Crank Lengths: Secrets Revealed
One of the most confusing aspects to finding the perfect bike set-up is determining the right crank length. If there was ever was an inexact science, this is it. We’ve had our best tech guys working round the clock for the past few weeks to help make sense confusing but critical issue, and here’s what we can tell you…
There are so many differences of opinion; it’s difficult and confusing to pick a bicycle cranks length that you feel most benefits your specific type of cycling. It’s a subjective call and sometimes just requires experimentation with different lengths to see which one most benefits your riding style. A widely held belief is that the optimum crank length also optimizes your pedaling “efficiency”. You want this.
Let’s start out with a definition of the word “efficiency” followed by some basic facts and observations, and give some final conclusions, leaving the ultimate choice up to you, the rider.
The word “efficiency” is used a great deal when talking about picking the correct crank size or pretty much anything that has to do with cycling. For example, “It is more “efficient” for me to use 172.5mm cranks than 175mm”. For the purpose of this article we will use the definition presented by PezCycling News science guru and all-around nice guy Stephen Cheung:
Efficiency defined – “The least energy consumed (e.g., oxygen uptake) per unit velocity (i.e., speed). In other words, if I need 2.2L/min O2 to go 30 km/h with one crank, and 2.0L/min to go 30 km/h with another, then the 2nd crank is more efficient because it uses less energy to produce the same speed. This is generally the main one that is used in scientific studies of the biomechanics and energetics of cycling”.
What Do We Know?
1. Longer cranks give more leverage and the ability to pedal a bigger gear given everything else being equal. The simple analogy I like to use is the old teeter-totter experiment. Put one person on the end of a teeter-totter. Now try to lift them from the opposite side closer to the center point and then farther from that location. Obviously, the outer most point is the easier, as you have more “leverage”, relative to the inner point. Same applies to cranks on a bicycle. This becomes especially useful in hill climbing, when we are pushing slower revolutions and of course more difficult to push a gear climbing.
Potential downsides of a longer crank include overstressing tendons and muscles at the more extreme ranges of motion – ie : the top and bottom of your pedal stroke.
2. Shorter cranks are easier to spin and thus better for acceleration and quickness, relative to something longer. Tradition always dictated that track riders used shorter cranks, as their riding style required those characteristics. Combining the advantages of greater leverage during the down phase with faster leg speed during the recovery phase of pedaling is the theory behind the variable cranks reviewed by Stephen Cheung last fall.
A shorter crank, if you focus a lot on climbing and also have a short seat position, may lead to overload of the muscles themselves and the knees. Again, you get the hint that there’s a lot of maybes here.
3. Shorter cranks require less flexibility. Think about it for a minute. By having a smaller circumference because the crank length is shorter, requires the muscles, especially at the top of the pedal stroke around the knee and hip area to be stressed less than using longer cranks. This is especially useful as you get older and have less flexibility. Hint – read this stretching article!
4. In general, the biomechanics literature is still in debate about whether an “optimal” crank length exists for a rider based on height and inseam. In terms of the “efficiency” definition above, the literature suggests that the most efficient crank length either does not exist or may be as low as 160 mm in some studies. However, the range of muscles used in pedaling vary quite a bit depending on seat height and crank length, and the main difference between crank choice appears to relate to muscle use and fatigue rather than energy efficiency.
5. There are 3 popular lengths in use today on the road. Most people are using 170, 172.5 or 175mm, while Miguel Indurain was known to hammer the opposition with gigantic 180 mm cranks. If you don’t know which length you are using, you can look on the crank and it is marked. For Shimano, check the inside of the crank near where the pedal connects. For Campy, it is printed on the outside of the crank arm.
So, how to choose the proper crank length and how do you know if you are absolutely riding the wrong length:
1. There is no rock-solid formula for determining crank length, saddle height, top tube length, or any of those things. There are general rules of thumb, but there are a lot of different confounding factors such as your preference for climbing making it possibly more beneficial to err on the side of a longer crank. It’s also very interactive between your seat height, seat fore/aft and crank length. Also, it’s a very organic process to set up a position, due to such things as flexibility and adaptation to a certain length. Therefore, often the best you can hope for is a rough indication, but you can get a better idea by using tools like the Varicranks from Computrainers or doing an SRM spin scan.
2. So, if you consider yourself short (<5’8”) or have a shorter inseam, then start out with a shorter (170 mm) crank length, if are of medium height, go for the 172.5mms, and if your talk and long – clip into some 175’s and see how they feel, work, etc. Try to ride a different length as well for comparison, and don’t make up your mind too fast, as even the smallest change to your set-up can feel like a big change until you get used to it – give it a few rides to adjust.
3. And finally, the best advice for experimenting with crank lengths is to see if you have a friend who can lend you a set for a few weeks. Test your normal length on a standard test on the trainer or on the road. Try out the new cranks for a couple of weeks and carefully note your subjective sensations. Finally, try your standard trainer or road test again prior to making any decisions on which set is most “efficient”.
In A Nutshell
The only real way to figure out what is / is not efficient is by testing
things out over a prolonged period of time with a metabolic cart to measure
oxygen uptake and/or a CompuTrainer. It’s somewhat tough to determine what
benefits are due to crank length changes and what is due to a new position,
period. If you don’t have a fully –equipped test lab at your disposal, then may the force be with you.
Bruce Hendler created AthletiCamps to provide cycling specific coaching and training to athletes and cyclists of all levels. Find out more at www.athleticamps.com.
Stephen Cheung is an Associate Professor of Kinesiology at Dalhousie University in Halifax, Nova Scotia, Canada, with a research specialty in the effects of thermal stress on human physiology and performance. He has been an avid roadie since beginning university in the mid-eighties, and still has non-indexed downtube shifters on his winter bike and wool jerseys hanging in his closet. He can be reached for advice or comments at [email protected]