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  #1  
Old 11-16-2019, 04:25 PM
scottcw2 scottcw2 is offline
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Seat tube angle effect on saddle height?

If all else is the same - reach to bars, drop, etc. - how does a slacker seat tube angle effect saddle height? Should saddle height be the same, higher, or lower? If higher or lower, what is a rough calc for each degree of angle?

Thanks!
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  #2  
Old 11-16-2019, 05:03 PM
mhespenheide mhespenheide is offline
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Most people measure "seat height" from the center of the bottom bracket up to the saddle along the line of the seat tube. So in that sense, changing the seat tube angle doesn't change the "height".

Now, in a real sense, the actual height of the saddle above the bottom bracket does change as you go to slacker angles. But basically no one thinks about it that way.
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Old 11-16-2019, 05:10 PM
marciero marciero is offline
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The height of the saddle, as measured vertically from the ground will decrease. But the distance from the bottom bracket will not change, assuming you have the saddle in the same position on the rails of the seatpost. That is what most people mean by saddle height.

If you imagine holding the saddle fixed while rotating the seat tube backward about the bottom bracket axis, your saddle would drop. The vertical distance is L(1-sin(alpha)), where L is the seat tube/seat post length and alpha is the seat tube angle. For saddle "height" of 74 cm, the difference from 73 to 72 degrees is about 4 mm. (This assumes that the saddle rails are horizontal and not tilted back.)

You might want to do this if, for example, you have your saddle all the way back on the rails and wanted the clamp somewhere in the middle.
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Old 11-16-2019, 05:20 PM
scottcw2 scottcw2 is offline
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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?
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  #5  
Old 11-16-2019, 05:39 PM
marciero marciero 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?
To clarify, what you are calling saddle height and leg extension are in fact the same thing. If the position of the saddle on the seatpost is not changing, the whole ball of wax changes together and there is no change. If you plan to move the saddle relative to the seatpost, see above.
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  #6  
Old 11-16-2019, 07:35 PM
scottcw2 scottcw2 is offline
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Quote:
Originally Posted by marciero View Post
To clarify, what you are calling saddle height and leg extension are in fact the same thing. If the position of the saddle on the seatpost is not changing, the whole ball of wax changes together and there is no change. If you plan to move the saddle relative to the seatpost, see above.
Yes, the saddle relative to the seatpost would move to keep the same reach and setback.
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  #7  
Old 11-17-2019, 06:21 AM
marciero marciero is offline
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Quote:
Originally Posted by scottcw2 View Post
Yes, the saddle relative to the seatpost would move to keep the same reach and setback.
Okay. The height drops as above. Then to raise it back you need to increase the height, but along the seat tube, so a bit longer. You need to multiply by cosecant of seat tube angle.
A formula is L2= L1sin(SA1)csc(SA2), This uses the sine and cosecant functions from trigonometry
where L1 is current saddle "height" at seat angle SA1 and SA2 is the new seat angle.
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.

Edit: Again, this assumes your saddle rails are horizontal. If they are tilted back the height increase could be significantly less.

Last edited by marciero; 11-17-2019 at 06:56 AM.
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  #8  
Old 11-17-2019, 11:06 AM
scottcw2 scottcw2 is offline
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Thank you.
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  #9  
Old 12-07-2019, 07:16 AM
Road Fan Road Fan is offline
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Quote:
Originally Posted by marciero View Post
Okay. The height drops as above. Then to raise it back you need to increase the height, but along the seat tube, so a bit longer. You need to multiply by cosecant of seat tube angle.
A formula is L2= L1sin(SA1)csc(SA2), This uses the sine and cosecant functions from trigonometry
where L1 is current saddle "height" at seat angle SA1 and SA2 is the new seat angle.
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.

Edit: Again, this assumes your saddle rails are horizontal. If they are tilted back the height increase could be significantly less.
Kudos for this discussion!

For me it's as follows: I adjust for leg extension, using the straight-leg heel contact method to find the maximum height without painful rocking. My bane as a cyclist is perineal and grain fold pain due to abrasion. That target height can, I suppose, be related to saddle design, but I set up the bike for one saddle at a time. I fine-tune for balance and fore-aft stability using setback, tilt, and rotation. I want setback so I can easily hover my hands over the hoods without falling forward, to set saddle tilt so I don't slide forward or backward without intending to, and rotation in case there is some asymmetry.

Simple geometry says that as my butt moves back on a level saddle such as a plank, my leg extension needs to be longer. I then lower the saddle to find a new height point, and see if my other factors are ok: tilt and rotation.

Some saddles have rails that lower the saddle as you slam it. This is good, but I still usually need to adjust it subjectively. I just got an older but good B17 Select that I'm working with.

Another fine point is the shape of the saddle looking down from the top. I prefer a saddle with the narrow section staying narrow and widening "suddenly." Examples are the Specialized Toupe, Brooks Swallow, the Rivet saddles, and the old Form E3 (reincarnated as Kontact). I'm not sure if Kontact is still on the market. If the transition between the nose and the platform is too gradual, I get pressure on the back of my thigh up at the hip, and tend to slide forward on the saddle, and fall off of the sit-bone platform.
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  #10  
Old 12-07-2019, 11:07 AM
ultraman6970 ultraman6970 is offline
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Kontact is still up and running, actually he dude is a forumite. years ago he was super kind to send a pass a long saddle which no idea where ended up.
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  #11  
Old 12-08-2019, 01:08 PM
giordana93 giordana93 is offline
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0 effect. as in, sta has no effect on saddle height. You should know your saddle set back and saddle height, and unless the seat tube angle is too extreme (either too shallow or too steep) to achieve the setback number, it doesn't really affect setting the height--the seatpost may go up a couple mm to achieve the same overall height, but you will have also moved the saddle rails in the seatpost to replicate your setback. Now, if you are experimenting with setback, you will have to lower the height a mm for 3mm more rearward setback or raise it if you push the saddle forward. For repeatability you could take saddle height measurement from the point where your sitbones hit and not at the seat tube intersection.

if you don't know your setback, it's more important than sta
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