We have built a variety of calculators that we use to help us during bike fits, or to help spec out the perfect frame for an athlete. This is a relatively simple example and the first one we have posted to the web.
This computer lets you play with stem lengths, angles, and spacer heights to find the right combination to achieve desired stack and reach changes. For example, let's say you have an aero bike, like the fit, but want to get rid of the spacers under the stem for better aerodynamics and structural integrity. If you have a 100mm, -6 degree stem with 2cm of spacers, the calculator shows you that simply flipping that stem and removing the spacers puts you in the same position. Obviously stems mostly come in 10mm length increments (even though the sliders let you move in 1mm increments) and the most common angles are 6, 10, and 17 degrees although there are oddball angles like 5, 12, etc.
We always try to use as few spacers as possible - especially with an aero bike, as spacers are aerodynamically dirty. Plus, it is structurally better to use fewer spacers. This all assumes you can do this with a "rational" stem - for tri this means between 70 and 120mm, and closer to the middle of the range is better for the best balance between rigidity and handling. That said, smaller frames/riders will typically use shorter stems and vice-versa.
The graph will move to show how both stems compare. The origin of the graph represents the center of your bike's headset cap. Note that all frame stack and reach measurements that we have seen published measure to the center of the head tube - so they must be adjusted to account for the headset cap which can be anywhere from 0.5-2cm high.
Drag sliders to change stem parameters - Mouse over terms for definitions - What we call a 6/-6 degree stem can also be called 96/84, etc.
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