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Now that you know how I feel about Dinah…
Let's talk about the difference a front-end makes. I wrote last week that tri bike makers have been doing the very opposite of road bike makers: road bikes have diverged from a single geometry, exploding into geometries of all types, while tri bike makers are coalescing around one geometry.
Let’s consider bikes with a reach of about 425mm. You’ve got a regular traffic jam here, but at 2 different heights. You can see from the graphic below that all these bikes have a reach of between 423mm and 425mm, that is to say, they are practically exactly the same length.
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Most are practically the same height as well. Most have a stack of 540mm, and those that don’t have lower or taller stacks to more or less normalize them at 540mm. The Felt IA frame is a little taller than the IAx, but the IA's integrated stem is a bit lower than the IAx's semi-integrated stem.
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Above is the mechanical drawing for the stem that comes stock on the IAx (IA 10, 16, etc.). The real fit difference between the two IA iterations is in length. The IAx stem has a pure run of about 90.5mm, and Felt’s fully integrated bike has a stem that measures more like 65mm in its run. I have run across people who just found the IAx slightly too long and the integrated IA was just a cleaner fit.
This illustrates the theme here: Front ends can significantly alter the way these bikes fit, even if the underlying frames are almost identical.
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Back to the original chart, highest above, The two frames not almost exactly the same in stack and reach are the Giant Trinity Advanced Pro and the Specialized Shiv, each practically the same length as these other bikes, but between 20mm and 25mm taller.
What is notable about all these bikes? First, look at the sizes. Some are M, some L, some XL, those with numeric nomenclatures are size 56 while another is 54. The take-away here is not to pay too much attention to the named size; rather look at the bike’s geometry (specifically the stack and reach), which are the truth tellers about a frame’s height and length for fitting purposes.
The second lesson we can see from the chart below. All the bikes on this chart have sizing systems that rely on Armrest X and Y, that is, they are superbikes or mostly so, they have integrated front ends, and each of these companies has generated a sizing chart using X and Y from the bottom bracket to the armrest. Note how different the size ranges are even though these bike frames are almost identical geometrically!