There comes a time in every bmxer’s life where they ask, are my cranks the right size? Crank length is vital for optimizing your race set up and is often overlooked in favor of trying new gearing combos. But seeing as cranks are your primary point of leverage, having the proper size can dramatically improve your overall speed and efficiency.
But where does one even start, and how do you know your cranks are the right size to begin with? Well look no further as we’ll outline what crank length is and the effects it has on your power production. We’ll also take a look at standard crank length sizing recommendations and discuss scenarios where you may want to go against those stock settings in favor for something more optimized. Let this serve as your new ultimate guide to all things crank length.
What is Crank Length
Crank length is the distance from the center of the pedal spindle to the center of the crank spindle. A set of 130mm cranks means each crank arm measures to 130mm in length. A larger number translate to a longer crank.
The image above showcases how cranks are properly measured. As you can see they aren't done from tip to tip but rather from the two contact points it shares with the rest of the drivetrain. The crank depicted features a 175mm crank arm which is the most popular crank size for full sized riders in today modern bmx era.
Effects of Crank Length
Fundamentally, crank length is a rather simple topic. But things have begun to get a little muddy with varying opinions and industry trends. So let’s clear some things while keeping it as practical as possible.
Longer cranks provide greater leverage, meaning the drivetrain (gearing) is easier to get moving. Shorter cranks may require more force during the initial start, but they produce a smaller circle which equals less travel between pedal strokes. Short cranks usually results in better rpm (Rotations per minute) numbers so racers will find these easier to ‘spin’ at high speeds.
Short & Sweet Version: Longer cranks will improve your starts while shorter cranks improve pedal cadence and the rider’s ability to maintain momentum.
With that info it’s pretty clear cut right? For a start dependent sport like bmx, you’d want the longest crank possible so you can hammer that start and get going. Well yes, but not really as there’s a bit more nuance to choosing crank length… And this is where things start to get muddy with crank length debates.
The benefits associated with crank length only operate within a certain range, meaning there is a certain point for each rider where it’s too long or too short. So let’s continue, but keep it simple!
Perfecting The Fit
People can get themselves into trouble when they start ask ‘what is the fastest crank length?’ I’m here to tell you that doesn’t exist. If it did, the industry would make one crank size and we’d all run it because science proved it to be the best. Now obviously that’s not the case as crank’s are highly customizable and come in all different sizes for a reason.
So really we need to reframe the original question from “what’s the fastest crank length” to “what’s the fastest crank length for ME”
More leverage is great, but if the cranks don’t align with the rider’s body mechanics then they’re working against them everywhere else on the track. Most crank length sizing is based off of inseam. This is a good place to start, but it’s not the end all be all as some rider’s vary in body position on the bike. However in most cases, measuring inseam is enough to get a solid base that’s effective enough for a good amount of people.
Remember that this is just a good place to start. If you're just looking for something that works and won't inhibit performance too much, then this chart will get you 90% of the way there. It's more the folks that are looking to gain each and every competitive advantage they can that will start to deviate outside of these numbers. With that being said, these are just that, rough numbers. So if you've found something else that works or is comfortable for you, then stay where you're at in terms of crank length.
Now for most fully grown riders, once you find a crank set that works with your body, there shouldn’t be too much more messing about. At very high levels of competition, riders may play with different set ups but it’s really just a few mm in either direction. For the typical bmx racer and even NAG plate earning experts, your cranks are likely fine.
It’s more the little guys and growing riders that will be swapping crank sizes the most. Seeing as these riders are quite literally growing each day, their equipment will need to keep up. So be sure to keep an eye on their inseam but let’s also look at some tale tell signs that indicate for new crank sizing.
When Should You Change Size?
One of the main things we watch when evaluating crank length is a rider’s hip motion throughout the pedal stroke. If you watch your rider and notice excessive bobbing then the cranks are likely too long. If there’s a considerable hip drop on the down stroke of the pedal, it’s because the crank stroke is longer than the rider’s leg or range of motion. In order to keep their foot on the pedal (or remain clipped in) the rider must drop with the pedal. This is not efficient and also causes instability as the rider will be swaying back in forth during sprinting. This bobbing motion is a rather obvious sign that adjustments need to be made to the crank length.
Based on how bad the bobbing is, that will determine how much shorter you need to go. Here at the shop we usually only recommend changes of +/-5mm. For example: 175mm to 170mm adjustments. There are smaller increments, but it’s usually really hard to get an impactful adjustment from such a small change in crank length. So in the situation of a rider bobbing up and down during the pedal stroke, you would drop a size or two in crank length.
When it comes to identifying shorter cranks, the signs aren’t as apparent. In most cases you’ll want to run something that’s just long enough to provide proper leverage but doesn’t cause that hip drop like we mentioned previously. But be careful of going too short as you’ll be putting yourself at a pretty big disadvantage on that first crank out of the gate. You can always adapt gearing to even out the new lack of leverage, but let this serve as more of a warning to avoid the idea of going as short as possible.
If you watch the rider and their pedal circles just look too tight, or it seems no matter what gear you put them on they look spun out. Then your cranks are likely too small.
The Sweet Spot
What makes a topic like crank length or gearing so engaging is the fact that’s there’s no true correct answer, yet people continue to seek out this magical number. Cranks are critical for performance, but it’s really more about find a comfortable range. We say prioritize comfort and fit first, then you can begin to consider leverage advantage second.
We put fit over leverage because a comfortable bike will always be faster than an ‘optimized’ bike. Just because the industry is hot on a new trend, or your rival is on a certain size, we don’t believe you should force yourself into something that doesn’t work with your body. Start with the sizing charts, they’re going to get you close. Then evaluate the riding over the course of time and make adjustments from there.
But notice we said TIME. Not one or two session!
The body is amazing and will adapt to most things you throw at it. You’d be surprised how many ‘bike fit’ issues can be fixed with a small tweak to riding technique. But still avoid swapping crank sizes on a weekly basis or right before a big race. While it may not sound like much 10mm (5mm on each crank arm) is a significant change to your pedal stroke. So there will be a period needed for adjusting. For that reason, when testing crank sizes give your body a few weeks to adjust to the motion before evaluating the technique. This is where video review can be very helpful.
Next Steps
By now, you should have a much clearer understanding of crank length and its associated nuance. But if you still can’t quite wrap your head around the concept, we’re here to help. We’re a rider owned and operated specialty BMX Race shop based out of Minnesota. So feel free to give a call and we can help you pick out the perfect crank length for you or your racer no matter the size.
You can get in touch with us here or shop our full line up of race cranks here.
