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I have done long rides with the bar 10+ cm below the saddle without getting a sore back, or anything. Thing is, when I do, my average speed is 2 - 4 km/h SLOWER than when I have the bar higher up.

I do know you lose power as you get lower. Weird thing with me, is that if the bar is lower than about 4 cm below the seat -- and I mean by just 5-10 mm -- my speed/power drops like a freakin' rock! (I'm talking as much as 3+ km/h slower up a 10% grade!) Any idea why such a dramatic drop in power?

My bar is only 53 cm from the seat, which seems awfully short to me, for someone who is 186 cm tall. (If I increase that reach just another 5-10 mm, I also have that same sharp power drop.) I do know my torso is shorter than normal, given my 91 cm inseam, but still. I think I have reasonable flexibility -- I can touch the floor with my fingers while bending over. I do know when I had the seat lower and/or further forward, that caused additional power loss when getting low, but I think I have the seat figured out now.

UPDATE: Today I fooled around -- yet, again!!! -- some more. Discovered that if I concentrate on flattening my back while leaning forward, I can actually go FASTER than the more "relaxed" position (when the sudden power loss occurred). (Sorry, not too familiar with anatomy . . . that's the best I can describe it.) As noted in one of the responses, this "leaning forward" motion did require moving the seat 5-10 mm forward to get to what seems to be optimal. I'll see how the "new and improved" position works tomorrow.

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I suspect it has to do with achieving an optimal angle for max output from the glutes. –  Daniel R Hicks Apr 18 '12 at 23:40
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Also, when you bend over too much you limit your lung capacity. –  Daniel R Hicks Apr 19 '12 at 0:25
    
3+ kph slower up a 10% grade from a 5-10 mm change in bar height is pretty extreme. What is your "base" speed from which this 3+ kph loss is observed? –  R. Chung Apr 19 '12 at 1:36
    
To clarify, are you saying that you can climb a 10% grade at, say, 10 kph while if you lower your bars by a quarter inch you climb the same hill at 7 kph? Without knowing your exact weight or that of your bike it's impossible to make an exact calculation but that would probably be in the neighborhood of 1 watt/kg difference (like, from 3.3 watts/kg to 2.3 watts/kg). That's a huge difference for a change in bar height of that size. –  R. Chung Apr 19 '12 at 1:50
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Flattening your back is accomplished by rotating your pelvis forward. That would indicate to me that this is in fact a skeletal flexibility issue. See a physio. –  zenbike Apr 21 '12 at 5:39
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4 Answers

Not sure this is the right way to do this . . . but, yes, I AM answering my own question! After extensive fiddling with my position, I've discovered that the power loss occurs when my seat is too far forward, and/or too low. This was the opposite of what I had expected. I can now get the bar as low as it goes -- about 8 cm drop on my bikes -- with no noticeable power loss . . . and it's actually MORE comfortable! (In my old higher & shorter reach combo, I now feel too scrunched up!) The seat has to be pretty far rearward -- about 10 cm tip behind BB -- but this puts me at KOP. (Had been using the "hands-off" test, but that put me at least 5 mm too far forward, thus the power loss.)

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External factors must be considered especially wind strength and direction. You didnt mention these.These make 1 off comparisons difficult as they are seldom the same on different days. Even a slight head wind will have a big effect on speed regardless of minor changes in bar height. Were you wearing exactly the same clothes? Loose flapping garments will slow you lots at high speed. An example -a racing bike (motorcycle) improved lap times by 5 seconds by changing from loose leather jacket and jeans to a 1 piece TIGHT leather suit. Dont expect that radical improvement on a cycle.The benefits show mainly over 60kph or more too so down hill courses have most gain. Never have a flapping number pinned to your back . It destroys laminar wind flow at higher speed.

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This doesn't answer the question. The OP has indicated he has carried out many tests. –  Rory Alsop Feb 20 '13 at 10:18
    
This would be better suited as a comment than an answer. –  jimirings Feb 20 '13 at 14:02
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The thing is, there are at least a half-dozen factors involved.

The two most obvious are positive factors -- up to a point:

  • A low angle reduces "frontal area", the primary factor in wind resistance
  • Leaning forward allows the primary muscles of the leg to be more effective, by improving the angle of pull and by utilizing the fact that muscles tend to be stronger when (somewhat) extended. (But both of these effects have an "optimum" angle that's likely somewhere between 25 and 45 degrees from the horizontal.)

Negative effects:

  • Leaning forward reduces cooling. At full power output ridding the body of excess heat is easily a limiting factor.
  • Leaning forward compresses the chest, reducing respiration effectiveness. Not only is the body less able to get sufficient oxygen and rid itself of CO2, it also experiences a further reduction in cooling.
  • Leaning forward compresses the blood vessels in the lower abdomen, groin, and upper legs, reducing blood flow to the leg muscles. (Interesting point: When you're in a squat blood flow to many of your leg muscles is completely cut off.)
  • Leaning forward, beyond a point, requires that significant additional effort be exerted by the arm and back muscles, to maintain the posture. This detracts from the energy available to power the legs, and also has a mental impact that detracts from being able to concentrate on pedaling.

Meanwhile, the two "positive" factors hit limits:

  • Beyond a point leaning forward produces little additional reduction in frontal area, and may (if tested in a wind tunnel) actually begin to have negative effects. (I suspect that the TdF teams have the wind tunnel data, but I'm not aware of anywhere that it's accessible.)
  • Beyond a point leaning forward begins to reduce leg muscle effectiveness, by over-extending the muscles, and by producing a less optimal tendon angle. And, as mentioned elsewhere, one can begin to run into skeletal limitations of leg angle.

All of these factors effectively multiply on each other such that a very slight change in posture can produce a dramatic drop in output. Also, many of the factors will be influenced to a degree by temperature, wind, the degree of fatigue, etc, so doing "tests" of one scheme and then the other can easily produce ambiguous results.

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I agree with the biomechanical hypothesis already mentioned by @DanielRHicks.

Every joint/muscle-group has an optimal range of motion and an optimal max-force/max-power position.

I am the same height as you, and the distance you mention is pretty short indeed, as measured on the bikes I ride.

There seems to be necessary to analyze the "cockpit triangle" of your bike: bottom-bracket/saddle/handlebars: in the hypothesis that, lowering your bar, you get an angle between torso and thighs that is too acute, then the correction of the problem would be opening that angle.

That could be made in two ways:

  1. Rise the handlebar (which apparently is the opposite of what you want to do);
  2. Move all your upper body front by putting a longer stem AND moving the saddle forward a bit, so that you feel the pedals "more behind" than in the current position.

As observations:

  • High power output (a.k.a FAST) bikes, with aggressive riding position, tend to throw the rider over the bars, so that the torso weight can counter-act the torque on the pedals, and also be more aerodynamic. It is common to see such bikes with high, forwarded and sloped seats (extreme example: time trial bikes);
  • If you just put a longer stem without bringing the saddle forward, you get in a more "superman" position, actually DECREASING the torso-thigh angle that we would want to increase.

Finally, there are mistery in finding the perfect position in a bike. Perhaps even sub-conscious factors of varied nature might contribute for a sensation of "not-right". Unfortunately, only self-analysis and trial-and-error are the guaranteed key to salvation in this case, although some professional advice would be always welcome (a.k.a. bike-fit).

Hope this helps!

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This. Plus the possibility of a biomechanical limit to the movement of your femur in the hip socket, which can have a drastic limiting effect on the angles at which you can ride a bicycle efficiently. You can be extremely flexible muscularly, and still have skeletal flexibility limitations. I expect, given a drastic drop in output with very minor fit changes, and relatively relaxed positioning required, that there may be a skeletal limitation in play. However, it is impossible to diagnose something like that over the the interwebs. See a professional, skilled bike fitter or Sports Physio. –  zenbike Apr 19 '12 at 3:35
    
I don't think the hip-torso angles explain it. He says a 5 - 10 mm change in height or in reach has a large effect on speed. I've seen small changes in height or reach affect aero drag and power -- but never as large as observed here. –  R. Chung Apr 19 '12 at 3:47
    
I've seen a guy who could ride at 6cm of drop, and output 250w for 20 minutes, but at 7cm the the rotation of his hip limited the available size of his hip socket, and the best he could do under the same conditions was 140-150w. It wasn't the size of the socket, but the angle of the opening and the limits placed by torso and leg angles limiting the motion of his legs. As I said, something like that is impossible to diagnose accurately here. But it is possible. Rare, but possible. –  zenbike Apr 19 '12 at 4:50
    
What about the change in reach? –  R. Chung Apr 19 '12 at 11:59
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@R.Chung I see. And I agree with everything said. Unfortunately, I think there is not much to do about that, these answers are only what we, as non-specialists from afar, can give, as already pointed out by Zenbike. –  heltonbiker Apr 20 '12 at 2:43
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