

Crank arms come in a range of sizes with the most common being 170mm, 172.5mm, and 175mm.

If a bike manufacturer sells three bike sizes (Small, Medium and Large) then typically Small will have 170mm crank arms, Medium 172.5mm, and Large 175mm.

So, should tall bike riders use longer cranks? Will folk with bad knees benefit from shorter cranks?
Unfortunately, there’s no clear consensus as to what the ‘perfect’ crank arm length is. However, if you think of the crank arms as levers, then any variation in the length of the lever can easily be compensated for by adjusting gear either up or down.

Which is a convoluted way of saying that, no, length doesn’t really matter all that much and there are other things that have a much bigger impact.

As is the case with so many things in life…

READ THIS NEXT What is a Groupset on a Bike?

Do bike cranks wear out?

The various components of the bike crank have a limited lifespan and will eventually wear out.

The main ones to suffer from wear and tear being the chain rings and bottom bracket.

It’s important to keep an eye on these as they can have a major impact on the performance of your bike.

On the chain rings, the teeth will become worn down and this will lead to the chain slipping as you pedal – look for the teeth turning pointed rather than having a square top.

The bottom bracket is difficult to diagnose faults with (as it can’t be seen) but listen out for clunking or clicking noises coming from the area – a sign that the BB is past its best.

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