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  #16  
Old 12-21-2019, 05:29 AM
marciero marciero is offline
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Originally Posted by giordana93 View Post
I respectfully disagree. OP asked whether seat height changes based on sta--when maintaining same reach, drop, and setback. And the answer is no (0) because saddle position--where you butt sits--depends on both height and setback, x and y from the bottom bracket. These numbers are independent of the seat tube angle. Your calculation, whereby seat height should drop with slacker angles, is correct if you do not maintain the same setback (x goes down as y goes up), but if you keep the setback the same, the height should remain the same. The position of the saddle rails on the post will move (forward if the frame is getting slacker) and indeed you may need a seatpost with more or less setback to get it in the right spot, but the height relative to the bb should not.
My calculation for drop in height does in fact assume the same setback. The point is that to maintain the "same height, drop, and reach" you do need to raise the seatpost, as well as slide the saddle on the rails. In the diagram, in order to keep the saddle at point A you need to raise the seatpost by an amount delta=L2-L1 and slide the saddle forward. If you dont do that, and only slide the saddle forward to achieve the same setback, the vertical height will drop by h and the distance to the bottom bracket will change by a bit more
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File Type: jpg sta.jpg (18.7 KB, 116 views)

Last edited by marciero; 12-21-2019 at 05:58 AM.
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  #17  
Old 12-21-2019, 10:34 AM
giordana93 giordana93 is offline
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agreed as long as we don't confuse "raise the seatpost" with change the seat height.

In a real world bike fit we aren't just pushing a fixed tube with an anchored seatpost that constitutes a fixed length line segment. When setting up a new frame (with possibly longer or shorter seat tube depending on sloping top tube, seat lug design, etc) you put the seat at the same height and setback relative to the bb as before.

When you write above
Quote:
With 77.7 saddle height at 75 degrees, I am getting about 78.5 cm. to go to 73 degrees and about 78.9 cm to go to 72 degrees.
you are actually showing how much leg extension increases from the increased setback of a shallower angle if you don't push the saddle forward while maintaining the same "seat height" (vertical measure above bb). Think about it, 78.9 at 72 degrees is waaay longer a leg extension than 77.7/75 because you've raised the saddle over a cm AND increased the setback. 78.9 vs 77.7 isn't the difference in seat height--it's the difference in leg extension resulting from the increased setback of a shallower seat tube and a static seat height and you would have to subtract, not add, the 1.2 cm (lower the seat) to maintain leg extension while increasing the setback because of the shallower angle. Anyway, OP seems to have gone missing for a few and I fear we may be going down a rabbit hole of just us two saying the same thing with different terms
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  #18  
Old 01-07-2020, 05:38 PM
Old School Old School is offline
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Quote:
Originally Posted by scottcw2 View Post
Maybe I should re-phrase the question.

With a saddle height of 77.7 from center of BB to top of saddle and a ST angle of 75*, what should the saddle height be for a 73* ST angle and a 72* angle if I want the same leg extension?
The difference would be 0.0053 of an inch. (seriously)
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  #19  
Old 01-31-2020, 09:03 PM
scottcw2 scottcw2 is offline
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Quote:
Originally Posted by giordana93 View Post
agreed as long as we don't confuse "raise the seatpost" with change the seat height.

In a real world bike fit we aren't just pushing a fixed tube with an anchored seatpost that constitutes a fixed length line segment. When setting up a new frame (with possibly longer or shorter seat tube depending on sloping top tube, seat lug design, etc) you put the seat at the same height and setback relative to the bb as before.

When you write above you are actually showing how much leg extension increases from the increased setback of a shallower angle if you don't push the saddle forward while maintaining the same "seat height" (vertical measure above bb). Think about it, 78.9 at 72 degrees is waaay longer a leg extension than 77.7/75 because you've raised the saddle over a cm AND increased the setback. 78.9 vs 77.7 isn't the difference in seat height--it's the difference in leg extension resulting from the increased setback of a shallower seat tube and a static seat height and you would have to subtract, not add, the 1.2 cm (lower the seat) to maintain leg extension while increasing the setback because of the shallower angle. Anyway, OP seems to have gone missing for a few and I fear we may be going down a rabbit hole of just us two saying the same thing with different terms
My head hurts. Here is what I am trying to accomplish:

Old bike that fit great... 75* STA, 4.8 cm setback, 77.7 cm saddle height from center of BB measured along the ST.

That bike is long gone. I want to replicate that on my current bikes. One has a 72* STA, the other two have 73* STA.

With the same setback of 4.8 cm, what should be my saddle height from center of BB measured along the ST??

If I'm understanding you correctly, the only thing that would make me want to raise/lower the saddle height is a change in setback. Less setback, higher saddle; more setback, lower saddle. Is this correct?

Thanks in advance.

Last edited by scottcw2; 02-01-2020 at 11:09 AM.
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  #20  
Old 02-01-2020, 04:52 PM
BdaGhisallo's Avatar
BdaGhisallo BdaGhisallo is offline
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Quote:
Originally Posted by scottcw2 View Post
My head hurts. Here is what I am trying to accomplish:

Old bike that fit great... 75* STA, 4.8 cm setback, 77.7 cm saddle height from center of BB measured along the ST.

That bike is long gone. I want to replicate that on my current bikes. One has a 72* STA, the other two have 73* STA.

With the same setback of 4.8 cm, what should be my saddle height from center of BB measured along the ST??

If I'm understanding you correctly, the only thing that would make me want to raise/lower the saddle height is a change in setback. Less setback, higher saddle; more setback, lower saddle. Is this correct?

Thanks in advance.
That’s not a question we can answer. With a different STA and a constant setback, the saddle will be in a different position in regard to a straight line drawn through the centerline of the seat tube. You will be measuring your saddle ht to a different point on the saddle surface. On a slacker STA your saddle will be further forward in the seat post cradle to maintain the same saddle setback.

What you need to do is measure from one consistent point on the saddle and measure a direct line from that point to the center of the BB, independent of any consideration of STA. The same for setback - measure from the nose of the saddle to a vertical line drawn through the center of the BB.
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  #21  
Old 02-01-2020, 05:43 PM
osbk67 osbk67 is offline
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Quote:
Originally Posted by BdaGhisallo View Post
That’s not a question we can answer. With a different STA and a constant setback, the saddle will be in a different position in regard to a straight line drawn through the centerline of the seat tube. You will be measuring your saddle ht to a different point on the saddle surface. On a slacker STA your saddle will be further forward in the seat post cradle to maintain the same saddle setback.

What you need to do is measure from one consistent point on the saddle and measure a direct line from that point to the center of the BB, independent of any consideration of STA. The same for setback - measure from the nose of the saddle to a vertical line drawn through the center of the BB.
I think of this question in terms of a circle with 77.7cm radius. If my maths is right that means one degree of rotation equates to more or less 1.35cm on the diameter of that circle. On that basis the point on the saddle in line with a 75 degree STA will be 4cm forward of the point on the same saddle when in line with a 72 degree STA, when setback and saddle height is unchanged. Or 2.7cm for a 73 degree STA. More or less, but within tolerance of measurement or feel.

If one is seeking to duplicate the position of a bike long gone, and especially with a different type of saddle, the above would provide at best a starting point...
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  #22  
Old 02-01-2020, 05:49 PM
osbk67 osbk67 is offline
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Quote:
Originally Posted by osbk67 View Post
I think of this question in terms of a circle with 77.7cm radius. If my maths is right that means one degree of rotation equates to more or less 1.35cm on the diameter of that circle. On that basis the point on the saddle in line with a 75 degree STA will be 4cm forward of the point on the same saddle when in line with a 72 degree STA, when setback and saddle height is unchanged. Or 2.7cm for a 73 degree STA. More or less, but within tolerance of measurement or feel.

If one is seeking to duplicate the position of a bike long gone, and especially with a different type of saddle, the above would provide at best a starting point...
So, to solve the problem - set saddle to 4.8cm setback and 77.7cm seat height at the point 4cm ahead of the point on the saddle top in line with the centre of the seat tube, on the frame with 72 degree STA.

Last edited by osbk67; 02-01-2020 at 05:52 PM. Reason: Clarification.
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  #23  
Old 02-01-2020, 07:47 PM
pbarry pbarry is offline
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The beauty of having the set back measurement is that it is independent of the STA. Set the saddle height, then drop your plumb bob or facsimile, and dial in the setback. Check saddle height again.
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  #24  
Old 02-01-2020, 08:56 PM
osbk67 osbk67 is offline
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Quote:
Originally Posted by pbarry View Post
The beauty of having the set back measurement is that it is independent of the STA. Set the saddle height, then drop your plumb bob or facsimile, and dial in the setback. Check saddle height again.
The set back measurement may be independent of the STA but the seat height is not when only known measured at a point on the saddle top in line with the centre line of the seat tube for a specific STA. Unless I'm missing something, again, this question is about where on the saddle seat height is measured, and duplicating that point on a new bike with a different seat angle. If the seat height was known in relation to a nominal point, say the midpoint of the saddle length or X cm from the nose, rather than in relation to a specific seat angle the question wouldn't arise.
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  #25  
Old 02-02-2020, 03:40 AM
scottcw2 scottcw2 is offline
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Quote:
Originally Posted by osbk67 View Post
So, to solve the problem - set saddle to 4.8cm setback and 77.7cm seat height at the point 4cm ahead of the point on the saddle top in line with the centre of the seat tube, on the frame with 72 degree STA.
That would be great if my saddles were flat(ish). I use Brooks saddles, which I set to be flat at the back making the nose point slightly up. Measuring 4cm forward of the center of the ST would hit an area on a slight up slope.
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  #26  
Old 02-02-2020, 12:56 PM
osbk67 osbk67 is offline
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Quote:
Originally Posted by scottcw2 View Post
That would be great if my saddles were flat(ish). I use Brooks saddles, which I set to be flat at the back making the nose point slightly up. Measuring 4cm forward of the center of the ST would hit an area on a slight up slope.
Whether or not that section is level, isn’t it still the point at which seat height was measured on the long-gone frame with 75 degree STA?

Admittedly the method I use applies a horizontal measurement to what is in fact a distance along the circumference of an imaginary circle centred on the bb axle, but this will still provide duplication of position within the tolerances _most_ people are able to measure.

6A61CFFB-B860-4CE0-A5C1-B9C4C9E7A74C.jpg

Photo from Road Racing Technique and Training by Hinault and Genzling c. 1985
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  #27  
Old 02-04-2020, 07:12 PM
giordana93 giordana93 is offline
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Quote:
Originally Posted by scottcw2 View Post
My head hurts. Here is what I am trying to accomplish:

Old bike that fit great... 75* STA, 4.8 cm setback, 77.7 cm saddle height from center of BB measured along the ST.

That bike is long gone. I want to replicate that on my current bikes. One has a 72* STA, the other two have 73* STA.

With the same setback of 4.8 cm, what should be my saddle height from center of BB measured along the ST??

If I'm understanding you correctly, the only thing that would make me want to raise/lower the saddle height is a change in setback. Less setback, higher saddle; more setback, lower saddle. Is this correct?

Thanks in advance.
sorry I missed the recent activity on this thread, but your last sentence is correct: if you start changing the setback, the height will change, but if you are able to duplicate the setback, the seat height would be the same: it's the same triangle being drawn between bottom bracket and your sit bones (assuming it's the same saddle because that is another variable, as the dip or sweet spot where you settle in can vary a good cm or so between saddles). I always think it's a good idea to measure to jot down the measurement of bottom bracket to the spot where I think my sitbones hit....

anyway, if you lay a ruler or torpedo level across the top of your saddle and see where that intersects your seat height measure along the seat tube, it should be ok
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