Originally Posted by
Dean V
There is also the possibility of using a different size frame. Maybe you ride a 56cm Tarmac but get the fit you want on a 54cm Roubaix.
It really has to be taken on a case by case basis.
This is entirely true. But it is usually thought that the size labels that a particular brand uses refer to what they see as equivalent sizing, so if we are going to talk about the intent by Specialized to offer a "race geometry" vs an "endurance geometry" there isn't going to be a more fair comparison than the same size for two models.
In the real world, people aren't a "size 56" because some 56s have tall headtubes and others do not, and that (along with other factors) makes a big difference in how you're going to fit on your bike.
Speaking of which, there is a general idea that seat to bar drop is indicative of comfort. From working extensively for a fitter it is very clear that this is not true. Most people would be uncomfortable sitting up too high, and there needs to be a reasonable bend at the waist for comfort on drop bar style bike - to the point that lower back issues are sometimes fixed by lowering the handlebar, not raising it.
In the Rando or touring world, bars are generally higher, but setback is much greater with the net effect of rotating the entire rider back around the axis point of the BB. They end up with a similar back bend to a racer in many cases, but the leg, hip, back and seat to bar angles are all rotated toward the rear tire. This is a very different fit than taking a racing bike (73° or greater STA) and raising the bars.