Originally Posted by
Puget Pounder
Keep in mind a couple of things about matching fit... For a bike with EQUAL top tube lengths:
1. Changing the angle by 1 degree, adds/subtracts 1 cm from the TT length. Steepening the STA by 1 degree, effectively makes the TT physically 1 cm shorter. Steepening the HTA by 1 degree, makes the TT physically 1cm longer.
2. Despite the info above, the one with the steeper seat tube angle ends up being being effectively longer if you are trying to achieve the same saddle setback (which is important for fit). The steeper STA puts you further forward, but you need to offset that by moving your saddle backward, or getting a more setback post.
Take a look at the angles between touring bikes and racing bikes. The touring bikes will definitely have more slack head tubes than the race bikes, decreasing the effective length of the TT. Don't go by TT length alone, bikes with different angles and TT lengths will fit more closely than at first glance.
As far as the head tube angle, this would seem to have relatively little effect on the feel of the top tube length and overall "reach". This because of the very short difference in height between the bars and where the top tube length is measured from.
The head tube angle can however effect the range of stem lengths that will give the bike good neutral handling.
The seat tube angle, otoh, is angled from the bb all the way up to where a top tube measurement is taken from, so a 1-degree steeper seat tube will push the top tube foreward about .8 or .9cm (depending on seat tube length).
Also, I kind of disagree with comment #1 in that the frame angles actually do not change the physically measured lengths of the tubes (I'm just struggling with your wording).
But of course a builder will have to match physical frame tube lenghs with chosen angles so that all tube junctions meet with no gap or overlap.
I totally agree with your last comment, after comment #2. You described the situation quite well imo.
I caution against over-generalizing about "touring" and "racing" geometry without looking at the spec's though, since many frames have a surprising choice of angles and because the smallest and largest frames have their angles adjusted almost radically in many cases.
Road bike frames built with multisport "tri" capability can for instance have very steep seat tube angles combined with a touring-style 72 degree head tube angle, while road bike frames built for stage racing or century riding can have somewhat the opposite combination of angles.
Case in point is the Peugeot UO8, UO9 and UO10 frameset, which from 1978 to 1979 (US models) underwent a geometry reversal from "Fast Century" (steep head tube, relaxed seat tube) to "Triathlon/Multisport" (steep seat tube and relaxed head tube).
The mid-range UO9 model took on the SuperSport moniker with that change, fwiw, and took on better tubing, new lugs, shorter stays and brazed (no longer spot-welded) cable stops.