Use of rear diffusers in modern cars

[Richard-G]

Hello fellow ph'ers

just a quick question to satisfy my odd fixation with car design and aerodynamics

Anybody got any examples of modern cars utilising a functional rear diffuser that actually contributes to the cars downforce?

lately the 'sporty' versions of cars (zetec for ford, s-line for audi, M sport for bm and 'sport' for merc etc) have a blanking in the rear bumper to denote them from the standard versions, now this to me is an obvios attempt at mimicking the rear diffuser style we see on the dtm, gt, f1 cars.

my own car (e92 m sport) has this blanking and some smooth tunnels under the rear bumper which i always assumed were for show, however my friendly local bm dealer (who is also a ph'er) told me to check the brochure, which indeed states that as part of the m sport pack 'the car features a rear diffuser to increase downforce pressure on the rear axle' to me this looks to be rubbish, but i am intrigued none the less

Certainly the s2 lotus Elise looks to have some sort of venturi tunnels

Yours inquisitively

Rich g

[rhinochopig]

A lot are largely cosmetic as they need to work with a flat underside to really work effectively. In some case they act as a parachute as they just catch air.

[ManOpener]

Many supercars have them, and more and more sports cars do too I think. I'm not sure how many are fully functional, I remember Renault going on about how there's was functional when the Clio 197 was released, though whether theres any truth in that I dont know.

[10 Pence Short]

I've seen a few current Focus with comedy rear 'diffusers'. Doris on her way to Morrisons can shave an additional 5 tenths from the journey, which is coincidentally the same kind of benefit you get having Alonso driving your car.

[RacingPete]

The purpose of the rear diffuser is to help create downforce by allowing the change in pressure below the car to dissipate effectively on the way out... the purpose is to make the air flowing under the car quicker than going over the car and thus pulling the car towards the ground.

For this to work effectively you need to have the car pretty close to the ground and to have a flat bottom on the car and to limit the amount of airflow coming in from the sides, thus I feel the modern cars with funny diffuser shape styling at the back are more for show than actually creating the effect.

[Richard-G]

RacingPete said:

The purpose of the rear diffuser is to help create downforce by allowing the change in pressure below the car to dissipate effectively on the way out... the purpose is to make the air flowing under the car quicker than going over the car and thus pulling the car towards the ground.

For this to work effectively you need to have the car pretty close to the ground and to have a flat bottom on the car and to limit the amount of airflow coming in from the sides, thus I feel the modern cars with funny diffuser shape styling at the back are more for show than actually creating the effect.

hmm, i see, which does make sense when i look back at the furore bought about when the the fia were looking at red bulls ride height adjustments. Saying that the clio 197 has a very prominent rear diffuser, must be one for the barry boys then!

as for the focus one! i think it has vents and all sort of jiggery pokery, looks sporty though!

[br d]

RacingPete said:

For this to work effectively you need to have the car pretty close to the ground and to have a flat bottom on the car and to limit the amount of airflow coming in from the sides...

Ah I wondered why my car has that. I bottomed out over a pothole once and decided to check underneath for damage, it has a flat cover the full length and width of the car (there were some minor scratches), if you turned it over onto it's back it would look like a one of those kids toy cars.

[RenesisEvo]

doogz said:

I've wondered about this previously, are production cars tested in wind tunnels? You'd imagine not, due to the expense, but if that is the the case, how are manufacturers able to state Cd?

I reckon these days it's much more likely that CFD is used to analyse airflow over and under vehicles, but i really don't know.

Windtunnels are used extensively by all the major manufacturers, and CFD is starting to become a serious analysis tool to support the wind tunnel programmes. Windtunnels are now even able to do hot/cold (Audi), as well as aeroacoustics and soiling. It's amazing how much goes into vehicle development nowadays, and yes, the cost is something - hence the rise of CFD as a cheaper alternative for prototype-level development, before expensive one-off models are made.

EDIT: someone mentioned the FOcus - the Focus Zetec S has a bodykit with a faux diffuser. I can't comment on its effectiveness but I'd imagine without many flat panels, the diffuser isn't going to drive the pressure low enough to make a huge difference to the lift.

Edited by RenesisEvo on Tuesday 6th July 13:36

[Richard-G]

br d said:

RacingPete said:

For this to work effectively you need to have the car pretty close to the ground and to have a flat bottom on the car and to limit the amount of airflow coming in from the sides...

Ah I wondered why my car has that. I bottomed out over a pothole once and decided to check underneath for damage, it has a flat cover the full length and width of the car (there were some minor scratches), if you turned it over onto it's back it would look like a one of those kids toy cars.

out of interest br d, what car is that?

my e92 is very smooth underneath, you can hardly see the rear diff, saying that its still a fair distance off the ground

[rhinochopig]

RacingPete said:

The purpose of the rear diffuser is to help create downforce by allowing the change in pressure below the car to dissipate effectively on the way out... the purpose is to make the air flowing under the car quicker than going over the car and thus pulling the car towards the ground.

For this to work effectively you need to have the car pretty close to the ground and to have a flat bottom on the car and to limit the amount of airflow coming in from the sides, thus I feel the modern cars with funny diffuser shape styling at the back are more for show than actually creating the effect.

Even better if you suck the air out. Brabham Fan-Car for example.

[Richard-G]

rhinochopig said:

RacingPete said:

The purpose of the rear diffuser is to help create downforce by allowing the change in pressure below the car to dissipate effectively on the way out... the purpose is to make the air flowing under the car quicker than going over the car and thus pulling the car towards the ground.

For this to work effectively you need to have the car pretty close to the ground and to have a flat bottom on the car and to limit the amount of airflow coming in from the sides, thus I feel the modern cars with funny diffuser shape styling at the back are more for show than actually creating the effect.

Even better if you suck the air out. Brabham Fan-Car for example.

i wont be standing up for a while

[br d]

Richard-G said:

br d said:

RacingPete said:

For this to work effectively you need to have the car pretty close to the ground and to have a flat bottom on the car and to limit the amount of airflow coming in from the sides...

Ah I wondered why my car has that. I bottomed out over a pothole once and decided to check underneath for damage, it has a flat cover the full length and width of the car (there were some minor scratches), if you turned it over onto it's back it would look like a one of those kids toy cars.

out of interest br d, what car is that?

my e92 is very smooth underneath, you can hardly see the rear diff, saying that its still a fair distance off the ground

Ferrari F430. It does sit pretty close to the floor, there are a couple of raised drain covers I have to drive around on my local routes.

[The Wookie]

I always thought they were there to effectively couple the vacuum created directly behind the car to the underside of the car?

And also, I believe the term is to 'reduce lift' rather than create downforce, although I'm happy to be corrected as I'm no aerodynamicist

[RacingPete]

The Wookie said:

I always thought they were there to effectively couple the vacuum created directly behind the car to the underside of the car?

And also, I believe the term is to 'reduce lift' rather than create downforce, although I'm happy to be corrected as I'm no aerodynamicist

The wings on a plane are also there to reduce downforce... It depends if it is creating it or reducing it I suppose

[The Wookie]

RacingPete said:

The Wookie said:

I always thought they were there to effectively couple the vacuum created directly behind the car to the underside of the car?

And also, I believe the term is to 'reduce lift' rather than create downforce, although I'm happy to be corrected as I'm no aerodynamicist

The wings on a plane are also there to reduce downforce... It depends if it is creating it or reducing it I suppose

And there's the thrust of it, isn't a diffuser there to reduce the pressure under the car and hence, reduce the axle lift?

I thought aerofoils were the only things that produce 'downforce' as such?

[Richard-G]

br d said:

Richard-G said:

br d said:

RacingPete said:

For this to work effectively you need to have the car pretty close to the ground and to have a flat bottom on the car and to limit the amount of airflow coming in from the sides...

Ah I wondered why my car has that. I bottomed out over a pothole once and decided to check underneath for damage, it has a flat cover the full length and width of the car (there were some minor scratches), if you turned it over onto it's back it would look like a one of those kids toy cars.

out of interest br d, what car is that?

my e92 is very smooth underneath, you can hardly see the rear diff, saying that its still a fair distance off the ground

Ferrari F430. It does sit pretty close to the floor, there are a couple of raised drain covers I have to drive around on my local routes.

ok then br d, even though your particular choice of steed is rare , the next time i see an f430 im going to study its bum in depth!

[RenesisEvo]

doogz said:

Thanks for the reply.

So do the "major manufacturers" use scale models, and small wind tunnels, as opposed to full size tunnels?

Here my knowledge starts to become a little stretched. I know that Audi's climatic tunnel is full-scale, and many others are - Pininfarina's tunnel, BMW have at least one, JLR have an interesting tunnel/CFD programme I recall reading about. I'm sure any volume manufacturer will have a full-size tunnel. I'm sure there must be some model testing, but because it's the small details that get fettled to assist drag (and thereby fuel economy) targets, along side things like cross-wind stability, stable separation points in yaw (the round rear of the Mk1 Audi TT was a big problem with this, ditto the early Ford Sierra, hence the strange protrustions on the final pillar), and as mentioned noise reduction, these are very hard to accurately model at scale. I have read about early research into using CFD to do some of this work, but it's very much early days. I'm certainly not the best placed person to answer your question (although aerodynamics, particularly CFD, is actually my occupation, but I don't deal with the mainstream automotive industry).

[RacingPete]

The Wookie said:

And there's the thrust of it, isn't a diffuser there to reduce the pressure under the car and hence, reduce the axle lift?

I thought aerofoils were the only things that produce 'downforce' as such?

You are right in a sense as the diffuser does not create downforce itself, and its purpose is to allow the air under the car to dissipate into the environment effectively, but the whole process does create downforce by making the whole underneath of the car one large aerofoil (in a sense).

[br d]

Richard-G said:

br d said:

Richard-G said:

br d said:

RacingPete said:

For this to work effectively you need to have the car pretty close to the ground and to have a flat bottom on the car and to limit the amount of airflow coming in from the sides...

Ah I wondered why my car has that. I bottomed out over a pothole once and decided to check underneath for damage, it has a flat cover the full length and width of the car (there were some minor scratches), if you turned it over onto it's back it would look like a one of those kids toy cars.

out of interest br d, what car is that?

my e92 is very smooth underneath, you can hardly see the rear diff, saying that its still a fair distance off the ground

Ferrari F430. It does sit pretty close to the floor, there are a couple of raised drain covers I have to drive around on my local routes.

ok then br d, even though your particular choice of steed is rare , the next time i see an f430 im going to study its bum in depth!

I can understand it when people say it's an ugly car from the back but that big diffuser is a thing of beauty, one of the details that first drew me to that car.

[300bhp/ton]

Richard-G said:

Hello fellow ph'ers

just a quick question to satisfy my odd fixation with car design and aerodynamics

Anybody got any examples of modern cars utilising a functional rear diffuser that actually contributes to the cars downforce?

lately the 'sporty' versions of cars (zetec for ford, s-line for audi, M sport for bm and 'sport' for merc etc) have a blanking in the rear bumper to denote them from the standard versions, now this to me is an obvios attempt at mimicking the rear diffuser style we see on the dtm, gt, f1 cars.

my own car (e92 m sport) has this blanking and some smooth tunnels under the rear bumper which i always assumed were for show, however my friendly local bm dealer (who is also a ph'er) told me to check the brochure, which indeed states that as part of the m sport pack 'the car features a rear diffuser to increase downforce pressure on the rear axle' to me this looks to be rubbish, but i am intrigued none the less

Certainly the s2 lotus Elise looks to have some sort of venturi tunnels

Yours inquisitively

Rich g

Bit of an oddball one, but I believe these funky little motors had quite a bit of aero design (and wind tunnel testing??), with input from one Gordon Murry. Might be wrong though.