Quote:
Originally Posted by simonov
Why wouldn't you. The STA will dictate the range of possible saddle positioning given a preferred saddle and post. It also informs the real world reach when looking at top tube length. Understanding the relationship between TT and STA will help inform options and guide decisions on seatpost setback, stem length and other reach related characteristics. A 55 TT with 74 deg STA and a 55 TT with 73 deg STA, both as most manufacturers measure, will have a roughly 1 centimeter difference in the "length" of the fit. Depending on where a rider fits in that range it could mean one bike won't get the saddle far enough forward without a compromise or that the other wont get the saddle far enough back. Sure it's just one number in a complex geometry, but it's an important one.
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This is all true enough, but real world it isn't such a great idea to presume that you calculate your stem length. Differences in brake hood and handlebar shape can have an unexpected impact on what kind of reach you're really going to need.
For most folks, a 25mm set back seat post will work with a standard saddle pretty well between 72 and 75° STA, which is why it doesn't come up a lot.