Somewhere in America a six foot one-inch man is walking into his LBS looking for his new tri bike. Now that his Christmas spending is behind and his annual bonus check is ahead he's smacking his lips and dreaming about that Cervelo P3C, or Kuota Kalibur, or Trek Equinox 9.9.
Except this man has a particular issue that, if not common, occurs with maddening frequency. He has no torso.... to speak of. His P3C will be longish in the 56cm size, and he might be an even better fit on the 54cm. That's his best horizontal fit: the 56cm bike's reach (the horizontal distance between the bottom bracket and the headset top) best fills this man's needs.
But if this 6'1" man has the torso of a 5'10" man, he has the legs of a 6'4" man. In a less strident case he might have the torso and legs of a 5'11" and 6'3" man respectively. You get the picture (as in the picture at left of Swedish pro Jonas Colting, and you probably didn't realize his legs were that long, did you?).
This man with his bonus check burning a hole in his pocket and with 6'3" or 6'4" worth of leg length ought, on paper, to ride a 61cm P3C. But he needs the top tube of a bike 2 or 3 sizes smaller. What to do, what to do?
It is not that often that a person needs a custom bike, but this is the most typical of circumstances when a custom is indicated. I'll explain the dynamics.
The problem with tri bikes, and their handling, is not apparent when you're riding down the road in the aero position. Tri bikes are at their worst when you're out of the aero position, hands on the pursuit bars, descending, braking, cornering, or executing any combination of these maneuvers. That's when tri bikes are fish out of water. You do your best to design the bikes, and place the pursuit bars in their appropriate positions for cornering and descending, but the fact is that the bottom bracket is sitting directly below the saddle on a true tri bike and that's problematic when you're whipping down mountain descents.
What frequently happens in these cases is that your center of mass is quite high off the ground, higher still in this age of flat pursuit bars. Let us differentiate between center of gravity -- a one-dimensional coordinate that sits somewhere along a horizontal line running fore and aft -- and center of mass, which sits on a two-dimensional plane that runs fore/aft and up/down. Your center of mass sits some distance above the ground, and if you can imagine a double-decker London bus coursing down the Paso Pordoi in the Italian Dolomites, trying to keep up with Il Falco Savoldelli, you can imagine the leggy man's center of mass problem I'm describing.
If you're 6'1" and you're riding a 61cm P3C, you might find your short torso means even a 5cm stem on that bike won't be short enough. But if you choose a 56cm bike you'll have a lot of seat post out, perhaps quite a few spacers under the stem, but it'll fit you. The problem with the 56cm bike is the ratio of the wheelbase to the distance your center of mass sits above the ground.
I do not know what that ratio ought to be, and I can't fathom how I would identify someone's center of mass. But to get an idea what I'm talking about, consider the image adjacent. A road rider's wheelbase is illustrated by the horizontal blue line, and it's the distance between the front and rear wheel axles. A road rider, seated, hands on the drops preparing for a turn, might have his center of mass represented by the ball on the top of the vertical blue line. But the triathlete in the aero bars (as this image depicts) will at some point come out of those aerobars and place his hands on the pursuits. When he does, his center of mass will be more representative of the ball on top of the green line. His center of gravity is forward of the horizontal blue line because he rides a steeper seat angle, and his two-dimensional, planar, center of mass is going to be not only further forward, but higher than the road rider's center of mass.
The problem gets worse yet when the rider's legs are longer and his torso shorter, because his 55cm or 56cm bike (his ideal size for our hypothetical subject's torso) has a shorter wheelbase (the horizontal blue line decreases in length). But, because his saddle height is higher than usual, the green ball sits even higher in the air, and that ratio of the vertical green line to the horizontal blue line gets out of whack.
The solution for this rider is to build a bike with more or less the same steering and handling as his 55cm or 56cm bike, but with a longer wheelbase. In broad strokes, that means stretching out that horizontal blue line.
That might be done through lengthening the chain stay, as shown in the adjacent image. But that may or may not be practical, depending on whether the custom bike maker employs a faired rear wheel design. Certainly, the wheelbase can be extended by shallowing the head angle and adding offset (rake) to the fork. When you employ both these tactics in tandem, you keep the trail constant. That's what we'll do.
Let's take Kestrel's size 54cm Airfoil Pro as an example. It's got a 54.5cm top tube, a 73.25 degree head angle, and 57mm of trail. The bike's front/center in the 54cm is 60.7, and that's the distance we really want to enlarge (this is the measure from the bottom bracket to the front axle. Let's say it's that 63cm front/center in the Airfoil Pro's 59cm we are looking for. If we shallow the head angle to 72 degrees, and add 6mm of fork offset, we still have 57mm of trail. But we've added more than a centimeter of front center. If we go to 71.5 degrees and add 4mm more of offset, we get very close to the 63cm we're looking for. Voila, there's this man's custom bike (assuming everything else about the Airfoil Pro is geometrically agreeable to him). This man's got the cockpit of the Airfoil's 54cm, and the wheelbase of the 59cm (the chain stay is the same for both the Airfoil's sizes).
I often speak of "touring front ends" when I talk about triathlon bike geomtry and, in fact, frame builders I've known for 30 years who make custom touring bikes are among those I talk to for guidance. When you build a bike that's going to have 40 pounds of panniers hanging on the front axle, that's got a lot in common with what a tri bike is like once you're laid out on the armrests, with your weight on the front wheel. A 72 degree, or even 71 degree, head angle is not at all rare in touring bikes.
This sort of sizing issue became evident to me when I was making wetsuits for triathletes. At Quintana Roo, 15 years ago, it became apparent that XS, S, M, L, XL were not close to taking care of our population. Adding medium-small and medium-large helped. But then we had to add a whole new layer of sizing: medium short, large short, medium large short, and these were for those who had long torsos and short legs. The converse is what we're discussing today, and if the long-torsoed person is rather easy to fit aboard a tri bike, but hard to fit in a wetsuit, it's the long-legged person who's the easier fit in the wetsuit but creates a damnable fit problem on the bike.
Certainly the great majority of folks can ride standard production tri bike sizes. But, because of the high centers of mass faced by triathletes when cornering and descending out of the aero position, the leggy types are the prime candidates for custom tri bikes.