Not a whole lof of new athletes think about it when they buy a bike. I can count the amount of times on one hand that I've sold a bike in the past year and have been asked what size crank arms come on the bike I am selling. And to be honest, I never would have either. But with more and more schools of thought coming out on who needs what in terms of the arm length, I'm here to somewhat divvy up what you should be thinking when going in to buy a bike (or change your arm lengths).
Let's start with the purchase first off. Say you're looking to get yourself on a brand new bike and have gotten it dialed down to a few and you're just not quite sure what to be looking for. If you're getting into a bike that is fitted the right size for you. Say you're 5'9" and getting on a 56cm frame, more than likely it will come with either 170 (most likely) or 172.5mm (less likely) crank arms. Yea, I know the difference isn't really much, but there is a difference. Let's take a look at that...
So you're riding your bike and it is PERFECTLY fit in terms of saddle height, saddle fore/aft positioning and so on, and everything is going great. Awesome. Good for you. But you just feel like you can't swing your feet around faster than 100 rpms for the life of you. Or let's say, you're riding down the road and when you watch your knees come up to the top of the pedal stroke, you begin to notice them popping out to the inside or outside. Neither of these things seem right, do they? Absolutely not.
The prior could just be part of lack of training at a higher cadence, but when thinking of the latter, there are two problems that could arise. One being that you have terrible range of motion in your hip area, namely your hip flexor. But another idea is that, for both problems, you have crank arms that are too long for you.
Why would that matter? Well, if you're crank arms are too long, your body will make up for it in other ways. Kind of like when you hurt one ankle running, you overcompensate for it by leaning more on the other. In turn, hurting the other as well. Same holds true here. If your crank arms are too long, your body will let you know. Just because your knee bend at the bottom of the pedal stroke is perfect dialed in, that doesn't correlate to a proper fitted bicycle. If you have range of motion problems, one way to solve that wobble at the top of your knees is a different carnk length.
Just think of your knees like pistons. Shooting up and down on a perfectly vertical path. Should they falter in any way from side to side, that is wasted energy and can lead to other problems down the road. Especially in the hip area.
Another reason for shorter crank arms is this: aerodynamics.
So you're a triathlete and you want to be as aero as possible on your bike. You've got everything you need on your bike, and have amazing flexibility in your hips and lower back. Great! But for some reason, you've got your 56cm bike with 172.5mm crank arms on them and you want to squeeze out a bit more aero if you can. So check this out. Swap them out for 170mm arms and you'll not only see your back get flatter as you raise your seat to accomodate the pedals not reachins as low to the ground as before, but it will also help to open up your hip angle. Something often times overlooked by many people.
But I'm 6'3" and my fitter said I needed 175mm carnk arms. FALSE. Carnk arm length should have little correlation to your height but more so your flexibility and abilities. Take for example this years Ironman Championships. I watched and heard many of the best athletes were riding on shorter crank arms for a few reasons. Some of them being in the 6 foot plus range. And that reason was to both keep a higher, more sustained cadence and become a slight bit more aero.
What does having a slightly higher cadence do? Well once you get off your bike and start running, you want your legs to move, and move fast on the run. If you're rocking 175mm crank arms, your body is moving your foot in a larger circumference around the center axis. Thus, making you use more torque to slame through each pedal stroke. Essentially, this will wear out your major muscles in your legs faster. Hence, brick legs on the run.
Now switch out the long arms for some 5mm shorter and you'll be spinning higher and using smaller muscles to acheive a greater performance. Essentially on the run.
So a lot of this comes down to just a few simple things. Carnk arm length is still a hot topic in both the biking and triathlon worlds. I have been told I should be riding 172.5-175mm crank arms, but feel best on 170 and maybe 172.5. And the best way I've found this out? I've ridden all 3 sizes. Each one of my bikes currently has a different size crank on it. So I've been around the block, per say, in terms of what lengths work for me. I'm somewhat felxible, but have trouble spinning faster on the longer arms and feel my legs drop out on them as well.
This is just some food for thought. Not any end all be all theory. Take these things into consideration before you go jumping on your next bike or are looking for some upgrades to your current bike.
-Dave
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