Bikes and Tech
Photo: Dan Cavallari | VeloNews

Technical FAQ: Finding the right crank length, more on cleat positioning

Lennard takes a deep dive on how crank length should relate to leg length, and how that affects overall bike geometry.

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Dear Lennard,
I’m just 6’ 2” and a zippy descender, but a recent holiday in Lanzarote and some poor performance relative to a descending partner left me wondering about my bike geometry and set up. I’ve been digging around for info and have come to some of the same conclusions that you have elucidated here in your article. I’ve subsequently gone back to a longer stem and steeper saddle position.

I’m considering a made-to-measure frame. I’m thinking of a 72.5-degree head tube after some speed wobble on my 73.5-degree, 59cm Raleigh Militis. After reading the Q that you provided the A to, should I aim for a saddle-axle setback similar to smaller bikes, so chainstays upwards of 420mm perhaps? I also suffer from seated wheelies on steep climbs.

About cranks, I’ve felt good with 165mm on my TT bike, since thigh does not come as close to my stomach and I can get shoulders a little lower. How would you balance the conflicting aero position vs. longer cranks argument on a road bike?

Can you recommend any other literature on the subject of tall riders, high-speed handling (descending, which I most enjoy) and geometry?
— Luke

Dear Luke,
Yes, longer chainstays better balance having the saddle way up high for a tall rider; 420-425mm is a good length for someone your height.

That said, a critical frame feature in preventing a tall bike from pulling wheelies when climbing is to build it for proportional-length cranks. The reason most tall bikes have such slack seat-tube angles — which play a HUGE factor in the bikes wanting to pull wheelies when climbing — is that they are designed so that the rider can position their knee over the pedal spindle with a standard-length (i.e., 170-175mm) crank. With a tall person, that crank is such a small proportion of their leg length that they need the saddle to be way back to accommodate it.

However, if the crank is instead in a similar proportion to the crank proportion used by a mid-size rider (around 21 percent of inner leg length), the seat-tube angle can be the same as (74-degree or so) on, say, 54cm frames of that model. This results in the saddle not moving back as far over the rear wheel when raised to proper height as it would be with the slack (71-degree or so) seat-tube angle found on the tallest sizes of many stock bikes.

When it comes to crank length on a time-trial bike, yes, shorter is often an advantage. Minimizing aerodynamic drag is more important in an individual time trial than gaining the small increase in power to be had by using a longer lever (crank) with long legs. The shorter the crank, the easier it is for the rider to pedal with a horizontal upper body, because the knees don’t hit the chest, the hip angles are not so tight, and the hamstrings do not yank on the low back as much.

On a road bike, however, the riding position should deliver all-day comfort and handling control more than optimal aerodynamics, and riding in a group should also be a design feature. So on a road bike, especially given that you love descending and hence will be doing a lot of climbing, optimizing crank length to your leg length will increase your climbing speed without adversely affecting you on the flats or when descending.

Accepting that there is a relationship between leg length and crank length begs the question of what constant of proportionality to use between leg length and crank length. While what number you pick as the multiplier of your leg length to determine your crank length is subject of debate, it is between 0.194 and 0.216 for top pros.

If you start using a constant anywhere in this 0.194 to 0.216 range, you will soon find that a high percentage of (maybe even most) riders will be outside of the standard 170-175mm crank-length range. However, don’t expect to see cranks built proportionally to rider leg length and frames to match them on anything other than a custom basis or specialty low production anytime soon. That’s because following crank-proportionality guidelines upsets the traditional bike-industry applecart, raising manufacturing and inventory costs for manufacturers and retailers to make and distribute cranks in sizes for everyone. It also complicates the jobs of bicycle-company product managers to determine what bottom-bracket heights (to optimize pedal cornering clearance) and seat angles (see above) to select for each size when designing the frames. It would also require retailers to think differently about crank length, something they are not likely to do.

Crank proportionality also demands that component companies think outside of the existing gear ratios, since short cranks naturally result in higher pedaling cadences than do long cranks, and vice versa. Try pedaling with your normal riding companions on 100mm cranks and see if it doesn’t result in you spinning comfortably at very high cadences — over 150 RPM. Do the same with 220mm cranks and see if it doesn’t result in you comfortably turning very low cadences — below 70 RPM. I know that this is exactly what happens because I’ve done both. Because Power = Torque X RPM, if torque is low (short crank), then RPM is high, and vice versa. I have managed to keep up fine on group rides (many years ago) while riding all sorts of length cranks ranging from 100mm to 220mm. It is amazing how much power you can put out on a little 100mm crank—but it requires low enough gearing. Similarly, riding a 220mm crank in a fast group requires higher gearing, because it is inefficient to spin that crank at 90-100 RPM.

As for your reading-material question, this book has a lot in it on the subject of tall riders, high-speed handling (descending), and frame geometry. It is out of print, but you can find used copies online.
― Lennard

Dear Lennard,
In reading, I find interesting the bit by:

…But in the practice, I do support the notion that recreational cyclists do need to move their cleats back slightly farther than originally thought: possibly 4mm behind the head of the first metatarsal. It works for forefoot problems and it takes tension off the Achilles tendon and posterior muscles of the leg. I have the luxury here in the office to take an x-ray of my foot in my cycling shoes so I can see exactly where all the bones fall over the center marker on the cleats; it makes micro-adjusting the cleats almost perfect.
— Alan G. Shier, D.P.M.
Foot Care & Surgery Center

I am a recreational cyclist riding 4,000km/yr. from May-Oct. (because Quebec/Canada has road weather blackouts periods!). I also ride indoors during the off-season for another 1,000km or so. Last year, an early season hilly fondo in June left me with an aching underfoot, with a sort of contusion (bruise) under my foot right at the cleat position. It was there for the remainder of the summer and off-season.

My pedal position is as stated above and never caused pain issues before, why all of a sudden was I walking with a limp, due to pain on and also off the bike!? My first knee-jerk reaction was to question my cleat position. Instead, and before any modification, I consulted a physiotherapist and found some issues with my posterior leg muscles, mostly at the hip level (ischium) and calf. The main effort was put into gaining flexibility with stretches, which has resolved 95 percent of my foot pain located at the pedal position. The remaining 5 percent is more about rehabbing the stresses of training which started in February. I feel reassured that I’m not starting the season with an injury.

Muscle imbalance and lack of cross-training may be an issue for recreational riders like myself. A health care professional is a good place to start to fix these problems, physiotherapists in particular.
— Germain
P.S. I’m not a physio or health care professional!