Cycling performance: long or short crank lengths for amateurs?

The past decade has seen a huge growth in the popularity of cycling, and along with it has come big strides in bike technology. Carbon fiber and super lightweight alloys based on titanium, scandium or aluminium are now de rigour in bike frame construction. Meanwhile, brake discs have replaced wheel rim braking (even on fairly budget bikes) tubeless tires are no longer considered exotic while electronic wireless gear shifting, which does away with cables, is becoming ever more common. However, the fact remains that when it come to mechanical propulsion, the principles remain largely unchanged; the cyclist applies force to the pedals, which is transmitted via the cranks to the chainring , driving the chain, which drives a cog on the rear wheel, thus producing forward motion.

Chainring design

In a previous SPB article, we looked at one aspect of the drivetrain that has been tinkered with, which is chainring design. Chainrings have traditionally used circular chain rings to drive the chain and turn the rear wheel. Despite this, some sports physiologists have questioned if the use of non-circular chain rings (containing a very slight ellipse or oval), might be better suited to the biomechanical requirements of pedalling, resulting in more force delivered to the crank for less perceived effort from the rider. The theory of non-circular chainrings is that by using a ring with its peak effective diameter when the cranks are in a horizontal position (3 o’clock and 9 o’clock), this provides a rider with maximum leverage over the crank during the power stroke, enabling the rider to develop more torque (force) or the same amount of torque but more efficiently.

The theory sounds good, but the research on non-circular chainrings is mixed at best. In a study on 12 male elite cyclists riding an incremental test to exhaustion, researchers found that during the short sprints, average power output was only slightly increased with ovalized rings, and the gains were not large enough to be considered statistically significant – ie they could have occurred simply as the result of a ‘statistical blip’⁽¹⁾. There were also no significant benefits in terms of blood lactate (a measured of muscle fatigue during exercise), power output, oxygen consumption or heart rate.

In another study, comparing circular and ovalized chainring use in well-trained cyclists, there did appear to be some benefits in terms of a slightly reduced level of blood lactate during high-torque efforts⁽²⁾. This correlated to reduced muscle activity at the point where peak crank torque occurred during each crank rotation – supporting anecdotal observations of some cyclists that pedalling using non-circular chainrings feels easier for a given pace compared to conventional circular chainrings. A third study comparing elliptical and circular chainrings during all-out sprinting found somewhat more positive results⁽³⁾; the maximal power output during each pedal stroke was significantly higher (+4.3%) when using the non-circular chainring. The researchers concluded that this improvement was likely explained by the mechanical advantage given by an elliptical ring, suggesting potential benefits on sprint cycling performance.

Leveraging the power of cranks

Overall, it’s fair to say that unless you’re a sprint cyclist, the jury is still out on the benefits or otherwise of non-circular chainrings – although for hill climbing and/or maximal effort riding, there’s slightly more evidence that they might help. However, changing the profile of the chainring is not the only way of manipulating torque output in the drivetrain. Because of the physics of levers and turning force (see box 1), a simple change to the crank length can affect torque output too.

Box 1: Levers, force and torque 

Torque is a measure of the turning force that causes an object to rotate about an axis, fulcrum, or pivot. It is defined as the product of the magnitude of the force applied and the perpendicular distance from the point of application of the force to the axis of rotation, known as the lever arm. Mathematically, torque is expressed as:

Torque = F × r ×sin(θ)

(where F is the applied force, r is the length of the lever arm, and θ is the angle between the force vector and the lever arm vector).

When the force is applied perpendicularly to the lever arm – for example pushing the pedal vertically down with the cranks in the 3 and 9 o’clock position, sin(θ)=1, simplifying the equation to Torque = F × r. When understanding crank length and torque, r is the crank length in this equation, so you can see that using a longer crank increases torque while using a shorter crank decreases torque.

Standard vs. non-standard crank lengths

In years gone by, the standard crank length was 170mm, and that was pretty much the only choice offered to cyclists. However, technology and consumer choice has moved on and many manufacturers now offer a choice in crank sizes on their chainsets. For example, the latest version of SRAM’s ‘Red’ chainset now offers cyclists crank lengths of 165mm, 167.5mm, 170mm, 172.5mm, 175mm, 177.5mm! Previous research has shown mixed results on crank length. Shorter cranks (around 145–165 mm) seem to be favoured for cycling efficiency (economy) and for helping to ease knee loading⁽⁴⁾. However, during harder efforts such as sprints or steep climbs where getting out of the saddle is required, the higher torque output generated by longer cranks (170mm and over) yields a significant performance advantage⁽⁵⁾. This is why pro cyclists tend to favor longer cranks, as sprinting and climbing performance often play a much bigger role in determining the outcome of races!

New research

If you’re an amateur who needs to perform well on longer, flatter rides (where an efficient and comfortable cycling action is essential), but you also compete in hilly events or race over shorter distances, what crank length is likely to give you the best all-round performance? Is there a sweet spot – ie good power for sprints and hills but comfortable and efficient to avoid knee injuries or fatigue in the longer term? This is a question that a team of Chinese researchers have attempted to answer in a very recent study published in the Journal of Exercise Science and Fitness.

In this study, the researchers investigated the influence of three different crank arm lengths - 165 mm (shorter than standard), 170 mm (standard), and 175 mm(longer than standard) on sprint power, perceived fatigue and cycling economy, Cycling economy is a measure of the amount of oxygen consumed per unit of work performed/distance covered at a steady sub-maximal intensity. Economy is an important determinant of endurance performance; better economy means less oxygen consumed per mile, which means less accumulated fatigue over longer distances!

The testing was conducted using the Garmin Neo Bike Plus bicycle ergometer (see figure 1), which is specifically designed to replicate the riding posture of road cycling. A feature of this ergometer is the ability to affix pedals at different points on the crank to give different effective cranks lengths. This design allowed participants to maintain a standard cycling position throughout the trials, regardless of crank length. In conjunction with this, the GARMIN Edge 1040 bicycle computer was used to record key performance metrics, including cadence, power output, and heart rate. Together, these devices ensured accurate measurement and consistent cycling posture, closely replicating outdoor cycling conditions.

Figure 1: Crank arm of the Garmin Neo Bike Plus ergometer