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Switching to shorter cranks
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I am planning on moving to a shorter crank from my 172.5mm that I have today. What considerations do you make regarding chain ring sizes before pulling the plug on this? Right now I have a 52/36 and it seems the more common setup is a 165mm with 50/34. Seems logical to go with a slightly smaller ring up front if using shorter cranks but I'd like to hear other thoughts on this. Regardless of 165 vs 160 etc, how much do you consider ring size?

Thanks!
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Re: Switching to shorter cranks [dave_o] [ In reply to ]
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The chainrings you will want to use depend on where you will be riding, not on crank arm length.
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Re: Switching to shorter cranks [jimatbeyond] [ In reply to ]
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Makes sense. Thanks.
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Re: Switching to shorter cranks [dave_o] [ In reply to ]
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All else being equal, shorter cranks should mean higher cadence. If you are at the limit of your climbing gears you may want to gear a bit lower.
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Re: Switching to shorter cranks [rruff] [ In reply to ]
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actually, if everything is equal, the cadence will be, too.
the one thing that changes is the circumference your pedal has to travel during one revolution. which means, if you have everything staying the same and only shorten the cranks, the only thing that changes is the speed at which your foot moves: shorter crank -> smaller circumference -> the foot/pedal travels slower if the cadence stays the same.
the effect is that you can up the cadence without the feeling you are spinning your legs fast.
worked perfect for me. i'm 1,67 m (which is about 5'5''). i'm riding 145 mm cranklength.
danny

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Re: Switching to shorter cranks [dave_o] [ In reply to ]
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It really depends what you're trying to accomplish.

To produce the same power you did before the change, you need the product of torque and rotational speed of the pedals to remain constant.
Torque is pedal force multiplied by crank length and rotational speed is proportional to cadence.

So there are 3 potential solutions for producing the same power with shorter cranks:
  1. You maintain the same torque by increasing pedal force and cadence stays as it is.
  2. You maintain existing pedal force and increase cadence to compensate for reduction in torque.
  3. A combination of increased pedal force and increased cadence.

I think many would argue, and I'd probably agree, that you can more comfortably maintain a higher pedal force through a larger portion of the stroke with shorter cranks since your leg is straighter for longer. It's often also suggested that higher cadence is facilitated by the reduced pedal speed for a given cadence.

So, back to chainring sizes:

If you tend to maintain existing cadence and increase pedal force then there is no significant impact on the chainrings, or any other part of your drive-train downstream of the cranks. You'll simply turn the same chainrings at the same rpm in the same gear for the same resistance. If, on the other hand, you increase your cadence so that you can avoid higher pedal force, then you'll be using a smaller ("easier") gear for a given resistance. In this case, you could change to 50/34 to maintain the chainline, but unless you have very high confidence your setup was perfectly optimised beforehand and you somehow know exactly how the shorter cranks will effect your pedalling, I have no idea how you'd know if this is a good idea or not.

So I would suggest sticking with what you've got unless you've some other reason to change, such as already finding 52/36 a bit too big before making the change, in which case I'd still view them mostly as separate mods to your setup.

Last year, I switched from 52/38 chainrings and 172.5mm cranks to 52/36 chainrings and 160mm cranks. I only changed from 38T to 36T because I had to change chainrings anyway and had always found the 38T a bit too large on some of my hilly routes. I wasn't trying to compensate for the crank change. If I was I'd have been looking for a 48T big ring! I haven't noticed any significant change in my cadence, I just push a bit harder. I'm happy with the change to shorter cranks, as I do find my position more comfortable and my stroke smoother. I haven't noticed any major change in performance, but I'm pretty sure I didn't get slower!
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Re: Switching to shorter cranks [Ai_1] [ In reply to ]
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Thanks for the details! This helps a lot.
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Re: Switching to shorter cranks [danny_t] [ In reply to ]
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danny_t wrote:
actually, if everything is equal, the cadence will be, too.

Cadence is variable and is self selected based on other factors. Power is the important thing to keep constant, and IME cadence naturally will rise a little less than the reduction in crank length. In other words the foot speed is about the same.
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