nope

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Tech writer/support on this here site. FIST school instructor and certified bike fitter. Formerly at Diamondback Bikes, LeMond Fitness, FSA, TiCycles, etc.
Coaching and bike fit - http://source-e.net/ Cyclocross blog - https://crosssports.net/ BJJ instruction - https://ballardbjj.com/

Thanks for the reply. Maybe I'm just overthinking this, but my thinking says if my saddle is further back in relation to the BB, my reach would be stretched out more if I kept the same pad X. Is there something I am missing?

Your fit determines the saddle position, the seat tube angle does not. On a bike with a slacker seat tube you'd need to slide the saddle further forward on the rails.

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So that is my mistake. I am thinking that the saddle would move proportionately rearward as the STA moved rearward. Ok, so I have to move the saddle forward to match the position on Bike A.
Thanks for the help. I knew I was overthinking it.

Seat tube angle ≠ saddle position, or SX/Y, and the 39mm you're deriving is within the range of adjustment allowed with most saddle rails, depending on where the saddle was originally positioned. Which isn't included in your set of variables. You can derive a virtual/effective seat tube angle from where the saddle is positioned in space - which you appear to have added as an edit - but there's a broad range of possible virtual angles depending on saddle type and seat post clamping mechanism. Seat tube angle does not directly correlate to effective seat tube angle, or actual saddle position.

So it's an issue of the terms you're using, and what you're actually trying to get at. If what you're trying to ask is "if SX moves in a negative direction, should we expect a corresponding change in padX", the answer is yes. We'd also probably expect to see an increase in PadY, but that's where things get complicated, and it's why we can't actually answer...


...because, maybe? Changes in SX result in changes in body mechanics, and padX/Y is affected by these changes in body mechanics. The basic trig will tell you what changes in padX/Y will correlate to changes in saddleX/Y to maintain a constant relationship between those points in space, but it won't capture the relationship of those changed points in space to the body that they are there to support.

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Tech writer/support on this here site. FIST school instructor and certified bike fitter. Formerly at Diamondback Bikes, LeMond Fitness, FSA, TiCycles, etc.
Coaching and bike fit - http://source-e.net/ Cyclocross blog - https://crosssports.net/ BJJ instruction - https://ballardbjj.com/

Thanks for the reply. My edit was much simpler, I accidentally put pad y in the title.
I appreciate the explanation, I certainly have much to learn. My basic point of this was to understand, if I was to leave the saddle position as is, and not move it forward would I then have to adjust my pad x measurement.
This question came to be because of a bike I am interested in, the Argon18 E-119. Using my 470mm pad X has me at the very end of range in the size large based on their mid pad measurements, and I am simply trying to think of ways to make it fit, otherwise I will cross it off the list if it won’t work.

From what I'm reading, pad reach range of a L is 44.4-53.1 on pad center. That seems like it will be just fine for you? Even if your 47 is from pad edge, we typically calc out the difference as +4cm, and you're still in the range.

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Tech writer/support on this here site. FIST school instructor and certified bike fitter. Formerly at Diamondback Bikes, LeMond Fitness, FSA, TiCycles, etc.
Coaching and bike fit - http://source-e.net/ Cyclocross blog - https://crosssports.net/ BJJ instruction - https://ballardbjj.com/

...but, yeah; don't push your seat back to solve for pad X disparity.

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Tech writer/support on this here site. FIST school instructor and certified bike fitter. Formerly at Diamondback Bikes, LeMond Fitness, FSA, TiCycles, etc.
Coaching and bike fit - http://source-e.net/ Cyclocross blog - https://crosssports.net/ BJJ instruction - https://ballardbjj.com/