If you roll back the calendar a decade, tri bike geometries were all over the map. Stack and reach Ė metrics first coined and described here in 2003 Ė quickly became the way that bike manufacturers, bike fitters, retailers, consumers identified how a frame would fit. These metrics exposed frames that fit oddly or badly, or size runs that were badly scaled.
Over the past 5 years tri bike makers have solved this by coalescing around an orthodox geometries. But that doesnít mean there is orthodoxy in how tri bikes fit. Now the frames fit but the aerobars donít.
Letís take a frame with a stack and reach of 540mm and 425mm respectively. This is a Quintana Roo PR 3/5/6 in size 54cm; itís within 3mm of stack of a Felt IAx in size 56cm; itís a Trek Speed Concept in size L; a Dimond Marquise in size M; and several other frames. See the chart below? The colored circles, squares, triangles in the lower left show where the head tube top terminates (where stack and reach are measured). As you see, 3 of these frames are identical, one has a bit more stack and a little less reach (the Scott) and the Giant has just a little less frame reach. The point is, theyíre all close to each other.
What this should tell you is: That there is a consensus around frame geometry; an orthodoxy; and secondly, that you should pay no attention to the stated size! Stack and reach are truth tellers.
But this doesnít mean all these bikes fit the same. Itís just that the blame or credit for how expansively they fit; and whether they fit long, short, high, low; now belongs to the front end that protrudes from the frame.
Frame geometries have converged. What is very different is in how these 5 bikes fit. The colored rectangles show the fit range ďrectangleĒ. The pads can sit anywhere inside of those rectangles (the color of the frame corresponds with the color of the rectangle). QR's frame terminates where the tiny green (obscured by the other bike's icons) square is, and the square rectangle is its pad range. And so on.
As you see, the Trek (blue dot, blue rectangle) has the most expansive range of pad placement (and Iím normalizing everything to pad-center); the QR is way up there in fit range and Felt is pretty good too. You should also note that even though the Giant, even though its geometry is pretty similar to the other bikes, had almost no overlap with any of these bikes except the Trek.
In other words, a Giant Trinity Advanced Pro with the same frame stack as a QR, a Felt or a Scott Plasma will fit an entirely different person than anyone riding these other bikes.
Can I back up for one short paragraph? For those who are lost? For those who havenít been reading along over the past several years, this cartesian graph above shows the rise and run of the head tube top of the frame from the bottom bracket (the place on the frame where the crank attaches). This rise and run is called stack and reach. Then thereís the rise and run from the BB to the armrest, on the top of the pad, midway between the leading and trailing edge of the pad. This is important because this is where you contact the bike.
Superbike companies that make their own aerobars
Giant makes its own aerobars. Scott kind of does, for its Plasma Premium. I have found as a general rule that if a company makes its own aerobars that these aerobars lack the sort of adjustability youíd find in a bar made by Profile Design or Zipp.
Yes, the Plasma Premium uses a bar that Profile Design makes for it, but there are some mods on that bar that make it a bit less fore/aft adjustable than standard PD bar. But it does have great up/down range. Giantís aerobar has the opposite: pretty good fore/aft, but a very narrow 30mm range of up/down.
Quintana Roo simply puts a PD bar on the bike. Wise move.
Trek & Felt
Felt has a pretty long history making its own bars and part of the reason it feels it can do this effectively is that one of its excellent engineers Ė Anton Petrov Ė spent several years engineering at Profile Design. Its bar has quite a bit of adjustability to it, both fore/aft and up/down. This allows Felt to (so far) offer only a single 93mm long stem for its IA10 thru 16. Iíd like to see Felt offer a second stem eventually, maybe in the 60mm range, but itís the expansive bar range that allows this bike to fit a lot of people.
Trek is in its own genus. It makes excellent aerobars and its fit range is so broad that I struggle to make a chart big enough to house it. But there is one downside to Trekís system: The range is much, much, much smaller if you restrict yourself to the single pedestal and stem that might come with a particular bike. Itís quite difficult, techie and time consuming to lower or raise the pads 10mm on a Speed Concept, versus the prior, original (2011) edition of the Speed Concept that simply pedestaled the pads. Still, you have to hand it to Trek Ė it made a bike with an extremely adjustable front end.
Itís the aerobar, stupid
This is the tri bike analog to ďItís the economy, stupid.Ē By far, the thing that matters today, versus a decade ago, is the adjustment scheme of the front end: the stem, and the bar, anything that protrudes above and in front of the head tube top. This is now how you choose a tri bike to fit; and this is why all the superbikes (bikes with integrated front ends) have these Pad x and y solvers, to help you choose the config that matches your position.
Because of this, if you say, ďIím going to choose my next tri bike based off stack and reach,Ē youíre a very wise man! But your wisdom is 5 or 10 years old. Now you should be saying, ďIím going to choose my next tri bike based on my Pad x and y.Ē (Or pad stack and reach if you prefer.)
This wisdom started with superbikes, however more and more weíre able to calculate your best size and config from Pad x/y even if itís a pretty standard tri bike with standard stems. Pad x/y are now the most important pair of metrics for choosing a bike that fits you well.