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Discussion Starter · #1 ·
Ok, I flunked most of my math classes. Through a bike fit I have ended up with my saddle very far forward and was curious if there was a formula for calculating how much change there would be from going from a seat tube angle of 72 to 74 and see how much this would move my saddle forward, all other things being equal?
 

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That depends on how high the saddle is too. Just use your basic trigonometry with that and 2 degrees.
However, it's a lot more than just how far forward, you're completely changing your balance on the bike and putting a lot more weight on the handlebars. If your reach seems too far, I'd recommend either flipping the stem or getting a shorter stem or both.
 

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I've seen a few members here quote 1 degree = 1 cm approximately. Just keep in mind this will also impact reach and top tube length. If all you are worried about is centering your saddle rails on the seatpost clamp, if you haven't already swapped to a zero setback seatpost, try that.
 

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krisdrum said:
I've seen a few members here quote 1 degree = 1 cm approximately. Just keep in mind this will also impact reach and top tube length. If all you are worried about is centering your saddle rails on the seatpost clamp, if you haven't already swapped to a zero setback seatpost, try that.
That's the idiot's guide to ST angles. The math guys'll be by in a bit to tell you its closer to .7634 or something.

I stick with 1cm per degree of ST angle change. Easier for me.

M
 

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MShaw said:
That's the idiot's guide to ST angles. The math guys'll be by in a bit to tell you its closer to .7634 or something.

I stick with 1cm per degree of ST angle change. Easier for me.

M
I'm fine with being an idiot. So you lose a cm for every 3 degrees. Good to keep in mind, but I'm ok with estimating.
 

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Discussion Starter · #6 ·
I will worry about the other issues in being moved forward when I come to that. At this point just trying to decide what bike will work with getting my knee over the axle. I guess I have short femurs for my size. Which of course is not the greatest but oh well.
 

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krisdrum said:
I'm fine with being an idiot. So you lose a cm for every 3 degrees. Good to keep in mind, but I'm ok with estimating.
nope. Every degree = pretty much 1cm. 3 degrees = ~3cm (prolly really closer to 2.5-2.75, but who's counting a few extra mm?!)

M
 

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MShaw said:
nope. Every degree = pretty much 1cm. 3 degrees = ~3cm (prolly really closer to 2.5-2.75, but who's counting a few extra mm?!)

M
Exactly, I was going off the number you provided earlier. I'm ok with being pretty close on the math.
 

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gh1 said:
I will worry about the other issues in being moved forward when I come to that. At this point just trying to decide what bike will work with getting my knee over the axle. I guess I have short femurs for my size. Which of course is not the greatest but oh well.
I have pretty short femurs too. Are you riding a seatpost currently that has setback or no?
 

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info...

gh1 said:
I will worry about the other issues in being moved forward when I come to that. At this point just trying to decide what bike will work with getting my knee over the axle. I guess I have short femurs for my size. Which of course is not the greatest but oh well.
The whole idea of KOP have never proven to be the "correct" way to set saddle fore/aft position. If you have short femurs, using KOP may put you too far forawrd for proper weight balance over the saddle.

Rather than worry so much about STA, you need to find a frame that fits and worry about getting the correct seatpost offset after that. For given size, it's not often that the STA varies by 2 degrees between brands, but 1-1.5 is common.

The formula for setapost setback changes is saddle height times (cosA-cosB) where A and B are the two angles. It's usually a little more than 1cm per degree.

STA also afffects the frame reach (and stem length required). The same formula applies, but the c-c frame size is used instead of the saddle height. A common mistake is to think that saddle height affects the change in frame reach. It does NOT.
 

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C-40 said:
The whole idea of KOP have never proven to be the "correct" way to set saddle fore/aft position. If you have short femurs, using KOP may put you too far forawrd for proper weight balance over the saddle.

Rather than worry so much about STA, you need to find a frame that fits and worry about getting the correct seatpost offset after that. For given size, it's not often that the STA varies by 2 degrees between brands, but 1-1.5 is common.

The formula for setapost setback changes is saddle height times (cosA-cosB) where A and B are the two angles. It's usually a little more than 1cm per degree.

STA also afffects the frame reach (and stem length required). The same formula applies, but the c-c frame size is used instead of the saddle height. A common mistake is to think that saddle height affects the change in frame reach. It does NOT.
What he said.

Being a slave to KOPS is asking for a repetitive-stress injury down the road, if not more immediate trouble. Sure, I like to check my knee-pedal relationship after making a fit adjustment or getting a new saddle or frame or something just to see where I'm at and then pay attention for weird pain that may correspond, but that's about it. Short femurs seem to indicate an almost certain behind-KOPS position, but nothing's ever certain in fit-voodoo:). Sheldon Brown breaks down the KOPS myth quite convincingly on his website.

Once you get the saddle/feet position reasonably close, then you can get around to choosing a seatpost and THEN you can adjust the stem--or frame if necessary--to match the correct saddle position.
 

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Ibashii said:
Being a slave to KOPS is asking for a repetitive-stress injury down the road, if not more immediate trouble.


got any data supporting that?
 

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Discussion Starter · #13 ·
krisdrum said:
I have pretty short femurs too. Are you riding a seatpost currently that has setback or no?
I ride a giant and it has a proprietary seatpost ,no setback but it does have a slack seat tube angle.
 

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Discussion Starter · #14 ·
Thanks for the calculation, was just looking for a guideline and the 1+ cm per degree is close enough for me.
 

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FatTireFred said:
got any data supporting that?
Not really, but you know that already:). The only data that supports my claim is the wide diversity of human shapes, sizes, levels of fitness, flexibility, etc. that want to put themselves on road bikes. Anytime you take two people of the same height or leg-length or whatever but different as snowflakes and press them into the same fit mold, you lose the ability to customize for things like the OP and his unusual femurs. How does KOPS account for that? I don't think it's much of a leap from there to the suggestion that blind obedience to KOPS or any other fit dogma is a recipe for repetitive stress injuries.

Of course these aren't my ideas: I learned them from reading Sheldon Brown and Leonard Zinn and Peter White and some other people. They do, however, match my experience and that of many others, so that's why I post stuff like I did when someone sounds like they're building their fit too tightly around a single element in a horribly complicated process.
 

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Ibashii said:
I don't think it's much of a leap from there to the suggestion that blind obedience to KOPS or any other fit dogma is a recipe for repetitive stress injuries.


I do... otherwise kops wouldn't be used much and would be thoroughly disproven and shown to cause such injuries, right? rather, it's a start and you gotta start somewhere... tweaks come later, as with all fittings
 

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FatTireFred said:
I do... otherwise kops wouldn't be used much and would be thoroughly disproven and shown to cause such injuries, right? rather, it's a start and you gotta start somewhere... tweaks come later, as with all fittings
Hey, it's impolite to quote only the second half of a syllogistic argument:).

In any case, if you're willing to admit that tweaks can and often do come later, then you're not really being dogmatic about KOPS and we don't have anything to argue about. Nothing wrong with using KOPS or any other generic fitting rule of thumb to start with; I was just pointing out that locking your seat fore/aft position at KOPS or sizing a frame based on potential KOPS without taking other factors into account isn't a very good idea, and in the OPs second post it sounded like s/he was moving in that direction.
 

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Ibashii said:
Hey, it's impolite to quote only the second half of a syllogistic argument:).

In any case, if you're willing to admit that tweaks can and often do come later, then you're not really being dogmatic about KOPS and we don't have anything to argue about. Nothing wrong with using KOPS or any other generic fitting rule of thumb to start with; I was just pointing out that locking your seat fore/aft position at KOPS or sizing a frame based on potential KOPS without taking other factors into account isn't a very good idea, and in the OPs second post it sounded like s/he was moving in that direction.


but then again how many people tweak? not all, certainly (unless something is hurting). yet how many repetitive stress injuries do you hear about, either on down the road or immediately? it may happen, but is kops a sure-fire recipe? doubt it... (again) otherwise there would be all these horror stories and kops would be disproven.

people size frames based on kops???
 

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Ibashii said:
Hey, it's impolite to quote only the second half of a syllogistic argument:).

In any case, if you're willing to admit that tweaks can and often do come later, then you're not really being dogmatic about KOPS and we don't have anything to argue about. Nothing wrong with using KOPS or any other generic fitting rule of thumb to start with; I was just pointing out that locking your seat fore/aft position at KOPS or sizing a frame based on potential KOPS without taking other factors into account isn't a very good idea, and in the OPs second post it sounded like s/he was moving in that direction.
It sounds as if you're over-analyzing the whole issue. With so many fit "rules" out there, it's confusing. It's best to advise to go to a quality, experienced fitter when getting a bike. But, some people would rather do this on their own.
What is advisable to the OP's questions: Don't judge these measurements by looking or measuring, but GO AND RIDE THE BIKE FIRST!!! See if it feels good! That's all the technical speak you need.:thumbsup:
 

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FatTireFred said:
but then again how many people tweak? not all, certainly (unless something is hurting). yet how many repetitive stress injuries do you hear about, either on down the road or immediately? it may happen, but is kops a sure-fire recipe? doubt it... (again) otherwise there would be all these horror stories and kops would be disproven.

people size frames based on kops???
First, I never said KOPS itself was going to hurt anyone: I said that sticking to it as an absolute measure of bike fit not to be violated no matter what strange biomechanical shyte one might have going on--short femurs in this case--is a good way to run into repetitive-stress trouble.

Second, in his second post the OP wrote "At this point just trying to decide what bike will work with getting my knee over the axle," so yeah, apparently some people do think about KOPS when they're looking at frame size.


Peanya said:
It sounds as if you're over-analyzing the whole issue. With so many fit "rules" out there, it's confusing. It's best to advise to go to a quality, experienced fitter when getting a bike. But, some people would rather do this on their own.
What is advisable to the OP's questions: Don't judge these measurements by looking or measuring, but GO AND RIDE THE BIKE FIRST!!! See if it feels good! That's all the technical speak you need.:thumbsup:
Hells yeah:). I only got into the over-analyzing as a response to my critique of KOPS as a valid-in-all-cases fit rule; I agree with every word you wrote, except maybe I want to point out that "quality, experienced fitter" is far from a well-defined quantity...but there are already plenty of threads on that subject.
 
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