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And, by the way, this is road or tri. Road is monumentally easy. Tri is a little tougher, you guys are smart, so we'll limit this to finding tri bikes that fit.
The time consuming part is figuring out what your position is. You can do this trial and error, over a period of years, or you can go to a fitter, hop on his fit bike, and discover it, or if you are confident you're in a good position now you measure it. You measure your saddle height and setback (or set forward), versus the bottom bracket, and you measure your pad height and pad reach or run or X or whatever you want to call the horizontal distance from the BB to the center of the pad. Measure this on your existing bike, if you're confident in your fit coordinates on that bike.
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Trek Speed Concept
I am horribly editing, truncating, bastardizing Carl Matson's fine, and clever, fit chart and system for the Speed Concept. It is below. I am doing this in order to simplify and zoom in on my specific case. I'm going to find "my" bike over the next several days, finding out whether BMC, Felt, Cervelo, et al, make superbikes that fit me. Then I'll go through a somewhat different process to discover which unsuper tri bikes fit me.
The chart below zooms in on the Speed Concept in frame size Large. This is a portion of the entire fit table, which includes sizes S, M and XL. Further, that fit table discusses pursuit bar positioning, but that's a little more advanced and we're not going to tackle that now.
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The Speed Concept is incredibly adjustable, so much so that it would be impossibly complex to try to put all the fit permutations in one system, although Carl Matson (Trek's engineer who designed the SC) and I have talked about it ad nauseum. Here's a hint. An awful lot about bike fit nowadays becomes easier when you look at these X and Y systems as plots on a Cartesian graph, just like you learned (or didn't learn) in high school geometry (I'll bet, in retrospect, you wish you hadn't been smoking pot under the bleachers when they taught this).
My Pad Y, or pad height, is 625mm, remember? And my Pad X is 500mm. If you scroll to Pad Y of 625mm on the left of that chart above, you can then move horizontally along that line and find my Pad X. It sits nicely inside the tightest range of a particular box, and above that box sits the notation: Low-Far. This is the stem. An SC stem doesn't look like other stems. Below is what the stem looks like. This is, I think, a drawing of a Low-Near stem. The Low-Far is the same height profile, just longer. This bike's stem is the thing under the pursuit bar that the bolts go through from underneath.
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On top of the stem goes the pursuit bar (base bar), and on top of that goes 1 of 4 pedestals, each successively taller by 10mm. This is a single pedestal rather than the pedestals we see underneath each armrest in most aerobar systems. This raises an interesting question: What is aerodynamically preferable, a lower front end with armrests pedestaled, or frame that is taller but with fewer pedestals? That's a question we would like to try to answer with more specificity this Spring. The big difference in bike set up is that the low front end (a bike with a lower stack), requiring the pads to be pedestaled, usually results in a low pursuit bar position, which you might like and you might not.
Me, that 625mm of Pad Y, that requires a Low stem with a 35mm pedestal. The "35 + 5" above means a 35mm pedestal + a 5mm spacer that sits under the pad wing, and that gives you 5mm of height adjustment all the way up or down the scale.
When you see the ranges in the chart above, you see three ranges per stem. Each range is just a wider version of the range above it. The center of each range is the same. What these ranges mean is that if your Pad Y happens to sit inside the uppermost of the 3 "range" boxes (the tightest range), everything about the SC's front end should be very close to centered in its range. In other words, the pads should probably sit in 1 of the 2 hole sets that are close to the pad center. The wing on which the pads sit should be swept back. The wing should sit right over the center of the pursuit bar.