Crank Arms
- Posted on
- By John McHenry
Crank arm length has been a big deal this year. Why now? Not sure, or my customers are just getting wiser. The impact of crank arm length on bio-mechanical function, power output, and performance has been the focus of quite a bit of attention and research over the past few years, both informally and anecdotally in the fit studio, and scientifically in the research lab. EVERYONE should evaluate the appropriate crank arm length in their assessment of the rider and their bike, and either confirm the suitability of a selected crank length or make a recommendation for a different length. Such a recommendation may be a “soft recommendation”, where the current crank may be satisfactory, but a different length may be more optimal, or a “hard recommendation” whereby a change of crank length is assessed as essential to overcoming a particular complaint or functional limitation. Increasingly, cyclists are more conscious that crank length can impact function and performance and are asking their fitter if their crank length is appropriate.
Why is Crank Arm Length Important?
Most people use cycling shoes with cleats that attach to a pedal. With the pelvis jammed on a saddle at one end of their legs, and their feet mechanically attached to the pedals at the other end of the legs, the bio-mechanical gait action of cycling is then largely defined by the (slightly elliptical) circle that the legs move through to deliver force to the pedals. The length of the crank arms determines the diameter of that circle. For the hip and leg muscles to contract through their optimal range to generate force, the diameter needs to be appropriate. Not too big, not too small. If the circle is too big, the knee and hip joints are forced through a larger flexion range than they may like whilst attempting to generate force. This can aggravate muscles, tendons, and ligaments as well as compress arterial blood supply and impinge on nerves. In addition, the pedal stroke can feel lumpy rather than fluid, and there may be a delay in force applied during the power phase of the stroke. Common symptoms of a crank that is too long are pain in the hip joint or nearby muscles (often on one side only), knee pain, and riding uphill seems unreasonably hard. A crank arm too short is far less common and has less downside. The sensation can be that the pedal stroke is over too quickly, and the cyclist was only just starting to get a good push. This will be more obvious to cyclists who prefer a low cadence.
What is Worse – a Crank Too Short or a Crank Too Long?
A crank arm too long is worse than one too short. A crank too short may result in a loss of performance (although research argues this is rarely the case) but is highly unlikely to cause any harm. A crank too long is a common cause of low-grade injury and futile trips to a physical therapist for relief. Futile because it seems to be a body issue, not a bike issue, but the PT makes little progress because every ride sets back any progress made from the therapy.
Why Does a Bike Come with a Particular Crank Length, and What are the Options?
Unless you are on a Strider, every bike comes with a crankset, featuring a particular crank arm length. Why this is what it is can be attributed to a mix of tradition (i.e. that’s the way it’s always been), manufacturing cost control, economy of scale, and a bizarre dance between the demand from bike brand product managers who specify the parts on a bike, and the supply options from component manufacturers. Bike fitters and cyclists alike don’t get a say in the matter.
Focusing our attention on adult road, gravel, mountain, and tri bikes, there are 4 common sizes available from the dominant suppliers (Shimano and SRAM):
- 165 mm
- 170 mm
- 172.5 mm
- 175 mm
That’s a tiny 10mm difference between shortest and longest, to suit riders whose legs may vary in length by 200mm! Do you see the potential for inequity here? My inseam is 76cm and on my road bike I installed 170mm cranks. I see cyclists in for a fit who are on the same crank length with legs 10cm shorter than mine. Relatively to me, they are turning a much larger circle, subjecting their joints and attending muscles and ligaments to a larger range of motion and forces. Why aren’t there more options? Product managers aren’t demanding them from the component suppliers (they are not bike fitters and aren’t thinking deeply about fit), and component manufacturers are not offering them due to perceived low demand and high production costs (i.e. poor ROI). Product managers try and do their best with what they can acquire and will generally scale the crankset to the bike size: smaller bikes will have shorter cranks and larger bikes will have longer cranks. Although this is conceptually a good idea, the biomechanical reality is there is frequently a misalignment between what is supplied on the bike and what would be optimal for the rider. This is especially pronounced at the smaller end of the bike size range. That’s because the crank arm size range available does not cater to the range of body types that actually want to ride a bike comfortably and effectively. The crank arms are not scaled down proportionally to match the frame size, or the cyclist on that frame size.
What is a Good Size Range for Cranks?
I’d suggest at least 155mm to 175mm in 5mm increments, and even better would be 145 mm to 185 mm. The most common default size in the market is 172.5 mm for no apparent reason, and although I’m sure it has its fans it’s sincerely a useless non-size in that a 170 or 175 would likely provide a better alternative for the cyclist. Shimano recently added 160 mm cranks for some groupsets, which have been a great problem solver, and an easy changeover where the current crankset is also Shimano. Also, several other manufacturers offer a wider range of lengths including Rotor (many choices), Speed and Comfort (JCOBB cranks down to 145mm), and Zinn (for those looking for longer than standard). Options for various crank lengths are also available from Croder Cycling and Appleman Bicycles. For mountain bikes and single chainring gravel bikes, Canfield Bikes and 5DEV offer a great selection of shorter crank options.