Does drafting drag?

Tim Downie said:

Silly newby question possibly but does drafting/slipstreaming/wheelsucking
another rider actually impose any extra drag on the lead rider?

No. Theoretically it actually reduces the drag on the towing rider
slightly but the effect is negligible.

--
Dave...

in message <[email hidden]>, Tim Downie

(') said:

Silly newby question possibly but does
drafting/slipstreaming/wheelsucking another rider actually impose any
extra drag on the lead rider?

Apparently not. Apparently there's aerodynamic benefit for the lead rider
as well.

--
[email hidden] (Simon Brooke) http://www.jasmine.org.uk/~simon/

;; Want to know what SCO stands for?
;; http://ars.userfriendly.org/cartoons/?id=20030605

Tim Downie said:

Silly newby question possibly but does drafting/slipstreaming/wheelsucking
another rider actually impose any extra drag on the lead rider?

Tim

physically , no

mentally, yes

--
"You can't see paradise if you don't pedal" (Fowler, Chicken Run)

Simon Brooke said:

in message <[email hidden]>, Tim Downie

(') said:

Silly newby question possibly but does
drafting/slipstreaming/wheelsucking another rider actually impose any
extra drag on the lead rider?

Apparently not. Apparently there's aerodynamic benefit for the lead
rider as well.

I'd like to believe this but I can't convince myself.

Here's a thought experiment.

Imagine a flat board lying on the floor with piece of string tied to the
middle. If you lift it quickly by the string, the force required will
initially be high until air can rush in underneath it. I magine now having
a second board underneath the first. This time when you pull the string, the
second board will also rise briefly. Will the force required be the same?
I stongly suspect that you will be doing extra work to briefly lift the
second board.

Now imagine a board being lifted by a winch at a constant speed. Now
imagine gradually bringing a second board close to the underside to the
first board. As you bring the second board close to the first winched
board, the energy required to do this will reduce. Work is now being done
on the second board which has to have come from somewhere. I can't see how
the the tension in the string (& load on the winch) *cannot* increase.

I realise that in real life these forces when dealing with irregularly
shaped objects like bicycles may be trivial but I'm sure someone somewhere
must have done the definitive physics and experiments.

Tim

Tim Downie said:

Simon Brooke said:

in message <[email hidden]>, Tim Downie

(') said:

Silly newby question possibly but does
drafting/slipstreaming/wheelsucking another rider actually impose any
extra drag on the lead rider?

Apparently not. Apparently there's aerodynamic benefit for the lead
rider as well.

I'd like to believe this but I can't convince myself.

Here's a thought experiment.

Imagine a flat board lying on the floor with piece of string tied to the
middle. If you lift it quickly by the string, the force required will
initially be high until air can rush in underneath it. I magine now having
a second board underneath the first. This time when you pull the string, the
second board will also rise briefly. Will the force required be the same?
I stongly suspect that you will be doing extra work to briefly lift the
second board.

Now imagine a board being lifted by a winch at a constant speed. Now
imagine gradually bringing a second board close to the underside to the
first board. As you bring the second board close to the first winched
board, the energy required to do this will reduce. Work is now being done
on the second board which has to have come from somewhere. I can't see how
the the tension in the string (& load on the winch) *cannot* increase.

I realise that in real life these forces when dealing with irregularly
shaped objects like bicycles may be trivial but I'm sure someone somewhere
must have done the definitive physics and experiments.

I'm not sure the board argument works.

The separation of the riders is much larger than their frontal area,
which isn't the case with your boards (at least in the view I constructed
in my imagination :-) ). I think you're right about the boards in that
the effect will be much as you predict, and the energy has to some from
somehwere, but I can't see any effect on the first board until the
rear one is in close procimity (something like a board width behind).
At that point it is going to affect the airflow around the first board,
specifically it'll break into the trailing edge vorticies and probably
reduce the pressure behind the first board (thus in effect sucking it
backwards).

Riders are separated by many times their width, though, and any advantage
to the second rider has to be more of a far field effect. The rear rider
will be inside a general forward moving mass of air behind the front
rider - that mass already contains an amount of energy imparted to it
by the forward rider which would normally be dissipated into the
environment. The second rider probably just becomes part of that
environment.

The second rider will have some effect on the first, for the same reason
that you can feel the pressure wave of a car passing you before the
car is alongside, because the air in front is compressed as it has to
in order to flow around the moving body. However that effect is likely
to be tiny for a bike rider trailing 4 or 5 feet behind another, but if
it is measurable it'll be a positive for the leading rider.

--
Nobby Anderson

Tim Downie said:

I realise that in real life these forces when dealing with irregularly
shaped objects like bicycles may be trivial but I'm sure someone somewhere
must have done the definitive physics and experiments.

Tim

I think it's generally accepted that the front rider gets a 5% benefit from
drafting. I guess the reason is that the two rider team is more
aerodynically efficient than the single rider, resulting in less turbulence
and less energy being wasted producing turbulence.
--
Michael MacClancy

Tim Downie said:

Quoted message said:

Quoted message said:

Silly newby question possibly but does
drafting/slipstreaming/wheelsucking another rider actually impose any
extra drag on the lead rider?

Apparently not. Apparently there's aerodynamic benefit for the lead
rider as well.

I'd like to believe this but I can't convince myself.

One theory goes like this:

The laminar airflow passing round the lead rider will break down into
turbulent flow behind her as it "closes up", that will cause drag. If
instead you get a second rider close behind, the airflow remains laminar
as it flows over the lead, starts to move back in (to the low pressure
region behind her), but then is forced back out round the second rider.
Hence, no turbulence, less drag.

R.

Tim Downie said:

Simon Brooke said:

in message <[email hidden]>, Tim Downie

(') said:

Silly newby question possibly but does
drafting/slipstreaming/wheelsucking another rider actually impose any
extra drag on the lead rider?

Apparently not. Apparently there's aerodynamic benefit for the lead
rider as well.

I'd like to believe this but I can't convince myself.

Here's a thought experiment.

Imagine a flat board lying on the floor with piece of string tied to the
middle. If you lift it quickly by the string, the force required will
initially be high until air can rush in underneath it. I magine now having
a second board underneath the first. This time when you pull the string, the
second board will also rise briefly. Will the force required be the same?
I stongly suspect that you will be doing extra work to briefly lift the
second board.

Here is another thought experiment:-
Imagine a cyclist riding along a flat track
at a constant /effort/ in still air.

A powerfull motorcycle catches up with the cyclist
and follows the cyclist at a fixed but short distance.

Can you be sure that the cyclist will not speed
up?

If you like, try substituting a large bus for the motorcycle
and re-compute.

I like this one, try imagining the motorcycle
following at a considerable distance, however
the motorcyclist is holding an aerodynamic
fairing that perfectly matches the cyclists rear :-))
Conveniently he has a long pole attached to the fairing
and can suspend the fairing arbitrarily close to, but
not touching, the cyclist.

I am convinced! A following cyclist could in principle
assist a leading cyclist.

Richard said:

The laminar airflow passing round the lead rider will break down into
turbulent flow behind her as it "closes up", that will cause drag. If
instead you get a second rider close behind, the airflow remains laminar
as it flows over the lead, starts to move back in (to the low pressure
region behind her), but then is forced back out round the second rider.
Hence, no turbulence, less drag.

Or, a simple practical example of how just messing about with length and
tail profile can help is a lowracer recumbent with a tail fairing, which
goes faster than an otherwise identical (but lighter, of course)
lowracer without a tail fairing. Nothing has changed until /after/ the
air is past the rider.

M5 quote 10% faster for their tailfairings on their lowracer model, just
from adding something behind the rider.

Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net [email hidden] http://www.dundee.ac.uk/~pjclinch/

in message <[email hidden]>, Michael

MacClancy (') said:

Tim Downie said:

I realise that in real life these forces when dealing with irregularly
shaped objects like bicycles may be trivial but I'm sure someone
somewhere must have done the definitive physics and experiments.

I think it's generally accepted that the front rider gets a 5% benefit
from
drafting. I guess the reason is that the two rider team is more
aerodynically efficient than the single rider, resulting in less
turbulence and less energy being wasted producing turbulence.

I think the truth is that there's /more/ turbulence but substantially
less than twice as much turbulence, and you have two riders pulling that
turbulence along. This is similar to a tandem's aerodynamic advantage,
after all. The wheel-sucker has to put in at least enough effort to stay
within the turbulence envelope, so he has to drag a proportion of the
turbulence, so the amount the lead rider has to drag is slightly
reduced.

--
[email hidden] (Simon Brooke) http://www.jasmine.org.uk/~simon/

;; 99% of browsers can't run ActiveX controls. Unfortunately
;; 99% of users are using the 1% of browsers that can...
[seen on /. 08:04:02]

Quoted message said:

I am convinced! A following cyclist could in principle
assist a leading cyclist.

The classic example is racing cars on oval tracks such as Indianapolis
Speedway where a pair of cars running nose to tail can go faster than
either of them alone. In cycling, however, I don't believe the effect
is significant otherwise racing cyclists would be generally aware of
it.

--
Dave...

in message <[email hidden]>,

dkahn400 (') said:

Quoted message said:

I am convinced! A following cyclist could in principle
assist a leading cyclist.

The classic example is racing cars on oval tracks such as Indianapolis
Speedway where a pair of cars running nose to tail can go faster than
either of them alone. In cycling, however, I don't believe the effect
is significant otherwise racing cyclists would be generally aware of
it.

Racing cyclists /are/ generally aware of it. What do you think
chain-ganging is all about? Have you never watched a team time trial?

--
[email hidden] (Simon Brooke) http://www.jasmine.org.uk/~simon/

Anagram: I'm soon broke.

"Simon Brooke" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

in message <[email hidden]>,

dkahn400 (') said:

Quoted message said:

I am convinced! A following cyclist could in principle
assist a leading cyclist.

The classic example is racing cars on oval tracks such as Indianapolis
Speedway where a pair of cars running nose to tail can go faster than
either of them alone. In cycling, however, I don't believe the effect
is significant otherwise racing cyclists would be generally aware of
it.

Racing cyclists /are/ generally aware of it. What do you think
chain-ganging is all about? Have you never watched a team time trial?

All that demonstrates is that the person behind gets an easier time. This is
different from the person in front going faster. The net effect is that
everybody goes faster, but that's just because the rest period means people
can put in more when they're at the front.
This is different to the racing cars thing, when without swapping the lead,
the cars can go faster as a pair.

cheers,
clive

Simon Brooke said:

in message <[email hidden]>,
dkahn400 ([email hidden]'😉 wrote:

Quoted message said:

Quoted message said:

The classic example is racing cars on oval tracks such as Indianapolis
Speedway where a pair of cars running nose to tail can go faster than
either of them alone. In cycling, however, I don't believe the effect
is significant otherwise racing cyclists would be generally aware of
it.

Racing cyclists /are/ generally aware of it. What do you think
chain-ganging is all about? Have you never watched a team time trial?

I don't think they are aware of it because I think the gain for the
lead rider is small enough to be insignificant. Chain-ganging is about
taking shelter to minimise the time spent in the lead allowing each
rider to put in a short effort followed by a long recovery.

Note also that in one of his books Lance Armstrong claimed that the
lead rider is actually slowed down by giving a tow. This makes it clear
that the theoretical gain for the lead rider is not generally known
among racing cyclists.

--
Dave...

dkahn400 said:

Note also that in one of his books Lance Armstrong claimed that the
lead rider is actually slowed down by giving a tow. This makes it clear
that the theoretical gain for the lead rider is not generally known
among racing cyclists.

I suspect any physical advantage is outweighed by the psychological
stress of knowing that the guy behind is getting a free ride on you -
causing the leader to tense-up and lose form.

As the fairing post demonstrates, it's basic aerodynamics that a
following rider gives the leader a push, but as the effect is
insignificant compared to the follower, the front is still not where you
want to be.

Tim Downie said:

Here's a thought experiment.

Imagine a flat board lying on the floor with piece of string tied to the
middle. If you lift it quickly by the string, the force required will
initially be high until air can rush in underneath it. I magine now having
a second board underneath the first. This time when you pull the string, the
second board will also rise briefly. Will the force required be the same?
I stongly suspect that you will be doing extra work to briefly lift the
second board.

Yes, but that doesn't correlate to the cyclists - because the extra
force is to lift both boards, that would be equivalent to towing the
second cyclist.

Quoted message said:

Now imagine a board being lifted by a winch at a constant speed. Now
imagine gradually bringing a second board close to the underside to the
first board. As you bring the second board close to the first winched
board, the energy required to do this will reduce. Work is now being done
on the second board which has to have come from somewhere.

That's where it goes wrong... There are two lots of work being done on
the second board: the work require to lift it, and the work require to
overcome the air resistance. When the boards are close together, you're
reducing the work required to overcome the air resistance of the second
board - but that doesn't have to come from anywhere else.

To extend your thought experiment...

Connect your boards together with a long length of string. The winch
will be supplying enough power to lift the weight of two boards, plus
enough to overcome the air resistance of the two boards. Now glue the
boards face to face. Now the winch is still supplying enough power to
lift the weight of two boards, but there's only one board's worth of air
resistance. You're putting less energy in overall. You're not getting
'free energy' from anywhere, you're just wasting less. The amount of
work done to raise the boards becomes potential energy and that's the
same whether the boards are together or apart. The work needed to
overcome air resistance mostly just warms the air up a bit. When the
boards are together you warm it less than when they're apart

--
Andrew

Ib said:

dkahn400 said:

Note also that in one of his books Lance Armstrong claimed that the
lead rider is actually slowed down by giving a tow. This makes it clear
that the theoretical gain for the lead rider is not generally known
among racing cyclists.

I suspect any physical advantage is outweighed by the psychological
stress of knowing that the guy behind is getting a free ride on you -
causing the leader to tense-up and lose form.

I think that's right, but if the towing rider mistakenly believes that
he is being slowed down by the towee that can only increase the
psychological stress. In a 2 man break, knowing that the man sitting on
your wheel is not only not pulling you back to the bunch but may
actually be helping very slightly to keep you away removes some of the
urgency to dump him as long as you are reasonably confident you can do
so eventually. It does not of course reduce the stress of knowing that
he's working less hard than you.

--
Dave...

In article <[email hidden]>,

() said:

Here is another thought experiment:-
Imagine a cyclist riding along a flat track
at a constant /effort/ in still air.

A powerfull motorcycle catches up with the cyclist
and follows the cyclist at a fixed but short distance.

Can you be sure that the cyclist will not speed
up?

If you like, try substituting a large bus for the motorcycle
and re-compute.

Something like this has happened more than once during HPV speed record
attempts, which is why at Battle Mountain the Organisators take great
pains to ensure that the riders' chase vehicles are several hundred
metres behind the HPV.

--
Dave Larrington - <http://www.legslarry.beerdrinkers.co.uk/>
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