“…For Any Other Reason.”
By Steve Auchterlonie
Cycling Performance Lab
Part 1 of this series busted the longer crank length = greater performance myth via a physics analysis and literature review. How useful is that? It’s easy to be a detractor; the challenge is to be a contributor. That is my primary goal in creating the LAB. I want to contribute through helping performance-oriented cyclists get faster. So, it’s time to contribute on the crank length topic.
The first article concluded with citing a Cervelo article, “The main thing is to realize that the choice of crank length doesn’t significantly affect power, so any length is now free to choose FOR ANY OTHER REASON.” What other reason? How about biomechanical for comfort (aka, joint health) and performance…how about performance depending on race type – sprinting vs climbing…how about bike handling?
a) Let us look at biomechanical first. The literature offers many theories on how to determine the optimal crank length (OCL) for a cyclist. Most are based upon leg length in some fashion…inseam…femur length…tibia length…or some combination. This logic passes the laugh test because the pedaling motion is a series of coupled levers including the femur/thigh, shin/tibia, foot, crankarm and so on. Efficient mechanical design would optimize the lengths of these levers, in relation to one another. The challenge is the wide ranging equations proposed to determine OCL, none of which are based on mechanical design. For yours truly, the various equations propose that my OCL should be anywhere from 120mm to 180mm…seriously. Who to believe and where to begin? In my opinion, the best sources are to know i) what crank lengths the pros are running, ii) personal experience, and iii) biomechanical testing:
i) Pros – A great measure because you know they have been fit by the best and are privy to the best research. Most pro men range in height from 5’6” to 6’3” (with exceptions outside that range) and the crank lengths used range from 170mm to 180mm, with a direct correlation between height and crank length. For pro women, the ranges are 5’3” to 6’ and 165mm to 175mm. Taller cyclists have longer leg lengths requiring longer crank arms, at first glance. What is not apparent is that pros are elite athletes for several reasons, including longer than normal ratio of femur length to tibia length. No, this is not true for 100% of the pros, but it is a common characteristic of the elite cyclists, same as high VO2 max values are common. From a bike fit view, this femur to tibia ratio expresses itself in saddles set back further in relation to the bottom bracket and the need for either setback seatposts and/or saddles pushed back on their rails. My experience with many amateur racers is a normal to below normal femur to tibia ratio, resulting in zero setback posts and/or saddles pushed forward on the rails.
There are many pros with similar stature to my own but I will simplify and focus on Alberto Contador (AC) mainly because there is a wealth of bike fit info on him available and he is one of the most consistent top pros for the last 10 years. It’s easy to find bike measurements on the web and to analyze photos to determine key measurements such as saddle setback and handlebar drop. AC has used a 172.5mm crank for many years. Without boring you (or scaring you with my obsessive quality) with the details, let me simply summarize my studies on AC vs Yours Truly (YT): 1) very similar total height and leg lengths based on saddle heights, and 2) very different leg geometries with AC’s femurs 2.5 cm longer than mine and my Tibias 2.5cm longer than his (as documented in saddle setback measurements). Longer femur to tibia ratio is ideal for cycling performance…dang it. It also impacts the ideal length of crank because longer tibias compress the knee angle and hip angle at the top of the pedal stroke. So, the question becomes should I use a 172.5 like AC since we are almost equal height and total leg length, or should I use a shorter crank since my femurs are shorter and tibias longer than his?
ii) Personal Experience – I have fairly long legs for my height (34 inch = 86 cm inseam for 5’9”) so my first professional bike fit (by a national renown fitter) recommended I run 175mm cranks. My first couple years racing produced decent results but inconsistent. I struggled maintaining 90+rpm on long TT efforts and long climbs typically broke down to 70 rpm despite trying to keep 80 rpm. Sprinting was always disappointing but I concluded that was due to my age (near 50 when I started), although that was inconsistent with my athletic background outside cycling. I always felt like I was fighting the bike and chronic lower back tightness. A new bike purchase came with a “shorter” 172.5mm crank and no change in performance. It wasn’t until one year ago when I purchased an adjustable crank for winter training that I could test various lengths…the result was stunning. I experimented with the entire range from 125 to 180 over a couple months during the winter. I could hang with the fast group rides regardless, but found it difficult to “jump” with cranks 175mm+ and to produce sustained power on long climbs with the cranks less than 155mm. Going into the 2015 race season, I “risked” the season by installing 165mm cranks and had the best season ever with excellent overall power – sprinting and climbing – and almost no lumbar tightness.
iii) Retul Measurements – The LAB owns a Retul system which measures many body position parameters dynamically. Two key parameters for performance directly impacted by crank length are closed hip angle and knee flexion angle. Both of these angles are measured at the top of the pedal stroke (TOS), so crank length directly impacts these measurements by moving the thigh at TOS for hip angle and by moving the foot at TOS for the knee angle. Retul offers normative ranges for these measurements but, through experience working/measuring over 50 racers, I have found that most top local amateur racers are performing best when the hip angle is not closed beyond 45 degrees at TOS when in the drops, and, more importantly, the knee extension angle (leg extended at bottom of stroke, BOS)) is 37-40 degrees with the knee flexion angle (TOS) is less than 110 degrees. For you without a Retul system, these angles are difficult to measure accurately statically with a goniometer, but you would use values 5 degrees lower due to the measurement technique. Greater performance is achieved with more knee flex at BOS but only if the crank does not push the knee flexion angle above 110 degrees. Many cyclists ride in the 110 to 115 degree range and I have done that personally. It is not going to injure the joint but pulling the pedal up the backstroke and over the top is more difficult. A knee flexion angle greater than 115 is not healthy or optimized for performance.
With 175mm cranks, my knee flexion angle was greater than 115 degrees, even with a higher saddle height (less optimal for performance). A 165mm crank achieves a knee flexion angle of 110 degrees for me with a knee extension angle in the performance range of 37-40 degrees. Performance has jumped significantly, both sprinting and climbing.
b) What about race type? Many examples found on the web are consistent with my own experience, in that if one uses a crank different than their OCL, then shorter cranks improve sprinting and longer cranks improve TT efforts involving long, extended climbing. Again, this makes sense because shorter cranks are easier to accelerate and sprinting is dependent upon pedal velocity. Yes, they are shorter and so they travel less distance per revolution resulting in a faster cadence, but, far more importantly, the shorter cranks open the hip angle and knee flexion angle (top of pedal stroke) which allow the cyclist to pull through the backstroke and over the top of the pedal stroke with much less joint restriction. Longer cranks for TT, sustained power efforts make sense…as long as the cyclist can maintain the same pedal velocity…there is a limit to what is acceptably “long” for each cyclist.
How is bike handling impacted by crank length? This was a surprise to me but should not have been. Bike position includes setting the saddle setback, which is the fore-aft position of the saddle and cyclist. As cranks get longer, the saddle position moves forward relative to the bottom bracket in order to maintain the knee over pedal spindle position (a completely different topic and not addressed in this article). Conversely, shorter cranks position the cyclist further back on the bike. Handling of the bike is affected by the center of gravity which is directly affected by the saddle setback. Handling is typically sacrificed as the position is moved further forward, especially in high speed cornering and descending due to additional weight on the front wheel.
From personal experience, I can share two examples of the impact on handling. First, I have never been able to balance myself on the bike without my hands on the handlebar and just thought it meant I had poor balance…again this is inconsistent with other activities off the bike. Moving to the 165mm crank has positioned me such that I can ride “no-handed” all day long. Secondly, the local Devil’s Den climb used in the Joe Martin time trial race is a two miler at 6.5% with several rounded 90 degree turns and one sharp switchback turn. Historically, I always had to brake in the switchback when descending at speeds above 30mph. With the 165mm, I can take that turn with complete confidence without braking.
The crank length dictates the optimum performance for a cyclist both biomechanically (pedaling) and mechanically (handling). There exists an Optimum Crank Length (OCL) for a cyclist for “all around” riding, which is determined by their unique leg lengths for the femur and tibia. Cycling performance can be enhanced for sprinting/handling by reducing the crank length and for extended climbing/TT efforts with a longer crank length. This “range” of crank lengths is limited due to biomechanical restrictions caused by the femur/tibia ratio.
Simply, if you feel like you are fighting the bike, battling chronic lumbar back pain, struggle to keep a higher cadence when climbing or long TT efforts, then your crank length is most likely too long. The crank length could be too short if you feel like you spin out frequently or struggle climbing with other cyclists that you easily ride beside on the flats. Two other indicators of incorrect crank length are a) poor high speed handling when descending and/or b) a saddle position which requires an unusual saddle setback including zero setback seatposts with saddle slammed forward on rail or the opposite with a huge saddle setback.
Alright, enough discussion…where’s the proof with some power data? Part three of the crank length series will provide power and biomechanical data to document performance improvement when using the OCL. In addition, I will provide a simple equation to calculate your own OCL. I hope to be timelier in writing Part Three, within the next few weeks.