Am looking around for a new bike, myself, these days.
I agree with you, that it can be really risky to pull the trigger on something you haven't yet ridden. Little difference can mean a lot.
Of course, if close, it might well just be "little" differences. Meaning, a seatpost with moderate rearward offset (layback), slightly longer steam, different handlebars, might make all the difference you need.
From the Trek website, the geometry difference for the X-Caliber 29er are:
- Effective Top Tube: 54.0cm in the 14.5" model, versus 57.0cm in the 15.5" model. That's a 1.2" greater length, in the 15.5" model, from the center of the seat to the head.
- Reach: 37.2cm (14.5"), versus 38.7cm (15.5"). That's a 0.6" greater length, in the 15.5" model, from the center of the BB to the head.
- Wheelbase: 107cm (14.5"), versus 109cm (15.5"). That's a 0.8" greater length, in the 15.5" model from hub to hub. I'm assuming this part doesn't feel too quirky, that the bike tracks fine and doesn't feel too point-and-shoot.
I've only required a fitting a handful of times, myself. But, like you, I have most of my height in one part of me. My length is in my
torso, versus
legs for you. Finding the perfect frame is a real bear, for me. Minor differences matter a lot, in terms of comfort. Generally, I when the frame is the right size for my legs, it generally jams me up with too short of a reach up top. I'm relegated to shoving the seat back and sending the bars forward via either bar and/or stem changes.
You're essentially having the same issue. The seat-to-legs position seems fine, but you've got too little reach up top.
As you might imagine, the balance of the bike
can potentially be affected by how forward or rearward your lean is, even by an inch or so. Depends on the bike, depends on how much the movement of your point of balance is. Meaning, you don't want to necessarily add all your reach gain via just a bar change, or just a seatpost change. To maintain reasonable balance, you might well need to do two or more of the changes in combination, else your balance could foul up. Otherwise, you could end up with too much weight on the hands, or too much on the seat. (Some of which is just preference.)
So, presuming
no handling quirks would exist, and presuming you'd want to keep a good balance of weight on the hands vs seat ... If I were experiencing that situation you're describing, I'd consider adding back another 0.8" to 1.2" in length top side. I'd want to have the shop consider the following:
- Rotating the handlebars forward/upward -- could perhaps gain back that 1" without a seatpost and/or stem change. But, that would shift weight forward to the hands by that much. Might or might not be what you want to do, at least not all of at 1".
- Different set of handlebars -- can shift the weight forward, but it gives you an opportunity to change the shape of the bar at the same time.
- An offset seatpost that would send the seat rearward by 0.5" or more. Could balance out the weight shift, by sending your butt rearward a bit, leaving the rest to the bar and/or stem change.
- A longer stem that would increase your forward reach by 0.5" or more. Might want to change the angle a bit, at the same time, if you're making this swap.
A question to ask yourself is, apart from the fit to your legs (position of legs vs seat), whether the forward reach was perfect in that 15.5" model. (That was 1.2" of greater length than the 14.5" model, as viewed from seat to head, or 0.8" greater length as viewed from the center of the BB to head.) By adding 1" to 1.2" length up top, you might end up just right. The seat-to-feet layout will match well enough, and you'll have the reach of the 15.5" sized frame.
Speak with the fitment person at the shop, to explore options. I'd think you'd want to consider the above possible changes:
bar movement; possible new bar; possible offset seatpost; possible longer stem. Likely, a combination of two or more of these changes. Since your shop's recommendation got you here, they might be completely willing to make the changes to accommodate.
I'm sure the fitment gurus could chime in with estimates on how much your weight might shift forward or rearward via such changes. In my experience, "feel" is such a subtle thing. The recommendations can be one thing, but only you can tell if the change in feel via that tweak is going to feel right.
Good luck to you.