Login required to started new threads

Login required to post replies

Seat Tube Angle
Quote | Reply
Please correct me if I'm wrong about this but my understanding is that one of the major components of an aero bike fit is a flat back meaning the riders back is parallel to the ground. The two factors that dictate back angle would be effective seat tube angle and hip angle. The slacker the seat tube angle the more acute the hip angle needs to be to achieve a flat back. So for even a fairly aggressive seat tube angle of 80 degrees would need a hip angle of 80 degrees to achieve a perfectly flat back. That hip angle is pretty far from the recommended 90-105 degree angle and for me at least would result in power loss and comfort issues.

What I'm trying to figure out is why/why not setup with a super aggressive 90+ degree seat tube angle so you can have a flat back and maintain an open hip angle. It seems like the best of both worlds, an open hip angle that feels relaxed but the aero benefits of a flat back. Has this ever been tried? Any data or opinions on it?
Quote Reply
Re: Seat Tube Angle [OpossumPosse] [ In reply to ]
Quote | Reply
A flat back doesn't work for a lot of people.
Quote Reply
Re: Seat Tube Angle [jimatbeyond] [ In reply to ]
Quote | Reply
I believe a flat back would result in the smallest cross section surface area so it should be more aero if not faster
Quote Reply
Re: Seat Tube Angle [OpossumPosse] [ In reply to ]
Quote | Reply
Let me rephrase...

It isn't possible for a lot of people to ride with a flat back.
Quote Reply
Re: Seat Tube Angle [jimatbeyond] [ In reply to ]
Quote | Reply
Sure, myself included. I'm not very flexible myself and ride with about a 95 degree hip angle and 78 degree seat tube angle which makes my back about 17 degrees above flat. I'm trying to figure out if there's any reason I shouldn't increase my seat angle to something like 90 degrees which would allow my back to be only 5 degrees above flat while maintaining the same hip angle. In theory I think it would feel the same but be much more aero.
Last edited by: OpossumPosse: May 22, 17 13:32
Quote Reply
Re: Seat Tube Angle [OpossumPosse] [ In reply to ]
Quote | Reply
the desire to achive this position is noble, but the steeper you get, the more you need to understand how the front end of a bike works.

caster, BB drop and load distribution are the factors in how a bike is designed to handle. Whether it is recumbent bikes or Stayer track bikes, they are all subject to basic physics.

Craig Calfee has a good little discussion on this.

Assuming that you are a man, to put you at that 90deg SA would move your weight distribution closer to a 60/40 (or more) out over the front center. Unless there is a corresponding change in the fork rake/trail, the bike will become quite unstable at any speed. There is nothing more fun than riding a bike in a position it is not designed to support. *sarcasm*

As a Fitter, I would not be driven by getting a rider to achieve a "flat back"; good pelvic rotation and proper saddle will certainly allow the rider to lay out, then having enough stack up underneath them for comfort. Aerodynamics can come in many ways.

Anne Barnes
ABBikefit, Ltd
FIST/SICI/FIST DOWN DEEP
X/Y Coordinator
abbikefit@gmail.com
Quote Reply
Re: Seat Tube Angle [ABarnes] [ In reply to ]
Quote | Reply
ABarnes wrote:
There is nothing more fun than riding a bike in a position it is not designed to support. *sarcasm*

The story of "tri" bikes in the late 80's and very early 90's...

Why Profile Fast Forward seatposts still exist is a mystery to me.

---------------------------------------------------------------------
Some are born to move the world to live their fantasies...

https://triomultisport.com/
http://www.mjolnircycles.com/
Quote Reply
Re: Seat Tube Angle [OpossumPosse] [ In reply to ]
Quote | Reply
OpossumPosse wrote:
I believe a flat back would result in the smallest cross section surface area so it should be more aero if not faster

Often not. I use dynamic frontal area analysis in fits to assess the impact of position changes on aerodynamics. For most triathletes there comes a point where going lower increases frontal area as they stick their heads up in order to see. Or arch their backs.

The aim is to have the most aero position that doesn't compromise power and comfort too much. Sometimes this means the rider is nowhere close to a flat back. Other times they look like a bullet.
Quote Reply
Re: Seat Tube Angle [cyclenutnz] [ In reply to ]
Quote | Reply
I used to ride far forward.
My seat tip was 80mm in front of the BB.
Kenny Souza was about the same.
We experimented the hell out of this in the 90's.
Once rules come in to limit forward seat to 50mm I had to lift my stack.

I had my own frame built with 82 deg seat tube, 60cm top tube, 74 deg headtube and a wheelbase that wouldn't be out of place on a semi trailer.
Best handling bike I ever owned. (Except for going down really steep hills)

As you go forward it is not as simple as seat goes forward bars come down.
When you go forward you are also going higher unless you build your frame to compensate.
Some of your flat back goodness is lost to a higher profile.
Your seat also stays reasonably flat to, so how the seat interfaces with you is different.
For me it required that the seat height be even higher, so my seat was now perched some 3 to 4 cm higher due to seat rotating about BB and then a couple of cm on top of that.

Now once I had a bike where I could play with things and not just have everything maxed out, it soon become apparent that for me 55mm in front was best if it was a high intensity event and wouldn't reccomend it for IM.

It is quite possible to get very aero sitting far back even with a very sloping back.
The position is much lower and what is gained through height makes up for loss of flat back.
Bloody difficult to run off though in short course.

If you want to experiment, just put your trainer on blocks until you achieve the position you want.
Find out how well you make power in different positions, then take photos and do a bit of critical viewing with imaginary ground lines to see how it effects your frontal area and you profile from the side.
Just be aware that your frontal area is usually far more important than your back profile.
You can compare frontal area with good pictures, just remember to allow for the block under the trainer.
Quote Reply