Crank Length Research. When do they help? How short should you go? (Video)

So what about shortening crank length? What are the up sides and the down sides? Is it for everyone? Is there enough crank length research to go on?https://youtu.be/gwoNCypfg08If you haven't seen part one of this, just click here to see learn how a rider was swindled by another bike fitter about their crank length.It's unlikely that everyone would benefit equally from going with a shorter crank arm length. That's a pretty safe assumption especially given in just the simplest scenario a 175 mm crank may not be "long" to someone who's 6'4" tall. That said, some of the research shows that optimal crank length can't be determined by leg length alone¹, so perhaps even a very tall person might benefit from a 160 mm crank.Interestingly the research also leads us to believe that in many cases opting for a shorter versus a longer crank is a safer overall bet. In this particular study they compared 175 mm versus 170 mm cranks and concluded that erring on the side of the shorter crank was a safer bet in nearly all cases.One of the main points regarding shorter cranks has to do with cadence and pedal speed.  As the speed of the pedal increases, the oxygen consumption goes up. So with shorter cranks and  maintaining the cadence you had with your longer cranks you will have a slower pedal speed and lower oxygen consumption. These are small gains measured across a wide range of crank lengths, so making a modest change in crank length (like from 172.5 to 170 mm) won't be of much benefit

• benefits - decreased knee flexion at top of the stroke; have the chance of better aerodynamics because you can lower the torso more because you're not moving into as much hip flexion
• if you want to know more about crank length and how it affects us you should do some research and read everything you can by Jim Martin (University of Utah) and Jim McDaniel (some links in the website article to their papers)
• simplest way to do this is to hit the main points that these researchers and many others have refined over the last roughly 20 years
  • there seems to be very little downside to going with the shorter of two crank choices when you can't decide
    • longer cranks can introduce too much knee and hip flexion at the top of the pedal stroke and there doesn't appear to be any major issues with opting shorter
  • you can still generate power with shorter cranks
  • while it may seem like leg length should determine crank length, it doesn't appear that this holds, so an optimal crank length for two people that have the same inseam may in fact be different
  • know that many of the differences between what is optimal and what isn't are often very  fine details - splitting hairs at times - especially when we consider the idea of aerobic efficiency as Jim Martin has said "we determined modest changes in oxygen consumption across the large variations in crank length (145 to 190 mm) that we implemented. So switching from 175 to 172.5 mm would likely have little measureable or even undetectable benefit. The main point is that you would be safe switching to shorter cranks for aerodynamic reasons....”
• things to know: as cranks get shorter, by just adapting the gearing power differences can be mitigated ; oxygen consumption increases as pedal speed gincreases not cadence - longer crank at same cadence = greater pedal speed
• takeaway: these studies haven't looked at long term outcomes - can you train yourself to be even better and more efficient with shorter cranks? (even if it initially feels funny)

 ¹https://www.ncbi.nlm.nih.gov/pubmed/9356762

• The effects of bicycle crank arm length on oxygen consumption.
• 1990, tested 165mm, 170mm, and 175mm cranks for oxygen consumption
• The results of the study suggest that each subject has a most efficient crank arm length, but it does not appear that optimal crank arm length can be predicted by leg length.

https://www.ncbi.nlm.nih.gov/pubmed/21311357

• Effect of crank length on joint-specific power during maximal cycling; 2011
• Previous investigators have suggested that crank length has little effect on overall short-term maximal cycling power once the effects of pedal speed and pedaling rate are accounted for.
• 150, 165, 170, 175, & 190mm cranks
• pedaled at most effeicient cadence for that length and also at 120 rpm
• substantiate previous findings that crank length per se is not an important determinant of maximum cycling power production = you can still generate power with shorter cranks

https://www.ncbi.nlm.nih.gov/pubmed/27484153

• Acute effects of small changes in crank length on gross efficiency and pedalling technique during submaximal cycling.
• tested 170-175mm
• Changes in crank length had no significant effect on heart rate (144 ± 13, 145 ± 12 and 145 ± 13 bpm, respectively) and gross efficiency (GE) (20.4 ± 2.1, 20.1 ± 2.2 and 20.3 ± 2.4%, respectively)
• maximum flexion and range of motion of the hip and knee joints were significantly increased (1.8-3.4° and P < 0.05), whereas the ankle joint was not affected.
• In conclusion, the biomechanical changes due to a longer crank did not alter the metabolic cost of pedalling, although they could have long-term adverse effects. Therefore, in case of doubt between two lengths, the shorter one might be recommended.

https://www.ncbi.nlm.nih.gov/pubmed/26559455

• Effects of Pedal Speed and Crank Length on Pedaling Mechanics during Submaximal Cycling.

 McDaniels and Martin Research

• https://www.ncbi.nlm.nih.gov/pubmed/11417428
  • Determinants of maximal cycling power: crank length, pedaling rate and pedal speed.
  • full text: https://link.springer.com/article/10.1007%2Fs004210100400
  • These data suggest that pedal speed (which constrains muscle shortening velocity) and pedaling rate (which affects muscle excitation state) exert distinct effects that influence muscular power during cycling. Even though maximum cycling power was significantly affected by crank length, use of the standard 170-mm length cranks should not substantially compromise maximum power in most adults.
• "During one particular study, Martin and I recorded athletes’ oxygen consumption while cycling on a stationary bike in the lab. We used crank lengths of 145, 170 and 195 mm; pedaling rates of 40, 60, 80 and 100 rpm’s; and intensity levels of 30, 60 and 90% of lactate threshold.“We found that the power produced (i.e. force applied to move the pedals) during exertion accounted for 95% of oxygen consumption (V02). Changes in crank length and pedal rate had the capacity to alter oxygen consumption or efficiency by about 3%.“Furthermore, this study demonstrated that the body requires more oxygen as pedal speed (speed of the pedal along its axis of travel – NOT cadence) increases. So, for any given pedal rate (cadence), pedal speed will therefore be slower with shorter cranks, resulting in a decreased oxygen requirement.“Let me emphasize, however, that we determined modest changes in oxygen consumption across the large variations in crank length (145 to 190 mm) that we implemented. So switching from 175 to 172.5 mm would likely have little measureable or even undetectable benefit. The main point is that you would be safe switching to shorter cranks for aerodynamic reasons, as proposed by Cobb, without fear of decreasing efficiency. In fact, research implies your efficiency would increase somewhat.”
• Martin has determined that shorter cranks can allow a rider to increase saddle height by 5-7 mm with a comparable lowering of the front end – i.e. lower overall rider position, since the more contained pedaling action was less inhibitive with regard to leg movement and breathing.“Subsequently I conducted a wind tunnel test with one triathlete in which the change to shorter cranks and a correspondingly more compact position yielded a 30% reduction in drag. That means a theoretical reduction in time of 25 minutes over the Ironman bike distance!”

Shorter crank leads to higher cadence but foot speed  and muscle shortening speed are the same