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Aero seat posts - are they aero?(23 posts)

Aero seat posts - are they aero?Pack Meat
Jul 24, 2002 2:05 PM
John Cobb, Lance's aero guy said the quote below in Velonews:

"...when we were working in the wind tunnel trying to figure out why aero' seatposts don't work, which they don't - ONCE would be a lot faster if they got rid of theirs."

There is no other info anywhere about why aero posts don't work. His statement makes it sound like the aero post slows you down. Anybody got any info on this?

Before everybody says "just ride your bike", "don't worry about it unless you're racing in the Tour" I'm building a time trial bike for the State TT championships and I already have both an aero post and a round post so there is no expense for me to put either on the bike. And I am training my ass off. The bottom line is if the post makes as little as a 15 second difference over 40k, I need that 15 seconds, after all it would be free time.
Mike Burrows might differspookyload
Jul 24, 2002 2:32 PM
I guess it is who you listen to. If you ride a traditional frame like Postal does, little seatpost is exposed. If you ride a compact frame and have lots of post showing, my guess is it would help. Mike Burrows is thought of by many as the king of aero. His Lotus carbon frames were cutting edge. John Cobb seems to be in vogoue for aero these days since he is Lance Armstrongs personal wind tunnel expert. So I guess it boils down to this question for me...if an aero seatpost doesn't matter then why does the Trek TT frame have the Aero extension coming up instead of just using a round post? LA is the epitome of aero anal retentive and he seems to think it is important.
re: Aero seat posts - are they aero?DougSloan
Jul 24, 2002 3:22 PM
I thought I read something to the effect that the seatpost does not "see" clean enough air for aero to matter. I did not take it that the aero post would be slower, though. I can't imagine how aero could be slower, either, particularly such that riders would be a lot faster if they got rid of them.

The seatpost on my P3 sure is an airfoil shape. There might be something on this at the Cervelo website.

Doug
More Surface Area = More Dragjose_Tex_mex
Jul 24, 2002 3:38 PM
If it's a thing that the post is not seeing clean air, it follows that air is hitting the post from the side as well as the front. If the post is aero and getting hit from the sides disproportionally, the extra surface area would IMHO be a liability.
re: Aero seat posts - are they aero?yeah right
Jul 24, 2002 3:41 PM
every moving object has (at least) two kinds of drag associated with it. one comes from the changes in pressure around the object (pressure drag) and the other is due to skin friction.

seatposts are really quite small, and do not move through the air *that* quickly, and therefore operate in a low reynolds number regime. basically, you can't save that much drag from using a airfoil due to the scale and nature of this(assuming you can't make the post thinner), but when you increase the wetted area (surface) of the post you do increase skin drag, therefore it is concevable that you can cost yourself drag counts with an aero post.

as with all things of this nature (i.e. small reynolds numbers) the best idea is to test it (run a course under same conditions if possible). however, the mike burrows design is clearly way too long to be truly optomized.
skin vs. pressure dragDougSloan
Jul 25, 2002 5:46 AM
Especially at lower speeds, isn't skin drag miniscule compared to pressure drag? We aren't golf balls going 150 mph.

Doug
skin vs. pressure dragyeah right
Jul 25, 2002 7:11 AM
to tell you the truth, i'm not sure how significant (quantitatively) skin drag is in this case, but i'm guessing it's a lot more than you think. you aren't a golf ball moving at 150mph, but you are an object many, many times the size of a golf ball moving at 30mph. look at botero moving quickly (faster than a lot of tt specialists)on a fairly standard roadbike, if pressure drag completely dominated, he wouldn't have a chance, but his set up does have less skin friction.

skin drag is significant enough that even when i've been on projects designing slow flying, small aircraft you don't make the fuselage or tail surfaces an inch longer or larger than you need, because at 30mph it makes a big difference. i suppose i could go run a simulation tonight on my computer and put the argument to rest.
what about disc wheels?DougSloan
Jul 25, 2002 7:39 AM
So disc wheels must be the slowest wheel you can use, right?

Doug
what about disc wheels?yeah right
Jul 25, 2002 7:57 AM
in the 1920's airplanes went from biplanes to monoplanes because the structures of the time required exposed wires to be used a crossbracing between two wing segments. This increased the drag of the airplanes up to 30%.

I will be very clear that I'm no expert on the aerodynamics of rotating objects, and low reynolds number unsteady flows are impossible to say much about, other than in a wind tunnel, but spokes are a little bit like like those wires on the biplanes. disk wheels as you know, are faster and have lower drag because they don't cause nearly as much pressure drag, but comparing a wheel with as many as 32 spokes churning up air, and one seat post is a different aerodynamic story.

interestingly, as a side note, i think i read something from lew, that said that he thought there were wheels more efficient than disks fwiw, that could be bs marketing though.
interesting articleDougSloan
Jul 25, 2002 8:18 AM
http://www.bicyclesports.com/technical/aerodynamics/watts_drag.html

All those little spokes are primarily creating pressure drag, right? Everything I've read shows the disc to be lowest drag, and there can be no doubt that its surface drag is enormous compared to any other wheel; the thing is, skin drag is miniscule compared to pressure drag. Sort of like rolling resistance compared to air resistance.

Doug
interesting articleyeah right
Jul 25, 2002 8:47 AM
http://www.lew-usa.com/pages/faq.html they used to have a better one, but i can't find it, not saying i agree with this

i'm going to respectfully disagree to your statement that skin drag is miniscule because in general, it's not. i can't find the article on the giro aerohead helmet, but i believe that wind tunnel tests indicated that lemond woud have been faster without it, due to highly increased skin drag of the helmet.

i guess my original point was this: an aero seatpost could be designed that is faster than a round tube. this is fact. the popular mike burrows design is not the best. in an attempt to look aero (this is my guess) he drastically increased the length of the post and probably increased the area of it by a factor of five. while it probably does a very fine job of reducing pressure drag, it has higher skin friction than needed, which when introduced to the unsteady airflow and interference effects of a bicycle probably adds a lot of drag, which is why cobb thinks they are slower.

my .02
LewDougSloan
Jul 25, 2002 9:21 AM
The only point I'd knock is that Lew does not make disc wheels, so it misleadingly states that disc wheels have enormous skin drag compared to spoked wheels. Yes, that's true, but disc wheel have almost no pressure drag, which is overwhelmingly dominant.

Every test I've seen shows disc wheels to have, on average, much less total drag than spoked wheels, even negative drag at certain wind angles.

Doug
re: Aero seat posts - are they aero?aliensporebomb
Jul 24, 2002 4:47 PM
A test:

Take your favorite bike with aero post and do a 25 mile ride and time the distance.

Then, take the same bike and put a standard post on it, same seat, same route and
time the distance.

Preferably the same day so the weather/wind conditions are similar.

Also, the Cobb quote is at odds with Lance's TT bike having an aero post. ???
Well...grzy
Jul 24, 2002 4:58 PM
To have any validity you'd need to run the test a whole bunch of time such that you could include statisticaly valid uncertainty analysis to account for all of the real world variation. Who says to time trials done back to back are even remotely similar given all the variables? Nope, gotta get a grad student at MIT to run it through a wind tunnel for you....a whole bunch of times.
Lance Armstrongs/John Cobbs take on it in picturesspookyload
Jul 24, 2002 5:54 PM
I found a decent pic of the bike Lance used on the prologue and it does apear to have a very very wide aero profile built into the frame where an aero seatpost would be. He seems to think it is important on Lance's bike I guess, but not important in theory?
a bunch of boolsheetStarliner
Jul 24, 2002 6:23 PM
Anybody who believes someone who claims a fu$%ing seatpost will save xx seconds over yy k's, well, there's a one-way ticket to Jonestown waiting for you, and you'll be served some real kool refreshments upon arrival.

Seatposts are located to the rear of other bicycle components which punch through the airflow first. Then, theres the torso, arms, and head of the rider which have a significant effect in breaking through the air before the seatpost comes into play.

Unless you're somebody (LA) who has millions of dollars on the line, where nothing must be left to chance, an aero seatpost is simply a cosmetic item to add to the overall list of components from which to choose when building up your ride.

INTIMIDATION
IF THAT'S THE RESULT YOU SEEK
BE A FOOL AND TRY
re: Aero seat posts - are they aero?feathers mcgraw
Jul 24, 2002 7:26 PM
Read something on slowtwitch.com, the gist of which was that it is better to fill the gap between your thighs (insert your own joke here). Skinny aero posts let too much air through, a regular round post is better, and a thick aero post is best.
http://www.slowtwitch.com/mainheadings/techctr/compact.html
re: Aero seat posts - are they aero?TonyR
Jul 24, 2002 8:00 PM
I don't have much to add to this other than to point out that 1.) Cobb does more windtunnel testing than anyone else, and is used by numerous TdF teams, and 2.) If you read the slow twitch article you see that there is a marked difference in aerodynamics between a bike, and in this case a seatpost with and without a rider present. My bet is that Burroughs may have made they mistake of not putting someone in the tube to sit on top of his rig. How much due these little tweaks help with TT's? Hell if I know, but it is fun to argue over.

Getting a frame that fits with components that allow for the best position are probably alot more important that worrying about what seatpost is on the bike.

Just my 2 cents.

-TR
re: Aero seat posts - are they aero?aliensporebomb
Jul 25, 2002 2:35 AM
If you wanted to be anal-retentive about it you could say:

"Ah, but the aero carbon seatpost I have smoothes out the ride somewhat, leading to less overall fatigue, that leads to better overall performance on the bike, that leads to a better potential for victory."

There's always going to be a naysayer who says "this won't work" yet who knows?

Maybe Cobb was trying to freak out ONCE into giving up their aero posts so that
Lance would have a secret advantage, maybe it's just a ploy.

Think about it.
I notice my fat inner thighs don't rub on an Aero post.bnlkid
Jul 25, 2002 5:15 AM
That alone makes me more effecient in my pedal stroke.
Ask the horseTurtleherder
Jul 25, 2002 6:11 AM
Instead of asking us igits what we think why not just E-mail Mr. Cobb at Bicycle Sports and ask him what he meant. jcobb@bicyclesports.com. Explain your situation and why you want the information, I hear he answers these type of questions.
I just emailed CobbPack Meat
Jul 25, 2002 8:36 AM
I'll keep ya'll posted if and when I get a response.
Here's the response from Bicycle Sports about seat postsPack Meat
Jul 25, 2002 9:07 AM
"Put on the round post. Turns out aero posts don't work. The air is too disturbed by that point. The aero post doesn't channel the air properly off the bike. We have preached aero post for years. Turns out we were wrong
after an in depth study we did 2 years ago."

That was a quick response, that's pretty cool. I didn't want to poke them in the eye and ask them about USPS using aero posts.