One of the most important factors affecting performance, comfort, and injury prevention in cycling is bike fit.
An often-overlooked element of bike fit, however, is crank arm length, which plays a huge part in the saddle height and knee angle, making a massive difference to your riding experience.
As a professional bike fitter, I’ve regularly had clients who pay a huge amount of attention to getting their handlebar and riding position dialed in – but when it comes to the crank length, they shrug and are happy just to stick with their new bike’s factory-fitted cranks or blindly follower a manufacturer’s chart. This is a mistake.
Finding the correct crank arm length for you can be very challenging, and there’s a lot that needs to be considered, such as height, inseam, injury history, and more.
I have bike-fitted many cyclists who have found that changing their crank length (usually going shorter) has made a huge difference in the management of chronic cycling injuries.
In my own experience as an ultra-endurance racer, I find the right crank length can make a massive difference to my ability to maintain consistent performance and power output too, particularly across longer riding stints.
How Is Crank Arm Length Measured?
The crank arm length is measured from the center of the crankset at the bottom bracket axle to the center of the pedal spindle (where the pedal attaches to the crank arm).
The crank arm length governs the circumference of the circle which the pedal rotation will take.
A larger crank offers a much larger circumference, while a smaller crank will offer a smaller circumference. This circumference can make huge differences to how a bike feels to ride and greatly affects movement in the knees and the hips.
A good way of understanding crank length is by thinking about leverage. The longer the crank, the more leverage you have when you pedal, but the larger the pedal rotation. The smaller the crank arm, the less leverage you have, but the smaller the pedal rotation.
Standard Crank Arm Sizes
| Taller Riders | 175 mm |
| 172.5 mm | |
| 170 mm | |
| 167.5 mm | |
| 165 mm | |
| 162.5 mm | |
| Shorter Riders | 160 mm |
Crank arms come in many different sizes. As a general rule, taller riders will use longer crank arms, and shorter riders will use shorter crank arms (though there’s a lot more to it than that, as we’ll explore further).
This is the size range you’ll most likely find fitted to bikes as standard, although 160 mm and 162.5 mm crank arms are becoming increasingly common and are now more or less considered standard too. SRAM also now produces 177.5 mm and 180 mm options.
You can get specialty cranks that go beyond the extremes of this range, (I’ve seen crank arms as short as 140 mm and as long as 190 mm), but these are often custom-ordered and used for cyclists with limited mobility.
How To Find The Length Of Your Crank Arms
The first and easiest way to find out your crank arm length is by looking at the back of your crank. Typically, most brands have it printed or stamped there, as in the picture above.
Failing that, grab a tape measure, start at the center of the bottom bracket axle on the crank, and measure to the center of the pedal spindle. That will give you the crank length you are using.
I highly recommend measuring both crank arms. You’d be surprised how often I’ve seen two different sizes combined in error on the same bike.
Why Does Crank Length Matter?
Crank arm length is one of the most important factors regarding your bike fit.
If you swap your cranks for another size, it does feel completely different, and even a change as little as 2.5 mm will feel like you’re on another bike entirely. Here’s why it’s important to have your crank arm length dialed in.
Comfort
The biggest and most important factor when it comes to crank length is comfort. The correct crank length will ensure the angle on the knees and hips is okay, keeping you pedaling comfortably and properly.
Injury Reduction
Another key element in the correct crank length is reducing injuries. The wrong crank length will cause unnecessary pressure on the hips and knees, which can lead to injury or hip and knee pain. If the crank length is too long, it’s impossible to achieve the perfect fit.
Pedaling Efficiency
The correct crank arm length will give you the pedaling efficiency you need. It will ensure that the pedal rotation is correct for your leg length, and this can not just increase power but also reduce fatigue.
Short cranks can also make you marginally more aero-efficient, as your frontal surface area is slightly reduced.
Pedaling Technique
Different crank lengths can help alter your pedaling technique – for better or for worse.
For many, a smaller crank is going to offer a tighter pedal stroke, which makes it easier to achieve a higher cadence, which can reduce fatigue when climbing, among other benefits.
While the “optimum pedaling cadence” is a bit of a grey area and is much debated among cyclists, elite riders typically utilize higher cadences than amateurs – so if you’re looking to bump your own cadence up, shorter cranks can be a helpful place to start.
That said, some cyclists just prefer grinding the gears, opting for a low cadence in a high gear. In this case, long crank arms can offer a little more leverage while riding out of the saddle.
Pedal Clearance and Toe Overlap
Shorter crank arms increase pedal clearance from the ground at the bottom of the pedal stroke.
This is particularly beneficial for crit racing, in which regular sharp corners and the need for constant pedaling increase the risk of a pedal striking the ground, and for mountain bikes to reduce the risk of the pedals hitting obstacles such as rocks or tree roots.
Shorter crank arms also reduce the likelihood of toe overlap (when the front of the foot touches the front wheel while cornering sharply), which is more prevalent with smaller frame sizes.
What Is The Best Crank Arm Length For Cycling?
As a professional bike fitter, I can 100% confirm that crank arm length comes down to the individual. There is no single crank length that is “best”, nor is there even a “golden ratio” between height and crank length that will work perfectly for all riders.
Generally, the accepted logic is that if you are taller, you will use a longer crank, and shorter riders will use a shorter crank. This is half right, but there’s much more to consider besides height.
Another factor is joint flexibility. If you’re very flexible and have a large range of motion in your joints, you will be able to work effectively with longer cranks. This doesn’t just come down to knee mobility but hip flexion as well.
More relaxed (upright) riding positions tend to work better with longer cranks, and in more aggressive riding positions, cyclists often benefit from shorter cranks allowing for wider hip angles.
A good example of this is that professional cyclists will often use shorter cranks for the ultra-aerodynamic positions required on a time-trial bike (or the similarly aero position on a triathlon bike) than they do on their regular road racing bikes.
In short, the optimal crank length is right for your body and the type of riding you plan to do. A lot of it also comes down to personal preference for the rider and the bike fitter’s advice. No one rule book fits all for cyclists.
If trying out multiple crank arm lengths, bear in mind that you’ll need to adjust your saddle height to compensate for the change in crank length.
How To Find The Best Crank Length For You
1. Go To A Bike Fitter
The best option is to go to a bike fitter. They will be able to test your flexibility, take your measurements, find out a little more about your riding style, and then recommend exactly what crank arm length you need to be on.
A good bike fitter will get you in a perfect position, but it will take hours and lots of adjusting your bike.
2. Use A Crank Arm Length Chart
The next option is to estimate the crank arm length you need based on your height. Above you can find our crank arm length chart relative to rider height, as well as the typical lengths that would come fitted as standard on a road bike.
However, note that this chart is only a very rough starting point. This is based on the crank length charts manufacturers would typically provide, but these ignore all of the individual variables described above.
For shorter riders in particular, these “standard” crank arms lengths listed in the chart above are likely to be longer than they should be for a perfect bike fit.
3. Use A Crank Arm Length Formula Based On Height or Inseam
Graeme Obree, the Scottish former hour record holder, proposed a formula of 0.95 x rider height, divided by 10.
For a 178 cm (5’10”) cyclist, this would provide a crank arm length of 169 mm, so you’d need to either size up to 170 mm cranks or size down to 167.5 mm cranks.
Alternatively, Machine Head Software suggests the following formula: [(rider’s inseam measurement in centimeters x 1.25) + 65 centimeters] divided by 10.
By this formula, a rider with an inseam of 80 cm would end up with 165 mm cranks.
However, these formulas are hardly foolproof. A common criticism is that they tend to suggest shorter riders, and female riders in particular, use cranks that are often far too long for them.
My Experience In Crank Arm Length As A Bike Fitter And Ultra Cyclist
As someone who doesn’t just look at the technical side of getting the perfect crank length but someone who has ridden a very wide range of sizes, my thoughts differ from a lot of information online.
Most advice online or manufacturer’s guidance will put you on a given crank size, which will likely be okay, but 90% of people I have fitted and worked with benefit from going slightly smaller.
Take me as an example. At 6 feet tall (183 cm), most online advice would have me on 175 mm cranks. However, when I tried out 172.5 mm cranks, I was more comfortable and pedaled more efficiently. Then, I found even further improvement when dropping to 170 mm.
It comes down to personal preference, but in my experience, most cyclists find that smaller cranks come with amazing benefits and are much better suited and much more comfortable.
It’s also a trend that’s being replicated in the professional peloton, with many riders using crank lengths shorter than the above formulas would suggest, or than they have done previously in their careers.
At the 2023 Tour de France, for example, GCN noticed that Tadej Pogačar had downsized to 170 mm cranks from the 172.5 mm ones he’d always used previously.
