Cervelo's new geometry for the S5 is a move toward the industry standard for stack and reach, with shortened head tubes reducing the stack and increasing the reach of each size.
I decided to see what this new geometry did for my fit on the bikes. Would I end up with a hideous tower of spacers?
Here's a chart I posted before, with Cervelo S5 2015 replacing the Cannondale Evo. The Cervelo S5 and Evo geometries are fairly close, and so I removed the Evo curve to reduce color. I also show the Trek H1, an example of an aggressive geometry, and the Fuji SL1, which is my bike.
My bike is small, with an 11 cm +6 degree stem establishing the position set for me by 3D Bike Fit in San Francisco. Then from those coordinates (to the right of the plot), I trace back to the frame using a variety of stem options, using the head tube angles of the Cervelo R-series bikes.
With the R-series (also S2-3, and also the 2014 S5), I could ride a 48 cm frame with a 13 cm +6 degree stem, a 51 cm frame with a -6 degree 11.5 cm frame (adding 1 cm of spacers), or a 54 cm frame with a -17 degree, 10 cm stem (with around 5 mm of added spacers). The 51 cm is the most likely candidate here.
To the S5, shown with dotted lines: now the 51 cm would require an 11 cm, +6 degree stem, removing around 5 mm of my present 1 cm stack height. But with the 54 cm frame, I'd be very close to my present fit using a 10 cm -6 degree stem. It would be even closer if I'd adjust for the fact that the 54 cm has a 0.9 deg steeper head tube and the displayed stem trajectory is just slightly too steep (upward).
The result: with a -6 degree stem, I can use the 51 cm R-series with an 11.5 cm stem, or the 54 cm S-5 with a 10 cm stem. Personally I'd prefer the 10 cm stem.
So really no spacer difference is involved: to the contrary, in fact. With the S5 I end up with fewer spacers than I would have for the R-series. And the reason is with the S5 I can upsize to the 54 cm frame with a -6 degree stem.