Originally Posted by
DaveSSS
Stack really does have some value to it. It totally eliminates fork length and BB drop from the comparison. If all you do is compare the head tube length, then BB drop and fork length differences could lead to unanticipated changes to the spacers required under the stem or the stem angle, to get the desired setup. Headset height still needs to be considered, since some frames may have a 20mm minimum headset top section and another may allow as little as 8mm.
And eliminating the fork length does what for fit? You can't ride a bike without a fork and the length of a fork is an important part of how far up the front end of the bike is pushed. If as cyclezen states the reach and stack comes from mountain biking, eliminating the fork length from the fit is even sillier. When you can have forks that runs from 80mm of travel to 300 mm, that's a lot of variance in the head height and it will have a huge impact on size frame the rider needs.
Originally Posted by
DaveSSS
The idea behind reach is to eliminate the seat tube angle from the fit comparison, which it does, but I've never seen a manufacturer explain that reach comparisons are only valid at ONE stack height. You can find frames that differ by 20mm in stack height and list the same reach, but they will not fit the same, when the handlebar height is set to the same distance above the BB. The "handlebar reach" of the smaller frame will be about 6mm shorter, with the same stem.
The fit of a bicycle isn't that hypercritical. If it were, we'd have stems that are available in many more lengths than what are currently available. We'd also find bikes with much more variance in frame dimensions which really don't vary all that much from manufacturer to manufacturer when comparing bikes for similar uses.
Originally Posted by
DaveSSS
Measuring the horizontal ("effective") TT lengths is fine, but a buyer has to understand how to correct for any difference in the STA, in order to figure the true difference in reach. Many buyers don't understand this and only compare TT lengths.
Again, if the seat tube angle varied much, you might have a valid point but the seat angle doesn't vary all the much within classes of bikes for similar applications.
In the end, fit is all about getting into the right ball park and then adapting the flexible part of the system...the rider...to the inflexible part of the system...the bike. Fit matters but only up to a certain point.