Alan Couzens wrote:
Slowman wrote:
somewhere in nature lurks the unifying theory of everything (in tri/tt bike position), where mechanical advantage, efficiency, aerodynamics and crank length live as values, and if you change 1 value you change the rest. if you change the crank length on your gravel bike you probably don't change the aerobar elevation. but on your tri bike, you do, if you want to normalize that bike for hip angle at TDC.Thanks Dan,
Your response brings back a very fond memory of you, me and Mat hanging out in Gordo's basement engaged in deep, friendly argument over the role of mobility in bike fit. Very good times.
Unfortunately, in this case, I deeply regret to have to say that I agree 100% with what you've written above.
With a couple of minor additions...
1. In triathlon, especially long course triathlon, for the vast majority of folks on the course, I would suggest that, with mean race durations of 12hrs and bike speeds of ~30km/h, efficiency greatly trumps aerodynamics when it comes to optimizing that 'balance of factors' that you're talking about. This is not reflected in fit marketing ('cause it's not as sexy as taking out more spacers :-)
2. The balance of important factors that you're talking about above are very rarely taken into consideration in a real world bike fit. IME, fitters are either over-fixated on aerodynamics to the exclusion of individual efficiency or, (less commonly) focused on coming up with a comfortable fit without assessing the aerodynamic cost. I have a hard time coming up with a name of a fitter who does both well. I know some sports medicine/physio guys who are very cluey on the comfort/movement efficiency side and some 'wind tunnel guys' who know the ins and outs of a truly aero fit but there are few who do a complete job of balancing the 2 IMO.
Frankly, it blows me away that in this day and age, most fits are being done without any actual data collection on either side of those equations, i.e. aerodynamic or physiological 'cost' of various positions.
As you suggested above, the answer to the 'right' crank length for an individual surely lies in collecting data from those metrics to come up with the optimal 'balance-point' for each individual.
Thanks, once again, for some great dialogue.
AC
thank you for the thoughtful response. yes, i agree, in practice, during a fit session, the drivers of the actions the fitters take are most often efficiency (not aero) specific (tho jim manton or brian stover may disagree). just, the metric that i use is RPE (rather than gas analyzers) because in practice, RPE always wins (when a fitter isn't present - which is all the time minus 2 hours per year). the rider, constantly feeling his seat is too high/low/forward/rearward will change it. hence RPE as the metric that matters most (besides, it's pretty reliable).
i only offer this gentle pushback - or maybe amplification is better - as regards what happens in a fit session. we have a lot of data on fit. no, we aren't going to measure VO2 during a fit and i don't think we should, because i don't think we'd get data more precise than we get from RPE. but we can draw conclusions based on the law of large numbers. when i wrote about orthodoxy, and i wrote about it again, this is what's often missing from fit sessions. but there are good fitters who do honor orthodoxy and its implications.
when we say that included knee angle at BDC should be 145° (or whatever) we're saying that this is the angle (plus or minus) that generates the best result. does it generate the lowest VO2? i don't know. i don't care. what's more important - and what drives a good fitter's behavior - is it generates the best result. why? because it's the knee angle most top riders use when they pedal.
in triathlon, we know this about hip position fore/aft, shoulder angle, knee angle, hip angle, handlebar elevation and so forth. are these positions the most aero? efficient? economical? don't know. don't much care. what's most important to know is that there is a pretty high correlation between top performers and their measured positions. my answer, then, to you, is that in the real world we neither test for pounds of drag nor oxygen consumed, but we are guided by proxies for both. it's like water quality testing. they don't test for leptospirosis in the lake where you'll compete in next week's triathlon. but they test for fecal coliform because there's a high degree of correlation. we test for the proxy.
finally, tho, there are ways to kinda sorta test for drag during a fit session and perhaps cyclenutnz will come in here with a fulsome explanation. but, to the point of crank length, i remember a fit session jordan rapp and i engaged in some years back, where he knew the saddle height (per a given crank length), the fore/after, cockpit distance, etc., and he knew the handlebar elevation. and he knew his hip angle. he knew all of that, because they were all proxies for either aerodynamics or efficiency. what he didn't know was crank length. this was the value for which we were solving: what would generate 97° of hip angle at BDC (using my landmarks) @ 165mm of handlebar elevation?
the answer we got (the crank length that our experiment yielded, which is of course possible only with a fit bike that delivers a reliable fixed resistance and allows all those values, including crank length, to easily/quickly adjust) leaves open the question of jordan's muscles' "desire" to work through a full range of motion, which was your point. how about some points for further study? (as our professor might say). such as: nothing close to a full range of motion of prime mover muscles happens in distance running. but i'll defer that discussion to your timing.
Dan Empfield
aka Slowman
