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Old 01-08-2018, 03:54 PM
Kontact Kontact is offline
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Join Date: Apr 2011
Location: Sunny Seattle
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Originally Posted by audiojan View Post
Not really... stack is measured center BB to top of head tube in a vertical line. Reach is measured same points horizontal line. Hence, stack is only partially related to leg length, but more so with core strength and flexibility (which also impacts reach...). Still height is a crappy indicator of frame size even if you do know the brand/model... You can take 3 people of exactly the same height, pick the same brand/model, have one fit a 52, another fit a 54 and the third fitting a 56. Take out the common denominator of the same brand/model and you're really adding too many variable to have any clear indication of fit...
"Stack" is a number that reflects how high vertically above our feet we need the handlebar to be to comfortably bend over to grasp it. That is dependent on our legs more than anything else.

"Reach" is a number that indicates how far horizontally the bars are going to be from hips, since we are bending he upper body toward horizontal to grasp the bars.

Three people of the same height but different proportions will often ride the same size frame for a given model. Take three people who are 5'10" on a 56:

A guy with short legs and long torso will have a low saddle height which makes a larger frame a bad idea, but his proportionately long reach makes a smaller size have too little reach. So he lowers the saddle and uses a stem one increment longer than stock and slammed.

The "normally" proportioned guy puts his saddle at the expected height for the frame and gets a good reach on the stock stem and a few spacers. No reason to make either stack or reach more or less with a different frame size.

The long legged guy puts his seat up higher than average and adds more stack spacers while going to a shorter stem than stock. His legs suggest going up in frame size to use fewer spacers, but his short upper body would require a very short stem. His short upper body suggest going down in frame size, but then the stack would be too low from his saddle height.

These are generalizations, but they illustrate why you can't ignore height to focus on one attribute or another, because stock bikes grow vertically and horizontally together. You can switch models for different fit, and tall head tubes have been great for leggy people. But when sizing people for a specific model bike, height makes the most sense as a starting point.
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