Optimal Crank Length Analysis
Modern bike fitting trends favor shorter cranks (165-170mm) over traditional 172.5mm for better joint angles, but if you're comfortable now, no need to change.
Prompt
I'm a 5'10" (178 cm) cyclist primarily doing indoor road racing. I'm currently using a 172.5 mm crank length and experience no knee, joint, or muscular discomfort. However, I'm curious about the current expert thinking on optimal crank length specifically for improved biomechanics, efficiency, and overall comfort.
Response
Optimal Crank Length for a 5’10” Road Cyclist (Indoor Racing)
Joint Angles and Biomechanics
Crank length directly affects your pedaling biomechanics, especially the knee and hip angles at the top of the pedal stroke. Using shorter cranks means you don’t have to lift your knees as high each revolution, which increases the angle between your thigh (femur) and shin (tibia) at peak flexion. In practical terms, a larger knee angle at the top of the stroke reduces the peak load on your knee joint (less strain)1.
Similarly, a shorter crank opens up your hip angle – your thigh doesn’t come up as close to your torso – which can alleviate hip impingement and improve comfort. This more open hip position is beneficial both for injury prevention and for allowing a lower riding posture without compression at the hips2.
Studies confirm these effects on joint range of motion. In one experiment with elite cyclists, reducing crank length from 180 mm to 150 mm cut down the total movement at the hip and knee joints significantly. The riders’ hips traveled through about 45° of motion with 150 mm cranks versus ~51° with 180 mm cranks, and their knees bent through ~67° versus 75° respectively3. In other words, shorter cranks mean the legs flex less at the top of the stroke. By reducing how tightly your knees and hips must bend, shorter crank arms can improve your biomechanics – minimizing knee shear forces and hip compression – even if you currently feel no pain. Many bike fitters consider crank length the second most important fit parameter (after saddle height) for exactly this reason, since it influences joint angles throughout the pedal cycle4.
Power Output and Cadence
A common concern is that shorter cranks might reduce leverage and, with it, power output. However, current research and expert consensus indicate that within the normal crank length range, power is essentially unchanged. A foundational study by Dr. Jim Martin tested cranks from a very short 120 mm up to 220 mm and found only a 4% difference in peak power between the extreme lengths. Within the typical range of 145–170 mm, the difference in max power was just ~1.6%5.
In fact, even in longer efforts (30-second all-out tests), riders showed no significant differences in power or fatigue rates across crank lengths ranging 120–220 mm6. The clear conclusion was that “cyclists can ride the crank length they prefer, without limiting power”7. In practical terms, a 5 mm change (e.g. 172.5 to 167.5 mm) is well under a 1% difference in leverage – virtually negligible for power generation8.
What does change with crank length is your optimal cadence and pedal stroke feel. Shorter cranks produce less torque for a given muscle force, so to maintain the same power you’ll naturally pedal at a slightly higher cadence9. Think of power as a product of force and pedaling speed – if the “lever” is shorter, you make up for it by turning circles a bit faster. Many cyclists find they can spin more smoothly with shorter cranks, and indeed Martin’s research noted that the optimal pedaling rate increased as crank length decreased (e.g. ~130 rpm optimal with 150 mm vs ~110–120 rpm with 200 mm+ cranks)10.
For most, this higher cadence requirement is a non-issue or even a benefit (especially for indoor racing, where quick leg speed can be advantageous in sprints and accelerations). Gross efficiency might also tick up slightly – one 2018 study found riders were about 0.5% more efficient with 150 mm cranks than 180 mm11, likely because the legs are moving through a smaller arc (less internal work). In short, shorter cranks tend to favor a “spinny” riding style.
Fit and Positioning Considerations
Beyond raw numbers, crank length plays a big role in bike fit, comfort, and riding posture. A shorter crank effectively lets you ride in a lower or more forward position without causing as much joint compression. This is why time-trialists and triathletes have widely adopted shorter cranks (sometimes 160–165 mm even for average-height riders) – it opens the hip angle at the top of the stroke, so they can drop their torso lower for aerodynamics without kneeing themselves in the chest.
For example, during preparations for the 2016 Olympics, British Cycling switched all their track endurance riders to 165 mm cranks to allow more aggressive aero positions. Sir Bradley Wiggins (who is ~190 cm tall) went from 177.5 mm to 165 mm cranks and was able to lower his front end by 30 mm, gaining a ~3.5% reduction in aerodynamic drag (CdA)12.
Even on the road, several pros around your height have recently downsized their cranks:
- Tadej Pogačar (177 cm) switched to 165 mm cranks for the 2024 season13
- Wout van Aert (190 cm) and others have also been spotted on shorter cranks for road racing14
The takeaway is that an open hip angle not only helps comfort, but can expand your fit “envelope” – giving you more room to get low or rotate your hips forward if desired, which is beneficial for aero or for a powerful upright sprint. (Aerodynamics aren’t a factor in indoor racing on a trainer, but the comfort and power benefits of an open hip still apply, especially if you simulate a race posture on the bike.)
Shorter cranks can also improve comfort and stability on the saddle during hard efforts. Riders on cranks that are slightly too long often exhibit telltale signs: their knees might flare outward at the top of the stroke, their hips rock, or they struggle to maintain a smooth pedaling circle. These are indications that the hip is hitting a biomechanical limit (hip flexion constraint) each revolution15.
By shortening the crank even by 5 mm, many cyclists suddenly find their pedal stroke feels smoother and more “free” because their body isn’t forced into extreme angles16. Phil Cavell (CycleFit co-founder) notes that when a rider has a hip-flexion limitation, dropping from (say) 172.5 mm to 165 mm can make their stroke fluid and eliminate the compensations their body was making17.
Potential Trade-offs and Adaptation
If your current setup feels good and you’re performing well, you’re correct to ask whether changing crank length is necessary. The truth is, there’s no one “perfect” crank length for a given height – it often comes down to personal preference once you’re in the acceptable range18.
A very flexible rider with no history of knee or hip issues might do just fine on a traditionally “long” crank and enjoy the slightly different feel of the pedal stroke. For example, a taller cyclist who naturally pedals at low cadence and has excellent hip mobility may prefer staying with 175 mm cranks, as for them that feels most powerful19.
There’s no need to change what isn’t broken. As frame builder Brent Appleman quips in his crank-length guide, “If you’re pain free and riding your best life…by all means, skip a crank length change. Save money and keep on keepin’ on.”20
Typical Crank Length Guidelines by Height
For reference, here are the traditional crank length ranges commonly seen for road cyclists by rider height (assuming average leg proportions). These are not strict rules, but they illustrate how bike manufacturers and fitters often size cranks21:
| Rider Height | Common Crank Length(s) |
|---|---|
| 5’5″ (165 cm) | ~160 mm cranks |
| 5’7″ (170 cm) | ~165 mm cranks |
| 5’9″ (175 cm) | ~170 mm cranks |
| 5’10″ (178 cm) | ~170–172.5 mm cranks |
| 5’11″ (180 cm) | ~172.5 mm cranks |
| 6’1″ (185 cm) | ~175 mm cranks |
| 6’2″+ (188+ cm) | ~177.5 mm+ cranks |
Table: Common crank length sizing by rider height.
In practice, riders around 5’10” (178 cm) often come spec’d with 172.5 mm cranks as an “average” size22. Shorter riders (<5’7”) frequently benefit from 165 mm or below, and very tall riders (>6’2”) may use 175–180 mm. Modern trends are shifting these numbers slightly downward, as experts realize many riders (even those of average height) do well with shorter cranks for the reasons discussed. Today, 165–170 mm cranks are no longer just for “small” riders – they’re becoming popular for average-sized cyclists to optimize fit. The industry’s current offerings reflect this: for example, Shimano now offers high-end road cranksets in lengths from 160 mm up to 177.5 mm23, whereas in the past 170–175 mm dominated. In the end, use these numbers as a starting point, but let your comfort and pedal stroke quality be the deciding factors.