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Old 06-16-2020, 05:48 PM
owly owly is offline
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Will going to a 5mm longer crank upset weight at bars?

Been using 172.5mm arms for a few years. A couple of years ago when hunting around for a suitable gravel frameset, the Competitive Cyclist fit calculator suggested in its numbers that I use a 177.5mm arm.

When I fit anything larger than a 700x33 tyre, I find the ride height too tall and haven't been able to get used to it / like it. So rather than shell out quite a bit of money for a new frame with a lower bb, I am thinking that longer crank arm will pull me down about 5mm into the bike, which should offset running a 35mm. (I'm hoping to be able to try a 38mm ideally; achieve a good ride feel/height).

I think I'm correct in thinking that longer arm length will pull me a bit forward in the saddle when the arm is pointing forward (parallel with ground), and thus at the bars my balance in the bike would now be too far forward. I would need to move my saddle back a bit to compensate, however I can't really move my position back as my cleat fore/aft position is locked into place in the shoe.

I have experimented with moving my cleats forward a mm at a time in the past. When they get to a little over 2mm forward, I start getting some numb toes.

So in summary is there any way to run a longer crank arm and still find the right fore/aft weight/balance feel when riding, by tweaking component position? or am I going to run into trouble?

Edit: Geometry/maths speaking, is a lower bb position the only correct (fit) solution here?

Last edited by owly; 06-16-2020 at 05:54 PM. Reason: txt
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  #2  
Old 06-17-2020, 12:48 PM
Old School Old School is offline
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Quote:
Originally Posted by owly View Post

Edit: Geometry/maths speaking, is a lower bb position the only correct (fit) solution here?
I believe this ^
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  #3  
Old 06-22-2020, 06:23 PM
owly owly is offline
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Thanks OldSchool.

I've decided to just suck it up and not spend more frame money at the moment.

Have finally got around to ordering some Steilacoom 38mm.
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  #4  
Old 06-23-2020, 07:40 PM
giordana93 giordana93 is offline
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I've tried to resist replying because there is so much going on in this post, but can resist no more.

So, you have too much going on and need to isolate some variables. 1st, if you can feel the difference in height between a 33 and 35 or even 38 mm tire, especially when there is lower air pressure in the fatter tires (as should be the case), I'm impressed (but very, very skeptical)--because it works out to less than 1/10 of an inch. That sounds snarky and is not meant to be, only to point out that you may/probably have something else going on. So here are some notes:

do make sure you are reducing air pressure as you go to fatter tires; a 35mm jacked up to even 90 psi will ride like a rock and bounce you off your line and feel unstable

saddle fore/aft certainly feels different/can vary with cleat fore/aft, but there is absolutely nothing that "ties" them together: you push your saddle back or forward and same for the cleats. You will definitely have to adjust saddle height based on moving either saddle or cleat back/forward, but the only thing limiting saddle fore/aft is the seatpost setback and length of your saddle rails.

Same for crank length. It may pull you forward if that's where your weight wants to be when pedaling, but it's not automatic. You can push saddle back or forward or leave it. Now, if you're already at maximum seat height and throw a longer crank on, you will naturally creep forward because you're being pulled closer to the crank centerline, simply because you can't reach that far forward. But drop the saddle height enough and that disappears.

In short, it sounds to me like you're still looking for your best balanced fore/aft saddle + cleat + seat height + crank length position (which can furthermore depend on the sweet spot of your saddle, especially if you like one with more dip/hammock than dead flat). Bottom bracket height and tire width aren't irrelevant, but they are a grain of sand compared to the pea in your princess and pea dilemma (again, no offense, I often use that analogy for folks who are sensitive to minor bike fit changes, including myself)

remember:you have joints at the hip, knee, and ankle. The angles on these will change as you make changes, but you're not a stick man. 5mm forward crank does not make you 5mm taller/more forward automatically; it's a dynamic pedaling motion/weight distribution equation, not simple plus/minus.
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  #5  
Old 06-25-2020, 05:56 AM
owly owly is offline
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^ thanks for your thoughts. Very helpful.

Yes I can feel the ride height diff between a 33 and 35mm.

90psi and yes; I'd be bouncing all over the place. Generally around 25-40psi with a 35mm for me; On and offroad.
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  #6  
Old 06-25-2020, 04:13 PM
giordana93 giordana93 is offline
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Quote:
Originally Posted by owly View Post
^ thanks for your thoughts. Very helpful.

Yes I can feel the ride height diff between a 33 and 35mm.

90psi and yes; I'd be bouncing all over the place. Generally around 25-40psi with a 35mm for me; On and offroad.
so I don't doubt that you're feeling something...but (and again, this is just to get you thinking, and for the sake of argument), that 2mm difference is in the tire width, not height. Even if it were the height, and even if that height were not reduced by the greater deflection from flattening out that happens with a fatter tire, 2mm is about the height of a US nickel. Is it the same brand/model of tire? because even that will have a greater variance. Anyway, no point in belaboring this one.

Realized I did not really answer the original question directly, though maybe indirectly I did. There will be a "weight at bars" balance that depends on everything I mentioned in my post: saddle fore/aft & height; cleat fore/aft, with crank length in there as well; saddle to bar drop; and finally rider effort and fitness. Rolling along at 12mph puts you in a different position than flying around a track (or time trial) at over 25. And I'm forgetting the shape and angle of your saddle. A saddle set up nose down does more to mess up weight at the bars than crank length every day of the week. Good luck finding your balance. I still think you need to play with saddle setback and height (and angle--go level), along with bar drop to find your sweet spot. I'm also curious about how CC came up with 177.5, which is pretty long. Really long legs, I assume. I would suggest reading Dave Kirk (here on PL forum) and Dave Moulton (on his blog, search bike fit) as well as your compatriot Steve Hogg on the issue of balance and setback.
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  #7  
Old 06-25-2020, 05:54 PM
owly owly is offline
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Thanks again.

I do have a fairly tall cycling inseam for height. 34" at 177.5. Plus long femurs.

Saddle angle/setback and overall bike fit is pretty good. (Had two thorough fittings with physio/bikefitter and another). Been doing about a 260km ride once a week for last three weeks. While these distances are fairly new to me, the body feels pretty good at the end.

Did do quite a bit of reading through Steve Hogg's site a while back. If I were in his city I'd definitely see him.
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