As Seen In The March 2016 issue of The Racing Post.
The topic of crank arm length is sure to get some feathers ruffled in any group of riders. Many believe in some of the myths that have been carried for many of generations of cyclist. In an attempt to dispel some of the myths, I want to share some of the some of the science and some real world scenarios of how crank arm length can positively and negatively affect ones pedal stroke.
So what are we talking about when we say “crank arm length”? Crank arm length is the length of the arm from the center of the crank to the center of the hole for the pedal. I have heard “shorter cranks” referred to as “compact cranks.” Crank arm length should not be confused with the terms compact cranks and standard cranks. The difference between the two mentioned cranks is the circumference of the bolts that hold the chain rings to the cranks arms themselves. It is possible to have all lengths of crank arms in both compact and standard bolt patterns.
Now that we have the hardware defined, let us take a look at the software – our body. If we examine the pedal stroke, we will find that the hip, knee and foot all extend and flex at some point in the pedal stroke. Extension is an unbending movement around a joint in a limb (as the knee or elbow) that increases the angle between the bones of the limb at the joint.1 Flexion is a bending movement around a joint in a limb (as the knee or elbow) that decreases the angle between the bones of the limb at the joint.2 When the pedal is at the bottom dead center of the pedal stroke, the knee and hip are extended. The opposite – pedal at the top dead center – has the hip and knee flexed. This is what I want to dive deeper into and where I find most problems occur.
If there is not enough room to bring the knee over the top of the stroke, the body will compensate. This compensation can come in the form of driving the heel down more, raising the hip, medial and lateral pelvic movement and/or moving the knee out and away from the bike. At the top of the stroke, the hip is closed more and the knee is at maximal flexion. A hip that is closed off, may not produce the same power as a hip that is more open. Try to think of yourself doing a full squat. How much weight can you lift from a full squat and how much from a half squat? You can lift a lot more weight from a half squat than a full squat. With a half squat, the hip is open compared to closed in a full squat. A full squat puts maximal tension on the patella and patellar tendon. This tension can result in patellar femoral pain that is typically felt in the front of the knee. In more extreme cases where the hip is closed off too much, Iliac Artery Endofibrosis can occur where the iliac artery can become kinked and choke off the blood supply to the leg.
The compensation of the body for a crank arm that is too long can result in knee pain, hip pain, saddle sores/discomfort and ankle/foot pain. I have had riders experience one or more of these issues and opting for a shorter crank arm resulted in pain free riding.
It is true that the majority of riders have a bike configured with an adequate crank arm length. However, in cases where the hip is closed off too much or where the range of motion of the hip, knee or foot are limited or compromised, crank arm length should be considered. The mass majority of my clients whom I have changed crank arm length have had positive results. While there is no cookie cutter approach to crank arm length, I have found that riders with saddle heights below 700mm tend to perform better with crank arm lengths less than 170mm. With that said, I have several exceptions on both sides of the 700mm line. Crank arm length should be evaluated on a case by case basis.
How does a hip get closed off in the pedal stroke? As your back gets lower and the pedal comes over top dead center, the hip becomes more closed.
Are there other ways to help with the hip and knee flexion other than changing crank length? Yes. Other ways include addressing saddle position, cleat position and back angle.
Is my height the only determinant in selecting crank length? No. Other things to consider are range of motion issues that limit flexion of the hip, knee or foot. Even when a shorter crank mathematically does not make a whole lot of sense – it can be a big help.
I have a time trial bike and would like to attain a lower profile, will shorter cranks help? With shorter crank arms, the hip will be more open and may allow for a lower profile. Again, this is a case by case determination but should be considered as an option.
Do you find this to be more of an issue based on type of bike? Yes, from a hip perspective, I find it less applicable with mountain bikes or bikes where the back angle is significantly higher. A TT/Tri bike would be the most susceptible to needing shorter cranks. The issues related to knee flexion – frontal knee pain – are probable regardless of the type of bike. I am 6’2” with a saddle height around 800mm. I switched from 175mm to 172.5mm cranks on my TT bike. It made a huge difference for me.
I found a formula online for calculating crank arm length, will that will work for me? I have seen a few formulas floating around but none of them address the body as a whole in motion on the bike. Some look at inseam, others height or a combination of. We have already learned that we can be the same height but have shorter/longer legs and our leg segment length can vary. You have to look at the knee, the hip and the foot in motion.
MYTH : I am 5’2″ and I will loose power. I was told by my friend who is 6’5″ on 175mm cranks that if I go from my 172.5mm cranks to 165mm I will loose power. —The published scientific studies do not show a rider will experience a loss in power. I have not seen one scientific study professing that going to a different crank length decreases power output. Research does not support power loss except at the extremes of function. What does that mean? Basically, there are EXTREMES that a rider could see decreased function on both ends of the spectrum of too short or too long. A 6’5″ rider on 120mm may be an extreme. “In comparing Maximum Power vs Crank Length, there is little difference in max power. Pedaling rate increases with shorter cranks and maximum power is achieved at higher cadence with shorter cranks.”3
MYTH : The stock crank that came on my bike has to be the optimal length or they would not have put it on the bike. —The manufacturers are looking at averages. I find that for the mid height range, the length that comes stock on the bike is usually adequate. With that said, the dominant 3 crank lengths (170mm,172.5mm,175mm) do not support the entire population. 165mm cranks are becoming more common now but only on the smallest frame sizes.
MYTH : Only riders with short legs should be concerned with crank length. —Crank arm length can be a factor for riders of all heights.
- “Extension.” Merriam-Webster.com. Merriam-Webster, n.d. Web. 21 Feb. 2016.
- “Flexion.” Merriam-Webster.com. Merriam-Webster, n.d. Web. 21 Feb. 2016.
- McDaniel, John. Ph.D. Professor of Exercise Physiology, Kent State University. “Myth and Science in Cycling.” Medicine of Cycling Conference, USA Cycling Headquarters, Colorado Springs, CO. August 2015. Conference Presentation.