I redid my sketches over lunch and have a very good model for experimentation now.
- This shows a Large size Epic Evo in
- Red at Unladen height.
- Black at Sagged height, with 20% sag at both ends just to show one example.
- This is with the fore-aft location of the BB held, so the frame is not “moving” forward or backward in this example. It does show the relative drop and angle change of the frame, along with wheel shift in the horizontal since the tires are held flat on the ground plane.
- This pic shows a measurement between the effective top tube at both heights.
- Note that the angular difference is out to the 3rd decimal place, so it is dead level for all intents and purposes. Meaning there would be no need to alter saddle angle between unladen and sagged heights.
I can quickly alter the sketch once I get your sag data (@HumpDiesel) in either % of total travel on each end, or actual distance sagged. Then we can see more of what you have in your use case. I can also change from restraining the horizontal movement at the BB to the front or rear wheel if those are closer to what you are doing in your evaluations.
But in short, unless you have a very different amount of sag front vs rear, I don’t think the delta you are getting above is from angular change. Something is more likely moving in the horizontal plane between your unladen and sagged states.
-
Evaluating horizontal shift with BB held vertical, we see a slight shift at the rear, with nearly 10mm at the front.
-
With an even 20% sag at both ends, there could be a horizonal delta at the saddle nose if you locked the front tire, which would mean the bikes moves “forward” at the saddle 10mm or so.