Wednesday, November 14, 2012

Crank length, What’s Your Size?

Zack Allison, BS
Source Endurance, Senior Coach

What length is your road crank? If you are less than 5 feet tall you are likely running 165mm cranks. If your height is 5’5” – 6 feet you are likely spinning 172.5mm cranks. Over 6 feet and the length “standards start to vary.” 175mm? 180? Custom length specially made? What actually makes these the right lengths for peak power and efficiency?

Just for fun I took a look at some of the “greats” of cycling and what lengths they ran during the hour record.  Some of these could be bogus but it’s interesting to think about before we go into the science of crank lengths.

Here is a chart of hour record holders over the years. Notice the crank lengths.

Indurain ran 190mm cranks. Huge for a track but there aren’t really any needs to accelerate fast. I couldn’t find any information on the rationale of such an outlier of crank length for his record. Boardman ran 170’s and is not a small man stretching to 6 feet 2 inches tall. The track hour record is relevant to this discussion as there are people of all different sizes holding the record for periods of time all running different length cranks not correlating to their height. Now that it seems like history has told us that crank length has very little if any correlation let’s dig deeper.

*We've been advised that Indurain actually ran 180mm cranks made by Campy, who have never made 190mm cranks.  Thanks for the correction Kris. 

Scientifically, “what is best and why” is a loaded question. When looking at crank length vs. power you take into account leverage. There are three forms of simple levers with a bicycle crank, and a wheel barrow being second class levers.  Check out this class two lever diagram to the right.

If you consider your crank/bottom bracket system as a second class lever, the length of the crank arm is the effort arm. The fulcrum is at the bottom bracket on the crank arm and the effort force of you pushing or pulling on the pedals is at the other end of the crank arm. Since this is a simple second class lever, the longer the effort arm, or crank arm, the larger the mechanical advantage, or more leverage. In the above picture the effort arm is from the line labeled effort to the fulcrum. The longer the effort arm is the easier it is to move the load, yet the further you have to move it.

MA= Fb/Fa= a/b

The law above is the “law of the lever.”  It shows that if the distance a from the fulcrum to where the input force is applied is greater than the distance b (from fulcrum to where the output force is applied), then the lever amplifies the input force. In our case the input force can be considered the power you apply to the pedals. If you simplify this and apply it to a bike, the longer your crank, the more your force is “amplified” to the cassette and forward movement. Just like if you are trying to take your bottom bracket out after 8 years. Its stuck, your normal 150mm bottom bracket tool isn’t working to you use a “cheater bar” to make the tool longer, increasing the effort arm, increasing the force, yet you are exerting the same force you had on the tool before the cheater bar.

Lets apply this to crank length on a larger scale. Let’s say you have 300mm cranks. This is overly long but for the sake of argument the same principles will apply for 160mm vs 175mm cranks. You have now extended your effort arm by almost 100% if you started with 170mm cranks. You could feasibly, on a climb for example, stick it in your big ring and power up 20% grades with half the force that you were applying with your 170mm cranks. Mathematically this works out in the equation above.
So why do we not all run 180mm cranks? There’s more to your pedal stroke than just force. There is the distance your foot has to travel and the force length curve of your muscles. Every muscle has the same force length curve. It looks like this. If you double your crank length, you also double the distance and speed your feet has to move to turn the crank, and your muscles experience a further stretch and contraction than with shorter cranks.

What this is showing is that as your muscle stretches its amount of force changes. Each single muscle fiber can produce a certain amount of force. The amount of force it can produce never changes; to produce more force your body recruits more fibers. As you see in the diagram your muscle, at its most lengthened position can produce no force. Your muscle at its most shortened position can produce no force. Think about if you are stretching your quadriceps while standing with your hand and you try to straighten your leg. You can give some force, but it’s not nearly as much force as your quad can produce when it’s only slightly bent. Same as if you are in a low squat, it is much harder to begin the lift, than to finish it out.

We can apply this to the crank length argument. If you run a crank length that is too long you will be brining your muscles though a range of motion that is not optimal for the muscle force length curve. As you are at the top of your pedal stroke with a crank that is too long you will be shortening your hamstrings, and stretching your quads so much that they are well out of their optimal length to produce force.

When deciding on crank length, if you are not willing to accept, “you get what comes on the bike” you have to consider all of these variables and how you fit on the bike. Too long of a crank and you are not efficient in your muscle contractions or the distance your foot has to travel each pedal stroke. Too short of a crank length and the amount of force you have to exert on the pedals out weights the smaller range of motion.
This is where we get into how to pick your specific crank length. There are a few variables. 1. Crank application. Are you climbing, sprinting, cross riding, all of the above? 2. How long are your femurs?
Track bikes will not have the same crank length as mountain bikes as the force’s produced are vastly different each different discipline. In track riding you have one gear, you need to be able to accelerate that gear but also spin the gear very fast to maintain top speeds. In mountain you need more torque to power over obstinacies and up steep climbs. You will run a longer crank, usually up 1 standard size for mountain than for road and usually two sized from track to mountain crank lengths. If you need more torque and force like in mountain you need a longer effort arm in your lever, a longer crank arm. You would not be able to spin this longer crank arm on the track. Road you need both qualities so you run the in-between based on your femur size.

Much of crank length choice is “standardized” to bike size. If you are running a 52-58 you are likely on 172.5mm Anything more and you are on 175’s anything less than a 54 and you are on 170’s. A better measurement of what your crank length should be is based on Femur length. Femur length will dictate most of which crank length will keep you in your force length curve during your entire pedal stoke. In most cases your height/ stand over height will dictate how long your femurs are, and therefore you are receiving the right size cranks on your bike automatically. Before you settle for this technique measure your femur length and decide for yourself. If you have a longer femur length for a 5 foot 2inch person you may want to move up to 172.5mm cranks over the 170s that came on your 50centimeter frame.

In the study “Influence of crank length on cycle ergometry performance of well-trained female cross-country mountain bike athletes” Trained female mountain bikers were tested at different intensities for efficiency of pedal stroke based on different crank lengths. The results fall in line with what we have discussed. Crank lengths do change the properties of power output and should be specialized for each discipline but in any one discipline for example road, there is not an advantage to running long cranks or short cranks rather cranks that optimize efficiency for your femur length will be the best.  Stay tuned as I discuss how to incorporate femur length into crank arm length choice.