Crank length why do we all use 175mm?

Now always looking for a bargain it seems theres always plenty of shorter length cranks in the bargain bins. The Duo FSA set are 170mm and I’ve had good success with them and no adverse reactions so I was wondering what the “reasoning” was behind why we all seem to religiously use 175mm cranks in the mtb world.

Macinehead who sem to be cycling engineering geeks have a calculator, putting my 32″ inside length in, gives an optimum of 166.6mm, more confusing?  The cycling guru Sheldon Brown  has a good info as usual and one phrase that caught my attention was “Too long cranks cause excessive knee flex, and can cause pain/injury if it causes your knee to flex more than it is used to” So what is the prinipal behind different crank lengths.

Bike fitting specialists Bike Dynamics have some blinding science all about knee angles too, and as I do have weak kness from early life abuse I’m drawn to going shorter on length. Another factor is that I like to spin the pedals as much as possible and while using single ring setups more recently it all seems to point to shorter being best. 

Will this reduce power though, you still have the slow grinding climbs, especially using 1×9 and running out of the smaller gears even in this relatively flat area of the country. The people who deal in power meters should know. Powercranks have a interesting study. They talk about crazy short lengths that I didn’t even realise people rode, and back them with power readings, but one section bears some truth.

So, using shorter cranks will usually give the rider: 1. More (or, the same) power. 2. More ground clearance. 3. Better aerodynamics. What is the mountainbiker not to like?

The Anthem with its definate low bottom bracket and 170mm cranks are the way to go for me, sold! the order is in.



2 thoughts on “Crank length why do we all use 175mm?

  1. Interesting read but not so sure about the aero qualities argument – I assume that was tongue in cheek. Disc and deep section wheels went out in the early 90s didn’t they? MTB riders don’t go in for all those pointy hats etc. The reason, or so I have gleaned is because MTBs do not go quickly enough for aerodynamics to be an issue.

    OK, so you have passed me in many an event at warp speed but, from my testing experience (back in the day), aerodynamics only really made an impact when you were hitting >20mph. Not many mtb riders average these speeds on a race course.

    The main point of the post is valid though and gives you grounds to question our lemming like rush for 175s. Interestingly, I was talking to a bike fit bod a little while ago and he was creating an equally cogent argument for 180s! The jury, it seems, is still out!

    1. Yes tongue firmly in cheek, it’s all just messing about. But as over 80% of your energy when cycling is used to overcome pushing through the air, aerodynamics is a big factor at any speed. Drafting another rider shows that at any speed, just seeing the drop on a power meter as you draft a rider is staggering.
      With no wind that 20mph speed might be valid, but as we often pedal into 10-15 mpg winds ourselves surely it doesn’t take much speed for aero to mater.
      Having raced on a 6 inch fork high barred mountainbike for a laugh I can tell you it’s a killer compared to a head down full on race stance.

      Singlespeed guys tend to use 180+ cranks for extra leverage and grunt, but as the emergence of these elliptical rings shows, the faster you spin through the deadspot, the quicker your pedal stroke is back on the power, surely shorter cranks will help that a bit.

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