PiLigand

So I've heard the usual "You can save up to 30% effort by drafting," and the basics of how paceline rotations and slipstreams work. And I know they vary depending on wind conditions speeds and hills. That's all well and good, but I'm still curious. I did a search for older posts, but nothing apparent popped up. Let me know if I missed something.

As you go further back in the paceline, it would SEEM to me that one's effort drops [marginally] further. Like the second person save 30% energy, the third save 34%, the fourth saves 35.5% etc. and so on. To your knowledge, has anyone looked into this as a possibility and even published results? Or is this some common knowledge that I've managed to avoid for this long?
It's something that's been sitting in the back of my mind for a while. Any info would be great, thanks!

Beaker

That migt be true if everyone in the line rode at the exact same speed and kept things super smooth. Things like wind, changes in pace, riders leaving gaps probably more than compensate for any additional benefit.

canam73

You are essentially thinking correctly, but your savings estimates might be a bit high. And once you 3-4th wheel I doubt there is much difference. What does make a difference is having riders on either side. When you are on the inside of a pack (think TdF peloton) you need to work less than if you are a rider in a single paceline.

Some studies also indicate having somebody draft you gives the leader a very small boost.

merlinextraligh

This. In general, there's more draft being the 4th or 5th rider back, than the second. Also there's more draft a bit back in the pack, if it's a pack of riders not a single line.

But the tradeoff is the further back you get, the more "yo-yo'ing" there is, which results in surges and lulls, and the need for little accelerations that take energy.

So there's a sweetspot, several riders back from the front, but not far enough back that the yo yoing becomes a problem that is the most efficient place to be. Exactly where that sweetspot is will vary depending on a number of factors, including how smooth the group is.

PiLigand

Beaker: Yeah, I imagine you're right, but I still wonder what those numbers would be if everything were ideal.

canam73: They very well may be high. I totally just made them up. I hadn't thought about the either side thing, but that makes a lot of sense given the shear forces.

Thanks for your input.

asgelle

https://www.ncbi.nlm.nih.gov/pubmed/10589873

asgelle

Is this a new publication? I don't know of any.

gsa103

Having a rider behind you eliminates the low pressure region behind you (effectively pulling you back). Its a well studied effect for avian flight and in NASCAR. If I recall the difference is only a few percent reduction in drag.

asgelle

There's a difference between conceiving of a mechanism and data*.

*If you run some numbers, you'd see A) the speed and size of a Nascar racer is so far removed from a bicycle to make it irrelevant. B) Pressure drag is such a small component of total drag that changing the pressure behind the rider could not create an observable change in drag.

canam73

No, I haven't seen anything cycling specific, only what gsa103 talked of. And I completely agree that if there is an effect it is most likely insignificant and/or immeasurable.

But hey, this is the 41 so it matters.

PiLigand

I'd bet that the rider behind actually does make a difference. Those low pressure zones can get surprisingly powerful. Now that's not saying it makes very much of one. But also, I cede that I haven't examined the data for myself, so I can't say for certain. But as for my experience in fluid dynamics, I could definitely see it making a difference.

Edit: "See it making a difference" = trust that there would be a difference, not "this happened to me and I felt it."

asgelle

It's never a good idea to put intuition up against data. I don't know what your experience is with fluid dynamics and it really doesn't matter. No amount of experience can overcome even a single data point. Be that as it may, if you looked at the data before reaching your conclusion, you would see that Broker et al. are able to fit their data without reducing drag on the leader and despite repeated attempts to measure the effect of trailing riders (and as power meters have become more popular, the number has grown rapidly), I know of only one instance where a difference was seen and the person who did this admits the results are far from conclusive. https://www.trainingandracingwithapow...-rider_23.html

canam73

Following some links from Asgelle's post I came across this to answer one of OP's first questions:

(in regards to a 9 man TTT)"The simulation results were illuminating. Compared with the lead cyclist, the drag of the rider in the second position is reduced by 21%. The third rider feels a further small decrease in drag over the second, but from the third rider back all other cyclists experience almost identical drag."

From this:

https://www.deskeng.com/articles/aaabey.htm

But also in there is this:

"Perhaps the most surprising conclusion is that, despite the full force of the oncoming air, the lead rider experiences lower drag than if he were riding an ITT at the same speed. The drag coefficient of the leading TTT rider is 0.277, while that of an individual rider is 0.285. This occurs because the second place rider reduces the influence of the lead rider’s wake, increasing his base pressure and consequently reducing the drag force."

gsa103

Well, a 30 sec google search actually turned up some CFD simulations of 2-rider configurations.
https://sts.bwk.tue.nl/urbanphysics/p...H_Preprint.pdf

"Compared to an isolated cyclist and for d = 0.01 m, the drag reduction of the leading cyclist is 0.8%, 1.7% and 2.6% for UP (hoods), DP (drops) and TTP (time-trial), respectively. Apart from the well-known drag reduction for the trailing cyclist, this study also confirms and quantifies the drag reduction for the leading cyclist."

There you go, cycling specific results. The answer is a few percent at most.

Homebrew01

Car racing, yes. Bikes, no

PiLigand

Boom. Done and done. This post is frighteningly efficient. It knowledge this concise even allowed on these forums?
Haha, thanks all who contributed. i appreciate your input.

canam73

Incorrect.

Since apparently you missed it 2 posts above:

https://www.deskeng.com/articles/aaabey.htm

But also in there is this:

"Perhaps the most surprising conclusion is that, despite the full force of the oncoming air, the lead rider experiences lower drag than if he were riding an ITT at the same speed. The drag coefficient of the leading TTT rider is 0.277, while that of an individual rider is 0.285. This occurs because the second place rider reduces the influence of the lead rider’s wake, increasing his base pressure and consequently reducing the drag force."

And note I said: Some studies also indicate having somebody draft you gives the leader a very small boost.

asgelle

I wouldn't put a lot of faith in a purely numerical prediction showing 3% reduction for the lead rider, when they predict a 20% reduction for number 2 compared to data showing 30%. (I also don't consider an unreviewed publicity piece a study, but that's just me.)

canam73

I'm not putting faith in any of, and never said I was.

What I did say was: "Some studies also indicate having somebody draft you gives the leader a very small boost." What exactly about that are you taking issue with?

PiLigand

Really? I'd say they probably have a good case, but being skeptical is never a bad thing. Except in vaccines. That's a different argument.

I was under the impression that the 30% thing was a number we just used to get into the ballpark. Do you have on better authority than this report that the reduction for rider number 2 should be higher?

asgelle

No issue, just expanding on the quality of the study for the general readership. I don't have time to read through the pre-print paper now.

asgelle

See the Broker paper. Their data is the source of the 30%. I know if it were me, I wouldn't publish any numerical results unless I had validation against their data.

canam73

Not sure if this has anything to do with the discrepancy, but there are a few ways to express the drafting effect. Common are either common are reduction of drag or reduction of watts required and these can be different.

big chainring 

In my best Jackie Stewart impression;
"They call it drafting here at Taledega. The drivers are literally sucked around the track"

RChung

Um, yeah, lots of people have looked into this as a possibility. I looked at some data last year in the run-up to the Olympics measuring drag for a pursuit team. I don't think I'm supposed to divulge which team but 1) drag continued to drop, though diminishingly, the farther back you go; 2) the amount of savings depends on who's in front of you in the rotation; 3) Olympic team pursuit has either 3 (women) or 4 (men) riders, so I don't know what happens if teams are larger than that; and 4) we didn't look at individual drag (we were looking at team pursuit, after all) so I don't know what happens to the lead rider in a team vs. individual situation.