sac02

I don't know why this popped into my head. Probably because I'm procrastinating studying for finals. Anyways...

We all know the advantages of drafting behind someone, but does having someone drafting behind you also increase your speed (or decrease your effort, however you want to look at it)?

In auto racing applications it definitely does: the tubulent low pressure area behind the rear bumper of the car produces a drag force - two cars drafting closely can move faster than one because the first car blocks wind for the second and the second car removes the drag force from the rear of the first; each does something to benefit the other (though obviously the benefit to the rear car is greater).

Is this effect noticed in cycling? In theory, a rider should be able to hold a slightly faster pace with someone drafting him, does this play out in reality?

Note that I'm not referring to the higher average speed that a pair of riders could attain by taking turns pulling, only the increase in speed from the second rider "helping" the first.

Mac

jmoody15

This is a pretty good question, though I admit I do not know the answer. You would have to eliminate the motivational drive of another person on your tail in order to get the answer

timmyquest

Explain the physics behind that one please.

531Aussie

dude, I not a pyhsicist; I'm just regurgitating something I've read. Funnily enough, I've also read contrary.

Percist

I'm guessing you watched the Nascar race at Talledega last weekend where it does, in fact, work to "push" other cars. Not so much with cycling (or most other Nascar tracks) where it's preferable energy-wise to be behind someone.

Disclaimer-I'm not an engineer so I can't explain the fluid dynamics of the process, I just listen to the commentators.

Fox5

I'm pretty sure it slows down the person being drafted from, I
want to say that the pocket of air that the
drafter is in is pulling on the front rider because of a vortex

Gee3

It makes the "puller" faster because you don't want to be the chump that slows down the line. So out of motivational determination you pull as hard as you can for as long as you can to avoid being called a wheel sucker! hahaha!

TheKillerPenguin

I've heard it provides a benefit of up to 5% for the person being drafted off of.

asgelle

It definetly speeds up the leading rider

Aboslutely not. In fact, all attempts to measure this effect have failed. By the way, reasoning by analogy between cars and bikes is dangerous because the nature of the flow (Reynolds number) is very different between the two.

Endox

It certainly should help the lead person very slightly, don't have any idea by how much but I can't see how it could possibly slow them.

Carus47

I have heard of the 5% benefit also

ElJamoquio

Yes, it should speed up the leading rider. There is a slight very slight high pressure in front of the rider, and a slight low pressure behind the rider. The person in rear will 'bridge' this high/low pressure zone, so that it will become high...<first rider>...medium...<second rider>...low pressure.

There will be a similar story with the apparent air velocity.

Since bikes go so slow, and are so far apart, (non-overlapped wheels), the first rider gets almost nothing, though.

ElJamoquio

It will pull on the first person whether someone is in it or not. In fact, it pulls harder if there isn't someone in it.

sac02

No, I'm not really a fan of the roundy-round stuff, though I don't hate it like some of my friends do. I follow F1 and various forms of sports car racing.

Heh, I am an engineer but I did not like my fluids classes. (getting my masters in materials) I know a fair amount of vehicle dynamics and such from personal interest, so I could actually describe the airflow around a car, but not much else.

Agreed, I was more trying to give an example that most here could relate to. I definitely wasn't saying "because it works on cars at 150mph, it should work on bikes at 20 mph". But it may, so that's why I was either asking those with more cycling experience if it is noticed in real-world situations, or perhaps someone with more fluid dynamics knowledge to explain why it would or wouldn't be noticable.

Mac

asgelle

This must be a gross exageration, but without a source it's hard to ell what it means. There have been many measurements with resolution less than 1% and none of them have been able to see this effect. See for example https://www.midweekclub.ca/articles/broker98.pdf

Sirrobinofcoxly

If there is a slight benefit for the puller, why do riders get pissed if you ride their wheel? They should be thanking you.

I'm talking about the random road rider you catch up to on a solo ride.

CdCf

Bicyclists probably can't get close enough for the front rider to experience any measurable benefit from having a "draftee" close behind.

If you want to check for yourself, I can think of one cheap and simple method to find out.
Get a ball of really light-weight knitting yarn and make 10-20 strands, each about 3-4 ft long.
Have the front rider wear a wind west or something similar that has a back with open netting. Tie the strands of yarn at reasonable intervals around the edge of the netting so that they're fixed at one end and loose and free to move at the other.

Have the front rider ride on a flat road straight into the wind and drive alongside with a car and observe how the strands move/stand in the airflow behind the rider's back. Then have a second rider move up behind the lead rider and get as close as possible. Check the strands throughout and see if/how their positions/angles (both vertical and horizontal) change. Have the trailing rider move in and out of the "bubble" to see the changes more clearly if there are any.

If a video camera is available, filming it would make comparisons easier.

And of course, the higher the speed at which this is done, the more likely it is that any change at all will be visible. 20-25 mph is probably minimum.

urbanknight

Because the increase of .001 mph isn't worth the risk of crashing because some lame @$$ didn't tell you he was there with his super quiet bike.

Most riders don't mind a wheel sucker, but they want to be aware of it. It's like being tailgated without a rearview mirror.

Oh, and +1 for the lead rider being faster because he doesn't want to look like a wussy. As for aerodynamics, I doubt it makes a measurable difference. We're not as wide as stock cars, nor are we going as fast.

austinspinner

From the Science of Cycling:

Drafting is an important technique in road racing. Exploratorium Senior Scientist Paul Doherty explained, "The bicyclist, as he moves through the air, produces a turbulent wake behind himself. It makes vortices. The vortices actually make a low pressure area behind the bicyclist and an area of wind that moves along with the bicyclist. If you're a following a bicyclist and can move into the wind behind the front bicyclist, you can gain an advantage. The low pressure moves you forward and the eddies push you forward."


Suprisingly drafting not only helps the bicyclist following the leader, but the lead cyclist gains an advantage as well. Paul explained, "The interesting thing is by filling in her eddy you improve the front person's performance as well. So two people who are drafting can put out less energy than two individuals (who are not drafting) would covering the same distance in the same time." While the lead cyclist gains some advantage in this situation she still needs to expend much more energy than the cyclist who is following.


In road racing, bicyclists group together in a pack known as the "peloton" or a pace line called an "echelon." Cyclists who are part of the group can save up to 40 percent in energy expeditures over a cyclist who is not drafting with the group. To be effective drafting, a cyclist needs to be as close as possible to the bicycle in front of him. Many professional cyclists get within inches of the the bicycle in front of them. The shorter the distance the larger the decrease in wind resistance.


https://www.exploratorium.edu/cycling/aerodynamics2.html

DannoXYZ 

The effect with cars does show that drafting helps the front car as well. If you take two cars with 150mph max-speed individually and have them draft each other, you'll find that their tandem speed is faster than what any one of them can do by themselves.

You can think of it as pressure-differences. With a single car, you may have this:

32kpa |<--| 28kpa

So you've got a pressure differential of 4kpa between the front of the car and the rear. By making the body artificially longer with a 2nd car behind it, you lessen the angle that the air has to come back around. You may end up with what looks like this:

32kpa |<--| 30kpa |<--| 28kpa

Now each car is facing only a 2kpa pressure difference between the front & rear bumpers. So there's less force pushing back on it. This is of course simplified with grossly exaggerated numbers, but that's to illustrate the effect.

As for bikes, I've certainly noticed from experience that having someone behind does allow me to go faster. Practice it yourself, do intervals a 32-35mph and see how long you can last. Having someone behind you will let you hold that same speed longer. Or for the same time-duration, do it at a faster speed.

asgelle

Since there have been numerous studies which have been unable to measure any change in drag on the lead rider, other than as a placebo effect, how do you explain this?

asgelle

Note he doesn't put any magnitude on this effect. Does he mean a 10% advantage, 1%, or more likely 0.000001%?

Psimet2001 

It will help the lead rider. Of course this has more to do with the hand that the wheel sucker is pushing the lead rider with more than it has to do with aerodynamics.

The Weak Link

It all depends on the design of the bike.

Here are the dimensions of mine: 15x61, 32xxxxxxxxxxxxxxxxxxx12, 62xz400, 18 72 39 70 42. ten seventeen. 48x83. 71xxxxxxxxxxxxxxx9m. Fourteen. Red dog, red dog, hut on two.

Hope that helps.

Keith99

The benefit is small. It also depends on someone being right on your wheel. Also it only deals with physics, any benefit is more than made up for if you have the slightest lack of confidence in the person behind you. The poenntial costs if the person behind you screws up when you brake or swerve to avoid an obsticle are large.

Again the benefit as far as physics goes is real.