I recently sold my '12 Cervelo R3 and bought a Cannondale CAAD 10.
I was professionally fitted on the R3 but won't have the luxury of the same on the CAAD.
The CAAD has a longer "reach" measurement but a shorter effective top tube measurement, only by .5mm though.
This would mean I need to move my saddle back by a hair to achieve the same "setback" measurement as the R3.
My question is,
Will I need a longer stem as well?
Forgot to add...the reach on the C3 Cannondale bars is also shorter than the
R3's 3T Ergonova bars.
Thanks in advance for any help.

First off size of the r3 and caad and also the stem length u had

Both 54...100mm stem on Cervelo ....stock on CAAD is 100mm.

The reach on the Caad is 5 mm more than on the R3. But the Caad has 9 mm less stack than the R3 so assuming you want the same bar height (relative to bb) the effective reach on the caad will be less than 5 mm bigger than the reach on the R3.

As for saddle setback, effective top tube length is a red herring (since it's dependent in stack). You should consider difference in sta. The change in saddle setback going from one sta to another is dependent on saddle height, but as there's only 0.5 degree difference between the two it will be less than 1 cm difference between the two ( (cos(73)-cos(73.5))*seat height).

Yes you need to move the seat back 7mm-ish to achieve the same relative position. Since your bar has less reach, you probably won't need a different stem, because assuming the different in bar reach is close to 5mm, you could end up with an identical HOOD POSITION.

What I would do, an I'd highly recommend to everyone else, is keep stems in a few sizes (grab cheap house brand stems from Scott, Felt, Cdale, etc for $10 on ebay). The time it takes to swap them is roughly the same as asking on a forum. Go for a ride and see which one works best. You could even keep two stems in your jersey pocket and change them on the fly.

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If only the "reach" was measured at some arbitrary height (same for all frames)... then you'd be able to compare directly.

The 9mm shorter stack of the Caad shaves off over half of the 5mm reach increase.

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'move saddle back...' no, assuming you want to replicate your postition 1:1, your saddle stays in the same position in relation to the bottom bracket

did you record the measurements on your R3 ?

then re the handlebar, what headset bearing cover (and spacers) did you use on the cervelo, which headset bearing cover will you use on the CAAD10 ?

do you want to replicate handlebar drop 1:1 ?

Reach values can only be compared directly, if the stack heights are the same. If not, then subtract 3mm for each 10mm of stack height difference, from the reach of the shorter frame for a more accurate comparison.

The saddle nose should be positioned at the same position as it was on the old frame (relative to the BB centerline). Any difference in reach is usually handled with a different length stem.