Observations from today's ride (aerodynamics related content)

fa63 · #1 08-17-2017, 10:19 AM

This morning, I met up with a triathlete friend to join her during her interval session. We both have power meters, so I thought I would take a look at the data post-ride and see what I could gather. Here are the profiles from Strava; the top one is me (220 lbs male), the bottom is my friend (120 lbs female):

I was in the front for the first 2 miles during warmup; she was ahead of me the rest of the time doing her intervals. The road is pretty much flat, essentially no wind. She was on a Cervelo P2 with aero wheels and a finely tuned position (she is pretty competitive in her age group, came in 29th out of several hundred at the nationals). I was on a Trek Emonda ALR with Reynolds Attack wheels, riding on the hoods. Using the speed-power data (and the Analytic Cycling website), I was able to deduce the following:

My frontal area, without the draft: 0.743 m^2
My frontal area, with the draft: 0.595 m^2

That is approximately a 20% reduction.

It took me 197W to average 21.2 mph while being in the draft essentially the whole time. Without a draft, it would have taken me 235W to achieve the same average. So I was saving about 16% by being in her draft. Value likely to be higher sucking the wheel of a less pint sized cyclist on a road bike

ptourkin · #2 08-17-2017, 12:28 PM

Aero is everything. I'm in the time of year where I have extensions on my road bike for a 500 mile race. I'm saving over 10% in the same flat sections when in the bars. Even in my draft, my coach was using 40 watts more than me in a flat/slight uphill segment this weekend.

Also, all of my PRs on the San Diego velodrome happened behind a motorcycle.

benb · #3 08-17-2017, 12:40 PM

Interesting but you need to try this without any drafting at all.

Was this pretty flat?

fa63 · #4 08-17-2017, 12:43 PM

Pretty much dead flat. I was in the front, not drafting, for the first two miles, from which I estimated my baseline frontal area. There might be a small effect from her drafting me for those two miles, but I doubt it is more than a couple watts.

weiwentg · #5 08-17-2017, 03:13 PM

Quote:

Originally Posted by fa63

...

It took me 197W to average 21.2 mph while being in the draft essentially the whole time. Without a draft, it would have taken me 235W to achieve the same average. So I was saving about 16% by being in her draft. Value likely to be higher sucking the wheel of a less pint sized cyclist on a road bike

You know, I've got maybe an inch and 10 lbs on your friend. I don't have very much drop, but if you get behind me, I'm still very low.

I think the conventional wisdom touts as the average power savings from drafting is 15-30%. I always got crap from my mates when I got took a pull - e.g. "I can see the whole road!", "wait, is it my turn to pull?", "go draft a recumbent and see what it's like!"

One way to think of it is that hey, my tiny draft is still saving you something like 15% of the watts you should be pulling! Another way to think of it is that I'm penalizing them somewhere between, say, 5% and 15%.

Just a thought.

shovelhd · #6 08-17-2017, 03:26 PM

Quote:

Originally Posted by weiwentg

I think the conventional wisdom touts as the average power savings from drafting is 15-30%.

That's been my experience.

benb · #7 08-17-2017, 03:31 PM

Yah I've seen that 30-35% range in my own data. Drafting is pretty huge.

I was actually surprised OP didn't save more. 200w for that speed on flat seems pretty high for a ride that is mostly drafting. Maybe the size difference + aero equipment on the lead rider explains that.

It's not flat here but I'll average 17-18mph at around 180-200w over rolling terrain not trying particularly hard to be aero. I'll average more like 140w in a pack if I mostly sit in to do the same thing. It's an enormous difference on long rides.

I've never tried drafting anyone with aerobars/tri-bike, etc.. they are way less common here than I get the impression they are in areas that are popular for Tri (FL, HI, etc..) And they are most likely to be seen on a fred going pretty slow. I don't think I'd try to hop on someone's wheel due to the reputation.

I've been to Kona 3X, it is amazing to see the contrast there. Where I live I'd guess it's < 5% of bikes seen on the road are Tri-Bikes/TT bikes or are otherwise equipped with aerobars, when I've been to Kona it looked like it was 90%+, and they've got plenty of winding roads with curves, and steep descents etc.. unless you stick to the Ironman route.

fa63 · #8 08-17-2017, 03:40 PM

Here is what I was on for the ride:

58 cm frame, ~8 cm saddle-to-bar drop so nothing too crazy. Me being wide doesn't help; 6'1", 220 lbs, friends say I have the best draft around town

Friend was on a Cervelo P2, I believe size 51, set up very low in the front so her back is completely flat.

fa63 · #9 08-17-2017, 03:46 PM

Quote:

Originally Posted by weiwentg

One way to think of it is that hey, my tiny draft is still saving you something like 15% of the watts you should be pulling! Another way to think of it is that I'm penalizing them somewhere between, say, 5% and 15%.

I usually think of the latter and not the former

But hey, life isn't always fair.

false_Aest · #10 08-17-2017, 03:56 PM

Bruh,

No need for a PM . . . just draft some Cat 1s or Pros.

Up PCH 2 weeks ago with some super fast folk. Every frickin' Strava segment I PRed on.

50km+ usually is reserved for beating red lights, intervals and town-lines.

fa63 · #11 08-17-2017, 04:10 PM

Was there any tailwind there? Either way, your HR suggests it was hell of an effort

pncguy · #12 08-17-2017, 04:33 PM

Quote:

Originally Posted by false_Aest

Bruh,

No need for a PM . . . just draft some Cat 1s or Pros.

Up PCH 2 weeks ago with some super fast folk. Every frickin' Strava segment I PRed on.

50km+ usually is reserved for beating red lights, intervals and town-lines.

Looking at your HR, my heart just exploded. I don't like being 50.

dddd · #13 08-17-2017, 05:06 PM

That Trek looks perfect.

And with SL pedals, did you perhaps build that for me?

fa63 · #14 08-17-2017, 05:34 PM

Thanks. You can ride it anytime you are in Atlanta

rnhood · #15 08-17-2017, 06:12 PM

Quote:

Originally Posted by benb

Yah I've seen that 30-35% range in my own data. Drafting is pretty huge.

I was actually surprised OP didn't save more. 200w for that speed on flat seems pretty high for a ride that is mostly drafting. Maybe the size difference + aero equipment on the lead rider explains that.

Well, he is also doing intervals and at his weight, this will probably explain some of the slightly high'ish average power.