Thanks for this (and the others who responded). Yeah, from what I'm hearing the 50cm is the size to go with.
It's still perplexing because the geometry of the 52cm is clearly closer - on paper at least - to the Synapse 51cm and the Trek Domane medium.
LCR018-D 50: ~12 mm lower and 5 mm shorter than Synapse 51
LCR018-D 52: ~2 mm taller and 1 mm longer than Synapse 51
But real world experience trumps numbers. But I've hacked my Aethos (528 factory stack) to a 570+ stack with spacers and a 17 degree stem. My entire goal with a new frame is get into a more upright position without a massive spacer tower and silly 17 degree stem. On paper at least the 50cm just doesn't do that quite as well.
Find a bike fitter local to you and see what they say. I had a bike fit, 172cm and went for size 50.
I think you need to look at it from the whole build. Consider if you have a huge stack of spacers, you reach is a lot less than on paper... Lots of assumptions here but its to get the point across regarding the aethos. Comparing a 100mm -7 stem with 20mm of spacer (pretty normal) to a 100mm +17 stem with 40mm of spacer (my guess to get 570+ stack) on a 72.5* headtube. The reach is 22mm shorter and 57.5mm higher with the +17 stem. You can't just look at individual numbers but the whole system.
If you used a bike you adapted to fit as your baseline, you could end of with a stem way too looooooooooooong by accident.
How many spacers, head tube angle and stem angle/length is on the synapse? Are you factoring in the LCR018 upper bearing cover adds 10mm? Did you consider the bar reach itself from where the stem ends to the shifter? There are many factors to consider beyond just the stack and reach of the frame.
The 52 may be correct, once you account for everything. It may solidify the 50 is correct. Also if you are going for more stack, consider the new HBR20 with 30mm of stack to the bar when ordering the LCR018. If you are not in a rush to get the frame. I think the HBR20 is supposed to be ready at the end of the year.
What bike do you use the most, how is it setup, what do you want to change with the LCR018. Asnwer those questions, then start to compare all the numbers together.
