Crank length, does it even matter?
For as long as I can remember, most road bikes have had cranks between 170 and 175mm long.
But in 2024, we are starting to see a change in the bike industry. Some brands are reserving the once-standard 172.5mm cranks exclusively for their extra-large frames.
Polygon is one brand that has reduced their crank lengths, equipping their large, medium and small frames, each with 170mm cranks, and their extra-small frames with 165mm cranks. Polygon’s chosen crank lengths are, on average, about two sizes smaller than the size bikes came with when I began fitting.
The slide toward shorter cranks is one of the prominent trends that we can expect to sweep through the bike industry over the next few years.
It is a trend that is not entirely new. It wasn’t too long ago that riders like David Miller and Miguel Indurain were using 180mm and 190mm cranks in time trials.
Despite a significant height difference, Lance Armstrong and Robbie McEwen both rode 175mm cranks. Marco Pantani is said to have ridden 180mm cranks when racing certain mountainous stages at grand tours.
Looking at the current generation of professionals, the last couple of riders to break the one-hour world record, Filippo Ganna and Dan Bigham, rode 170mm cranks. Both riders are taller than six foot.
Two-time Tour de France winner Tadej Pogacar rode 170mm cranks at last year’s Tour de France, while last year’s Strade Bianche winner Tom Pidcock has been trialling 165mm cranks ahead of his 2024 season.
Nevertheless, most of the pro peloton still run conventional crank lengths, including Jonas Vingegard, who has won the last two Tours de France on 172.5mm cranks.
But the trend towards shorter cranks is clear. Most crank manufacturers no longer produce 180mm cranks (Shimano being the exception), and cranks specialist Rotor Bike produce cranks ranging from 150mm to 175mm.
What is interesting here is that Rotor is a company with pedalling efficiency at its heart.
The company began in the 1990s with the vision of producing cranksets and chainrings that improved pedalling efficiency. This might partly explain why the company’s range of crank lengths is drastically different from other manufacturers.
Below is a table showing the current crank lengths available from the major manufacturers.
Crank length options in 2024
So, should we all follow the lead of Ganna, Pidcock and Co. and swap our cranks out for shorter versions?
From a fitter’s perspective, the primary considerations for crank length are,
· Power
· Aerodynamics
· Comfort
· Injury prevention or management
Let us take a closer look at each of these.
Power
Aswith most areas of bike fit literature, there have only been a small number of high-quality research studies into crank length and performance, but from the literature available, there are a couple of takeaways for us to consider.Paul Barrett from Brunel University studied the effects of different crank lengths on joint-specific power at maximal effort.
Barrett found no significant difference in power generation when riders rode a range of crank lengths between 150mm and 190mm.Longer crank lengths increased power generation through the gluteal muscles, but this was offset by a reduction in power by the quadriceps muscles. Likewise, short cranks led to less power from the glutes but increased quadriceps power. So, we know that some physiological changes occur with different crank lengths. Still, the overall power difference does not change much.
Another study by Martin and Spirduso examined how crank length affects power using cranks from 120mm to 220mm. They found that the 140mm and 170mm crank lengths were significantly more efficient than 120, 190 and 220mm cranks. They also highlighted the long-held assertion that shorter cranks require a slightly higher cadence than longer options when refining power output.
The big takeaway from Martin and Spirduso’s study is that, yes, cranks can be too long. Yes, you can go too short, but when we compare their findings to the average-size cranks that are coming on bikes, we can see that traditional crank lengths sit at the higher end of the efficient range. Alas, there is little scope for most riders to run longer cranks and a lot of scope for riders to run shorter cranks whilst maintaining efficient power generation.
It is unlikely to make much difference to power if most riders run a 175mm, 170mm or 165mm crank. In fact, based on my experience in my clinic, many riders run different lengths of cranks on their bikes without ever noticing a difference.
A rider could have 175mm cranks on their MTB, 172.5mm on their road bike and 170mm on the track bike. The rider adapts to the slight difference, which is usually insignificant compared to other differences between bike setups and riding positions.
Based on my clinical experience, shorter cyclists are most likely to have problems associated with cranks being too long. A rider using an extra-small road bike can have a saddle height that is 20–30cm lower than some of my tallest clients. I think these shorter riders need cranks that are more than 5mm shorter than the industry average.
If short riders have cranks that are too long, they struggle to produce any power over the top of each pedal stroke and often have a very pronounced dead spot where no power is generated.
It is normal for all riders to have a low-power section of the pedal-stroke when the hip and knee are in flexion, but riders with particularly short legs can develop a dead spot where they genuinely struggle to get their foot over the top of the pedal stroke.
The pronounced dead spot can lead to a couple of issues.
Firstly, it causes lumbopelvic instability throughout the pedal stroke, where the pelvis is more likely to rock from side to side, contributing to chafing, saddle sores, and unnecessary activation of lumbar spine stabilisers, leading to back pain.
These riders can also struggle to ride with anterior pelvic tilt, which amplifies hip flexion and pelvic rock.
From a neuromuscular coordination perspective, long cranks force joints to work through a broader range of motion, making it harder for riders to ride smoothly at high cadences.
You are likelier to bounce around in the saddle if your cranks are too long.
This is one reason that short cranks have been popular for track cyclists who can smoothly pedal at very high cadences.
Aerodynamics
For racers, reducing the crank length may provide some marginal gains from an aerodynamics perspective, and this is one of the reasons we see riders opt for shorter cranks for time trials and world hour record attempts.
Theoretically, running a shorter crank allows the rider to increase their saddle height, effectively lowering the rider’s torso and front end, which may help aerodynamics. It is important to note that as with all things’ marginal gains, validation of proposed aerodynamic advantages must be used with aero lab or validated field testing.
Comfort
A lot of my clients are not overly concerned about marginal aerodynamic gains. Comfort is a big concern for nearly everyone, including many professional athletes.
Many larger riders have issues where their thighs hit their belly at the top of the pedal stroke, which can be pretty uncomfortable. Saddle position can improve this in professional riders, but for riders with larger tummies, there can be no avoiding some level of thigh-to-belly contact and discomfort. However, reducing the crank length and raising the saddle and bar heights can minimise the degree of hip flexion at the top of the stroke and give these riders some more clearance.
For elite riders who adopt particularly low torso angles to improve their aerodynamics, shorter cranks can also be beneficial to reduce excessive hip flexion.
Injuries
Pain or injury is the most common reason I will prescribe shorter cranks for riders in the clinic.
Anterior hip impingement conditions can be exacerbated by positions and cranks that promote high degrees of hip flexion.
Riders suffering from these conditions get the top of the pedal stroke and have bone-on-bone-related discomfort where the neck of the femur contacts the acetabular rim. In these instances, we do all we can to reduce the degree of hip flexion. Using shorter cranks is a simple method to achieve this that does not lead to significant compromises throughout the rest of the pedal stroke.
Degenerative hip and knee conditions such as osteoarthritis or chondromalacia are other instances where shorter cranks can allow riders to ride with less pain and irritation. As a rule, older riders will benefit from cranks that reduce the range of motion that knee and hip joints move through, and we know that running shorter cranks is unlikely to significantly reduce performance.
In complex cases, we sometimes consider crank arms that vary in length from left to right, but I will not discuss those complexities here.
Take-Aways
As with all things bike fit-related, our clinical decision-making is based on a complex range of considerations that will vary depending on the athlete.
For most athletes, changing the crank length by 2.5 or 5mm is unlikely to make a significant difference to anything.
Some marginal gains are available for professional athletes seeking seconds or fractions of seconds in time trials.
Older riders and those with persistent injuries can benefit from shorter cranks if the end crank length stays within the range that major manufacturers currently produce.
The days of world hour records completed on 180 and 190mm cranks are certainly behind us.
Expect to see Shimano, SRAM, and Campagnolo follow Rotor’s lead and expand their range of crank length over the next few years.
Do not lose sleep if your next medium road bike has 167.5mm crank arms instead of 172.5 mm. You likely won’t even notice the difference after a week of riding.