In a companion piece to this one, what I've hoped to show is that the defining fit characteristics of bikes are no longer solely contained in the frame. A "56cm" bike is no longer adequate or even an applicable description of the size of bike you ride. At least, not in triathlon.
Rather, timed race (aka triathlon) bicycles are not just mechanical, but geometric constructs of frame and aerobar. What I mean is, not only do these aerobars support your upper body, they define—equally with the frame—the geometry you're riding.
What you're reading here is the final piece: that which goes in between the frame and the aerobar.
If you look at each of the bikes at left (Scott Plasma Premium and Specialized Shiv), you can see what goes in between the frame and the aerobar: nothing. There is no stem to speak of. These frame makers have decided, based simply on their latest superbike frame designs, that the stem, and any steer column sticking up above the head tube top, are aerodynamic liabilities.
This ought to be a clue to you. The question is, how will you react to this clue?
As you look at the images further down, you'll note that not all bike makers have decided to take this tack. Trek's Speed Concept doesn't mind a stem sticking up above the head tube top. What Trek seems to mind, however, is the traditional stem. Again, the culprit seems to be that traditional stem affixed to the traditional steer column sticking up above the frame's head tube top.
And, if you think about it, this makes pretty good sense. The one shape you just don't see on a bike any longer is round.
If you have one of these bikes pictured on this page—or one of the very few bikes like them—your bike doesn't have "round." Otherwise, it does, and the round shape sticking up above the head tube is no less round now than it was 50 years ago. In fact, it's bigger around, since steer columns are now larger. Further, in place of quills sticking inside of steerers, we now have spacers around the outside steerers, making them bigger yet. If you have a bike with a lot of steer column sticking above the frame, with several centimeters of spacers under the stem, this apparatus is much less aerodynamic now than a hi-rise quill stem of 40 years ago.
There's another difference between the bike of today and the bikes of a generation or two ago: Frame material today is much stronger, much lighter, and much more aerodynamic than the appendages that protrude from the frames (stems and steer columns).
What this demands is that bikes be built today with more frame and less appendage. More frame means, in most cases, a taller frame size than that which convention (or your retailer) might have suggested. It means a frame probably spec'd differently than it came from the manufacturer.
Everything that sticks up above that frame that is round needs to be gotten rid of. That means: tall headset top caps (which are really just spacers in disguise); steer columns plus their spacers; and upturned stems. The more you can eliminate these bad actors, the stronger, lighter, more aerodynamic, and better handling your bike will be.
This means stems ought to be horizontal, aka -17°, aka parallel to the ground. Or, if they're not, they need to be as flat as they can be. This would mean the elevation comes from the frame, rather than the stem or the fork steerer and spacers.
Stems can (and should) also be shorter on tri bikes than on road race bikes of a corresponding size. How much shorter: 2cm or 2.5cm on average. The stems arriving to retailers on built bikes tend to be too long.
The practical application of this is that the frame size you thought you needed is probably a size too small. If you replace a -6° stem with a -17° stem, and you take out all but perhaps 10mm or 15mm spacers, and you remove that horrible 2cm tall headset top cap and replace it with a 5mm top cap, you just lowered the front end of the bike a ton. You replace all that with frame!
Likewise, if the frame came with a stem 15mm to 25mm too long for you, you need a frame that's proportionally larger, that is, a frame that's both taller and longer.
I find that there are two sins often committed today in bike shops: Customers are fitted inexpertly to their bikes (no secret to most of you); and customers are placed on bikes a size or even two sizes too small. Even if you're fitted correctly aboard the bike, make sure it's a bike with the right mount of frame underneath you!
All that established, there is a need, from time to time for high-profile aerobars, or, low-profile aerobars retrofitted to be high-profile. Shivs and Plasmas are bikes made by companies that typically make narrow-and-tall bikes. Plasma 2 frames, and Transition frames, fit that geometric pattern. The designers at these companies correctly assumed they needed to get rid of the stem (and the part of the steerer sticking above the head tube top). What they didn't do was build up the size of the frame to accommodate the loss of height associated with getting rid of the stem and steerer.
They simply built down the front end of the bike, integrating it into the frame. In so doing, they took what was a narrow-tall bike and make it into a long-low bike. What if, as a result, you need a taller aerobar pad? These two companies diverge in their approaches to this.
Companies like Specialized and Giant seem to prefer to pedestal the armrests, figuring that pedestals can be made in an aerodynamic shape. Scott, on the other hand, created a set of stems that pitch the rider up. In this way the Plasma 3 starts to look—when outfitted with these pitched stems—a bit like the Speed Concept.
The difference in all these cases, however, is that you won't find a round steerer sticking up, and you won't find a round stem protruding forward. What this ought to tell you is that you shouldn't have these appendages on your bike either. Or to look at it another way, you should minimize them as much as possibie. To the degree you do minimize them, you turn a more typical bike into a superbike.
To bring this discussion full circle (and hopefully in for a successful landing!), bicycles are constructs. They are not only mechanical constructs, but fit constructs. The bicycle's length is a combination of its frame reach, and its stem length. The bicycle's height is a combination of its frame stack, and its stem pitch, and the combined height of the elements around the steerer sticking up above the head tube top (the headset top cap and any spacers); and the profile of the aerobars.
Because the height of your position is alterable in so many ways (high- versus low-profile aerobars, stem pitch, spacers); and because the length of the bike is only alterable by changing the stem; it's generally best to fit yourself to a bike using reach as the most important metric.
That established, if you're looking for the perfect fit aboard the lightest, strongest, most aerodynamic config that bike offers, you're looking at a bike built with a flat stem, a low-profile headset top cap, and few headset spacers.
The easiest way I know to discover this, is to pick an aerobar I want to ride; place it on my fit bike; also place on my fit bike a -17° stem in the length I think best suits me for handling and weight displacement reasons; place about 20mm of spacers underneath that stem that acts as a proxy for the total height of spacers + headset top cap; and then I create on that fit bike my fit coordinates.
Of course, I already know what my fit coordinates are. If you don't, then, you can find that out. If finding that out includes getting a fitting aboard a fit bike like the one I use, then, that fit bike needs to be set up as I describe above.
My fit bike reads out stack and reach of the frame that sits underneath the position created as a result of the fit session. Tri bikes made with a stack and reach close to the metrics indicated on my fit bike are my best fit options. These "ideal" tri bikes still, of course, have other duties to fulfill: they have to be good handlers; aerodynamic; they have to fit inside my price range; they have to be structurally sound, straight, trouble free; but, if there is trouble, they have to be made by companies with good warranties that honor their warranties.
What I want you to realize is that if I placed a different stem length and pitch on my fit bike before starting the fit process, my fit bike would have produced a different output—it would have indicated different frames as my "ideal." The aerobar's profile (armrests sitting high above, or only just above, the pursuit bar centerline), and the stem pitch, and the number of spacers under the stem, and the height of the headset top cap, all determine what the height of the frame is that sits up underneath me. The length of the stem determines the length of the bike that sits up underneath me.
Therefore, best to take all this into consideration. And, in general, what you want is a stem a little shorter than you have on your road race bike. And, as we have seen, you want a flatter stem, and fewer spacers underneath it. If you start with a flatter, shorter, stem as in "input" you'll get a better list of frames that match your fit coordinates as an output.