Fast TT/Tri Geometry Part Two
Last time, I pretty much just documented what was available out there on the internets with respect to how different manufacturers make and sell bikes.
If the goal is to somehow take a stab at saying which manufacturer has "faster" geometry than another, well, one could simply take a look at the demand side of things. What's the demand side?
From a simplified perspective, the demand side on a flat TT course or triathlon venue pretty much boils down to total package aerodynamics (there are other variables such as weight, crr, mass, drivetrain losses, dark matter...). Now, I'm not able to say definitively one way or another how the frames themselves will behave in a wind tunnel, but this blogseries isn't about the frames...it's about the geometry they employ and the resulting way it forces folks to sit on bikes that is being evaluated.
The first way I chose to look at this whole deal was to simply say that I, as a rider, have a saddle height that I need to hit and a single stem that I'm going to put on all the frames out there. So I asked the dataset the question: "How would the resulting CxA I would have whilst sitting on these bikes, as measured in the tunnel turn out on average?"
Now, I understand that this might not be the best way to take on this challenge, but you'll have to give it to me that it's _one way_ to git 'r dun, eh? ;-)
So, the above is what I did... Along the way, I did some fancypants math to take the publicly available stack/reach values and transform them into the variables that I measure in the tunnel (I've got probably 1200+ data points at this time) and then run them through the stepwise, multiple, linear regression model I constructed that relates those measured position variables to CxA/aerodynamic resistance.
If the manufacturer offered a seatpost with an adjustable seat tube angle, I averaged the predicted CxA values from both ends of the STA spectrum.
Anyway here's how this method wound up ranking things:
So, what's everyone else think is a better way to do this sort of evaluation of TT/Tri bike geometry.
I'm happy to give your ideas a go when I get the chance!
Next time, we'll see how the recent trends of the manufacturers may have influenced performance at the world championships in Kona.
If the goal is to somehow take a stab at saying which manufacturer has "faster" geometry than another, well, one could simply take a look at the demand side of things. What's the demand side?
From a simplified perspective, the demand side on a flat TT course or triathlon venue pretty much boils down to total package aerodynamics (there are other variables such as weight, crr, mass, drivetrain losses, dark matter...). Now, I'm not able to say definitively one way or another how the frames themselves will behave in a wind tunnel, but this blogseries isn't about the frames...it's about the geometry they employ and the resulting way it forces folks to sit on bikes that is being evaluated.
The first way I chose to look at this whole deal was to simply say that I, as a rider, have a saddle height that I need to hit and a single stem that I'm going to put on all the frames out there. So I asked the dataset the question: "How would the resulting CxA I would have whilst sitting on these bikes, as measured in the tunnel turn out on average?"
Now, I understand that this might not be the best way to take on this challenge, but you'll have to give it to me that it's _one way_ to git 'r dun, eh? ;-)
So, the above is what I did... Along the way, I did some fancypants math to take the publicly available stack/reach values and transform them into the variables that I measure in the tunnel (I've got probably 1200+ data points at this time) and then run them through the stepwise, multiple, linear regression model I constructed that relates those measured position variables to CxA/aerodynamic resistance.
If the manufacturer offered a seatpost with an adjustable seat tube angle, I averaged the predicted CxA values from both ends of the STA spectrum.
Anyway here's how this method wound up ranking things:
So, what's everyone else think is a better way to do this sort of evaluation of TT/Tri bike geometry.
I'm happy to give your ideas a go when I get the chance!
Next time, we'll see how the recent trends of the manufacturers may have influenced performance at the world championships in Kona.
Labels: triathlon, TT Position, Wind Tunnel
posted by kraig at
6:37 PM
5 Comments:
Nice work! But if I assume that the chainstays are a fixed length, and rider COM is fixed relative to the bottom bracket, and the rider picks the frame based on the fore-aft weight distribution, and the stem is customizable, then the characteristic measure of frame size would be front-center, as opposed to reach, right? Then measuring stack versus front-center allows determination of how low the bars can go. If stem length is fixed, then it seems the rider is forced to pick the frame based on reach, rather than front-center. Or maybe I'm all terribly confused, which is usually the state of things :).
Sorry -- one more comment! For sicko-short head tubes on a time trial bike, look at Chris Boardman's brand. 8cm on the 53 and 55 cm top tube versions, jumping to 12 cm @ 57.5 cm TT. It should score well on your ranking.
Hi Dan,
thanks for the kind words. I don't know how good the work is, but it's kind of interesting (to me anyway ;-) )to attempt to indirectly determine what manufacturers perceive as "fast" geometry. I like your ideas regarding alternate ways to do this!
I struggle a bit with allowing things to be "adjustable" for this type of evaluation. Simply because, IME, with a look ergostem, I can get the reach/drop combination I want (based on tunnel data) with either a 48cm frame, or a 56+ cm frame.
I'm not sure I'm explaining that quite right, but that's the personal experience I'm wrasslin' with in the ol' melon right now! :-)
If I'm following you correctly, I could basically specify a saddle setback (which I think would be a wt distribution proxy for a given front center?), which would eliminate certain frame sizes and maybe bias things.
FWIW, I think there is more to things than front center - STA/saddle setback affects things - I like to refer to "reach" as more akin to "effective" reach. IOW, front-center is only a part of "effective" reach.
oh, crap... I don't know - I'll have to give it some more thought, I reckon. What I want is to answer the question "which frame manufacturer has the fastest geometry", and not "which frame manufacturer can get the bars low".
I've got this big database of tunnel runs that I think I can provide insight into the question, but I'm not quite sure about the best way to do it. Know what I mean??
Hi Kraig - been a while - this is interesting stuff - what strikes me is the range of numbers you get, there's a few with order of magnitude variability from the average - with riders being the overwhelming majority of the drag, and presuming a fixed rider position, I think you can compare these fairly, but to some degree aren't we talking about net deltas of less than 1-2 percent of total drag, even between the two outliers?
Second question would be the role of vortices and shedding and other nonlinear drag contributions - we're at a point where manufacturers are using details to discriminate in marketing - next steps may be work on the trailing edges and even more work on leading edge shaping... but in this relatively low-speed world, how much does it matter??
At what point do you look at two bikes and say potato, potato...?
Cheers! -Don Gabrielson
Hi Don,
Thanks for posting. Yeah, the numbers are really just a rank order numbering kind of deal, in order to gain insight into what manufacturers think is appropriate (or fast/slow as it turns out) geometry for tt/triathlons. I'm not claiming that an "8" is 4 times better than a "2" on the list; I think I'm just simply trying to show that the "8" has a faster geometry than the "2" based on the way I'm doing the analysis.
The methodology isn't perfect, but it's a start.
Regarding your second question, I'm not evaluating the frames, per se, but rather, the geometry they have. IOW, how the geometry forces the rider to sit on the bike and not the bike itself. So, that's kind of a "punt" on your question...
Tube shaping matters, but in all honesty, the first thing to get sorted out is how you are going to sit on the bike - which is why I recently did a years worth of TT's on a 1983 1" round tubed steel Ciocc for quite some time. I put down some of my best TT times on that bike - that sucker was fast!
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