Crank length question

[ambal]

I have a question about road cycling crank length. I am currently in the market for a new road bike and I'm trying to decide on the appropriate crank length for my riding style. I have heard that crank length can affect power transfer, comfort, and overall efficiency on the road.

I was wondering if any of you have experience with different crank lengths and how they have impacted your riding experience? Also, are there any general guidelines or formulas for determining the best crank length for a specific rider?

Any input or advice would be greatly appreciated!

Thanks in advance.

 

[kcjc]

I used a 172.5 on my home trainer setup for around 5 years and didn't notice much difference from the 170 that I used almost exclusively since college some 35 years ago. I was probably doing up to 150 hours a year on my trainer due to time constraints and I hate the cold. At my last fit, 2017, my fitter suggested a 165 to open my hips more but I didn't want the expense reequipping three cranksets. I switched pedals/n+1 and had a hard time dialing in the fit based on my old setup. Theoretically, longer arm will give you higher torque but that's tempered by likely decrease in the angular velocity. Most studies I have heard says it's a wash and really a function of fit and comfort. I haven't seem any general guidelines or formulas but you could look to manufacturer section on bike sizing as a guide. If the purchase comes with a pro fit, check with the fitter.

 

[cobbwheels]

The calculated crank length for my inseam length is 170mm. My gravel bike has 170mm crank.

However, my MTB I used for 1 year mostly on paved roads prior only had 150mm crank length and my stationary bike has 140mm. All of them have correct saddle heights based on max leg extension or min knee angle with pedal between 5 and 6 o'clock position.

You would expect I'd be most comfortable and faster with the 170mm. But for me, it's actually the 150mm crank I find best and the 140mm a close 2nd! Not only I was more comfortable with the short 150mm crank, I was also faster and made more power. In fact, I couldn't beat the strava records I set with MTB with 150mm than with my gravel bike with 170 mm crank! The shorter crank made it significantly easier to spin and the higher cadence, better comfort even resulted to improved performance despite the lower torque at the pedals.

However, that's only true when I'm pedaling seated. When I'm out of the saddle (pedaling standing), I made more power with the 170 mm crank. It's also harder to spin smoothly with 170 mm at >100 rpm cadence but smooth and easy as butter in 150mm up to 120 rpm cadence.

Longer crank is only better when trying to max out your power out of the saddle and possibly in a sprint so it might be advantageous in a peloton race.... But for recreational riding and time trial, you'd do better and more comfortable with shorter crank.

If you're struggling at the climbs. A longer crank may not do you any favor. A much better solution is installing smaller gear ratios (or better training, your choice!).

 

[steve]

The trend these days with elite riders is to run shorter cranks. It wasn't long ago, taller guys like Big Mig were using 180mm cranks

 

[cobbwheels]

The trend these days with elite riders is to run shorter cranks. It wasn't long ago, taller guys like Big Mig were using 180mm cranks

At first I didn't liked 150mm. Before, my MTB with 150mm crank, I had another bike, a CX bike with 170mm (calculated crank length for me) I had used for one year.

When I first used 150mm. It felt like I lost a bit of power. But I had one year to adapt my pedaling technique to the shorter crank and at the end of the year I set personal records I couldn't beat when I went back to 170mm.

2 years now riding mostly 170mm crank, I still couldn't beat the records I set with 150mm. My legs are normally proportioned...So I guess the calculators are probably using obsolete formulas and needs to be updated.

The problem with many riders, they immediately dislike moving to shorter crank because of the reduced torque. But give it enough time to adapt, it took me a year and I'm riding faster than I ever did on both flats and climbs on 150mm vs 170mm.

I think the reason for this is the most efficient application of pedaling force is at 1 o'clock to 3 o'clock. It's not really 1 o'clock to 5 o'clock so the range is smaller and in a position where the knees are more bent. If you're using shorter crank, the 1 to 3 o'clock pedal position is in significantly more optimal leg extension for maximizing efficiency in producing power. If using longer crank, your knees will be more bent at the 1 to 3 o'clock position and that's not good, both for power and comfort. Ofc, to make the same or more power with shorter crank, you'll have to pedal at higher cadence. But as I've come to find at the end of one year, the higher cadence wasn't really a penalty since you're pedaling a smaller circle with shorter crank. You won't even notice the difference once you've adapted. What you'll notice is the improved comfort and the increased power you're adapted to the shorter crank.

 

[alicehosten]

I also think that there is not much difference between 172.5 and 170! They are similar in their characteristics!

 

[cobbwheels]

I also think that there is not much difference between 172.5 and 170! They are similar in their characteristics!

Some riders seem able to tell the difference. But many would advice, if you feel like you need to go shorter or longer, you need a difference of at least 10mm to get substantial change in feel.

Also using oval chainrings give the effect of using shorter crank since the pedal will only begin to present significant resistance between the 2 and 3 o'clock pedal position where your knees will be more extended than the 12 or 1 o'clock position if using circular chain rings. It's a good thing and many pros these days are switching to oval chainrings. They also make pedaling out of the saddle easier too.