Why Your Mountain Bike Crank Length is Likely Too Long

Why Your Mountain Bike Crank Length is Likely Too Long

With few exceptions, most XC and Trail oriented mountain bikes shipped today come with 175mm cranks. While some brands spec 170mm cranks, it is often only on their smallest sizes. However, the evidence leans towards mountain bike crank lengths being too long in general. Why are current lengths not optimal for many riders and why isn’t anyone doing anything about it? Good questions. I only have answers to the first.

Why are Short Cranks Better on a Mountain Bike?

Here are some reasons why shorter mountain bike cranks should become the norm.

  • You don’t have to be Rachel Atherton to know how often cranks and pedals can strike the ground on high sag/low BB height trail/enduro based mountain bikes. With wide platform pedals, it isn’t hard to touch down once or twice even on some of the easier mountain bike trails around me in Vermont. Cranks striking the ground is not a good thing. You may not need a higher bottom bracket as much as you need a shorter crank…
  • Shorter cranks provide an effectively lower climbing gear and accelerate faster than longer cranks.
  • There is no tangible maximal power loss to shorter cranks. One of the most respected researchers in cycling mechanics, Dr. Jim Martin, studied this very topic and published the results in 2001. https://www.cervelo.com/en/engineering-field-notes/a-new-spin-on-crank-length.  The bottom line is that most riders will not give up any power riding a shorter crank and many will actually gain a little.
  • Muscle Balance. I know I’m likely to get arguments on this one in particular, but if you truly want to maximize your power and efficiency, you want to develop a high cadence. Don’t want to believe me, talk to an exercise kinesiologist about the cadence characteristics that tend to define the most powerful and efficient cyclists.

Shorter cranks don’t increase your power on their own, but they do make developing a powerful high cadence stroke and accessing a broader range of muscle groups easier. In turn, this makes developing higher levels of sustainable power easier and creates more consistent traction.

http://www.pezcyclingnews.com/toolbox/power-output-and-cadence-learning-from-the-pros/#.Wb1FSciGOUk

https://breakingmuscle.com/fitness/big-power-does-not-require-big-gears

  • Reduced Joint Load. Regardless of your height, shorter cranks put less torque and load on joints and muscles, thus reducing the chances of knee, hip, back and other pain while riding your mountain bike.
  • The first mountain bikes were built from pirated road bike parts. Mountain bike crank length was simply transferred from whatever road bikes were using in the ‘80’s and haven’t changed much since. Road bike cranks are available in a lot more lengths than they were in 1985…

So, Why Aren’t There Shorter Cranks on Mountain Bikes?

  • Bike manufacturers need to start asking/insisting on more crank length options to be available if component manufacturers are going to start building them. I don’t think any/many bike manufacturers are asking. They should. I bet a bike where the crank doesn’t strike the ground on a demo ride sells better than one that does.
  • Not enough riders are asking for it. Just because we get used to something (like a certain length crank) doesn’t mean it is optimal.

Pro riders and bike manufacturers need to be the impetus that drives the change to shorter crank possibilities for XC and trail mountain bikes. While a few months of muscle adaptation may be required before the full benefit is realized, I encourage many pro mountain bike riders to try a shorter crank. I would not limit this to just smaller riders either; a number of Team Sky’s pro road riders used shorter cranks this year.

And to bike manufacturers, I ask why a 170mm crank on a 48cm road bike is almost universally considered unacceptably long today and the same is not true on mountain bikes?

Dogma is the Death of Innovation

Treating things as dogma goes against the spirit of innovation that has been a hallmark for the bicycle industry since before the Wright Brothers. A spirit that always looks for ways to improve the experience and the performance of riders.

For an industry that has put so much effort into changing axle or bottom bracket dimensions by a few mm, the passé faire attitude on something that can potentially improve how a bike and rider perform as much as crank length seems strange. The time is now for shorter XC/Trail MTB crank length options.

About Ian

From first time riders to Olympians, Ian has helped thousands of athletes achieve their cycling and triathlon goals. Ian develops much of the Fit Werx fitting and analysis protocols and is responsible for technology training and development. He is regarded as one of the industry leaders in bicycle fitting, cycling biomechanics and bicycle geometry and design. He is dedicated to making sure the Fit Werx differences are delivered daily and provides Fit Werx with corporate direction and is responsible for uniting our staff and initiatives.

Find out more about Ian Here

Permalink

Recent Posts

6 Responses to Why Your Mountain Bike Crank Length is Likely Too Long
  • Allison Murray

    As always, an excellent article with terrific information explained in a thoughtful manner. I always learn something from Ian.
    Thanks!!

    • Ian

      Thanks for reading Allison!

  • Brian Nystrom

    I think it’s important to point out that not everyone will benefit from shorter MTB cranks. Taller riders who use longer cranks on the road are one example. Matching your MTB crank length to your road crank length (assuming the latter is optimal) is a good idea, as it eliminates the need for adaptation when switching bikes.

    • Ian

      Hi Brian,

      Thanks for the comments and thoughts. I think what you said about matching optimal length is a good point. I don’t think the article stated that short cranks were immediately beneficial to everyone, but it touches on the point that the way crank length and rider height have historically been correlated is not what it has been made out to be from a power perspective. From a power perspective, regardless of rider height, the research shows that there is little to no maximal power difference across a wide range of crank lengths – people can train their muscles to produce power a number of different ways. However, from a biomechanical perspective, what crank length affects is the ease of access you have to specific muscle groups through the stroke. If you have been riding 180mm arms for years, 165mm could feel weak and slow initially, but this is more a reaction to an effective change in gearing and unconditioned muscle activation than the length not being optimal. Once muscle adaptation has happened, quality power can be produced across a wide array of crank lengths. In many instances, and for triathlon and TT in particular, we’ve put riders well over 6′ on 165mm and 170mm cranks and had it improve their comfort and performance. “Optimal” really depends on what you are trying to achieve when it comes to muscle recruitment and joint load. Going longer tends to bias the front of the stroke more and going shorter tends to help riders transition through the top and bottom more actively. This link offers more detailed information on some of the findings.

      Based on the muscle adaptation potential some taller riders may want to consider a shorter crank on their mountain bike because bigger bikes usually don’t have any more ground clearance than smaller bikes.

      Keep on riding and thanks for taking the time to post.

  • Lisa Saunders

    I switched to 145cm on my road bike due to knee arthritis (5’8″) and my fitness and speed have improved while keeping my joints happy thanks to a fitwerx fit by Dean Phillips.

    • Ian

      Thanks for working with us Lisa and glad your fit and changes helped your joints. As you mentioned, shorter cranks tend to put less strain in joints in general.

Leave a comment

POST COMMENT

Fit Werx

Fit Werx