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Old 05-08-2017, 01:56 PM
ghammer ghammer is offline
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From 175mm to 172.5mm

So my left knee had been bothering me for a while, and after reading I decided to go with shorter cranks. I'm 6' and ride a 56cm ctc frame with a seat height of 77.5cm. I ride an average of 6-10hrs a week (on a good week) and have been riding/racing forever.

I have ridden 2x already w the new cranks, but have been reading conflicting info:

1. some folks claim i should keep my position as is
2. others say i must increase saddle height by 2.5mm
3. others say it should be split between saddle height and sliding the seat backwards a bit to account for the extra room
4. and finally, some say i should *lower* the saddle (am not sure the rationale for this last one).

I have not altered the bar height. I have felt relief on hips while on the drops and noticed my hips are looser after riding. So there's a good point there. Plus, knee discomfort is a no longer an issue.

But I have been struggling a bit w height and fore-aft. I have not given too much adaptation time as it's allegedly a small difference (2.5mm), but at the same time I'm the type of rider who's a fuss budget about fit and i *do* feel minor adjustments.

I have no extra pain/discomfort outside of the ordinary. but indeed the hip area got a relief (who knew), and i do stretch frequently.

From the collective experience, what is the consensus regarding adjustments after crank arm length swap? Any and all input are welcome.

Thanks,

G
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  #2  
Old 05-08-2017, 02:48 PM
Peter P. Peter P. is offline
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Go by feel. Make one change and if that doesn't work, put it back and change the other one.

I'm a fussbudget too; 2.5mm isn't that difficult to iron out.
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Old 05-08-2017, 04:04 PM
tv_vt tv_vt is offline
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Well, if you're wanting to keep your leg extension the same as it was with 175 cranks, then raise your seat 2.5mm. With the 172 cranks, you are bottoming out 2.5mm higher than you were with 175s. Raising the seat an equal amount compensates for the shorter crank.

That's the theory anyway, you may find you're comfortable with some variation on it.

I did the same exact thing on my bikes over the last 6-8 months. No complaints so far. No knee issues (didn't have any before either...); performance seems about the same, or maybe a bit better.
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Old 05-10-2017, 06:52 PM
aki aki is offline
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All make sense depending on where in the pedalling stroke was the limiting factor. Also keep in mind that some discomfort can come from changing the relative position of the saddle to the handlebar. When I switched to 165 from 170, I started with slightly higher saddle and handlebar, but for 2.5mm difference, I would not change and see how it feels.

Quote:
Originally Posted by ghammer View Post
So my left knee had been bothering me for a while, and after reading I decided to go with shorter cranks. I'm 6' and ride a 56cm ctc frame with a seat height of 77.5cm. I ride an average of 6-10hrs a week (on a good week) and have been riding/racing forever.

I have ridden 2x already w the new cranks, but have been reading conflicting info:

1. some folks claim i should keep my position as is
2. others say i must increase saddle height by 2.5mm
3. others say it should be split between saddle height and sliding the seat backwards a bit to account for the extra room
4. and finally, some say i should *lower* the saddle (am not sure the rationale for this last one).

I have not altered the bar height. I have felt relief on hips while on the drops and noticed my hips are looser after riding. So there's a good point there. Plus, knee discomfort is a no longer an issue.

But I have been struggling a bit w height and fore-aft. I have not given too much adaptation time as it's allegedly a small difference (2.5mm), but at the same time I'm the type of rider who's a fuss budget about fit and i *do* feel minor adjustments.

I have no extra pain/discomfort outside of the ordinary. but indeed the hip area got a relief (who knew), and i do stretch frequently.

From the collective experience, what is the consensus regarding adjustments after crank arm length swap? Any and all input are welcome.

Thanks,

G


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  #5  
Old 05-22-2017, 01:21 AM
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BobO BobO is offline
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Quote:
Originally Posted by ghammer View Post
I have not altered the bar height. I have felt relief on hips while on the drops and noticed my hips are looser after riding. So there's a good point there. Plus, knee discomfort is a no longer an issue.
I'm late to the game here, but, if the reason for the change was to alleviate that specific discomfort, aren't you at mission accomplished? If you adjust the saddle to the same saddle-pedal height, aren't you putting yourself right back where you started in terms of leg extension, which may or may not have been the issue to begin with?

I am hyper sensitive to leg extension with regards to my knees. My legs are different lengths and if I get my extension wrong on the high side my left knee hurts in the back, if I get it wrong low my right knee hurts behind the kneecap. A couple of mm is all it takes to make a big difference. The crank length makes no difference at all as long as the saddle-pedal height is right.
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Old 05-22-2017, 07:16 PM
giordana93 giordana93 is offline
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Quote:
Originally Posted by BobO View Post
I'm late to the game here, but, if the reason for the change was to alleviate that specific discomfort, aren't you at mission accomplished? If you adjust the saddle to the same saddle-pedal height, aren't you putting yourself right back where you started in terms of leg extension, which may or may not have been the issue to begin with?

I am hyper sensitive to leg extension with regards to my knees. My legs are different lengths and if I get my extension wrong on the high side my left knee hurts in the back, if I get it wrong low my right knee hurts behind the kneecap. A couple of mm is all it takes to make a big difference. The crank length makes no difference at all as long as the saddle-pedal height is right.
I'd lean towards this attitude, realizing also that we are talking about one tenth of an inch.
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  #7  
Old 05-31-2017, 08:16 AM
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rwsaunders rwsaunders is offline
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I have nothing to offer from experience as I have a 172.5mm crank on one bike and a 175mm crank on the other and I haven't been able to tell the difference. Saddle heights are the same and the reach and drop are pretty close.

Like someone else has stated, if the discomfort is gone, something must have worked for you, right? Anyway, you may enjoy reading this article.

http://bikedynamics.co.uk/FitGuidecranks.htm
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  #8  
Old 05-31-2017, 09:43 AM
ghammer ghammer is offline
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True, and thanks for the article. The change was positive and I feel like I can spin bigger gears easily, if that makes any sense. The drawback, if i can call it that, is that i have an isp bike and the topper has cracked. So am converting it to a standard seatpost. But i did have to increase saddle height as my knees and left glute got sore after rides longer than 90mins.
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  #9  
Old 07-29-2017, 04:03 AM
Splash Splash is offline
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Rwsaunders- Excellent linked article. Thank you very much.

All, Those specific angles stated in that linked article (in pictures)
relating to knee, hips and ankles - are those stated values what should be aimed for to achieve optimal bike fit and pedal stroke?

If not, how do I obtain or know the optimal angles for these 3 x different body areas?



Splash

Last edited by Splash; 07-29-2017 at 04:44 AM.
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  #10  
Old 07-30-2017, 02:43 PM
bigreen505 bigreen505 is offline
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Quote:
Originally Posted by Splash View Post
Rwsaunders- Excellent linked article. Thank you very much.

All, Those specific angles stated in that linked article (in pictures)
relating to knee, hips and ankles - are those stated values what should be aimed for to achieve optimal bike fit and pedal stroke?

If not, how do I obtain or know the optimal angles for these 3 x different body areas?



Splash
Yes, and thank you Splash for bumping it. That was a great read.
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