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Wind Tunnel Data on Road Bike Positions

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Wind Tunnel Data on Road Bike Positions

Old 12-20-12, 03:34 PM
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Nate552
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Wind Tunnel Data on Road Bike Positions

I recently went to the wind tunnel with John Cobb and while there we tested some road bike positions. Cobb is a proponent of the "slammed" road position where the rider can handle it and wanted to get some data on it, when compared to a more upright position. So we ran some tests with me in the drops, with my stem slammed (like I've been riding for years) and then on the hoods. We then put some spacers under the stem, flipped my stem to a pretty upright position, and ran the same tests. For the positions, I tried my best to choose what "felt right" and what I felt like I would naturally do with my arms. It would be unreasonable to attempt to run enough tests to accommodate all the different arm bends, hand positions, etc. Usually we measure the gain at the tunnel as "time saved" because we mostly deal with Time Trialists and Triathletes. But since you don't really care about that in road races, in general, we measured it with "watts saved" instead. This was the average drag across all yaws compared to the other stem position.

Drops:




Riding in the drops with a lowered stem saves on average, approximately 46 watts at 30 mph.


Hoods:





Riding on the hoods with a lowered stem saves on average, approximately 29 watts at 30 mph.
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Old 12-20-12, 03:48 PM
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Thanks for doing that, and sharing the results...
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Old 12-20-12, 03:49 PM
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now if you could just lose a few lbs...
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Old 12-20-12, 03:58 PM
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Originally Posted by hammy56 View Post
now if you could just lose a few lbs...
He's skinny compared to me.

The pictures look like "new racer" vs "experienced racer".

Can you go further forward with your bars? Meaning with a longer stem or top tube.

Also, out of curiosity, what length cranks do you have and about what length leg (inseam or whatever)? They look like longer cranks.
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Old 12-20-12, 04:19 PM
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Thanks for sharing. Good data. It must be fun to have access to such testing and advice.

A(nother) teammate and I did some aero testing a while back, where we were each-other's control. Control rider was locked elbows on the hoods, neck in a repeatable position. The other rider would ride next to the control rider at the same speed (about 10' to the side, not right next to him). We would then share power files to line up and compare power in Excel.

I discovered that my deep drop position (horizontal forearms) is actually lower drag than IAB, but only 5W or so. It was also about 20W between my relaxed (slight bend) drop position and my deep drop position. Now, when I'm bridging, I always get as low as I can, because it made such a difference on that test day. It would be cool to get in a tunnel sometime and see what the real numbers are. My back is almost completely flat in my "comfy" drop position. The lower position actually puts my back down, and not flat, but I think blocking my thighs with my head and arms closes off that "cup" a bit and helps.

Everyone is different though.
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Old 12-20-12, 04:20 PM
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imagine how many watts nate would save if he actually had a jersey, instead of that orange parachute.
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Old 12-20-12, 06:30 PM
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Originally Posted by carpediemracing View Post
He's skinny compared to me.

The pictures look like "new racer" vs "experienced racer".

Can you go further forward with your bars? Meaning with a longer stem or top tube.

Also, out of curiosity, what length cranks do you have and about what length leg (inseam or whatever)? They look like longer cranks.
Dead on with the "new" vs "experienced." That was something we were trying to show. The high stem position represents the "fit included with bike purchase" fit you see.

I've thought about a longer stem. Recently I've been scooting forward on my seat and hence moving my seat forward. That's resulted in the "cramped" looking position. But, I like it. Cobb made the same comment. As for cranks, those are 170's. I rode with 165's last year, but Quarq only does 170 (except for Rotors, which I didn't want). They might look long because I'm right over my BB instead of behind it. Oh, and I'm ~5'8".

Originally Posted by botto View Post
imagine how many watts nate would save if he actually had a jersey, instead of that orange parachute.
Ain't that the truth! v And that's a Canari small jersey! When I saw the profile pic I cringed. I tried a XS a few seasons ago but while the torso fit nice, it was way too short. Makes a good case for a skinsuit in a crit doesn't it?

Last edited by Nate552; 12-20-12 at 06:39 PM.
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Old 12-20-12, 06:37 PM
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Originally Posted by waterrockets View Post
Thanks for sharing. Good data. It must be fun to have access to such testing and advice.

A(nother) teammate and I did some aero testing a while back, where we were each-other's control. Control rider was locked elbows on the hoods, neck in a repeatable position. The other rider would ride next to the control rider at the same speed (about 10' to the side, not right next to him). We would then share power files to line up and compare power in Excel.

I discovered that my deep drop position (horizontal forearms) is actually lower drag than IAB, but only 5W or so. It was also about 20W between my relaxed (slight bend) drop position and my deep drop position. Now, when I'm bridging, I always get as low as I can, because it made such a difference on that test day. It would be cool to get in a tunnel sometime and see what the real numbers are. My back is almost completely flat in my "comfy" drop position. The lower position actually puts my back down, and not flat, but I think blocking my thighs with my head and arms closes off that "cup" a bit and helps.

Everyone is different though.
Pretty cool test. That's interesting that your horizontal forearms were better. I suspect your frontal area is more reduced, or the open arm stance allowed the air to enter your torso area cleanly, and therefore exit quickly and cleanly. Forearm and bicep shape can greatly influence this. When bridging, those extra 20-30 watts matter!
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Old 12-20-12, 06:41 PM
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thanks for sharing. i echo WR's remarks in that i'd like to know the difference between drops, forearm parallel on hoods, IAB, and deep drop.

Also, what about savings at 20 and 25mph? i know there's a equation, and it's somewhat linear w.r.t. to ground speed (or is it)
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Old 12-20-12, 07:07 PM
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Originally Posted by Nate552 View Post


Ain't that the truth! v And that's a Canari small jersey! When I saw the profile pic I cringed. I tried a XS a few seasons ago but while the torso fit nice, it was way too short. Makes a good case for a skinsuit in a crit doesn't it?
maybe it's me, but i don't mind short jerseys.

as far as skinsuits for crits, hell yes.
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Old 12-20-12, 08:37 PM
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I was looking at your knee angle at the top of the pedal stroke. I think your saddle is lower than, say, mine, so you have a similar knee angle at the top of the pedal stroke as I do, but I'm on 175s with a 29" inseam (size S Giant, 52 cm Cdale).

The skinsuit thought popped into my head too. Unfortunately for me I fill those wrinkles in a size L jersey.
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Old 12-20-12, 09:24 PM
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Where was you power levels in these these tests? Were you holding a constant power across all tests? I guess I'm wondering if you held power in the lower positions or was there some drop-off which would offset some of the savings. Thanks
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Old 12-20-12, 09:43 PM
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Originally Posted by Nate552 View Post
Ain't that the truth! v And that's a Canari small jersey! When I saw the profile pic I cringed. I tried a XS a few seasons ago but while the torso fit nice, it was way too short. Makes a good case for a skinsuit in a crit doesn't it?
I had similar issue with my last Garneau team kit...the medium was just a tad too big in some areas, but the small was too small. Stuck between sizes.
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Old 12-20-12, 11:22 PM
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I'm doing pretty well with a medium fit for our race-fit jerseys. A little short in front for a victory salute, but not floppy anywhere.
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Old 12-20-12, 11:58 PM
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Originally Posted by echappist View Post
Also, what about savings at 20 and 25mph? i know there's a equation, and it's somewhat linear w.r.t. to ground speed (or is it)
Drag is just a squared function. Nothing tricky. Mr. Bernoulli said so.

x = speed of interest (mph)
Dx = Power at speed of interest (grams)
D30 = Power at 30mph (grams)

Dx = D30 * (x^2/900) (FWIW, 900=30^2)

also:
x = speed of interest (mph)
Px = Power at speed of interest (watt)
P30 = Power at 30mph (watt)

Px = P30 * (x^3/27000) (FWIW, 27000=30^3)


This will give you the aero side of the equation. There will be a rolling resistance component as well. My guess is that as speed goes up, the rolling resistance gets kind of overpowered by aero, but that's just speculation.

Last edited by Fat Boy; 12-21-12 at 12:54 PM. Reason: Changed due to WR pointing out I was screwed up.
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Old 12-21-12, 01:37 AM
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Originally Posted by Nate552 View Post
Riding in the drops with a lowered stem saves on average, approximately 46 watts at 30 mph.
...
Riding on the hoods with a lowered stem saves on average, approximately 29 watts at 30 mph.
thanks for posting!

interesting that the differences in these positions is on the order of magnitude of the TOTAL drag from teh frame of a super bike (TT frame).
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Old 12-21-12, 06:30 AM
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Thanks for posting, Nate. Don't mind Hammy. He thinks a wind tunnel is the back room at the Doll House.

So, using your numbers and my back of the envelope scientific calculator, would it be safe to say that riding on the hoods versus riding in the drops, for you at least, is around a 50 watt difference at 30mph? If so, that's good clinic material.
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Old 12-21-12, 09:34 AM
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Dead on with the "new" vs "experienced." That was something we were trying to show. The high stem position represents the "fit included with bike purchase" fit you see.
Not necessarily. Even when I was a semi-studly runner...40 years ago...I had a hard time touching my toes. I was professionally fit a couple of years ago and ended up with a 6cm bar-stem drop; I'm due for another fit session and will probably be fit with more drop, but not a whole bunch more, probably about 2cm or so. According to Chris Carmichael's The Ultimate Ride, riders my height (6'2") usually end up with 6-10cm of drop.

There are both noobs and guys my age (61) who have trouble keeping their nose off the front tire. It's not necessarily a factor of experience.

Also, for a math-challenged guy who took his last math class during his first semester in college (Fall '69), what is the "^" sign? This is especially confusing for a guy with a degree, graduate work and work experience in French.
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Old 12-21-12, 09:45 AM
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Originally Posted by shovelhd View Post
Thanks for posting, Nate. Don't mind Hammy. He thinks a wind tunnel is the back room at the Doll House.

So, using your numbers and my back of the envelope scientific calculator, would it be safe to say that riding on the hoods versus riding in the drops, for you at least, is around a 50 watt difference at 30mph? If so, that's good clinic material.

Hammy is just jealous of my ankles. Good back of the envelope math Shovel, in the slammed position going from hoods to drops, on average, saves 51 watts at 30 mph. And like Fat Boy said, you can scale it down to whatever speed you want. Flo Cycling has blog entry devoted to this math if you are so inclined. edit - here it is https://flocycling.blogspot.com/2012/...odynamics.html

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Old 12-21-12, 09:50 AM
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Originally Posted by revchuck View Post
Not necessarily. Even when I was a semi-studly runner...40 years ago...I had a hard time touching my toes. I was professionally fit a couple of years ago and ended up with a 6cm bar-stem drop; I'm due for another fit session and will probably be fit with more drop, but not a whole bunch more, probably about 2cm or so. According to Chris Carmichael's The Ultimate Ride, riders my height (6'2") usually end up with 6-10cm of drop.

There are both noobs and guys my age (61) who have trouble keeping their nose off the front tire. It's not necessarily a factor of experience.

Also, for a math-challenged guy who took his last math class during his first semester in college (Fall '69), what is the "^" sign? This is especially confusing for a guy with a degree, graduate work and work experience in French.
Sure, of course some guys have good fits quickly. But if you show up to a cat 4/5 race and counted position types, I suspect you would find guys like you are in the minority. And ^ means "to the power of"
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Old 12-21-12, 09:56 AM
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I wonder how many miles per hour are lost upright compared to slammed
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Old 12-21-12, 10:03 AM
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Nice post, thanks for sharing.

I recently met Cobb and even knowing his reputation I was still very impressed. The weirdest part was when he said he'd been hearing about me. That was a stunner.
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Old 12-21-12, 10:32 AM
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Originally Posted by RChung View Post
Nice post, thanks for sharing.

I recently met Cobb and even knowing his reputation I was still very impressed. The weirdest part was when he said he'd been hearing about me. That was a stunner.
LA Track I assume? We've discussed the "Chung Testing Protocol" at length. I actually plan on doing some Chung testing next week while on vacation with a new helmet.
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Old 12-21-12, 10:38 AM
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The Velodrome set up will be interesting, and certainly convenient
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Old 12-21-12, 10:50 AM
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Originally Posted by Nate552 View Post
LA Track I assume? We've discussed the "Chung Testing Protocol" at length. I actually plan on doing some Chung testing next week while on vacation with a new helmet.
Yup. Thanks for that. It was completely unexpected so I was pretty jazzed.
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