Differences in crank length

Recently while riding my fixed-gear bicycle I experimented with crank length. With a fixed-gear you have only one gear and you have to pedal constantly all the time. This means that when you go around a corner you have to pedal and as a consequence it is possible to hit the pedal on the ground during the turn. Hitting a pedal on the ground can have some major impact on your cycling day so it's something you try to avoid as much as possible.

My first set of cranks on this bike were 175 mm in length while the new set are 165 mm. Even though the testing was performed on a fixed-gear bike it was useful because it gave me a chance to really see first hand the difference that 10 mm makes when riding a bicycle regardless of the type.

The 175 mm cranks had the advantage of giving me more leverage when climbing hills. It seemed to be easier to go uphill as long as I maintained forward momentum. When descending I found that my hips ended up moving further and pedaling smooth circles was much harder.

The 165 mm cranks made pedaling fast downhill incredibly easy in comparison. On flat ground I also found that I was pedaling more smoothly without moving my hips as much. Uphills were indeed harder but not to the point where it was impossible to ascend.

I should also mention that I've been told that knee problems might be caused by extreme angles when pushing the pedals. The 175 mm cranks put the knees at a much greater angle then do the 165 mm ones. I don't know how true this is but it is something worth looking into. It's likely also worth noting that many mountain bikes come with 175 mm cranks and people seem to use those successfully in more difficult terrain then that encountered during the average bicycle road tour.

A second opinion on the importance of using the right size crank can be found on Peter John White's web site in the "How to Fit a Bicycle" article, crank length section.

Peter's article is very interesting because he presents his personal experience with different crank lengths and includes a very useful formula that he believes works well for his customers.

In his article Peter recommends measuring the distance from the top of your femur to the bottom of your bare feet. Figure out what 18.5% of that measurement is to determine your optimal crank arm size.

I haven't performed this calculation yet but once I do so I will post the result here for comparison purposes.

So how does this relate to my bicycle touring bike?

My touring bike has 170 mm cranks so it's setup mid-way between the two cranks I've used giving me slightly more leverage when climbing while also giving me the benefit of being able to spin easier circles while pedaling.

I intend to continue to experiment with this further. Please feel free to contact me if you have your own viewpoint on this topic.

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