I have a few question regarding bike fit in general and the P3 in particular.
First, I am curious to know what is more important how “aero” one is or how steep one’s seat angle is? Taking as a given that comfort is not a factor. Does it really matter if I ride my bike at 80 degrees but am less aero than if I ride it at 75 degrees. How much of an advantage does one really get from riding steeper. Is it quantifiable. Clearly the more aero one is the more “slippery” he is and less energy is required to go a given speed. How does steepeness relate.
Moreover, it seems to me by necessity at least on the P3SL the steeper one rides the longer ones stem needs to be, or one will be hunched up.
Instead of all this rules about knee over pedal and seat tube angle etc., it seems to make more sense from a physics standpoint to fit the bike so as to minimize one’s CdA, given the constraints of comfort and power generation.
Is this the by product of the FIST method of bike fitting? I don’t know. But clearly there are very fast and successful IMers who ride more upright(Macca for one). The difference I think is that even though he rides upright (meaning seat tube angle not steep) his CdA is low.
As for the P3SL: How does one go about fitting the seat post? Meaning do I fit it in the “relaxed” position and the turn the seat post forward without adjusting anything else, so that now I am in the “aggressive” position or do I just start with the forward facing seat post, set it to a steep angle, adjust the height to maintain 150 degree angle at the bottom stroke then adjust the cockpit so that I can be as low as possibly comfortable. It seems to me the adjustable seatpost just extends the fore-aft tweaking of the seat itself. Wouldn’t a seat with long adjustable rails effectively do the same thing if I just slammed the seat forward?
I have a few question regarding bike fit in general and the P3 in particular.
First, I am curious to know what is more important how “aero” one is or how steep one’s seat angle is? Taking as a given that comfort is not a factor. Does it really matter if I ride my bike at 80 degrees but am less aero than if I ride it at 75 degrees. How much of an advantage does one really get from riding steeper. Is it quantifiable. Clearly the more aero one is the more “slippery” he is and less energy is required to go a given speed. How does steepeness relate.
Moreover, it seems to me by necessity at least on the P3SL the steeper one rides the longer ones stem needs to be, or one will be hunched up.
Instead of all this rules about knee over pedal and seat tube angle etc., it seems to make more sense from a physics standpoint to fit the bike so as to minimize one’s CdA, given the constraints of comfort and power generation.
Is this the by product of the FIST method of bike fitting? I don’t know. But clearly there are very fast and successful IMers who ride more upright(Macca for one). The difference I think is that even though he rides upright (meaning seat tube angle not steep) his CdA is low.
As for the P3SL: How does one go about fitting the seat post? Meaning do I fit it in the “relaxed” position and the turn the seat post forward without adjusting anything else, so that now I am in the “aggressive” position or do I just start with the forward facing seat post, set it to a steep angle, adjust the height to maintain 150 degree angle at the bottom stroke then adjust the cockpit so that I can be as low as possibly comfortable. It seems to me the adjustable seatpost just extends the fore-aft tweaking of the seat itself. Wouldn’t a seat with long adjustable rails effectively do the same thing if I just slammed the seat forward?
Thanks.
How steep your seattube angle is is just a result of your fit, not the other way round. In most cases, a proper fit for tri will put people in a fairly steep position (equivalent to a steep position). I fail to see the root of your question of which is more important, the seattube angle or the aerodyanmics. The two key factors are biomechanics and aerodynamics and very rarely are those two at odds anyway so maximizing aerodynamics while maintaining comfort then becomes the name of the game.
Yes, as you go steeper you will usually need a slightly longer stem on the same frame.
Just forget about KOPS altogether, it’s nonsense.
Of course there are people who are aero and shallow, but there are many more people who are not aero and shallow. Most people cannot get into a very aero position with a shallow set-up, simply because it requires a compressed hip angle. This is not just true for MOP triathletes, even lots of cyclists have trouble with this. Ivan Basso had a position like parachute when he rode on his old TT bike with a shallow seattube. He simply did not have the flexibility to go lower in the front and reduce his CdA. Now, he rides at a 77 degree seattube angle and is very low in the front (and hence a low CdA).
for fitting a P3, it all depends on how much you know going in. But changing the seatpost position has the potential to change several things, so blindly flipping the post and thinking all will be good is too simple. As for a seat with a longer rails, you are correct that a flippable seatpost has the same effect, but rail dimensions differ by a few mm from saddle to saddle, the flippable seatpost gives you 2" to work with, so much more.
The root of my question stems from reading about the FIST method of bike fitting. It seems to me that the prime emphasis in a lot of Slowman’s articles re: bike fitting is the seat tube angle. Rather, it would seem to me that the proper method of bike fitting would be to maximize efficient power output. This would seem to entail a tradeoff between aerodynamics, power output in a given position and comfort. While I am sure that is the goal of Slowman’s bike fitting, it is not so explicitly made clear (at least to me). In my reading of his articles, Slowman emphasizes the metabolic advantage of a steep position (i.e., less recruitment of fast twitch muscle fibers), as opposed to pure aerodynamic advantage. Just how much of a metabolic advantage does one have.
Also it seems to me that a proper bike fit would entail the use of a dynamic fit on a ergometer such as a Computrainer or SRM on a trainer or PowerTap on a trainer, etc, with video capture technology as well as measuring one’s frontal area. Thanks.
Well, there are several studies to show that seattube angle has virtually no effect on power output. Kyle for example has shown a total spread of 6% for the range all the way from -90 to +90 degrees for the seattube angle, as long as the hip angle stayed the same (they strapped in the cyclists, and then rotated them all the way. And most of that effect can probably be attributed to the harnass the rider was wearing, which in the prone position limited breathing somewhat. There can be greater variations in maximum power, but that value is so much higher than you will ever achieve in a triathlon or time trial that it is irrelevant.
So that pairs it down to aerodynamics and comfort. You certainly can get aero in a slack position, but to get the same lower back flatness with a 73 degree seattube position would require a 5 degree more compressed hip angle than a 78 degree seattube angle would, and for many that would pose a problem (MOP’ers and Ivan Basso alike, to stick to that example). That doesn’t mean nobody can, obviously there are athletes as you have mentioned who can fold over that far.