"Fast" TT/Tri Geometry - Part One

Thanks to the guys over at Slowtwitch.com, us regular/caveman folks have access to a database of TT/Triathlon frame geometries. This is pretty cool, and I'm glad those guys have taken the effort to put this together.

It's always intriguing to me when datasets like this are published, as I think a closer look can reveal an additional layer of insight. But, I'm never really sure about this until folks other than the original publishers actually dive into the data.

It's great that these guys publish this data, don't get me wrong, but I think it stops short on the whole global "TT/Triathlon" positioning deal - I mean, a bike manufacturer has a definite "fit philosophy" when they ship a give frame geometry to the general public, don't they?

For all intents and purposes, it seems, that each bike manufacturer thinks their own "fit/geometry philosophy" is the "best" way to make their customers perform optimally (or at least I hope the mfr's are not hoping to make their customers slower!). So, which mfr is "correct"?

That's really the question I have, when it gets down to the core of the whole deal... What does the mfr frame geometry say about the mfr, and in the end, which frame geometry is faster than the rest.

As a first step in this frame geometry journey, let's just look at the raw data as provided by slowtwitch.com, but in a much more digestable, non-tabular format:



yeah, so it seems to me that there's a bit of spread with this data, eh? It's interesting to note that for a given slowtitch "reach", (which is kind of, but not completely, related to how I define "reach") or distance from bottom bracket to the top of the head tube at the centerline of the head tube longitudinally, you can see that some manufacturers have pretty diverse opinions on the whole deal of "fast geometry" from a positioning perspective. Who'd a thunk it, eh?

I mean, these days consumer products thrive on diversification, so it is to be expected that mfr's will attempt to "be different" than their competition and subsequently make claims about how their approach is superior to their competition...

But I digress.

Having had the opportunity to test lots of normal and elite folk in a wind tunnel while measuring different things about how their bikes are set up, I'm in the kind of unique position of trying to attempt to say: "these bike geometry variables will tend to improve, or hurt a rider's aerodynamic position". To put it another way, I've accumulated a lot of wind tunnel data over the years that suggests a thing or two about what is responsible for making a rider aerodynamic.

We'll get to that later, but IME, CxA(a measure of aerodynamics of a given position/geometry)from a positioning/bike geometry perspective, can be boiled down to the following variables:

saddle setback (tip of saddle relative to bottom bracket)
saddle height
reach (distance from tip of saddle to where the aero bars are grasped)
drop (top of saddle to elbow pads/elbows)
pad width

There are, of course, some other things that affect the whole deal, but they aren't really a function of a bike frame's geometry.

It seems like there might be an opportunity here, then, to attempt to draw a connection between how mfr's design and sell bikes, how consumers set-up these bikes, and how the resulting positions are measured from a CxA perspective in a wind tunnel.

In my next blog entry, I'll try to step through the logic of how I would rate mfr geomety decisions along the "fast-ness" spectrum.

Be well,

-k