Help me wrap my head around Seat Tube Angle

[Davey Two Shoes]

Hey Gang,

I can't get a grip on this. I am what we call "seat post challenged" In that I have a very short PBH resulting in less than expected exposed seatpost per size per my height. Hope that makes sense.

So I'm wondering how the amount of seatpost is affected by the seat tube angle going from 71.5 degrees like on my Hillborne to something with a more "aggressive" STA like 73.5. If we assumed the seat tube length were to remain the same length and be measured the same,  would I gain or lose exposed seat post?

Sorry to be wordy, I have trouble explaining what I don't understand!

[Mathias Steiner]

>>  how the amount of seatpost is affected by the seat tube angle going from 71.5 degrees like on my Hillborne to something with a more "aggressive" STA like 73.5.

Short version, it isn't.
The simplifying assumption is that we don't change the fore-and-aft position of the saddle as we change the seat tube angle.

To visualize, imagine you're getting on the bike to set the initial seat height. The classic method that always gets me close is to fully extend my leg and put the heel on the pedal in the downward position. Then seatpost-seattube-crank form a straight line, and the geometry does not change when the STA changes.

On the other hand, if you imagine a frame with adjustable STA -- ridiculous, but this is our gedankenexperiment, so we get to do whatever -- then there is a slight lengthening of the exposed seatpost as the STA decreases and the saddle slides forward  on its rails to stay in the same place relative to the handlebars and bottom bracket.

As always, you have to fit your body to the entire bike, there's no way to change only one variable.

[Garth]

You may find you need to adjust your saddle height some going to the steeper STA. Fom my experience in a drastic move forward over the BB on a 62cm c-c frame, I've changed saddles and posts many times in getting used to it. Different saddles have different stack heights, from rails to the top of the saddle. Different posts allow for varying amounts of expsore. If anything my overall saddle height is slightly lower now as I've altered my pedal style some.  Assuming you're using the same saddle and post, it wouldn't be more that a cm. difference, I'd guess. If such would make or break a frame from fitting you, as in no exposed post at all, you're looking at the wrong size frame to begin with. Otherwise it's no big deal on a frame where you have say a fistful of post. That your fist size, so it can vary from rider to rider ! 

Bottom line with any such change, there's always those unforeen consquences in doing so. 

There's also this about how a steeper STA alters saddle setback, and thus one's reach to the bars. 

https://groups.google.com/g/rbw-owners-bunch/c/lelFFs9VBCM/m/UBFEL7FTCAAJ

On Wednesday, September 24, 2025 at 11:42:03 AM UTC-4 Davey Two Shoes wrote:

[David Ross]

It is unaffected if you assume you're using the same seat post. The seat tube length and your saddle height are both measured along the line of the seat post, regardless of its angle. The only way I can think of that would slightly lower the amount of exposed seat post would be if you went from using a laid back seat post to using a seat post with zero set back. This would very slightly increase the amount of exposed seat post a given level of saddle height. 

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[Mathias Steiner]

This is good, Garth, I hadn't seen that before. 
Here is my version -- really just an example I wrote up so I wouldn't have to do the math again.

I was comparing the geometry of two Cannondales, my '87 ST600 in 25" and my '95 T400 in the 23" size, to the Hillborne, which has a slacker STA; 71.5 degrees vs. 73.5 for the 'dales.

The text in the lower left has the difference in effective top tube length -- IF we leave the saddle position alone. 

Mathias "nothing exceeds like excess" Steiner in East Lansing, MI

[Ben Miller]

"So I'm wondering how the amount of seatpost is affected by the seat tube angle going from 71.5 degrees like on my Hillborne to something with a more "aggressive" STA like 73.5. If we assumed the seat tube length were to remain the same length and be measured the same,  would I gain or lose exposed seat post?"

In the described situation you'd neither gain nor lose exposed seatpost. Generally, you're trying to keep your "saddle height" constant across bikes. "Saddle height" is the measurement from the BB to the top of the saddle along the seat tube. In others words, it's the length of the ST + exposed seatpost + plus the saddle's height (different from our saddle height... Suffering from some overlapping terminology).

So unless you change the ST or the saddle, the exposed seatpost remains constant. Steepening the ST angle will bring the saddle further from the ground. But the meaningful distance is to the pedals, and you want that to remain constant. 

[Zachary Cannon]

I think the issue here is that you want to be careful using your seat tube angle to raise your saddle because of how that alters other aspects of your fit.. 

If a steeper seattube results in you having less saddle setback (usually measured as the distance the tip of the saddle is behind the bottom bracket [everything changes with two different saddles]), you will gain a marginal amount of seatpost exposure. A couple of mm if you move your saddle forward a cm.

My concern is that you want to optimize your fit by positioning your saddle relative to the bottom bracket. In the same way that you don't want to move a saddle foward to compensate for too much reach, you don't want to move a saddle forward to gain more exposed seatpost.

As a 6'2" rider, I personally favor very slack seat angles to achieve the position I prefer/feels best. However, I've found my 5'1.5" wife and 5'5.5" daughter both felt better with a steeper seat angle and less saddle setback.

I'd recommend experimenting with how much saddle set back feels best to you, and then use that to decide on what seat angle you want. I wouldn't be surprised if you prefer a steeper angle, but it's best not to assume as they are too many variables at play such as femur length and flexibility. Another good starting point is measuring your most comfortable bike relative to the bottom bracket to get a starting point for things like saddle setback and once that's established the reach to your bars and how far you like your bars above or below your saddle.

Good luck finding a good fit! Zach (in ABQ)

[Mr. Ray]

Davey,

Here is a diagram illustrating the differences with STA's.  You will notice when saddle height doesn't change (inseam length), as the STA gets slacker, the saddle moves away and lower relative to your bottom bracket (and thus your handlebars).  Your peddling position relative to your bottom bracket changes.  In general, a bike with a slacker STA (i.e. 71.5) is for more upright riding and a steeper STA (i.e. 73.5) is for more aggressive riding (i.e. handlebar grips not behind head tube and handlebar height level or lower than saddle height).  If you get a bike with a steep STA like 73.5 and have the handlebars high and swept back, you will likely need a very laid back seatpost to push your saddle back far enough to be comfortable when peddling.  The problem you might run into is finding a seatpost with enough setback adjustment.   

On Wednesday, September 24, 2025 at 11:42:03 AM UTC-4 Davey Two Shoes wrote: