Originally Posted by
cormacf
Could my balance issues when descending (which don’t exist on a hardtail) just be because I’m tossing my weight so far forward because of my short femurs?
A forward fit puts your weight forward, yes.
This is the same reason that TT/Tri bikes tend to have long front-centers. When aerobars first came about in the 1980s, and triathletes were attempting super-forward fits on road frames, they had a tendency to feel squirrely and like they were "out over the front wheel." Designing the bikes with the front wheel farther forward solves this.
Mountain bikes are a bit different: MTB fits aren't generally more forward than road fits, but mountain biking creates a lot of situations (due to its technicality and steep descents) where it's much harder to keep your weight comfortably behind the front-end than road cycling does. So, mountain bikes tend to have very long front-center.
A shallow headtube angle can be used to create a long front-center, but it affects steering geometry as well (along with fork offset). Frame reach also contributes to front-center.