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Old 12-09-18, 01:38 PM
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Carbonfiberboy 
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Originally Posted by Kapusta View Post


Making the STA steeper itself does NOT move the bb farther from the front wheel.

Yes, it moves the bb farther back relative to the saddle (as you mention above). Hovever, it also moves the entire rest of the frame further back (relative to the saddle, if it is not slid fore/aft) including the front wheel. The bb to front wheel distance is unchanged, UNLESS you also increase the fame reach, increase the fork offset, or slacken the HTA.

In the diagram above (with the red and blue frames), The STA and the reach are both changed. But it is the frame REACH that changed the bb to front wheel distance, not the STA.

Also, the frame with the steeper STA in that diagram would actually fit a larger rider, not a smaller one, because you fit bikes by reach (and stack) not STA. A taller rider would more likely ride the frame in that diagram with the steeper STA becuase it also happens to have a longer reach. They would just be more likely to need to slide the saddle farther back on the rails.

Has anyone other than you ever mentioned keeping frame reach the same? There are two kinds of reach, "rider reach" and "frame reach." Rider reach refers to the distance from saddle to bar and is one of the two main elements of bike fit, the other being saddle to pedal axle distance. Riders buy and set their bikes for the reach they prefer. Using frame reach and stack as a fit definition does, as you point out, regard STA as a fixed quantity. Under that theory with unvarying STAs, buying a bike by frame reach will translate into expected rider reach. Except as we see by the frame geometry charts I posted, STA frequently changes with frame size. IOW, buying a bike by frame reach only works if you are replacing a bike with an identical STA. Otherwise people by bikes by rider reach and let the manufacturer take care of toe overlap by means of STA and HTA. Arguing that it works the reverse way, that riders must accept a fixed BB position and the toe overlap which goes with it doesn't make any sense and is not, obviously, industry practice.

@HTupolev's posted diagram makes that very clear. If rider reach is maintained, and STA is increased, the BB moves aft, along with the rear wheel, and of course frame reach increases because STA is increased.
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