I recently switched out my 165mm crank for a 155mm crank (per the recommendation of my bike fitter) and raised my seat up 1cm. What is the purpose of buying a new crank if my seat also gets adjusted? Couldn’t I have just adjusted my seat differently with the 165mm crank and had it produce the same effect? I’m genuinely curious as to why we couldn’t just play with the seat post heights and had it produce the same effect as (a seemingly insignificant) a 1cm difference in crank length.
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Re: Purpose of Shorter Cranks [Xavier500]
[ In reply to ]
If you think about it, that 1 cm in seat height gives you 2 cm of extra distance between your foot and your seat at the top of your pedal stroke..... (1 cm of seat height plus 1 less cm in crank length). This lets you get slightly more aggressive in your overall position on the bike.
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Jason
None of the secrets of success will work unless you do.
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Jason
None of the secrets of success will work unless you do.
Re: Purpose of Shorter Cranks [Xavier500]
[ In reply to ]
At the bottom of the pedal stroke your foot is now 1 cm higher or closer to your body (giving you more knee bend) and at the top of the stroke your foot is now 1 cm down or farther away from your body which opens up your hip angle. To get your leg extension (knee angle) back to how it was before you need to raise the saddle by 1 cm which in turn also opens up your hip angle even more.
Re: Purpose of Shorter Cranks [FuzzyRunner]
[ In reply to ]
FuzzyRunner wrote:
At the bottom of the pedal stroke your foot is now 1 cm higher or closer to your body (giving you more knee bend) and at the top of the stroke your foot is now 1 cm down or farther away from your body which opens up your hip angle. To get your leg extension (knee angle) back to how it was before you need to raise the saddle by 1 cm which in turn also opens up your hip angle even more.The two previous comments pretty much sum it up. You've opened up the hip angle and are now in a more aero position. Sounds like two win-win's to me. Hence why I also run 155mm on my tri bike.
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Re: Purpose of Shorter Cranks [Creatre]
[ In reply to ]
Re: Purpose of Shorter Cranks [Xavier500]
[ In reply to ]
No you cannot just adjust your seat height and get the same affect as shorter cranks. Assuming you are currently in an optimized position with 165 mm cranks, adjusting your seat height (say raising it), will negatively affect your knee angle at the bottom of the pedal stroke (6 o'clock), AND open up your hip angle at the top of your pedal stroke (12 o'clock). So by adjusting just your seat height, you are trading off having a more open hip angle to having too high of seat (higher knee angle.
Shorter cranks allows you to optimize seat height AND hip angle together. A more open hip angle can produce more power if you were too closed off with the longer cranks and gives you the option to lower the front end (potentially more aero) without closing off the hip angle too much.
At the end of the day, bike fitting is mostly a mathematical geometry problem. Shorter cranks are a potential solution to help you solve that problem.
blog
Shorter cranks allows you to optimize seat height AND hip angle together. A more open hip angle can produce more power if you were too closed off with the longer cranks and gives you the option to lower the front end (potentially more aero) without closing off the hip angle too much.
At the end of the day, bike fitting is mostly a mathematical geometry problem. Shorter cranks are a potential solution to help you solve that problem.
blog
Re: Purpose of Shorter Cranks [Xavier500]
[ In reply to ]
Xavier500 wrote:
Couldn’t I have just adjusted my seat differently with the 165mm crank and had it produce the same effect?
Re: Purpose of Shorter Cranks [HTupolev]
[ In reply to ]
HTupolev wrote:
Xavier500 wrote:
Couldn’t I have just adjusted my seat differently with the 165mm crank and had it produce the same effect?I wish this forum had a "like" button.
Matt J wrote:
HTupolev wrote:
Xavier500 wrote:
Couldn’t I have just adjusted my seat differently with the 165mm crank and had it produce the same effect?I wish this forum had a "like" button.
Yup. Like for sure.
Adjusting hip angle at the stack height changes your aero penalty. Adjusting hip angle via the crank length does not. It's to some reasonable limit of going too short.......free aero via stack you can go down to the same hip angle or power gained by opening up the hip angle. I personally think you can probably go too short and start to run into biomechanical "leg lever length" and power application issues. Where that point is, I've no idea and am zero expert in that.