Another trend that I just don’t get…why are more and more frames going to seat masts with very little adjustability, rather than sticking with seat posts? Plus you have to saw off the top part after you get the thing fit to you (talk about sweating that moment…“please, please let me have measured right!!”). And its unlikely you will be able to fit one in a standard travel case. Are the advantages of the mast really that much better than a seat post setup? Really don’t get this…
Spot
Aerodynamics my friend.
OK, so maybe part of it is looks 
how about “vendor lock in” ? you want or need a new seat mast? its not coming from anyone but the frame maker (more or less)
I wonder…why is a mast more aero than an aero post ? The post is typically narrower than a mast which has to house the post.
Aerodynamics my friend.
OK, so maybe part of it is looks
Given that none of the integrated masts has the aero properties of a cervelo/felt/trek post in road or tri I think looks is the main reason. Or point of difference marketing.
Small argument for stiffness but you can do that with a larger diameter post. Weight often ends up the same as several designs have miniposts inside the shaft anyway,
Read some of this stuff on aerodynamics:
http://www.princeton.edu/~asmits/Bicycle_web/bicycle_aero.html#intro
http://www.damonrinard.com/aero/aerodynamics.htm
http://www.slowtwitch.com/mainheadings/techctr/compact.html
.
They look cool, and are perhaps good for pros and their mechanics (no worries about a slipping post), but the biggest advantage is that the manufacturer can charge more for a product that doesn’t cost any more to manufacture.
After reading the linked articles, I will grant that there might be a small advantage in terms of aerodynamics, but I would think that in this era of aero seat posts, the advantage would be very, very small, and not enough to justify the very real disadvantages that come with an integrated seat mast that has very little adjustability and is a pain to travel with.
Spot
What does any of that have to do with the aerodynamics of a seatmast vs seatpost??
John Cobb:
" Using tall aero seat posts on a compact frame––though it seems like it ought to be faster––seems to actually cost time. How much slower is it? My results suggest that it might be about 45 seconds slower over 25 miles. Aero posts on traditional frames aren’t as bad but still aren’t as good as round posts."
Not sure about the comparison to a round Seat Post vs an Aero Foil Seat Post I think the AF post wins.
The seat mast thing is a biproduct of a frame manufacture getting t’ed off at a Seat Post manufacture and saying screw them our frames do not need your seat post. So says the Giant rep at my LBS. Not sure if this is just spin but makes for a funny topic.
John Cobb:
" Using tall aero seat posts on a compact frame––though it seems like it ought to be faster––seems to actually cost time. How much slower is it? My results suggest that it might be about 45 seconds slower over 25 miles. Aero posts on traditional frames aren’t as bad but still aren’t as good as round posts."
What proof does he base that on?
I’ve heard (from a friend at a bike manufacturer) that aero seat posts only measure better without a rider, and that a round post deals with the turbulence caused by the rider better. Maybe it has something to do with changed yaw angles due to the rider?
Steve
That was my understanding of it as well. Bikes designed and tested in isolation would appear very different to those tested with a rider…
There is an other argument than aerodynamics - better bike construction:
Frame makers aims to make the optimal frame regarding weight, stiffness and comfort/compliance. A junction with two overlapping sections (frame and seat post), where the overlap might vary makes it impossible to get the optimal stiffness & comfort for the seat tube/post:
Large overlap/long visible seatpost makes a very large stiff section, small overlap/short visible seatpost = very comfortable).
In addition, there is high stress at the point where the post leaves the frame. Since we don’t know where on the seatpost this point will be (due the adjustability), the entire post needs to be made extra solid and stiff, resulting in not optimal comfort.
Integrated, non-adjustable seatpost makes the best frame!
But is it worth it?
True but to be honest, with the state of optimisation of design using carbon fibre these days that isn’t so much an issue. Having to rely on two overlapping areas fro extra strength is a bit ‘Heath Robinson’ in todays age.
When you look at the market its notable that Guru have remained with a traditional post arrangement despite a unusual frame design…
Most likely he is using wind tunnel data…he spends a lot of time there…just a guess …
Still not buying it. Even with the argument with or with out a rider. From what we know a round seatpost is not aero especially when compared to an aero foil type shape. And almost all of the seat mast that are out there keep the same diameter all the way to the seat clamp. Why not taper it once it gets past the top tube. From what I have been told is that they have to keep the same dia so the mast will have strength to support the rider. I will keep the adjustability of the Seat Post just because the gains if any (still not sure) are not worth being stuck in a position that I might have to adjust due to injury or new position trials.
What happens if you want to sell your frame and the mast size that suits you is different than that of the buyer?
The weight savings and aerodynamics is minimal. Stupid idea IMHO.
http://www.slowtwitch.com/...techctr/compact.html
Finally a good explanation of the best flow of wind around the rider! Can’t believe that is a 6 year old article, because I have been asking that question here for years, “What is the best path for the wind, “around” the rider or “through” the rider?” and this, combined with gleamings from a Cobb post earlier this year (where, in reference to the “praying landis” position, he mentions something about how the wind is best channeled up and over the torso or under the arms and around the hips. I.E. the idea is that the rider should present a solid “bullet” shape to the wind, vs. an open “cup” shape, gives us a good idea that the best aero is to have the wind flow around the body and not through it (legs, seatpost, arms).
So a “wider” seatpost actually inhibits the wind “upstream” from entering the “cup” of the rider by preventing an exhaust channel so the air has a harder time flowing into the cup from the very beginning. F-1 aerodynamicists spend a lot of effort to determine how the “exhaust” flow of air effects the inflow of air upstream, and this is similar.
So net-net, the idea is to close off the flow of air between arms and legs, and arms and head, as much as possible to route the air around and under the arms and over head and torso, narrowing the profile as much as practical in all dimensions (knock-kneed and low head and back). An aero seatpost can act as an open “exhaust flap” but that is not what one wants. If the seatpost has the ability to prevent turbulence at the back of the bike, when the air comes back together, that is good, but less important than actually preventing too much air from flowing between the riders legs. Too much air between the legs actually draws/redirects flow (upstream) into the torso, where it is not wanted.
One can see how a very big drop, where the arms are a big vertical distance below the shoulders, causing a big “Cup” to catch the wind, can actually be less aero than less drop, as long as the head & torso are closer to the arms.