Drafting

[keno]

Yesterday, a caller raised a question about drafting on Car Talk, a public radio station show that is extremely entertaining to me and a show I expect many of us listen to from time to time. The issue arose with respect to cars, but should apply to bikes, as well.

The question was what effect does drafting have on the lead car? One of the brothers, whether it was Tom or Ray I forget, felt strongly that the lead car would expend extra energy, however miniscule, to maintain its speed as a result of being drafted. He seemed to think that it was a basic physics principle, although he couldn't state which principle.

Any knowledge out there about the fine art of drafting and its physics?

keno

[Smiley]

Migrating geese do it all the time but they draft from the lift created from their wing tips. In sailboat racing we cover other sailboats while racing by throwing them dirty air while they are down wind from the lead boat, if up wind one can sail of their lift. So the drafting thing is not new and can be used effectively . I think the lead bike will expand more energy than the drafting bike or car. Both working together would go a longer distance though.
Where is the freaking professor AhniedaRide when you need him most.

[Bruce K]

Keno;

For the purposes of the discussion lets assume a 2 car train.

It is my undertsanding that drafting benefits both participants.

The following car gets the benefit of not having to "break open" the air. The lead car gets the benefit of extending the envelope that surrounds it and not having the suction effect of the air closing behind it.

The 2nd car gets the greater benefit but there is benefit to the leader over running by himself.

It would be my guess that the benefit to the leader on a bicycle is negligible as the speeds are relatively low and therefore the envelope that surrounds a rider is fairly small. The greater benefit is not having to open the air.

BK

[Johny]

http://www.insidetri.com/train/bike/articles/1320.0.html

Dear Lennard Zinn;
Everyone knows you don't have to work as hard if you're drafting off the person in front of you. How does your work effort change if someone is drafting behind you?
Here is why I ask: I always thought that if you were riding by yourself, then you had to work harder than if somebody was drafting behind you because as you move through the air, it flows past your bike and body and creates turbulence as it swirls around in the void of space behind you. This almost creates a suction behind you pulling you back against your forward progress (if my logic is correct). However, if somebody pulls in behind you to draft, they fill this void of space behind you, the movement of air flows past you and then continues to flow past the second person, resulting in less turbulence and swirling air behind you. Thus, if this is right, like for some reason I've always believed it to be, then you can ride faster (or easier at the same speed) if someone is drafting off of you, than if you are riding alone.
Though I've always thought this was the case, when I was reading Lance Armstrong's book, "It's Not About the Bike," last year, he made some statements that led me to doubt this logic. He said something to the effect that when another cyclist might try to attack, Lance would get on their wheel so they would have to work harder to pull him along. I don't know if this was just a psychological tactic against his opponent, or if the actual physics and science of moving objects created the front rider to actually "pull" the drafter. Kind of like tying your bike to the guy behind you and towing him down the road. So I thought I would let you provide the definitive answer:
If you are on flat terrain with no wind going 25 mph and someone pulls in behind you and starts drafting, does it get easier or get harder for you to keep the speed at 25mph? Does it make any difference and the work effort stays the same? Would the answer change if you were going up an incline at 20mph? What if there was a 5mph headwind?
Thanks for your time. I hope you find this to be an interesting and challenging question and I look forward to seeing your answer. I think this is a particularly good question for triathletes. At a recent sprint distance race, I heard a friend claim, "some guy drafted off me for almost the entire 10 mile bike."
Based on your answer, this either benefited my friend and boosted his average speed, or it made him work harder and left him more tired for the run. Thanks again.
--David,
30-34 age grouper
Answer from aerodynamics professor and Project 96 and 1984 U.S. Olympic Team bike designer, Dr. Chester Kyle:
We measured this in the General Motors Wind Tunnel in 1996, and on the track using the SRM crank dynamometers. The lead rider in a 4-man pace line uses about 2 to 3 percent less energy than they would if riding solo.

The next in line needs about 71 percent of the lead rider's power, and the third and fourth riders about 65 percent. See "Racing cyclist power requirements in the 4000-m individual and team pursuits", Medicine and Science in Sports and Exercise, v31, no.11, pp 1677-1685, 1999. J.P. Broker, C.R. Kyle and E.R. Burke.
--Chet Kyle
Answer from aerodynamics guru and advisor to the U.S. Postal team, John Cobb:
About two days after this question I was going to the wind tunnel already to do some testing so I took a few minutes to set up this test. I couldn't find that the rear bike had any affect on the front bike in any wind angle. I tried a disc wheel and a standard wheel on the lead bike. I don't think bicycles go fast enough for the air compression that cars get to happen on bikes. When the bike is drafting it's drag was cut virtually in half out to 10 degrees and was about 60 percent when set up as an echelon out to 30 degrees.
-- John Cobb
Answer from chief wheel builder at Hed Design:
Theoretically you should get some benefit from the rider behind because he would reduce the turbulence of the air coming off you. But in practice you would have to go really fast, maybe 200 mph to make it measurable. I think it works for something like stock car racing, but they are bigger, faster, and probably more aero to start with.
--Andy
Hed Design

[TmcDet]

I would think that looking at Nascar running Daytona would answer this question. The cars run about 10 MPH qualifying when they can not have any drafting help than they do in the race.....the car in the front actually gets a push effect from the car behind, this can also be seen by how hard is for a car to pass another car without help from a car behind them.

Now if you are asking which of the 2 car are expending the most energy then there is no doubt the front car is....the front car would be pushed harder to maintain the speed than the car behind would be using.

[M_A_Martin]

The only time I've felt any affect of someone drafting off of my bike was while riding a tandem with an excellent tailwind on a road with a dead grade....We went from sailing right along by ourselves to working hard in mere seconds. Like we had a freak'n anchor on the bike. Couldn't convince the guy to go away either.

Future note for those who ride tours: Just because someone's riding a tandem, doesn't mean they want to lead a train of strangers all the time.

I've always heard that somewhere between 119 and 125mph is the magic number for aerodynamics "mattering" for a car.