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Aero question

Old 11-25-19, 11:35 AM
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woodcraft
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Aero question

When descending & approaching the choice between coasting and continuing to pedal (i.e. ~35mph, drops, reasonably trim size, position & gear),

how many watts are needed to overcome the added drag from pedaling vs coasting with cranks level?
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Old 11-25-19, 11:51 AM
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Originally Posted by woodcraft
When descending & approaching the choice between coasting and continuing to pedal (i.e. ~35mph, drops, reasonably trim size, position & gear),

how many watts are needed to overcome the added drag from pedaling vs coasting with cranks level?
It appears to depend slightly on the cadence, but the effects are relatively small. Look up a paper from maybe 10 years ago done in the MIT wind tunnel by Blair, Chew, and Cote on static leg position and cadence.

(From a practical point of view for aero drag testing, the lesson is to soft pedal while doing tests even while coasting because the precision of drag measurement is better. This doesn't matter much for people who don't do drag testing).

[Edited to add:] Here's a link.

Last edited by RChung; 11-25-19 at 11:57 AM.
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Old 11-25-19, 12:24 PM
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Interesting.

With all the fine-tuning of TT position and emphasis keeping still, one would think that pedaling

would produce much more drag than coasting, but that appears to not be the case.

"The drag measured while the rider is pedaling turned out to be a little smaller than the average of measurements taken with the rider's feet stationary at various positions around the pedaling circle."

(and close to the lowest stationary position measurement)

So from the article, I take the answer to my question to be an insignificant amount (although IME coasting makes it easier to hold a tighter tuck position).

Thanks!
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Old 11-25-19, 12:40 PM
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Originally Posted by woodcraft
With all the fine-tuning of TT position and emphasis keeping still, one would think that pedaling

would produce much more drag than coasting, but that appears to not be the case.
The emphasis on keeping still is more to not lose the wind tunnel optimized position than because movement causes tons of drag in and of itself.

(although IME coasting makes it easier to hold a tighter tuck position).
This is the main reason that tucking and coasting is sometimes recommended on fast descents. A good pedaling tuck can be a difficult and uncomfortable position to hold.

And because the speedup that you get for your pedaling efforts on super-fast downhills is poor anyway. If you want the descent KOM you might need to hammer through, but if it's just part of a long solo effort, it's not always so terrible to go a few mph slower for a minute while the pedaling muscles and lungs rest.
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Old 11-27-19, 06:57 PM
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A recent rehash of GCN's classic vs modern bikes/kits in a wind tunnel showed Ollie, the test cyclist, pedaling slowly during the test. Presumably the lab is sticking with the current info indicating that pedaling may be less inefficient. Or at least more consistent than trying to hold the legs stationary in an aero tuck.


I was mostly interested in the theory that, beside an aero position on the bike, an aero kit -- jersey, helmet, etc. -- may be a bigger factor than the bike shape when we're actually mounted on the machine. I recently got some long sleeve aero jerseys for cool weather rides and a cheap aero helmet just to play around with. Seems to help, although I'll need several rides on the same routes over time and various conditions to be sure.

I usually pedal at least a little on descents because my legs cramp if I try to hold the most aero tuck position for more than a few seconds, especially after cresting a hard climb. Gotta keep moving my legs to clear the lactate buildup, otherwise the legs feel mushy after only 30 seconds of inactivity.

Same reason some local clubs split into different camps, mostly over disagreements about full stops at stop signs and traffic lights. Some folks prefer to keep moving so their legs don't get mushy because they're essentially sprinting from intersection to intersection. I don't trust drivers or some cyclists that much so I treat city rides as more casual and practice the traditional Idaho Stop at intersections -- including full stops and waiting for red lights at busy intersections. If I want an uninterrupted workout ride there are plenty of places outside town for that.

During a club ride this week the group split into the usual sub-groups -- those of us who don't blast through intersections gradually fall behind and reform our own group. We mostly chatter anyway. On a fun fast half-mile 4% downhill I noticed the other fellows were coasting so I didn't blast down that hill pedaling like I usually do. I figured I could hold an aero tuck for the minute it took to descend. And I was wearing a new aero jersey (not quite custom fitted, a little flappy around my toothpick forearms). So I just coasted by the other folks. Actually shaved 5 seconds off my usual pedaling approach. Although variables due to wind (light tailwind, maybe 5 mph), wet road from light rain, etc., may have been factors. The tailwind on that downhill is pretty consistent, though, so I'll give more credit to the aero kit and tuck, and maybe a bit to the damp road and low barometric pressure.
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Old 11-27-19, 07:42 PM
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Originally Posted by woodcraft
When descending & approaching the choice between coasting and continuing to pedal (i.e. ~35mph, drops, reasonably trim size, position & gear),

how many watts are needed to overcome the added drag from pedaling vs coasting with cranks level?
The drag difference between pedaling and pedals level is negligible.

The bigger issue is being able to tuck in tighter when you don't need to pedal.

With a 75kg combined rider + bike weight and 0.004 Crr:

You could coast down a 4.63% grade at 34.4 MPH in the "back down 2" position, or pedal in the drops at 141 Watts. In the same position that would produce 19.7 MPH solo on flat ground which would be a bit much for a recreational rider at an endurance pace.

You could coast down an 8% grade in the Alps at 46.3 MPH, or pedal in the drops at 341W which is professional territory on long rides.

https://www.linkedin.com/pulse/new-r...scent-blocken/
Previous articles from the same author validated their Computational Fluid Dynamics calculations using wind tunnel measurements.

Last edited by Drew Eckhardt; 11-27-19 at 10:02 PM.
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Old 11-27-19, 08:16 PM
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Originally Posted by canklecat
I was mostly interested in the theory that, beside an aero position on the bike, an aero kit -- jersey, helmet, etc. -- may be a bigger factor than the bike shape when we're actually mounted on the machine.
Yeah. Clothing and helmets make a pretty big difference. From the data in that video, the old Merckx bike costs about .025 m^2 in drag area compared to the Pinarello, but the retro clothing costs about .07 m^2. We talked about the big effect of clothing in the Cyclingtips Nerd Alert podcast here.

[Edited to add:] Ballpark, a rough rule of thumb is that a difference of .01 in drag area is worth around 1 sec/km at racing speed, so a difference of .025 is roughly 2.5 secs/km, retro clothing is costs about 7 secs/km, and a round tube bike with retro clothing costs about 10 sec/km compared to tight clothing on a modern bike. If you're not racing, the differences are a bit larger than that, not smaller.

Last edited by RChung; 11-27-19 at 08:26 PM.
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Old 11-28-19, 08:49 AM
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Originally Posted by Drew Eckhardt
The drag difference between pedaling and pedals level is negligible.

The bigger issue is being able to tuck in tighter when you don't need to pedal.

With a 75kg combined rider + bike weight and 0.004 Crr:

You could coast down a 4.63% grade at 34.4 MPH in the "back down 2" position, or pedal in the drops at 141 Watts. In the same position that would produce 19.7 MPH solo on flat ground which would be a bit much for a recreational rider at an endurance pace.

You could coast down an 8% grade in the Alps at 46.3 MPH, or pedal in the drops at 341W which is professional territory on long rides.
Thanks a bunch for posting this. Most of the folks I ride with try to descend in a position as close to position 4 as possible. We just found this to be fastest based on experience, but never had anything to quantify. Based on trial and error, we have agreed 32-33mph is the cutoff when we should just get in the tuck based on the wattage required to go faster. Obviously this is based on what we feel is an acceptable power output going downhill.
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Old 12-01-19, 08:17 PM
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There’s another great pictorial chart out there, which I can’t find, that compares numerous hand positions on the bar. Maybe some knows what I’m talking about and can post it?!

If I recall correctly, the comparo chart claims that Nibali often positions his hands tight against the stem when descending. It’s faster than the drops because (I think) it minimizes how much wind catches the torso. I frequently adopted this technique and found myself descending a couple 2-3mph faster than in the drops. Regardless of hand position, I still position my butt behind the saddle with my xiphoid process nearly on the nose of my saddle.
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Old 12-02-19, 06:22 PM
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Originally Posted by woodcraft
When descending & approaching the choice between coasting and continuing to pedal (i.e. ~35mph, drops, reasonably trim size, position & gear),

how many watts are needed to overcome the added drag from pedaling vs coasting with cranks level?
I do not have a number. But I noticed it did matter. My kid had to use a pedal and coast technique because he was gear restricted. I think the speed up, coast down speed up repeat worked pretty well. The cadence at 40 was too high to hold with a 52X14.

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Old 12-02-19, 07:01 PM
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Originally Posted by Drew Eckhardt
The drag difference between pedaling and pedals level is negligible.

The bigger issue is being able to tuck in tighter when you don't need to pedal.

With a 75kg combined rider + bike weight and 0.004 Crr:

You could coast down a 4.63% grade at 34.4 MPH in the "back down 2" position, or pedal in the drops at 141 Watts. In the same position that would produce 19.7 MPH solo on flat ground which would be a bit much for a recreational rider at an endurance pace.

You could coast down an 8% grade in the Alps at 46.3 MPH, or pedal in the drops at 341W which is professional territory on long rides.
I'm missing the responses to the OP question.
Originally Posted by woodcraft
... added drag from pedaling vs coasting with cranks level?
Same position. Pedaling vs coasting with cranks level. I have seen little data on this, although a wind-tunnel would certainly do the trick.
I understand that if you give up pedaling, you can also assume another more aero position. But I'd be curious about just the drag increase pedaling - say 90 RPM vs cranks level.

I do think some of this could be used a bit for a TT, but the TT bike would not support many of those positions.
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Old 12-02-19, 08:14 PM
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A couple of things IME: the drawings showing very low positions don't allow for real roads. I always have 2" between my stem and my chin. Any lower is really risky unless you've been down that descent a few times and have all the potholes and bumps memorized. The other thing is that I get a bit of additional speed in the drops position by pulling my elbows under my stomach as far as they will go. In testing that was as fast as the hands-near-stem-elbows-down position plus it's good to have one's hands near the brakes. The first command is thou shalt not get injured. I'm talking about only 40-55 mph speeds, which is all we get around here.

As I read the above, the OP was satisfied that pedaling need add only a few watts to make up for the additional drag. Of course that's the power meter data. If we look at the physiological stress of pedaling at 120 rpm to add a a few watts, now that's significant. Way better to work on position. Though it's nice to know that simply rotating the pedals while coasting to keep the legs functional might actually be more successful in the long run. Just don't lose that draft.
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Old 12-02-19, 11:38 PM
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Originally Posted by Doge
I'm missing the responses to the OP question.

Same position. Pedaling vs coasting with cranks level. I have seen little data on this, although a wind-tunnel would certainly do the trick.
I understand that if you give up pedaling, you can also assume another more aero position. But I'd be curious about just the drag increase pedaling - say 90 RPM vs cranks level.

I do think some of this could be used a bit for a TT, but the TT bike would not support many of those positions.
6W at 37 MPH which is negligible as I noted.


Unfortunately, Specialized interprets their data as speed.

Base case: pedaling 90 RPM descending a 6% grade at "a tick under" 59 kph (36.7 mph)
One leg down: -1 kph
Feet level: +0.3 kph

Working backwards, with a 75kg bike + rider and 0.004 Crr that would mean a CdA of 0.248 m^2 yielding 16.47 meters/second.

A decrease to 16.39 meters/second from pedaling without power would increase CdA to .250 m^2 and require an extra 6W to achieve 16.47 m/s

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Old 12-02-19, 11:59 PM
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Originally Posted by Drew Eckhardt
6W at 37 MPH which is negligible as I noted.
...
I will look closer tomorrow. My initial reaction is 6W is a lab thing and not close to real outdoor situations with speeds of 40-60mph. I'd guess closer to 50W-100W equivalent. I have strava of a 130# 15 year old going 55mph on 52X14 gears (nationals) - because he wasn't using them - i.e. coasting. Nobody pedaling could keep up. The only kids going 55+ were coasting. I understand that a 53X11 might have made that different, but 6W is margin of error stuff.
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Old 12-03-19, 12:18 AM
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Originally Posted by Doge
I will look closer tomorrow. My initial reaction is 6W is a lab thing and not close to real outdoor situations with speeds of 40-60mph. I'd guess closer to 50W-100W equivalent. I have strava of a 130# 15 year old going 55mph on 52X14 gears (nationals) - because he wasn't using them - i.e. coasting. Nobody pedaling could keep up. The only kids going 55+ were coasting. I understand that a 53X11 might have made that different, but 6W is margin of error stuff.
The wind tunnel tests answer the question of whether pedaling adds appreciable drag with an otherwise identical position. 6W at 37 MPH and 20W at 55 aren't significant.

At a minimum in the real world, you'd tuck your knees against the top tube.

While many people would stop short of a pro's tuck, I can't see not getting lower when you don't pedal.

That sort of thing shows up in CFD models, wind tunnels, and just rolling past people still pedaling furiously.

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Old 12-03-19, 01:45 AM
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Originally Posted by Doge
I will look closer tomorrow. My initial reaction is 6W is a lab thing and not close to real outdoor situations with speeds of 40-60mph. I'd guess closer to 50W-100W equivalent. I have strava of a 130# 15 year old going 55mph on 52X14 gears (nationals) - because he wasn't using them - i.e. coasting. Nobody pedaling could keep up. The only kids going 55+ were coasting. I understand that a 53X11 might have made that different, but 6W is margin of error stuff.
If people pedaling were going slower, it's likely more because they were spending more time in (higher-CdA) pedaling-suitable postures than because pedaling was adding that much more drag in and of itself.
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Old 12-03-19, 08:23 AM
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I've posted this before in a similar topic. It's all about learning and knowing when and what to do, not just having the perfect tuck.

Watch this to understand.........Alaphillipe dropped on the climb and catches them on the descent this summer:

As for TT? My limitation is usually whether I want to die or not. We don't have mountains here, just steep punches out of creek beds. Sometimes curvy. So usually I'll do what Doge said at the crest then get on the bull bars until if straightens out. Or until after the inevitable bump over the crap bridge road transitions.

But, usually, on that bike around here the 99th percentile range of speeds encountered on a TT bike ride is between 12 and 40mph. The 95th percentile probably between 15 and 35mph.
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