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  #1  
Old 11-16-2019, 03:25 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, 04:03 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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  #3  
Old 11-16-2019, 04:10 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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  #4  
Old 11-16-2019, 04:20 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, 04:39 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, 06:35 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, 05:21 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 05:56 AM.
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  #8  
Old 11-17-2019, 10:06 AM
scottcw2 scottcw2 is offline
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Thank you.
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  #9  
Old 12-07-2019, 06:16 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, 10:07 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, 12:08 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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  #12  
Old 12-19-2019, 06:49 AM
Road Fan Road Fan is offline
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Quote:
Originally Posted by ultraman6970 View Post
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.
That's good to know - I was chatting with him a few years ago on another forum. Then he left it. I recently found the company's website, but there was no way to tell if it is current. I've liked the concepts behind the saddle since I read Joshua cohen's book about it. When Performance locally had them blowing out for $20 to $30, I bought two.

On that saddle I really like the shape of the narrow part - if the horn is that narrow (Toupe 143 shares this feature), I find it easier to spin fast. I also like the shape of the leading edge of the wide part - if it's too wide I get excess pressure and bruising behind my thighs. I wish the platforms were a cm or so wider. Like the Toupe 125 (123?) I sometime feel like I'm falling off the edge of the platform.

I have a vintage Brooks Pro on the trainer bike, and I have a modern Brooks Swallow to try out. We'll see!
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  #13  
Old 12-19-2019, 07:26 AM
Road Fan Road Fan is offline
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Quote:
Originally Posted by giordana93 View Post
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
Your technique sounds like what I use!

For me the end goal, say moving the same saddle between two bikes, is for both of them to have the same saddle height, same setback from BB plumb line, and same tilt angle.

How you achieve that on the two bikes is affected by seat tube angle. Saddle setback from BB is influenced by seat tube angle, the setback of the seatpost, and the design of the flat sections of the saddle rails. Not all "racing" saddles are the same, and not all models of Brooks leather saddles are the same - you can get more setback with a Swift or a Swallow for a given seatpost
than you can with a B17 or a Professional. Similarly I can get more with a Specialized than with a Selle Italia, at least old ones.

Example: you can get more setback with a Professional on a vintage Clubman frame due to its STA of 71 degrees than you can with that saddle on a late-model Mondonico with STA about 75 degrees (in my small frame sizes). To make the same total setback I needed to shop my feet to the bones for a high-setback seatpost to use on the Mondo. The Clubman may be able to use a straight seatpost, but the Mondo will need something like a Nitto S-84 with the funky "lugged" design.

In my opinion the saddle height that really matters is the hypotenuse of a right triangle from the BB axis to the sit-bone contact point (its centroid, actually), and the horizontal intercept between the sit-bone contact and the vertical (plumb) line through the BB axis.

It can affect how the saddle feels and the suitability of your pedaling position. If the saddle moves backward along that horizontal intercept, the hypotenuse will become longer, according to the Pythagorean Theorem (sorry, the math is necessary to mansplain this). In my opinion and my riding, that lengthening matters and I need to compensate it by lowering the seatpost in the seat-tube a little amount. Then I may need to slam the saddle back a bit more to restore the setback. This process can involve some iteration. My hope is always not to have to fine tune it while I'm riding with Mrs. Road Fan.
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  #14  
Old 12-19-2019, 12:19 PM
marciero marciero is offline
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Quote:
Originally Posted by giordana93 View Post
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
This is precisely what the OP was asking about. The formula I gave in post 7 gives the amount-it's not 0.
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  #15  
Old 12-20-2019, 11:30 PM
giordana93 giordana93 is offline
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Originally Posted by marciero View Post
This is precisely what the OP was asking about. The formula I gave in post 7 gives the amount-it's not 0.
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 point then is that OP needs to know his seat setback as well as height and that those numbers should both be duplicated when changing to a frame with a different seat tube angle--and be prepared to deal with seat post hassles if there is a large change in sta and/or (s)he is on either end of bell curve. There's a reason why small frames are steeper than big ones.
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