Correct because seat angle varies on each size.

yeah, they explain that oddity as a way to avoid toe overlap, and you just use shorter stems on the smaller sizes

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Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
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Freelance sports & outdoors writer Kathryn Hunter

There should be something wrong with the smallest size reach. Because the smallest 2 sizes have the same front center, the only way they have the same reach is with the same head tube angle. Since the Front center for the smallest two sizes is identical, it is impossible that the reach is the same given that the head tube angle for the smallest size (49) is slacker and the head tube is shorter. The smallest size, given the same front center, slacker head tube angle and shorter head tube should have a shorter reach.

Between the 52 and 54 at first I thought there has to be another mistake, because you can't have the same reach if the front center is different and the head tube angle is the same. Unless of course your head tube delta is tall enough to "move back" far enough to overcome the 7mm front center difference....so I had to crack out the calculator and sin(17 degrees) is 029....multiply that by the 2 cm difference in head tube height, and you get 6mm....which is prettty close to the 7 mm front center difference. So it seem that it is correct!

Now having gone through the second exercise, I just realized, that on the first comparison I may have missed something. Because of the delta in head tube height, the 52 frame has more vertical to get you back to the same reach given the the steeper head tube angle....not feeling like drawing out the triangles right now...but I must be wrong in my first statement.

All that to say, that the guys at Specialized magically gave all these three sizes the same reach by slightly changing front center, head tube angle and head tube length!

Just to be clear, seat tube angle has zero bearing on frame reach. Front center, head tube angle and head tube length come into play on where the stem goes into the head tube. The seat tube angle could be whatever you want because you just shrink or grow the top tube to end up to your point in space where the stem goes into the frame (stack-reach intersection point)

You can define reach either by:
* front center, fork rake, head angle head tube length
OR
* Seat tube, seat angle, top tube.
The calculations should concur.
As OP has observed, what initially look like longer frames are actually just slacker. A refinement of reach and stack is to calculate reach at a given stack height.

edited -- I figured it out.


Andy