This might be hard to describe without any pictures but some of you might understand what I am asking…
Looking at TT and Triathlon bike positions, there are 2 different back positions regardless of how low they are. Lance for example has a hump on his back looks like crunched, but most triathlon riders have flat streched backs.
Is there any advantages/disadvantages as long as wind resistance?
If I’m not mistaken, John Cobb addresses this exact question in some of his work. Maybe he’ll chime in and tackle your question.
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Do you have a link if there is any article about this?
Just mention of “B” style or rounded back, not an explanation
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When I first started doing wind tunnel testing I soon realised that the riders had two very different back shapes. I labeled these & to simplify my comparison studies. I found that A riders generally had lower drag but that B riders often made more power. I have never been able to get a B rider to look like a A rider back but it is very common to see a good A rider look like a B shape back because of saddle issues. Hip rotation is a big part of achieving this A shape back. As I continued to test, I found two distinctive shoulder shapes that riders with both back shapes can have. The shoulder shapes are what effect aero helmet choices more than back shape. Shoulder shapes effect elbow widths greatly, B shape riders generally need the elbow pads mounted more forward on the forearms for better airflow management thru the chest/hip area.
Thank you very much! Thats what I exactly got.
The shoulder shapes are what effect aero helmet choices more than back shape. Shoulder shapes effect elbow widths greatly, B shape riders generally need the elbow pads mounted more forward on the forearms for better airflow management thru the chest/hip area.
I’ve seen what you’ve written on the two back shapes, but the shoulder shape/pad placement is new to me. Have you elaborated on these findings somewhere? Sounds very interesting. Thanks for how much info you’ve shared here…much appreciated!
Link to more from Cyclingnews.com:
http://www.cyclingweekly.co.uk/fitness/How_to_improve_your_aero_position_article_170293.html
But we need to start somewhere, so we may as well start with getting lower. How low you can go without losing power or comfort will depend on how flexible you are, and also on where you are flexible. “There are two basic shapes of riders,” explains Cobb. “Group A, with good lower back flexibility right at the waist line, and Group B who have back flexibility located in the upper back at shoulder blades.
Riders with the A style backs can get very low for longer distances. Group B riders will always have a big ‘hump’ in their back. When they try to go too low at the front several bad things happen. They sacrifice breathing ability , they roll unwanted shoulder area into the wind, increasing drag, and their helmet won’t sit in a good position.”
After you’ve established how low you can comfortably be, the next step is to look at making yourself streamlined, because an aero position isn’t just about having less air hit you, it’s also about finding ways to get the air to flow smoothly over and past you as you ride. “As a general rule, having the hands as narrow as possible is always best,” explains Cobb. “Having the elbows no wider than the hips or upper legs is good, although shoulder width will ultimately dictate how narrow the elbows can be. Knees that are thrashing around in the wind are always bad, so while it’s not critical that the knees brush the top tube, they should be within a centimetre or so.”
Once again, however, the fact that everyone is shaped differently complicates the picture. “A rider that has large, square shoulders may need more open elbows,” explains Cobb. “But that same combination with a curved, Group B, back will need narrow elbows. There are just a lot of combinations, and they all require small trade-offs to give the overall best position.
David K
triathletes are wrong 
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The guy in the bottom picture is UCI illegal.
Isn’t that most likely the difference? If you are relegated to a slack seat tube angle (seat nose 5cm behind BB) then you must round your back further to obtain the lower front end. You have a tighter than 90 degree hip angle…but if you have lower back flexibility you can rotate the pelvis backwards to a normal seating position for more comfort and power by rounding the lower back. If it weren’t for UCI rules, then the pro cyclists might gravitate, as a whole, to the more triathlon based position.