OPTIMAL CRANK LENGTH | PART 1
By Steve Auchterlonie
Cycling Performance Lab
Matching the bike position to the rider and his/her talent(s) is my focus. Saddle height, setback, and tilt, and handlebar extension and drop are the main parameters to be adjusted to develop an optimal position unique to each cyclist. I will be writing a series of articles on various bike position parameters, for readers to consider. As with most aspects of life, this will not be a dialogue based on distinct right/wrong choices; instead, my goal is to invite a wide-open discussion and I invite your participation through the OCA website.
Part 1 – Optimal Crank Length
Oh no, another crank length article which regurgitates what’s already on the web and concludes with a wishy-washy recommendation to not change from the norm…wrong. I promise this will be fresh and progressive, with the goal to help everyone improve his/her performance and comfort on the bike. Finding the Optimal Crank Length (OCL) is a research interest of mine and will require at least two articles: Part 1 – a physics review and myth-busting adventure, and Part 2 – presentation of field results for performance and a proposal for determining OCL.
Why start the series of position articles with crank length? Simply, it is the center of the bike upon which everything depends. Change the crank length and one must change the seat height, setback, handlebar drop and extension. Consequently, changing the crank length has a dramatic affect on how the cyclist pedals and how the bike handles…dramatic.
Where to begin is easy. Historically, the cycling community (racers, bike shops, etc) has always maintained that longer cranks produce greater performance due to greater torque. Let’s explore if this myth is real or requires busting.
MYTH: GREATER CRANK LENGTH = GREATER TORQUE = GREATER PERFORMANCE
Investigating this myth reminds me of the old adage, “one must step back out of the trees to see the forest.” Physics defines torque = (lever length) x (force applied at end of lever). The crank is commonly thought of as the lever in cycling, so simple math indicates that greater torque is produced with a longer crank, at a given force.
No question whether this is true. The question for me is whether the crank is the true lever in the equation for cycling?
Why do we focus on the crank arm assembly only? Step back out of the “trees” and look at the “forest” which includes the cyclist and bicycle. The cyclist and bicycle combination (system) includes more than just the crank arm assembly when looking at levers: a) femur length, b) tibia length, c) foot length, d) crankarm, e) chainring diameter from the center of the bottom bracket to the teeth of the chainring, f) chain length from the chainring to the cassette, g) diameter of the cassette (specific to the gear engaged) and h) wheel diameter from the rear hub center to the tire contact patch on the road. Whoa, that’s a lot of levers which, for simplicity sake, can be added to calculate a “system” lever length: 450mm (avg femur) + 400mm (avg tibia) + 150mm (avg foot length from ankle to metatarsals) + 170mm (crank) + 100mm (52 chainring diameter) + 500mm (avg chain length) + 40mm (16 cog diammeter) + 335 mm(wheel diameter to contact patch) = 2145 mm. That’s the “forest.” One “tree” is the cranklength. Now, let’s get radical and change the crank arm from a 170 to a 175mm. This crank change is equal to a system change of 5mm / 2145mm = 0.23% increase. Thus, the torque increased 0.23%. Uh…seriously…0.23% performance gain….
For argument sake, there are those which would say all the levers up to the bottom bracket are primary (power producing) and downstream of the bottom bracket are secondary…power absorbing. I don’t agree but let’s do the math. The primary lever system = 450 + 400 + 150 + 170 + 100 = 1270mm. Changing the crank to 175 produces a 5/1270 = 0.39% increase in torque. Either way, less than 0.5% is not an increase in performance affecting race/riding results.
There have been several quality studies which have validated the minimal influence of changing crank length by millimeters, even centimeters. Summaries:
- Determinants of Maximal Cycling Power: Crank Length, Pedaling Rate and Pedal Speed, C. Martin and W.W. Spirduso, 2001 – Sixteen (16) trained cyclists performed maximal inertial load tests using crank lengths of 120, 145, 170, 195 and 220mm. Maximum power ranged from a low of 1149w for the 220 mm to a high of 1194w for the 145 mm. Power produced from the 145 and 170 cranks was significantly greater than that produced with the 120 and 220 cranks.
- The Bio-mechanical Effects of Crank Arm Length on Cycling Mechanics, E. B. Sprules and D.J. Sanderson, 2000 – Sixteen (16) avid cyclists, spanning a wide range of heights and leg lengths, performed sub-maximal tests using crank lengths from 120 to 220 mm. At a fixed power level, heart rate was measured to determine pedal efficiency and corresponding optimal crank length. The results mirrored the Martin and Spirduso study in that longer cranks did not produce greater performance. In fact, shorter cranks produced lower heart rates.
In addition, here are highlights from several key industry players:
- Simon Cycling wrote a crank length article in 2010 which cited that Andrew Coggan, well-known and respected cycling scientist, stated at a webinar that optimal crank length for most cyclists is probably under 170mm in length.
- Cervelo article on crank length summarizes the Martin and Spirduso study and writes, “… 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.”
- An article on FSA website reviews crank length, entitled “ Crank Length – Does it matter?” A couple excerpts are a) “If you’re under 6ft, 165 or 170mm cranks may improve both performance and comfort.”; b) “It may seem counter-intuitive; longer cranks mean more leverage and more power, right? Not quite. In fact, longer cranks give you more torque. Torque and power are often confused, because the difference is subtle. Torque is a measurement of twisting force. Power is a rate of work – it’s the energy consumed during a unit of time. The more energy you put in, the faster the work is done, and the more power you measurably have. Longer cranks can give you more leverage (torque), but they can’t give you more energy with which to pedal! Why not have the torque anyway? Because there are downsides. Longer cranks make it harder to sprint, as it’s harder to maintain leverage for as much of the each revolution as with shorter cranks.”
- Uli Shoberer, the entrepreneur and engineer who created SRM, discussed crank length in an interview with Slowtwitch.com in 2012. Key excerpts: a) We tested crank lengths a lot in cycling with several teams. We could not find any increase in power with shorter or longer cranks…it doesn’t matter…in general, with a shorter crank you spin faster…with a shorter crank you choose a higher cadence, and a longer crank you go to a lower cadence, but the power to heart rate relationship is the same. b) Our last test with T-Mobile we did tests on L’Alp d’Huez, and some cyclists rode it 6 times with different crank lengths, ovalized and round chainrings…we measured lactate, heart rate, power…we found that only what counts is power to body mass.
Say it with me…MYTH-BUSTED. Yes, longer cranks increase torque but not performance.
Where is the Torque podium? I have yet to attend a race where points were given to the racer producing the greatest measured torque. The winner is the person whose bike goes the fastest. In physics terms, the winner completes the work (race) in the shortest time = power = force x velocity (simplified).
Currently, the cycling industry provides a measure that is predictive of performance and race results through use of a power meter. The power meter format winning the retail game is typically measured at the crank but that is only due to marketing forces and convenience to the cyclist. Power can be measured in the pedals, in the chain and in the rear hub, just to name a few. The main point is that power is predictive of performance and it is determined by measuring the force and the velocity of the moving part.
When measuring power at the crank, the velocity of importance is the crank arm velocity or typically referred to as cadence…but not really…there is a difference between cadence and pedal velocity…and the difference is the crank length. At a given cadence (revolutions per min), the pedal moves further using a longer crank length. Ah hah!! The myth is true because the velocity is faster with a longer crank, right!!?? Here’s the rub. At a given force on the pedal, a longer crank will produce greater power if the cyclist can pedal it at the same velocity as a shorter crank. The reality is we cannot because our system of joints (ankle, knee and hip) have limited ranges of motion, and because each of us are “wired” with a unique combination of fast twitch and slow twitch muscles which control the firing of the muscles involved in cycling. Simply, we are each uniquely limited in how fast we can accelerate the pedals.
What does all this mean? Longer cranks would produce greater performance due to greater torque and greater crank velocity if we were machines unaffected by the resistance of our joints’ ranges of motion and our unique DNA. That’s not reality. Reality is when crank length increases then velocity (measured by cadence) decreases. This is exactly what was proved in the studies summarized earlier.
With the crank length/performance myth busted, the next step is best summarized by the previous citation by Cervelo, “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 bio-mechanical for comfort and performance…how about performance depending on race type – sprinting vs climbing…how about bike handling?
These are the areas which I have been studying for the past several months. Part 2 will present those results along with a protocol which can be used to determine your own unique optimum crank length. Stay tuned.