# Does drafting *literally* suck?



## Teuthis (Jul 29, 2015)

So I rode my first ever group ride today, a local sportive, and I unintentionally found my way into a lump of riders (I had planned on riding solo, but in the course of moving through the field, I kinda got stuck in a lump and decided to stay there). 

Anyway, I knew from lotsa YouTube that drafting saves a bunch of energy, but I noticed something unexpected (to me) that perhaps you guys could explain: whenever we were cruising on the flat (or mostly flat), I couldn't simply maintain an easy cadence without creeping up on the rider ahead of me. Most of the time, I was braking to maintain distance, while the rider ahead was spinning consistently. It seemed distinctly like I was being pulled into the airstream of the rider in front of me. This effect didn't happen under, say, 15mph-ish on the flats or at all on climbs, and because of my noobishness, I stayed far away from other riders on big descents, so I dunno if this phenomenon would apply then too. 

So to summarize: going slow, I could select a gear and spin smoothly while maintaining a consistent distance from the rider ahead, and going fast, the rider ahead would constantly be pedaling while I was coasting and braking to maintain distance. 

So what's up? Does drafting literally suck (a rider forward)?


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## JCavilia (Sep 12, 2005)

No. Eventually you learn to soft-pedal, and occasionally coast, so you don't overspeed and have to brake. You were creeping up because you were pedaling harder than necessary, without realizing it. You will learn to notice more quickly when the gap starts to close, and back off the pedaling before you have to brake.


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## Teuthis (Jul 29, 2015)

JCavilia said:


> No. Eventually you learn to soft-pedal, and occasionally coast, so you don't overspeed and have to brake. You were creeping up because you were pedaling harder than necessary, without realizing it. You will learn to notice more quickly when the gap starts to close, and back off the pedaling before you have to brake.


I had considered that on the road when I first noticed this happening, and I deliberately put less and less power into the pedals. Tailwind or some other aero effect? Seriously, this wasn't simply easier pedaling. After 25 miles, I felt like I had been sitting on the couch the whole time. 

And I repeatedly noted the guy in front of me (a college team racer) kept a smooth and consistent cadence while I was coasting and braking (in case it was a downhill false flat or something, and that's why I was gaining).


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## tednugent (Apr 26, 2010)

there is a science to it, it's been too long since I took Fluid Dynamics in undergrad.

It works somewhat like a car, using a hatchback as an exaggerated example:









So, in the area that don't have those lines, there's a vacuum or area before the upper and lower flow meets back up. When you enter in, you're going from a high pressure to a low pressure, thus you get "sucked" in.

For simplicity sake you can call it Bernoulli's principle, but it's actually a lot more complicated than that, as there are a lot more variables to take into account that affects flow.


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## PJ352 (Dec 5, 2007)

Drafting (Urban definition) 

"What the uneducated and under skilled drivers call Tailgating. *Its when you follow closely behind another* car traveling at a high rate of speed* to decrease the amount of air resistance *on your awesome sports car so as to save a little money on fuel *or get up more speed to slingshot around him*". 

Group riding skills come into play here and (with practice) you'll be able to 'read' conditions and tailor your output (momentum) accordingly.


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## Teuthis (Jul 29, 2015)

tednugent said:


> When you enter in, you're going from a high pressure to a low pressure, thus you get "sucked" in.


K, item (1) There's waaaay too many golden opportunities for ribald humor there *snort, chuckle* but I shall resist. 

And (2) That's kinda what I was thinking, but I never took Fluid Dynamics. It certainly felt like I was being pulled, rather than merely saving energy. Anyone else actually experience this? Or am I having a Bernoulli dream?


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## Doug B (Sep 11, 2009)

If you were following an 18 wheel semi truck, or a full size van, you might get a bit of suction. And you would need to be stuck to the bumper. But not from following another rider.


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## tednugent (Apr 26, 2010)

Doug B said:


> If you were following an 18 wheel semi truck, or a full size van, you might get a bit of suction. And you would need to be stuck to the bumper. But not from following another rider.


It doesn't require a lot of speed to take advantage of drafting. When birds are migrating, the they naturally do the V-pattern, and they don't go as fast as semi's on the highway.


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## JCavilia (Sep 12, 2005)

> I repeatedly noted the guy in front of me (a college team racer) kept a smooth and consistent cadence while I was coasting and braking


I repeat: He was smooth, but he was soft-pedaling. You were pedaling harder when you pedaled, even if you didn't realize it. So you were surging, at least slightly. Accelerating to a speed greater than the rider in front, even if by the tiniest amount, will start to close the gap. And as you get closer the drafting effect will increase, so soon you'll have to brake. But you weren't literally getting pulled.

It really does take practice. You have to be very aware of how the gap is changing. It becomes instinctive, eventually.


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## Doug B (Sep 11, 2009)

tednugent said:


> It doesn't require a lot of speed to take advantage of drafting. When birds are migrating, the they naturally do the V-pattern, and they don't go as fast as semi's on the highway.



I agree. You get a lot of advantage when drafting. I draft any chance I can. But, you do not get sucked forward into the guy in front of you.


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## ibericb (Oct 28, 2014)

Not pertinent to a discussion about technique, but more than you ever wanted to know about the benefits of drafting, and cooperative effects, from none other than Coach Alex Simons, complete with pertinent references and real, measured data.


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## Pirx (Aug 9, 2009)

Keeping it really simple, it is clear that under certain conditions (headwind+slight downhill grade) you can have a situation where the rider in front needs to pedal to maintain the chosen speed, but the rider behind him, who is subject to reduced aerodynamic drag in the wake ("slipstream") of the front rider, may need no power at all, or actually has to feather his brakes so as to not run into the rider in front. None of this has anything to do with "sucking". If by "sucking" you are referring to the idea of the drafting rider in fact experiencing _negative_ aerodynamic drag, no, that does not happen.



tednugent said:


> It doesn't require a lot of speed to take advantage of drafting. When birds are migrating, the they naturally do the V-pattern,


Different story altogether; with those birds, it's all about the induced drag associated with the lift generated by the wing. Completely different physics compared to the drafting situation, since most cyclists don't have wings.


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## Teuthis (Jul 29, 2015)

JCavilia said:


> I repeat: He was smooth, but he was soft-pedaling. You were pedaling harder when you pedaled, even if you didn't realize it.


I was thinking about this as I drifted to snooze last night. 

I had assumed some kind of vacuuming was going on because of the cadence of the rider ahead of me. More specifically, I had assumed he was putting some power down. And if he were pedaling very softly, then indeed it would've been an easy mistake for me to pedal not softly enough and consequently close the gap. So question answered. 

But now I have another one: at what low power output does a cyclist riding in a group simply stop pedaling? Or is it the point to maintain cadence despite really low pedal effort in order to maintain a consistent speed? Is learning to pedal at really low effort levels one of those things I'll get with practice?


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## Pirx (Aug 9, 2009)

Teuthis said:


> at what low power output does a cyclist riding in a group simply stop pedaling? Or is it the point to maintain cadence despite really low pedal effort in order to maintain a consistent speed? Is learning to pedal at really low effort levels one of those things I'll get with practice?


No, you're completely confused. This is really simple: Assuming you want to maintain a given speed, if the required power at the wheel is zero (well, pretty much, anyway, ignoring bearing losses in the rear hub and freehub), there's no need to pedal. If it's negative (because you're going downhill) you have to brake, if it's positive, you pedal. The cadence you choose has nothing to do with any of this. That choice only depends on you personal preference, possibly determined by considerations of biomechanical efficiency.


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## ericm979 (Jun 26, 2005)

With practice you'll be able to just ease off the power a bit to keep the bike in the right place. You need to do it early, before you're closing the gap too quickly. It just takes practice. If you can find people who ride steady you can practice not using the brakes. (BTW using brakes more than a tiny amount screws up the riders behind you. I will yell at riders who brake in the paceline, or just get ahead of them.) 

If there is a slight downhill- the kind that you still pedal on- then the draft seems stronger, possibly because speeds are higher.

I sometimes stop pedalling, and sometimes keep pedalling at a low power output. It depends on how much I want to slow down. Also if I want to keep my legs turning to keep them warm and ready to go or want to rest as much as possible. You can pedal at zero power if you want- you'll hear the freewheel clicking slowly.

When you're in a group or paceline keep your attention up to the front. Don't fixate on the rider in front of you. That's a good way to get hurt- when the pack/line swerves around something, you won't see it until it's too late to do anything about it. And don't look off to the side. I had my front teeth smashed in because I was looking to the side when a rider on the front slammed on the brakes.


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## rm -rf (Feb 27, 2006)

Teuthis said:


> I was thinking about this as I drifted to snooze last night.
> 
> I had assumed some kind of vacuuming was going on because of the cadence of the rider ahead of me. More specifically, I had assumed he was putting some power down. And if he were pedaling very softly, then indeed it would've been an easy mistake for me to pedal not softly enough and consequently close the gap. So question answered.
> 
> But now I have another one: at what low power output does a cyclist riding in a group simply stop pedaling? Or is it the point to *maintain cadence despite really low pedal effort *in order to maintain a consistent speed? Is learning to pedal at really low effort levels one of those things I'll get with practice?


*
Soft pedaling to ease the group dynamics*

In a group ride, if the rider in front stops pedaling and I hear the bike coasting, I have to focus more attention on what's happening there. Is there something in the road ahead? Will the group start braking hard? What's going on?

Some group riders never notice they are doing this. (And some of them are the ones that close up a gap too fast, stop pedaling or lightly brake, let the gap re-open, and repeat it all again. That's annoying.)

If we are starting down a hill, then it's normal to stop pedaling. I can ignore it. Rolling up to a stop sign? It's normal, and a heads up for the group slowing down.

Otherwise, the rider may have stopped pedaling due to a slight downhill, a really great draft, or going a little too fast for the riders ahead. Now it's something the rest of the riders behind need to pay attention to.

I don't like to do fake soft pedaling, where I'm* turning the pedals with no resistance at all. * It's actually kind of hard for me to keep the right cadence to not make coasting noises, but not speed up. (It's often a fast cadence, kind of tiring with no resistance.)

So, instead of fake soft pedaling, I often shift up to one of my hardest combinations, 50-12 or 50-11, and pedal very lightly at that very slow rpm. That's easier for me.


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## Roland44 (Mar 21, 2013)

ibericb said:


> Not pertinent to a discussion about technique, but more than you ever wanted to know about the benefits of drafting, and cooperative effects, from none other than Coach Alex Simons, complete with pertinent references and real, measured data.


Great article, clears a lot of things up. Thanks for sharing.


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## MerlinAma (Oct 11, 2005)

The other factor on downhills is weight. 
If you weigh much more than the guy in front of you, pure coasting can have you gaining on him - quickly if you are drafting too.


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## nsfbr (May 23, 2014)

So, I haven't read ALL of the previous posts, but I figured someone needs to explain why you actually have to learn to draft without moving into the person in front of you.

It is actually quite simple. The closer you get to the person in front of you the less power it takes to maintain a given speed. So, any equilibrium you achieve, exactly the same speed and close but not too close, is going to be unstable. The faster you are going in a draft, the bigger the benefit and the more unstable it is. As a fraction of total power, uphill is less unstable, downhill more so. 

What I do, and I'm NO expert, is to ride slightly to one side of the person I'm drafting when I do. That way, I can move slightly more or less into the slipstream to counteract this effect. If you do this in a group ride, be conscious of the others around you. I'm more or less a solo rider or riding with one or two others, so this is not really the same as riding in a group of 10 or more.


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## Teuthis (Jul 29, 2015)

rm -rf said:


> *
> Soft pedaling to ease the group dynamics*
> 
> In a group ride, if the rider in front stops pedaling and I hear the bike coasting, I have to focus more attention on what's happening there. Is there something in the road ahead? Will the group start braking hard? What's going on?
> ...



So maybe "sucking" was a bit too dramatic a term for the effect. "A really great draft" sounds better anyway. And I did try the 50-11 thing. It seemed to work well for me too.


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## Doug B (Sep 11, 2009)

MerlinAma said:


> The other factor on downhills is weight.
> If you weigh much more than the guy in front of you, pure coasting can have you gaining on him - quickly if you are drafting too.


Sir Issac Newton would argue this. Likewise, when converting potential energy at the top of a hill to kinetic energy at the bottom, mass cancels out. 

My riding buddy weighs about 65 pounds less than me. I used to pull away from him on down hills. But, by pure dumb luck, we both unknowingly bought new wheel sets....and just happened to buy the same brand and type of wheels. Now, our down hill speed is virtually identical. 

However, when drafting, you will typically accelerate faster than the guy in front of you, due to having lower aerodynamic drag. It's a drag thing, not a weight thing.


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## tednugent (Apr 26, 2010)

Doug B said:


> Sir Issac Newton would argue this. Likewise, when converting potential energy at the top of a hill to kinetic energy at the bottom, mass cancels out.
> 
> My riding buddy weighs about 65 pounds less than me. I used to pull away from him on down hills. But, by pure dumb luck, we both unknowingly bought new wheel sets....and just happened to buy the same brand and type of wheels. Now, our down hill speed is virtually identical.
> 
> However, when drafting, you will typically accelerate faster than the guy in front of you, due to having lower aerodynamic drag. It's a drag thing, not a weight thing.


Algebra would argue.

If m1 does not equal m2
Where m1 = mass of rider 1
And m2= mass of rider 2

In order for mass to cancel out, m1 has to equal m2, or close enough for government work.

If you had 65 lbs on the other rider, there's no way m1=m2

But then you're also correct, drag comes into play between the 2 riders, which complicates the basic laws of physics (which are based in huge estimations to simplify the equations, ie 2nd law is based on the assumption of constant mass)


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## ibericb (Oct 28, 2014)

tednugent said:


> Algebra would argue.
> 
> If m1 does not equal m2
> Where m1 = mass of rider 1
> ...


Sorry, but Doug B is correct. This is an age old problem that was explained centuries ago. It got Galileo Galilei in a bunch of trouble. The acceleration due to gravity is the same for both bodies. The higher mass body has greater potential at the top of the hill due to his mass, and at the bottom will also have higher kinetic energy, again due to his mass. However, in the absence of drag the acceleration and resultant velocity will be the same for both riders. The force of acceleration (gravity) is the same, and is independent of mass. Maybe this will help.


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## tednugent (Apr 26, 2010)

ibericb said:


> Sorry, but Doug B is correct. This is an age old problem that was explained centuries ago. It got Galileo Galilei in a bunch of trouble. The acceleration due to gravity is the same for both bodies. The higher mass body has greater potential at the top of the hill due to his mass, and at the bottom will also have higher kinetic energy, again due to his mass. However, in the absence of drag the acceleration and resultant velocity will be the same for both riders. The force of acceleration (gravity) is the same, and is independent of mass. Maybe this will help.


Yes, acceleration due to gravity is a constant, like I said about assumptions: lots of assumptions are made with your NASA link. 

Newton's 3 laws are based on a lot of assumptions go to narrow it down to core principles 

But reality is, once you start adding back the things that were assumed as zero, this start to become complicated again

Imagine if the requirement for a NASA researcher is just a HS diploma or GED. No advanced degrees required.


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## ibericb (Oct 28, 2014)

tednugent said:


> Yes, acceleration due to gravity is a constant, like I said about assumptions: lots of assumptions are made with your NASA link.
> 
> Newton's 3 laws are based on a lot of assumptions go to narrow it down to core principles
> 
> ...


Now you're rambling. Did you have a point?


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## tednugent (Apr 26, 2010)

ibericb said:


> Now you're rambling. Did you have a point?


basic HS physics is not sufficient to understand the science at hand here.

Another case in point:

Remember the rock vs feather in a tube capable of holding vacuum?

With air inside the tube, rock lands first.

with air removed from the tube, mass "cancels" out and they land at the same time.

https://www.youtube.com/watch?feature=player_embedded&v=AV-qyDnZx0A


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## ibericb (Oct 28, 2014)

tednugent said:


> basic HS physics is not sufficient to understand the science at hand here.
> 
> Another case in point:
> 
> ...


You're deviating from point. The point in contention, raised by MerlinAma, was simply mass. 

When it comes to acceleration due to gravity mass makes no difference. It's not that m1 has to equal m2. It's about converting the potential energy to motion and resultant kinetic energy, and comparing that for the two different m's. Converting potential energy to kinetic energy, mass cancels:

mgh = m(v^2)/2
or
gh = (v^2)/2

Aerodynamics (drag), as was noted by all, accounts for the differences. In that vain, the smaller object/rider - that is the one with the lower CdA (assuming Cd's are the same) - would actually have the real-world advantage for acceleration. So in the context of drafting, and making gains in a downhill situation, it is the one with the smaller CdA that stands to be the one prone to greater acceleration. Often that will be the lighter rider.

BTW - the original experiment with the feather in the vacuum tube was done by Robert Boyle, and I believe he used a coin, not a rock.


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## ibericb (Oct 28, 2014)

Adding ...

Where mass does make a difference for matching speed in a drafting situation is in the amount of rider supplied power required to accelerate to maintain a pace or the amount of braking needed to reduce velocity to keep from over-running the guy in front. In those situations the rider at a greater mass must either provide more power to comparably accelerate when increasing speed, or brake more severely to comparably decelerate. Again I'm setting aside the aerodynamic differences for simplicity. Those would be additive to the mass effects.


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## Lombard (May 8, 2014)

Teuthis said:


> It certainly felt like I was being pulled, rather than merely saving energy. Anyone else actually experience this? Or am I having a Bernoulli dream?


It sure does feel that way, though I think the feeling is relative.


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## Teuthis (Jul 29, 2015)

ibericb said:


> Not pertinent to a discussion about technique, but more than you ever wanted to know about the benefits of drafting, and cooperative effects, from none other than Coach Alex Simons, complete with pertinent references and real, measured data.


Great article. It and many of the responses here have engaged my noggin, and thus I speculate: most of the "sucking" to which I originally referred was likely a combo at different times of my pedaling too hard, maybe a slight downhill and more mass on me in the draft, maybe a tailwind here or there, maybe a bit of nervous noob imagination, and perhaps some slight and unexpected legitimate vacuum--Coach Simons was a bit surprised by some of his data, no?

Yes, common sense would seem to argue that a cyclist isn't going to produce nearly the tailvacuum a semi would, but I enjoy contemplating the improbable. Next time extraterrestrials are drafting me, I'll ask if they're experiencing any suck. 😆


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## Kerry Irons (Feb 25, 2002)

Doug B said:


> If you were following an 18 wheel semi truck, or a full size van, you might get a bit of suction. And you would need to be stuck to the bumper. But not from following another rider.


This pretty much sums it up. You obviously gain advantage from following a cyclist or group of cyclists but they are not big enough to create the vacuum shown in the stream line graphic of the car. To get true suction you need a big surface in front of you. You can't predict the effect very well, but when you look at the drafting bicycle land speed records you see that the rider is spending less energy than just rolling friction (tires, chain, bearings) would require. Those riders are being sucked along.


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