If a rider's center of mass is either too far forward on the bike, or too high above the bike, the best way to fix this on a tri bike is usually to increase the bike's front center. This is explained a bit in our article on short-torsoed riders. The chart below will give you some idea of the change in the bike's front/center you get by shallowing the head angle a half-degree combined with an increase in the fork's offset so as to achieve the same trail. In this case, I'm using certain given wheel radii for 650c and 700c that are subject to change, based on the profile of the tire you use. I'm also arbitrarity choosing an amount of trail typical (but not universal) in tri bike design. As you see, you'll add about 1.5cm of front/center for every degree you slacken the head angle.

My purpose in highlighting the above relates to the long-legged, short-torsoed athlete described here. In this case, it's often observed that the rider's center of mass sits too high above the ground relative to the wheelbase of that rider's bike, should he be fitted on a bike that corresponds to his cockpit distance. This is more problematic in triathlon than it is in road race, because of the problematic weight displacement steep-angled riders face, and because the "pursuit" position tends to place the rider higher than the road rider's hooks position. Should the rider want to keep his position intact, and should the fitter/designer want to keep his steering geometry intact, the best fix is to add front/center. The table above roughly quantifies the delta when executing that change.

ACCOUNTING FOR EXTERNAL HEADSETS AND SPACERS

A frame's stack & reach are calculated to the head tube top. What of bikes that use external headsets? (also see integrated headsets.) How do you account for this? You can't simply say, "Add 2cm of stack," because the steering axis isn't 90 degrees, it's positioned at 72 degrees or thereabouts. Plus, if you're adding spacers into the mix, you must account for these. Let us say your proposed bike uses an external headset plus you'll want 1.5cm of spacers underneath the stem. Most headsets average about 2cm in stack above the head tube top. So you've got 3.5cm to account for.

The sine of 72 degrees is very close to . The cosine of 72 degrees is very close to . These numbers are good enough to use for every head angle, because you're looking for precision within 1mm or 2mm, nothing more. So, in the case of 3.5cm of "angled stack," you multiply this times the sine and cosine of 72 degrees, and you get 3.3cm and 1.1cm respectively.

Let's try an example. Let's say that your fit simulator, with stem right down on the simulator's plate (analogous to your stem sitting right on your head tube top), shows you need a frame with a stack and reach of 58.7cm and 43.5cm respectively. Let's say you're considering a Calfee for your customer. You have to account for the Calfee's external headset. This headset, plus perhaps 1.5cm of spacers you might want under the stem, is that 3.5cm of "angled stack" described above. This 3.5cm of stack "angled back" at 73 degrees drives the stack of the frame you'll need down 3.3cm (3.5cm x sine73 = 3.3cm). So the frame's stack you'll need is really 54.4cm, not 58.7cm. This 3.5cm of stuff above the head tube top "shortens" the rider's cockpit by 3.5cm x cosine(73) = 1.1cm. So the actual reach you'll need on your hoped-for Calfee is 44.6cm instead of 43.5cm.

So now you're looking for a frame with a stack and reach of 54.4cm and 44.6cm respectively, not the 58.7cm and 43.5cm you thought you'd need prior to accounting for the "stuff" above the head tube. Calfee offers a production geometry with a 54.7cm and 44.5cm stack and reach, its 58cm tri bike in 700c. Assuming you outfit this frame with the same length and pitch of stem, and same aerobars, as you're using on the fit simulator, take out a 5mm spacer and sub-in a 2mm spacer, and you'll replicate the fit achieved on the simulator to within 1mm.

INTEGRATED HEADSETS: Keep in mind that the bike's published stack must often be added to, because there are head parts that rise above the head tube top even with integrated headsets. This may be 2mm, and it may be 10mm and more.

WEB RESOURCES

There are plenty of good (and even free) resources on the web. The best (that I've found) among the parametric programs that would aid a framebuilder or designer is an Excel-based framebuilding tool by Martin Manning, called Body Geometry 101. Note you'll have to register to join the Framebuilders Forum in order to find and download it.