Lightcarbon LCR020 35c clearance aero frame

[benevelo]

My thought based on the information is the following. Virtual STA of 74.5* and Actual STA of 79.5. But as you point out, the actual STA does not cross through the bottom of the BB but offset behind.

My thought is taking the distance of the offset and you would have to subtract that from what the actual STA pushes you forward. IF 79.5* moves the saddle 60mm forward and the offset is 40mm, then the saddle actually only moves 20mm forward.

Note I do not know if that offset is 40mm, I picked a number for my example.

The offset for your size looks to be around 95mm (horizontal distance between 79,5° line and BB)

[JensenMacD]

The offset for your size looks to be around 95mm (horizontal distance between 79,5° line and BB)

How did you get to that number? I just did the math and I get 40mm for the 470 frame.

This is triangle we are solving for the bottom length. The top is where the two STA intersect. Bottom right inner angle would be 74.5* STA, bottom left outer angle is 79.5* meaning inner angle is 100.5* (180-79.5). Leaving the top angle as 5* ((180-(100.5+74.5)). Right side length of the triangle is the middle of BB to top of seat tube, 452mm. That is all we need to solve for the bottom length of the triangle, I marked as C. That would be the total length of the offset of where the 79.5* line would be level with the 74.5* at the BB.

[KRoser]

JensenMacD:
Thanks to that seat post offset dimension from center of bb you can not calculate these triangles to top of seat tube, but to the top of your actual seat height.

These are my calculations of STA in fusion 360, difference is not that masive as it seams at first glace.
At size 54cm and saddle height 740mm with geo chart stated STA 73.6° actual STA will be 74.8° (+1.2° difference) and saddle will be 15mm further forward than on bike with standard STA of 73.6°

Hope this makes sense.

[JensenMacD]

JensenMacD:
Thanks to that seat post offset dimension from center of bb you can not calculate these triangles to top of seat tube, but to the top of your actual seat height.

These are my calculations of STA in fusion 360, difference is not that masive as it seams at first glace.
At size 54cm and saddle height 750mm with geo chart stated STA 73.6° actual STA will be 74.8° (+1.2° difference) and saddle will be 15mm further forward than on bike with standard STA of 73.6°

Hope this makes sense.

It makes sense but that is why I subtract the offset from the bottom of the triangle, from the total saddle movement forward. In my case, at a 690mm bb to saddle height, the 79.5* is 60mm further forward, subtract the 40mm offset puts total of 20mm further forward. That is what I expect at my saddle height on the 470, with my triangle calculation.

Using your example the difference of 73.6* vs 78.4* at 750mm is 60.8mm further forward. Doing the same triangle method, the bottom of the triangle is 45.3mm. That gives 17.8mm 15.5mm(60.8mm-45.3mm) further forward for your saddle. Not perfect compared to your model in Fusion, which I expect to be more accurate. But for a quick calculation, I think my triangle works for ballparking how much further forward the saddle will be. Given the few mm can easily be adjusted by the saddle rail.

All this assumes a 0 offset post, which is what I was quoted when getting info on the LCR020. Offset would change my calculations.


Edit my math - 60.8-45.3 is 15.5 not 17.8...

Also checking the diagram, it shows a saddle height of 740mm not 750mm. Once that is adjusted, it gives me 14.7mmm Even closer to the 15.1 from Fusion. This gives me more confidence my idea is pretty close.

[benevelo]

You are completely right with ~40mm...
I used the drawing and forgot to use my own scaling in the final approximation. So my approach should have yielded 47.5mm