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Old 10-23-02, 08:07 AM   #1
jge
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Drafting guestion

I need help from a bike techno wizard to settle a debate. I have an associate arguing that by drafting behind a rider you actually slow that first rider down. That doesn't seem possible to me What would be the physics behind that?

They are supporting there argument by this quote out of Lance Armstrong's book:

"Kevins' job was to get behind Zulle and stay right behind his wheel, making it harder for Zulle to pull up the hill. It's called "sitting on him." While Kevin "sat" on Zulle's wheel and slowed him down,....."

Please tell me this a psychialogical ploy not an actual physical phenomenon.
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Old 10-23-02, 08:17 AM   #2
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Drafing behind somebody does not slow that person down. In fact, it should speed the person in front up a very small amount (I do mean very, its probably not percievable). The reason for this is that there is less turbulence over the rear wheel with another rider right behind.

I hope this settles your bet.
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Old 10-23-02, 08:19 AM   #3
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I'm not quite sure how it works in Biking, but in Auto racing, 2 cars together are significantly faster that either one alone. This is becomes more evident as speeds increase. In addition, 3 cars are faster than 2, 4 are faster than 3, etc. I'm not real sure about the physics, but I'd think it would be the same on a bike.

The idea of "sitting" on someone does work to slow someone down, but I think it is more of an emotional effect, than a physical one. You tend to become this "ominous" presence dogging the every move of the "leader".

Just my thoughts.

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Old 10-23-02, 09:16 AM   #4
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This should help, it is from NASCAR. Same applies to cycling.
Slainte

Basic Dynamics of Drafting

"As a Winston Cup racecar rushes through the air, the design of its body - with all wheels enclosed, a cab around the driver, and a "spoiler" across the rear deck - creates an air pressure bubble in front and a minor vacuum behind it. And this poses a drag on how many extra miles per hour it can speed at full throttle. Aerodynamic detailing can reduce this drag, but not by much, because NASCAR's rules require that the custom-made bodies match approved templates, notably to replicate the looks of a Chevrolet Monte Carlo, a Pontiac Grand Prix, or a Ford Taurus (and also, next season, a Chrysler Intrepid, as Chrysler returns to stock-car racing).

Drafting, which looks like tailgating or slipstreaming, occurs when a second car tucks closely behind the first, filling part of the vacuum. The car in front loses some of the drag at its rear. The second car still has a vacuum at its rear, but now has less air resistance in front. As a result, both cars quicken a bit - the two combined speed a few miles per hour faster than either can alone. This push-pull effect is stronger the closer the second car gets to the first. Indeed, the second may even touch and push the first in a tricky maneuver called "bump drafting." But slipstreaming so closely hinders airflow into the trailing car's radiator and can cause its engine to overheat. Most drafting calls for a half to a full car length between the two cars. (In contrast, open-wheel, motorcycle, and bicycle racers do not gain extra speed by drafting, because a trailing racer does not generate the forward aero push that stock cars do.)"
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Old 10-23-02, 09:29 AM   #5
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So, theoretically then, on a bicycle, drafting will only help the rear rider. Likewise it would neither help nor hurt the front rider (measurably, anyway). Hmm. OK

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Old 10-23-02, 09:44 AM   #6
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The Lance reference of "sitting on the wheel", meant that Kevin didn't "Pull Through". IOW, Kevin slowed him down, by not helping him.

It is courtesy in cycling to take your turn at the front and "Pull Through" allowing the previous leader to rest. Kevin did not do that, he "sat on his wheel", while Lance's teamates did with Lance.

That's what happened!

L8R
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Old 10-23-02, 10:45 AM   #7
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Quote:
Originally posted by jge
arguing that by drafting behind a rider you actually slow that first rider down. That doesn't seem possible to me What would be the physics behind that?
You're right. Poor selection of words by the author.

But now you've got me curious why does a drafting cars/bikes/whatever reduce drag on the leader? My guess is that there is a low pressure area behind each rider, but it is greatest on the last in line, where the "envelope" closes. However, the drag is insignificant relative to the frontal resistance (and insignificant, period, at cycling speeds).

Anyone? Anyone?
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Old 10-23-02, 01:09 PM   #8
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On a bicycle, the faster you go the more aero drag has an effect- or if there is a headwind. When wind is not a factor (like climbing) than the responsibility of setting tempo at the front wears you down.

the reason drafting reduces drag is like the others said, it reduces turbulence behind the first rider while "punching" a hole in the air for others to ride in the draft of that first rider.

Drafting has HUGE effects at cycling speeds. Aero drag is significant which is why body position, aero helmets and bikes are so important for time trials. All designed to minimize turbulence and frontal area.
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Old 10-23-02, 02:32 PM   #9
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Quote:
Originally posted by roadbuzz

But now you've got me curious why does a drafting cars/bikes/whatever reduce drag on the leader? My guess is that there is a low pressure area behind each rider, but it is greatest on the last in line, where the "envelope" closes. However, the drag is insignificant relative to the frontal resistance (and insignificant, period, at cycling speeds).
A much better way of wording what i mean't. There will be a reduction for the person in front, but it will be so small, it will be hardly noticeable.
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