Why Crank Length Matters

Cyclists buying a new bike, look for the correct frame size but just as critical is the optimum crank length. This article explores why.

Crank length is the length from the centre of the bottom bracket to the centre of the pedal axle. Most bikes are fitted with a 170-175mm crank. The size range available can stretch from 140-190mm with a little searching amongst different manufacturers.

Last year I did a cycle fit for a triathlete visiting us for a training week. Their coach wanted them to optimise their position, and improve their potential run off the bike.

I asked if the crank length, had been deemed optimal by their coach and previous fitter. “I think so, it’s what has always been on the bike.” was the reply.

Watching as they pedalled, I saw their hips move to allow the knee space to clear the ribs and I watched as their hips dropped to facilitate the bottom of the pedal stroke.

The solution was not just a bike fit but a change to a shorter crank.

Speed = Comfort + Power + Aerodynamics

Comfort in cycling terms can be defined as the ability of a rider to sustain their optimum position for the duration of a ride.

Power is the ability to apply force to the pedals without restriction within a range of accepted biomechanical norms.

Aerodynamics are achieved by optimising frontal surface area and shape to minimize resistive forces of air.

Crank length is all too often overlooked. Over time the crank length on a given size frame has become accepted as a norm. There is no particular basis for the current default standards applied by cycle manufacturers.

Size ranges that are readily available and supplied to bike companies are based on what is popular. It would be better if they are based on the specific science and research.

One major bike manufacturer works on a basis of no options available in crank length, on a given size. If you buy a frame of a given size say a medium you get a crank of xxx mm. If you want a different crank length, you need a different size frame. That now means that the geometry may be wrong for you. Alternatively you can purchase a new crank set of the correct length for approximately €300.

We were impressed when Kate bought a new BTB bike last year from a manufacturer that did offer a choice of two crank sizes for her size of frame.

The Difference Crank Length Makes

The crank arm creates the pedalling radius from the bottom bracket. A shorter pedal crank will make a smaller circle. The implication is that the foot won’t move so far up and down. Less range of motion for the leg to travel through, this affects all the key joints in the lower body.

There are optimum ranges of motion for these joints depending on the bike you are on. Opening the hip angle can lead to improved power and aerodynamics. Some research indicates that for triathletes, this may lead to a better run off the bike.

Athletes have three touchpoints on the bike: saddle, handlebars, and pedals. Many athletes can recognise the difference that saddle and handlebars make on their position. They often think about pedals as simply about their brand or type of choice. Many don’t think about the part that connects the pedals to the bike: the crank arm.

What Crank Length Is Right For Me?

As with all talk about bike fit, these are individual changes. There is not a one size fits all solution. Personal mobility, flexibility, biomechanics, and experience all impact any decision about crank length.

Research indicates that 20% of leg length or 41% of tibia length is a starting point. This is more specific than a manufacturers’ default, of length by particular frame size.

Additionally consider the crank length, compared to the saddle height, to the bottom bracket. This ratio is likely to be specific to the rider, their proportions, and their mobility.

To date, there is no ‘standard’ answer. For triathletes, consider how efficiently and effectively you can run off the bike.

Check what cranks your bike came with. Consider whether you have enough space at the top of the stroke. Can you move cleanly through, without excess load or changes of angle to the hip? If not, possibly consider that switching to a shorter crank length may help.

If you’re about to buy a new bike, discuss with your local bike shop as to your options. Take time to consider what different manufacturers offer.

Possible Problems With Cranks That Are Too Long

• Seat position must be lower in order to not overextend the leg at the bottom of the pedal stroke.
• If the seat position is not lower, excessive hip rocking can occur with instability at the saddle contact point.
• Inability or difficulty in spinning a high cadence.
• The knee has to go higher at the top of the pedal stroke just to get over the pedal.
• The angle between the knee and the chest is reduced with the knee coming closer to the chest. This may be impacted by individual athlete mobility issues and physical dimensions.
• Difficulty breathing.
• To assist with increased leg motion, the ankle may compensate (or increase range of motion) to facilitate movement.
• Compensation may occur at the hip to enable the knee to clear the chest.
• Some riders may experience a limited difference other than overload on associated soft tissue. Triathletes may benefit by reducing or minimising any excessive loading to avoid any issues caused on the bike from showing up on the run.

Using A Shorter Crank Can Offer Advantages

• Comfort: a shorter crank length reduces range of motion at the knee (extension and flexion), hips, and low back.
• Power: shorter cranks alone will not increase power output but they can reduce restrictions through the top of the pedal stroke. This is by opening up an impinged hip angle and/or reducing knee flexion.
• Aerodynamics: a shorter crank, subject to individual mobility, may allow you to ride at a lower angle. In turn this may minimise frontal surface area improving aerodynamics.
• May reduce pain or excess discomfort in hips, knees, or lower back. This is due to decreased loading of specific joints, especially the hip flexors.
• Mountain bikers may benefit from a decreased time to reach peak power and increased ground clearance.
• Research on the topic of crank length showed negligible difference in power produced. However with shorter cranks there was significantly less oxygen requirement.

To summarise there is no one size fits all. There is not a specific length for an individual or an ideal for certain disciplines. If there is nothing wrong with your current crank size, then don’t change them.

As a rider be curious and consider a review of your bike fit. Exploring the option of crank size is part of a good bike fit.

Potentially the biggest downside is the cost of replacing your cranks if you decide you need to go smaller. The benefits of an improved ride position, experience, and results may happily offset this.

If you do change, remember to make the necessary adjustments to saddle height and fore and aft position.

Give yourself time to adapt to a new position before your next race, event, or big cycling adventure.

Enjoy your cycling comfort and efficiency … and have fun!