Drafting a rider decreases the lead rider's drag also.....?

Bhima

I know this is a fact, and I know it's something to do with the drafting rider filling the turbulence gap behind the leader, smoothing out the airflow but I just can't understand how this works...? An aero helmet smooths out the turbulence behind your head, so I think a drafter would do something similar but i'm having trouble getting my head round it...

I've always thought that drafting was like running down a corridoor full of closed doors - if you're following someone, they're opening all the doors and you can follow them through the opened doors, saving energy because you don't have to touch the doors yourself. I'm trying to work out how the statement in the thread title applies to this (or a similar) analogy...

Can someone PLEASE explain how this works in simple terms?

doyler78

Don't know if this makes sense to you:

http://www.cd-adapco.com/press_room/dyn ... rance.html

This is the relevant explanation:

This rare example of "something for nothing" 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 that he experiences.

djaeggi

Right, I'll try and explain this simply!

First, rid your head of the notion that drag is caused by some notion of "turbulence". It's not, although that's not to say that turbulent wakes aren't related or symptomatic of drag.... But it's not the key concept here, so forget about it for now :-)

No, drag is down to two things. Your body feels drag in two ways. I have a mug on my desk. If I want to exert a force on it, I can do two things: I can push it with my finger; I can stroke it with my hand. By pushing it with my finger, I'm applying a force perpendicular to the surface. By stroking it, I'm generating a force due to friction between my hand and the mug.

Aero drag essentially works the same way.

Viscous drag (the friction one) is generated by air moving over a surface and having viscosity, and the surface being rough.

Pressure drag (the pushing one) is generated by air pressure pusing the surface. If you take a complete object, the sum total of all the pressure around the body, acting on the body might produce a net force, and if this force is "backwards" we might call it drag. (If you're mathematically inclined, this is a surface integral).

Pressure drag is what we're talking about here in the context of drafting. You feel drag on a bike because the sum total of the pressure forces in front of you are greater than those behind you. In the wake behind you, there is a region of low pressure - this is called the base pressure - and this is what causes the bulk of the drag. This is also what the drafter uses - the base pressure of the lead cyclist reduces the pressure differential front-to-back of the drafter, thus reducing his/her drag.

So, how do we reduce the drag of a cyclist? There are a few ways. Reducing the size of the wake is a good one, because we reduce the size of the low pressure region behind the cyclist (think aero positioning, pointy helmets, etc.). But we can also try to increase the base pressure - although the wake region will be the same size, the sum of the pressure times the area will be more. How do we do this?

The simple answer is that the drafting cyclist influences the pressure field in front of him. (Imagine standing at the side of a road as a truck passes - you feel the "push" of the air from the truck before the front of the truck passes you.)

The more involved answer is that pressure is related to the curvature of the streamlines. A blob of fluid behaves pretty much like a solid, in that it behaves according to Newton's laws. If you want to move something sideways, you have to push it sideways. To make a weight on the end of a string go round in a circle, the string has to pull it towards the centre. Same with our blob of fluid. Except, where does the force come from? Pressure! To make a blob of fluid move in any direction, there has to be a net pressure force acting on it, which means there has to be a difference in pressure across it.

Apply this principle to a blob of fluid moving along a curved streamline, and you'll see that there has to be a high-to-low pressure difference across is, with the low pressure being towards the centre of the radius of curvature. (think of the low pressure being the hand that's holding the string and pulling the fluid towards the centre).

So, base pressure is related to the curvature of the wake "bubble" behind the cyclist. (The wake is bubble-shaped, and not cyclist-shaped off to infinity, because the turbulent region between the wake and the free-strem flow mixes things up, a process called entrainement. And this is where turbulence does affect drag - more turbulence means more entrainement means higher curvature means lower base pressure.).

Right, do you see where this is going? Putting a second cycling in the wake of the first, means the curvature of the wake bubble is reduced - the air passing the first starts to curve in, then has to move out to pass the second. This effectively straightens out the boundary of the bubble, reducing the radius of curvature of the streamlines, reducing the high-to-low pressure difference. The high pressure is fixed - this is simply the pressure of the free-stream flow far away from the cyclist - so the low-pressure (the base pressure) increases. Thus, the net force felt by the first cyclist due to the pressure difference front-to-back, reduces.

Bingo!

DaveyL

Mr "E=mc2" Bhima will love all that!!!!

andrewgturnbull

djaeggi wrote:

[snip]

Right, do you see where this is going? Putting a second cycling in the wake of the first, means the curvature of the wake bubble is reduced - the air passing the first starts to curve in, then has to move out to pass the second. This effectively straightens out the boundary of the bubble, reducing the radius of curvature of the streamlines, reducing the high-to-low pressure difference. The high pressure is fixed - this is simply the pressure of the free-stream flow far away from the cyclist - so the low-pressure (the base pressure) increases. Thus, the net force felt by the first cyclist due to the pressure difference front-to-back, reduces.

Bingo!

Hi there.

Could you quantify this effect, say in terms of watts saved by the front rider at 25mph?

Cheers, Andy

Bhima

Cheers for all the info - sorry for the late reply, not ignoring you, just still trying to take it all in!

Alex_Simmons/RST

andrewgturnbull wrote:

Hi there.

Could you quantify this effect, say in terms of watts saved by the front rider at 25mph?

Cheers, Andy

See this item:
http://forum.simwe.com/archiver/tid-818221.html

Quote:
Perhaps the most surprising conclusion from the CFD simulation is that, despite feeling 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 [drag coeffient is measure of the force each rider experiences corrected for differences in size]. This rare example of "something for nothing" 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 that he experiences.

If that change in apparent CdA is accurate then an 80kg bike + rider would drop power required to ride a dead flat road in windless conditions at 25mph from ~ 282W to 275W (air density 1.191 kg/m^3, Crr 0.005). Or be able to ride at 25.2 mph for same power.

But it requires the drafter to be exceptionally close, as one would ride in a velodrome. Certainly it is substantial enough level to be readily measureable / detectable with power meters.

I know Andy Coggan has some data from his wife's velodrome efforts (with and without Colby Pearce drafting her) that show a similar effect.

andrewgturnbull

Alex_Simmons/RST wrote:

andrewgturnbull wrote:

Hi there.

Could you quantify this effect, say in terms of watts saved by the front rider at 25mph?

Cheers, Andy

See this item:
http://forum.simwe.com/archiver/tid-818221.html

Quote:
Perhaps the most surprising conclusion from the CFD simulation is that, despite feeling 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 [drag coeffient is measure of the force each rider experiences corrected for differences in size]. This rare example of "something for nothing" 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 that he experiences.

If that change in apparent CdA is accurate then an 80kg bike + rider would drop power required to ride a dead flat road in windless conditions at 25mph from ~ 282W to 275W (air density 1.191 kg/m^3, Crr 0.005). Or be able to ride at 25.2 mph for same power.

But it requires the drafter to be exceptionally close, as one would ride in a velodrome. Certainly it is substantial enough level to be readily measureable / detectable with power meters.

I know Andy Coggan has some data from his wife's velodrome efforts (with and without Colby Pearce drafting her) that show a similar effect.

Thanks for the reference Alex.

7 watts? Surely that figure is only as accurate as the model they put into the flow analysis in the first place. I also note that the info comes from a company press release, rather than a published paper.

Do those 7 watts get past your bullsh*t filter? I find it hard to believe that an effect of that magnitude could be replicated in the real world....

Cheers, Andy

Alex_Simmons/RST

I suspect that in an indoor velodrome (stable wind conditions) it is plausible. Hard to comment on the magnitude. I don't have the data to really say and I doubt that my own data from team pursuiting would have enough signal to noise ratio to ascertain such an effect.

But one can certainly test for it if one was really interested to find out.

Regression analysis and virtual elevation methods of testing aero drag using power meters are definitely sensitive enough to detect such things.

andrewgturnbull

Alex_Simmons/RST wrote:

I suspect that in an indoor velodrome (stable wind conditions) it is plausible. Hard to comment on the magnitude. I don't have the data to really say and I doubt that my own data from team pursuiting would have enough signal to noise ratio to ascertain such an effect.

But one can certainly test for it if one was really interested to find out.

Regression analysis and virtual elevation methods of testing aero drag using power meters are definitely sensitive enough to detect such things.

Hi Alex.

I certainly pay more heed to a study performed using real world, measurable parameters. The model in cfd analysis can be tuned to produce whatever result the operator thinks he's looking for.

Also the quoted analysis was for 9 riders in a line, not just two.

My scepticism was based on two experiences: Firstly anyone who's ever done a ramp test knows that byou would certainly be able to feel an abrubt jump of 7w. Secondly, how many times have you been riding hard on the front (track or otherwise), flicked your elbow to pull off and found the the following rider had dropped off - and you never noticed!

Cheers, Andy

Alex_Simmons/RST

When you're pulling 450+W on the front of a team pursuit, you sure don't notice 7W.

I have no problem with the skepticism though since I have no data to suggest one way or the other.

nolf

Very interesting. Great to see a forum where you have people who know things make educated statements with the appropriate caveats. Good job guys.

Interesting point, is there a way that you could manipulate this to help the lead rider more. For example, if the second and third riders in a team pursuit were less aero (e.g sat up) would that benefit the lead rider more as they're creating a larger low pressure area?

I realise that the benefits would be pretty small, but in something like the team pursuit where every watt counts, it might be useful.

Alex_Simmons/RST

nolf wrote:

I realise that the benefits would be pretty small, but in something like the team pursuit where every watt counts, it might be useful.

I can assure you that's the last thing you want to do when riding behind lead rider in a TP. You deperately want to save as much energy as possible so you can do a good turn when on the front and get back on the train when done. The energy cost of sitting up would be far far higher than any minor benefit granted the lead rider.

eh

As I understand it the only time this scenario will get close to working is if both riders were running full discs and riding very close, so the flow off the first rider is smoothed out (I guess the 2nd rider is kinda acting like a diffuser on a car). But with standard open wheels required for road racing etc. then the gap isn't filled, the air just flows off the 1st rider to be chopped up by the spokes of the second.

So in answer to Andy's question in the 'real world' it won't have any effect (unless you happen to be team pursuiter on a closed track).

djaeggi

Very briefly...

First off, that CD Adapco study is, erm, not very good. Think of it as a marketing tool! Drag prediction using CFD is very hard to get right and they've almost certainly not done enough to have any chance of getting an accurate simulation. What you can possibly say is that they've got the sign the right way round (i.e. drag of solo rider > drag of drafted rider), but that's about it. Don't believe the numbers at all! All the assumptions they've made about the flow, and their modeling simplifications will all serve to reduce the magnitude of the effect. In other words, you won't see anywhere near as much change in the real-world, particular if you're on the open road.

If you're interested in doing some quantitative testing, one way of doing it would be to rig a bike up with pressure taps - this gives you hard and real-world aerodynamic data. It's hard to pressure tap a rider (!) but a single tap at the back of a saddle (it's the wake region right behind the rider's bum that's going to be the region of interest - while the wheels are a significant drag contributor, they're not going to influence a change in drag by the presence of a drafter), or possibly a slightly modified saddle with a small vertical rear extension, might produce some interesting data. Assuming the magnitude of the effect was sufficiently large and not swamped by sensor noise (you'd have to do this in a velodrome, but even then noise would be something challenging to deal with), a drafted vs non-drafted difference in pressure would tell you something about whether the effect is large enough in real-life to make a difference, and what kind of speeds and separations are required.

MrChuck

Maybe a little OT, but I'm sure I remember a motorbike racer saying he could tell when somebody was tucked in right behind him- he could feel his leathers being lifted away from his back a little due to the change in airflow.

Obviously push bikes are something else again but interesting anyway.

Alex_Simmons/RST

djaeggi wrote:

Very briefly...

First off, that CD Adapco study is, erm, not very good. Think of it as a marketing tool! Drag prediction using CFD is very hard to get right and they've almost certainly not done enough to have any chance of getting an accurate simulation. What you can possibly say is that they've got the sign the right way round (i.e. drag of solo rider > drag of drafted rider), but that's about it. Don't believe the numbers at all! All the assumptions they've made about the flow, and their modeling simplifications will all serve to reduce the magnitude of the effect. In other words, you won't see anywhere near as much change in the real-world, particular if you're on the open road.

If you're interested in doing some quantitative testing, one way of doing it would be to rig a bike up with pressure taps - this gives you hard and real-world aerodynamic data. It's hard to pressure tap a rider (!) but a single tap at the back of a saddle (it's the wake region right behind the rider's bum that's going to be the region of interest - while the wheels are a significant drag contributor, they're not going to influence a change in drag by the presence of a drafter), or possibly a slightly modified saddle with a small vertical rear extension, might produce some interesting data. Assuming the magnitude of the effect was sufficiently large and not swamped by sensor noise (you'd have to do this in a velodrome, but even then noise would be something challenging to deal with), a drafted vs non-drafted difference in pressure would tell you something about whether the effect is large enough in real-life to make a difference, and what kind of speeds and separations are required.

Interesting. However you really only need a power meter to detect such things. If you couldn't detect the changes with the use of power meter, then it isn't significant enough to consider.

As a first test, in an indoor velodrome, just take two riders.
Rider A circulates.
Rider B waits on the outer rail.
Rider A does 3-4 minutes alone.
Then Rider B comes down off fence and tucks in behind for 3-4 minutes.
The Rider B goes back up to fence for 3-4 minutes.
Repeat several times.

A Virtual Elevation analysis on the power meter data will readily show any changes to the apparent CdA of the lead rider. It is very sensitive to minor variations.

djaeggi

Alex_Simmons/RST wrote:

A Virtual Elevation analysis on the power meter data will readily show any changes to the apparent CdA of the lead rider. It is very sensitive to minor variations.

I take your word for it, I've no experience with power meters. It would be very interesting to see the data if you do do it!

bendertherobot

Take this scenario:

We have a group of 7 riders.

Rider 7 does no work over an 85 mile section of sportive. Actively sits at the back.

Riders 1-6 do work on the front at regular intervals.

Rider 8, who does not ride, claims that rider 7 provides riders 1-6 with a positive effect during said 85 miles.

What level of wattage does he provide?

And, does he only provide this to the rider in position 6?

Pokerface

Nothing like resurrecting a 2 year old thread!

rdt

bendertherobot wrote:

Take this scenario:

We have a group of 7 riders.

Rider 7 does no work over an 85 mile section of sportive. Actively sits at the back.

Riders 1-6 do work on the front at regular intervals.

Rider 8, who does not ride, claims that rider 7 provides riders 1-6 with a positive effect during said 85 miles.

What level of wattage does he provide?

And, does he only provide this to the rider in position 6?

I'm afraid your scenario is fundamentally flawed from the get-go.

If you're a quick-ish rider in a sportive who's prepared to work, you'll find yourself at the front of a group of 50 riders, not 7, and none of riders 2-50 will do any work on the front whatsoever, such that you'll be forced to attack them in order drop the lazy [email protected], else face dragging them around the entire course without them even speaking to you.

At least post a realistic scenario

rolf_f

Pokerface wrote:

Nothing like resurrecting a 2 year old thread!

Are any of the original posters still alive?

bendertherobot

rdt wrote:

bendertherobot wrote:

Take this scenario:

We have a group of 7 riders.

Rider 7 does no work over an 85 mile section of sportive. Actively sits at the back.

Riders 1-6 do work on the front at regular intervals.

Rider 8, who does not ride, claims that rider 7 provides riders 1-6 with a positive effect during said 85 miles.

What level of wattage does he provide?

And, does he only provide this to the rider in position 6?

I'm afraid your scenario is fundamentally flawed from the get-go.

If you're a quick-ish rider in a sportive who's prepared to work, you'll find yourself at the front of a group of 50 riders, not 7, and none of riders 2-50 will do any work on the front whatsoever, such that you'll be forced to attack them in order drop the lazy [email protected], else face dragging them around the entire course without them even speaking to you.

At least post a realistic scenario

It happened on Sunday, so I'd say it's quite realistic.

I was, of course, as usual, always Rider 1

freehub

Rolf F wrote:

Pokerface wrote:

Nothing like resurrecting a 2 year old thread!

Are any of the original posters still alive?

Yes, and no.

Herbsman

bendertherobot wrote:

Take this scenario:

We have a group of 7 riders.

Rider 7 does no work over an 85 mile section of sportive. Actively sits at the back.

Riders 1-6 do work on the front at regular intervals.

Rider 8, who does not ride, claims that rider 7 provides riders 1-6 with a positive effect during said 85 miles.

What level of wattage does he provide?

And, does he only provide this to the rider in position 6?

If rider 8 doesn't ride in this pointless scenario you've dreamed up then he's an observer, not a rider. And 'what level of wattage does he provide'? How is it even remotely possible to answer that question?

bendertherobot

Herbsman wrote:

bendertherobot wrote:

Take this scenario:

We have a group of 7 riders.

Rider 7 does no work over an 85 mile section of sportive. Actively sits at the back.

Riders 1-6 do work on the front at regular intervals.

Rider 8, who does not ride, claims that rider 7 provides riders 1-6 with a positive effect during said 85 miles.

What level of wattage does he provide?

And, does he only provide this to the rider in position 6?

If rider 8 doesn't ride in this pointless scenario you've dreamed up then he's an observer, not a rider. And 'what level of wattage does he provide'? How is it even remotely possible to answer that question?

I'm not sure why it's pointless as it happened last Sunday. Rider 8 did **** all. Riders 1-7 were mates and this **** just tagged on.

Now, another "mate" claims we shouldn't be upset by this because he still benefitted us.

Understably we think "mate" is a **** as well.

P_Tucker

No-one should be upset by anything FFS - its not a race, and even if it was, no-one has to work if they don't want to.

bendertherobot

P_Tucker wrote:

No-one should be upset by anything FFS - its not a race, and even if it was, no-one has to work if they don't want to.

I'd normally agree, but this guy was plain odd. He tagged along with us from the start (the 7 riders are all mates), couple of us said hello, he ignored us. Sat on the back wheel, rider 7 dropped back a few times behind him, slams on the brakes, re-occupies 8th position. Waits at feedstations for us to depart, asked to come to the front, just blanks us. Just the oddest thing I've seen really.

But, that's not really the question, the question is, how much benefit can a rider provide versus how much more work would we have done had he not been there. My view is, on a sportive, that the answer is so close to zero as being statistically dismissable.

P_Tucker

bendertherobot wrote:

P_Tucker wrote:

No-one should be upset by anything FFS - its not a race, and even if it was, no-one has to work if they don't want to.

I'd normally agree, but this guy was plain odd. He tagged along with us from the start (the 7 riders are all mates), couple of us said hello, he ignored us. Sat on the back wheel, rider 7 dropped back a few times behind him, slams on the brakes, re-occupies 8th position. Waits at feedstations for us to depart, asked to come to the front, just blanks us. Just the oddest thing I've seen really.

But, that's not really the question, the question is, how much benefit can a rider provide versus how much more work would we have done had he not been there. My view is, on a sportive, that the answer is so close to zero as being statistically dismissable.

Dunno. Most studies around cycling aerodynamics probably assume a speed of higher than 18mph.

torms

remember reading an article about lance armstrong, saying whenever his opponents attacked he"d send one of his team mates to sit on there wheel to slow them down, as it had an impact on there speed

Pokerface

torms wrote:

remember reading an article about lance armstrong, saying whenever his opponents attacked he"d send one of his team mates to sit on there wheel to slow them down, as it had an impact on there speed

I think that's more of a mental thing - when you're trying to make a break, there's nothing worse than some fooker sitting on your wheel doing nothing. It messes withy our head.