All values are in millimeters!
The calculator above is designed to help fitters trained in the F.I.S.T. tri bike fit protocol to generate geometries for custom triathlon bikes. Note that the inputs required to use this calculator include "stack" and "reach." These values are described at length in our tri bike fit articles on Slowtwitch.com.
The F.I.S.T. fitting protocol is a two-step process: First, the fitter finds the rider's "fit coordinates," aka his "points in space." The second part of the protocol focuses on finding the right production bike to match those fit coordinates or, in this case, generating a custom geometry that matches the fit coordinates. But, know this: It is remarkably easy to calculate a "bad" custom geometry for a rider. The culprit is most often a badly executed "Part I" of this protocol, that is, the fitter executed a poor fit session. If this happens, the calculator below will faithfully generate the geometry for a bad bike to perfectly match the bad fit. Forewarned is forearmed.
Even if the fit is performed correctly, pitfalls loom. You must have the tools to accurately find the stack and reach of the bike that fits under the rider. The fitter is required to account for the stem length, stem pitch, height of the stem clamp, headset upper stack, spacers under the stem if any, and of course the choice of aerobar. He then must have an accurate way of generating the stack and reach measures, using either a fit bike (best is the bike made by Exit Cycling) that accurately generates these values (taking into consideration negative reach if applicable), or he must use an external tool, such as Serotta's X/Y tool.
Anyone who uses this calculator can change any of the defaults below. Toggling from 700c to 650c, or from standard to long-legged morphology, will change the defaults, but you can still override our defaults and insert your own values. There are explanatory notes below, and you are encouraged to heed them.
All bike makers, custom makers included, feature a minimum head tube length for the bikes they can build. Find this value out, from your chosen custom builder(s), because this calculator doesn't know what those minimum values are for each builder. Keep in mind that if your custom builder builds with external headsets, you must change the default lower headset stack in the calculator, and you must account for the potentially significant upper stack.
Morphologies: The calculator generates a geometry for long-legged, short-torsoed riders by employing two tactical changes, designed to grant the rider a lower center of mass, and a longer base of support for his otherwise high center of mass. When you select "Long Legged" the calculator drops the bottom bracket 10 extra millimeters below the bike's centerline; and it also shallows the head angle one degree, and adds 6mm of fork offset in order to keep the trail a constant value (our default settings generate a trail of about 61mm, more or less). Of course, you can override these defaults, and if your rider wishes (let us say) a higher bottom bracket notwithstanding his high center of mass, so be it. Just change the default.
Yes, lengthening the front/center of the bike while keeping the cockpit distance and trail constant can be achieved through shallowing the head angle while adding to the fork offset. The calculator will employ this tactic when you choose the long legged morphology option. This may result in a fork with an offset greater than 45mm. Not many fork manufacturers make forks with offsets greater than 45mm, so you'll want to check availability before you build or cause to have built a bike requiring this fork offset.
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