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Question about position

Old 07-02-11, 07:29 PM
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Gios78
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Question about position

Which is more aerodynamic being able to get into a position where your back is flat or getting your body lower over the bars?

thx
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Old 07-02-11, 11:04 PM
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Might be one, might be the other. Depends on a lot of factors.
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Old 07-03-11, 04:25 AM
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Originally Posted by Racer Ex
Might be one, might be the other. Depends on a lot of factors.
Figured it may come to that.

I was watching a circuit race on Sat and was looking at the positions of the guys in the open race. I noticed that the guys who seemed to size down a lot for a large saddle to drop didn't seem as stretched out as the guys on larger frames who seemed to be able to get into a flat back position. They seemed to get the front of the body lower but with a curved back and straight arms while the guys on larger frames seemed higher but had a nice looking flat back position with elbows bent a lot more in the drops.
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Old 07-03-11, 04:45 AM
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They're obviously running too short of a stem

https://www.bikeforums.net/showthread...he-pro-peloton

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Old 07-03-11, 06:14 AM
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eyes =/= wind tunnel data
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Old 07-03-11, 10:36 AM
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Originally Posted by Gios78
Which is more aerodynamic being able to get into a position where your back is flat or getting your body lower over the bars?

thx
Both?
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my race videos
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Old 07-03-11, 10:51 AM
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Originally Posted by Grumpy McTrumpy
eyes =/= wind tunnel data
True. However, careful field testing with the right equipment can == wind tunnel data.
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Old 07-03-11, 11:17 AM
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Originally Posted by RChung
True. However, careful field testing with the right equipment can == wind tunnel data.
do still need to find an indoor velodrome though, and those are far and few in between.

btw, can you tear me apart on this position?

11-12cm of seat to bar drop, maybe another 1-2 cm of stem to hood drop.



above is my preferred position, one in which i can sustain for hours at a time



i have no idea what i look like in the IAB
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Old 07-03-11, 02:00 PM
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Originally Posted by echappist
careful field testing with the right equipment can == wind tunnel data.
do still need to find an indoor velodrome though, and those are far and few in between.
With the right equipment maybe not.

btw, can you tear me apart on this position?
Hard to tell, especially from the angle the photos were taken. In general, it's best to have a video where you're pedaling rather than a static photo, and the video shots should be at zero and 90 (or 270) degrees. Even then, the eye != wind tunnel.
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Old 07-03-11, 04:24 PM
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Originally Posted by RChung
True. However, careful field testing with the right equipment can == wind tunnel data.
At zero yaw. And I have yet to do a race or TT at 0 yaw. Not that field testing doesn't have it's merit though, I have some slated as soon as the wind dies down around here. And yes I know that the "Chung on a Stick" will someday be released.
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Old 07-03-11, 04:30 PM
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You can make you own wind tunnel by using a bunch of fans and hanging a pencil by a wire from the ceiling ... there's some youtube videos on it check it out.
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Old 07-04-11, 09:02 AM
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Originally Posted by Nate552
At zero yaw. And I have yet to do a race or TT at 0 yaw.
If you ride faster the yaw will approach zero. But, yeah, that's why I said "with the right equipment."
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Old 07-04-11, 09:33 AM
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Originally Posted by RChung
If you ride faster the yaw will approach zero. But, yeah, that's why I said "with the right equipment."
I ride so fast I go to negative yaw.

Ba ha ha ha.

After looking at less tests than Nate but more than a lot of folks, there's no substitute for the tunnel if you're looking for small differentials or data for different yaw angles, and everybody is different so what works for one person is what works for one person.

You can do a lot with field testing though, at least get in the ballpark at the smaller yaw angles. I can PE position to some extent at this point, especially on the road bike.
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Old 07-04-11, 09:44 AM
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i posted this in the stickied TT thread but received no answers.

How much does Cobb charge for fitting and does that include tunnel time? Feel free to PM me if such information is confidential.
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Old 07-04-11, 09:53 AM
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Originally Posted by Racer Ex
After looking at less tests than Nate but more than a lot of folks, there's no substitute for the tunnel if you're looking for small differentials or data for different yaw angles
Hmmm.

and everybody is different so what works for one person is what works for one person.
With this I totally agree.
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Old 07-04-11, 10:35 AM
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Originally Posted by RChung
If you ride faster the yaw will approach zero. But, yeah, that's why I said "with the right equipment."

How fast do think people ride TT's? Let's take look at this. We'll use a rider speed of 25 mph, which is often an amateur racer's goal speed for a 40 K. A 5 mph cross wind will create a 11 -12 degree angle of wind on the rider. Now, let's look at a faster rider. Let's use a speed of 29 mph, a speed some fast guys can do like Racer Ex. The same 5 mph crosswind will create a wind angle of 10 degrees. So how fast do you have to go to make a 5 mph cross wind "0"? Well, lets just say that even at the speeds the guys in the TTT in the Tour de France went yesterday, it'd still be 7 degrees.

Now, let's look at a situation that is quite common. 0 yaw can lead you astray. What is fast at 0, is not necessarily fast at, 7, 10, 15, 20 etc. Here is some actual wind tunnel data of someone testing helmets. If you did some field testing, and did it well enough to call it " as good as tunnel data" you would look at the green helmet and say it was easily the fastest.



Then, you go out to a TT course that is out and back, and runs East/West. There is a north wind of 7 mph, which is very common at this course. For simplicity sake I will round the drag and math to make even numbers, but you just gave up 10 watts. I think Field testing has it's place, but I don't think it's correct to say that it can equal wind tunnel data.
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Old 07-04-11, 10:37 AM
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Originally Posted by echappist
i posted this in the stickied TT thread but received no answers.

How much does Cobb charge for fitting and does that include tunnel time? Feel free to PM me if such information is confidential.
Missed this, sorry. I'll ask John what he's charging for a fitting these days.
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Old 07-04-11, 11:01 AM
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Originally Posted by Nate552
How fast do think people ride TT's? Let's take look at this. We'll use a rider speed of 25 mph, which is often an amateur racer's goal speed for a 40 K. A 5 mph cross wind will create a 11 -12 degree angle of wind on the rider. Now, let's look at a faster rider. Let's use a speed of 29 mph, a speed some fast guys can do like Racer Ex. The same 5 mph crosswind will create a wind angle of 10 degrees. So how fast do you have to go to make a 5 mph cross wind "0"?
You need to calibrate your amusement threshold.

I think Field testing has it's place, but I don't think it's correct to say that it can equal wind tunnel data.
Two things:
1) only an on-bike wind sensor will tell you the "true" (i.e., weighted) exposure to yaw.
2) I've measured the sd of a carefully done field test estimate of CxA at .0006 m^2 for a CV of 0.3%. Compare that with the CV of a wind tunnel estimate.
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Old 07-04-11, 11:13 AM
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Originally Posted by RChung
You need to calibrate your amusement threshold.
Sorry, didn't catch that you were kidding.


Originally Posted by RChung
Two things:
1) only an on-bike wind sensor will tell you the "true" (i.e., weighted) exposure to yaw.
2) I've measured the sd of a carefully done field test estimate of CxA at .0006 m^2 for a CV of 0.3%. Compare that with the CV of a wind tunnel estimate.
1) Vaporware
2) Why worry about an estimate of CxA? Drag, in grams, no estimation.
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Old 07-04-11, 11:37 AM
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1) Ibike?
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Old 07-04-11, 01:30 PM
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Originally Posted by Nate552
1) Vaporware
2) Why worry about an estimate of CxA? Drag, in grams, no estimation.
1) Well, in a commercial sense, yes, but I'm not really talking about any particular commercial product. There are several working sensors out there that I know of and it's possible (though tedious) to make one yourself. The sensors I know of will, of course, differ in their sensitivity depending on who made them and the design decisions that were made. And, in a deeper sense, as far as wind tunnels go they may as well be vaporware to me and to most of us -- the nearest wind tunnel is far away and way out of my price range.

2) CxA is independent of v and rho. [Edit:] https://forum.slowtwitch.com/gforum.c...417364#3417364

Originally Posted by ridethecliche
1) Ibike?
iBike doesn't measure yaw.

Last edited by RChung; 07-04-11 at 01:51 PM.
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Old 07-04-11, 02:13 PM
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Originally Posted by RChung
1) And, in a deeper sense, as far as wind tunnels go they may as well be vaporware to me and to most of us -- the nearest wind tunnel is far away and way out of my price range.
Not directed at you specifically, but there may be a bit of false economy in this thinking for a fair number of people, both in time and in dollars. It's pretty easy to drop $1000-$1500 in equipment upgrades or hours field testing and come away with a lot less than dropping $1500 and a couple of days to go to a tunnel. Granted it will be out of some people's time/dollar budget, and gains will vary according to who's running the show, but my experience has been on a $/w basis, it's a very competitive investment.

And there's still a McGyver creativity factor if you go off piste and start fabbing up your own bits. Trust me, there's nothing quite spending hours cutting, grinding, and drilling only to find you went completely backwards on that base bar.
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Old 07-04-11, 02:37 PM
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Originally Posted by RChung
1) Well, in a commercial sense, yes, but I'm not really talking about any particular commercial product. There are several working sensors out there that I know of and it's possible (though tedious) to make one yourself. The sensors I know of will, of course, differ in their sensitivity depending on who made them and the design decisions that were made. And, in a deeper sense, as far as wind tunnels go they may as well be vaporware to me and to most of us -- the nearest wind tunnel is far away and way out of my price range.
A trip to the tunnel costs a lot less that some fancy wheels. Of course, if you add cost to the equation, things get more speculative. But that's not the discussion. The reality is, most people are not going to build this sensor. Plus, how would you validate the results of your sensor's reported wind angles? A trip to the tunnel?


Originally Posted by RChung
2) CxA is independent of v and rho. [Edit:] https://forum.slowtwitch.com/gforum.c...417364#3417364
I understand what CxA is, but like Damon, I prefer grams because that's what is discussed at the tunnel. CdA is never measured directly, only derived.
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Old 07-04-11, 03:06 PM
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Originally Posted by Racer Ex
Not directed at you specifically, but there may be a bit of false economy in this thinking for a fair number of people, both in time and in dollars. It's pretty easy to drop $1000-$1500 in equipment upgrades or hours field testing and come away with a lot less than dropping $1500 and a couple of days to go to a tunnel. Granted it will be out of some people's time/dollar budget, and gains will vary according to who's running the show, but my experience has been on a $/w basis, it's a very competitive investment.
Oh, certainly. But I think of field testing not as a replacement for wind tunnels but as a supplement that can be pretty cost-effective. Depending on where you're living and what your needs are, a wind tunnel can be either overkill or out of (financial or distance) range. I was only clarifying that the claim that wind tunnels are more precise than field testing doesn't appear to be true, and that even extends to non-zero yaw.

Originally Posted by Nate552
A trip to the tunnel costs a lot less that some fancy wheels. Of course, if you add cost to the equation, things get more speculative. But that's not the discussion. The reality is, most people are not going to build this sensor. Plus, how would you validate the results of your sensor's reported wind angles? A trip to the tunnel?
Yes, but you can do that in the kind of desktop wind tunnel that high schools own plus some calibration tricks. Powering up a full-size wind tunnel to calibrate a sensor is kind of wasteful.
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Old 07-04-11, 03:56 PM
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Originally Posted by RChung
I was only clarifying that the claim that wind tunnels are more precise than field testing doesn't appear to be true, and that even extends to non-zero yaw.
That wasn't really the statement, just that there wasn't a substitute for the tunnel right now for finer measurements and past zero yaw data.

I'm sure we could develop field test equipment that is as precise as a wind tunnel, given that most wind tunnel equipment was generated off of field technologies. And I know there are days when field testing yields very tight results. The problem with field testing (especially beyond zero yaw) is consistent external conditions. And by time you've invested the time and money into the equipment you'd need to offset this problem, you could likely go to the tunnel a bunch.

But more than external conditions is rider input. Even in the tunnel with the most disciplined rider, there's noise. You get a swing and give it the old WTF? Look at the camera overlays and you see what happened. Moved the hands/fingers/head a little. I'm doubtful a person pedaling out on the road or even coasting maintains near the position consistency they would in the tunnel, and the logistics to even verify this are another matter.

And like proximity to a tunnel, proximity to a location that affords favorable conditions is a bug in the rug. I had to wait over a month once in Texas for a day where the wind wasn't blowing and the temps were stable.

I like field testing BTW. It's a heck of a lot better than nothing.
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