How much downforce does the 197 diffuser give?

[_WILL_]

It will do something, i have studied this sort of thing at uni, although i was using water not air but its similar enough to make the same assumptions, but i wouldn't like to say how much without looking at the underneath of a 197/200.

I would imagine in perfect conditions it will get the broacher quoted levels, but in practice i would be very surprised, even simple things like uneven tyre pressures could have an effect on its efficiency. Adding a completely flat bottom to the car would improve it significantly.

Plus it may be creating 40kgs of down force, but we don't know if the cars design/shape does the opposite so negating the diffusers over all effect and so making it a necessity for stability at high speed (i would imagine this is unlikely tho).

As quoted before the angle makes a difference that's why I can't see the VXR's being much good but i may be wrong

The hanging 40 kgs of it isn't an accurate representation, either in its effect on the cars handling or the force exerted directly on the diffuser.

This is my understanding anyway, but im not a qualified engineer yet so who knows????

 

[bmit]

Loving this thread! - I have one - it looks nice and mean, but the back end feels as light to me as any other quick FWD car even at 'very high speed' especially if you lift off mid corner

But I think i've just had a revelation!! -

You've all answered the question already - the 40kg of 'downforce' or 'reduced lift' or whatever else 'aero fandangoschizzle' adds even more 'weight' to the whole car which is why the mpg is so bad!!

disclaimer - i'm in no way baseing any of this on fact nor am I technically quallifed whatsover in this field. I also totally fell for the 33mpg combined in the brochure....

 

[Tom]

It will do something, i have studied this sort of thing at uni, although i was using water not air but its similar enough to make the same assumptions, but i wouldn't like to say how much without looking at the underneath of a 197/200.

I would imagine in perfect conditions it will get the broacher quoted levels, but in practice i would be very surprised, even simple things like uneven tyre pressures could have an effect on its efficiency. Adding a completely flat bottom to the car would improve it significantly.

Plus it may be creating 40kgs of down force, but we don't know if the cars design/shape does the opposite so negating the diffusers over all effect and so making it a necessity for stability at high speed (i would imagine this is unlikely tho).

As quoted before the angle makes a difference that's why I can't see the VXR's being much good but i may be wrong

The hanging 40 kgs of it isn't an accurate representation, either in its effect on the cars handling or the force exerted directly on the diffuser.

This is my understanding anyway, but im not a qualified engineer yet so who knows????

Sounds about right to me.

 

[bradharding24]

Lotus claim the exige produces 48 pounds of rear downforce at 100 mph.

That has a massive wing, a sealed flat floor and a five element diffuser.

There is absolutely no way the Clio produces 40 kgs of downforce from its rear. No way. I doubt it produces anything. It's for show. The air wouldnt reach the rear of the car in any fit state to do anything.

 

[bradharding24]

It will do something, i have studied this sort of thing at uni, although i was using water not air but its similar enough to make the same assumptions, but i wouldn't like to say how much without looking at the underneath of a 197/200.

I would imagine in perfect conditions it will get the broacher quoted levels, but in practice i would be very surprised, even simple things like uneven tyre pressures could have an effect on its efficiency. Adding a completely flat bottom to the car would improve it significantly.

Plus it may be creating 40kgs of down force, but we don't know if the cars design/shape does the opposite so negating the diffusers over all effect and so making it a necessity for stability at high speed (i would imagine this is unlikely tho).

As quoted before the angle makes a difference that's why I can't see the VXR's being much good but i may be wrong

The hanging 40 kgs of it isn't an accurate representation, either in its effect on the cars handling or the force exerted directly on the diffuser.

This is my understanding anyway, but im not a qualified engineer yet so who knows????

I think 5 to 10 degrees is the often quoted angle for the diffuser. Or there abouts. 15 to 20 degree is f1 stuff. A flat floor wont just help a diffuser, but is essential.

The low pressure area under the car is caused by the increased speed of the airflow (which is Bernoulli's principle). The less resistance, ie the flatter the under body, the faster the airflow and the lower the pressure. This low pressure generates downforce. All the diffuser does is take the lower pressure airflow and funnel it back and up, creating an opposite reaction in the car that pushes it down (air goes up, car goes down), creating more downforce. However, without the flat underbody providing the surface for the initial low pressure, the diffuser wont be able to work effectively and in reality wont do much at all.

Without that flat floor generting the low pressure to start with, how are you going to get the diffuser to channel low pressure air upwards to get a reaction in the car body?

 

[Pinkorstink]

Without that flat floor generting the low pressure to start with, how are you going to get the diffuser to channel low pressure air upwards to get a reaction in the car body?

You seem to be the expert you tell us.

 

[_WILL_]

The low pressure area under the car is caused by the increased speed of the airflow (which is Bernoulli's principle). The less resistance, ie the flatter the under body, the faster the airflow and the lower the pressure. This low pressure generates downforce. All the diffuser does is take the lower pressure airflow and funnel it back and up, creating an opposite reaction in the car that pushes it down (air goes up, car goes down), creating more downforce. However, without the flat underbody providing the surface for the initial low pressure, the diffuser wont be able to work effectively and in reality wont do much at all.

Without that flat floor generting the low pressure to start with, how are you going to get the diffuser to channel low pressure air upwards to get a reaction in the car body?

Yeah this sounds good to me but i would argue that the underside of the car, although not flat, is flat enough for Bernoulli's principle to apply(obviously its not perfect conditions and like i said i aint ever looked under a 197/200.) Hence why i said in perfect conditions it will prob get the quoted levels but that in practice it is unlikely.

I personally can't understand why they would quote figures if they weren't "true" when they could have just said "it increases down force."

Please note that i don't own a 197/200 so i am not being bias, this is just my opinion.

If someone has a 200 cup, in yellow with recaros, i can have then i promise to do a little wind tunnel testing and then i can post the results here, altho it may be a while as i would prob go for a very very long drive round and round north wales

 

[c__w]

Lotus claim the exige produces 48 pounds of rear downforce at 100 mph.

That has a massive wing, a sealed flat floor and a five element diffuser.

There is absolutely no way the Clio produces 40 kgs of downforce from its rear. No way. I doubt it produces anything. It's for show. The air wouldnt reach the rear of the car in any fit state to do anything.

At 100mph and "at very high speed" is probably the difference here. I am of the opinion the "at very high speed" = the car's top speed (ie around 140mph). Without knowing this speed we're just guessing but I think if it was a quotable speed of 100mph or less it would have been, otherwise they will just say "very high speed" as it is the maximum speed, therefore they can quote the maximum effect from the diffuser.

Taking into account that air resistance sqaures with speed I think it's safe to assume the efficiency of a diffuser might follow a similar pattern so comparing something at 100mph vs 140mph is a huge huge difference.

So at 100mph the 197 diffuser might be barely doing anything, maybe 5kg/10lb vs the 25kg/50lb of the Exige. At 140mph the Exige will therefore be loads more than this and of the Clio!

 

[bradharding24]

Can someone tell me the angle of the rear diffuser from the lowest point level with the underside to the top lip of the outer most part?

Also someone put up a pic of the clio underfloor and i will tell you if the diffuser produces downforce or not (IMO).

 

[Pinkorstink]

Can someone tell me the angle of the rear diffuser from the lowest point level with the underside to the top lip of the outer most part?

Also someone put up a pic of the clio underfloor and i will tell you if the diffuser produces downforce or not (IMO).

You stated already it doesn't, and now from a picture you're going to tell us whetehr or not it does but that will only be an opinion anyway??

 

[theduck]

Brad other than the exhuast recess its pretty much flat

 

[Marv-C]

Can someone tell me the angle of the rear diffuser from the lowest point level with the underside to the top lip of the outer most part?

Also someone put up a pic of the clio underfloor and i will tell you if the diffuser produces downforce or not (IMO).

Some pictures are on Page 6 of this thread.

 

[alanc1000]

Can someone tell me the angle of the rear diffuser from the lowest point level with the underside to the top lip of the outer most part?

Also someone put up a pic of the clio underfloor and i will tell you if the diffuser produces downforce or not (IMO).

out of genuine interest what is your technical knowledge this arena that will enable you to make a reasonable judgement that is SUPERIOR to the Renault development team?

 

[bradharding24]

the 197 one is pretty damn big tbf.

goes a long way under the car, this is mine

The problem is the diffuser can only work if the airflow under the car is smooth, as this accelerates the air and produces downforce. Without that speed the low pressure area cannot form, and without any low pressure the diffuser cannot channel that air upward causing an equal and opposite reaction (air goes up, car goes down.). You only have to look at the underside of that car and compare it to the flat sealed floor of an exige (or any proper sports car) to see that the lotus would indeed accelerate the airflow as it is completely flat (no sills tunnels or protrusions), whereas the protrusions from the clio would decelerate the air preventing the formation of low pressure.

The diffuser in effect just increases the surface area of the downforce produced by the floor of the car, but if the floor doesnt produce low pressure downforce, neither can the diffuser. The diffuser must work with the floor of the car.

To think that a hatchback car can produce downforce from airflow under the car with all the crevaces and protrusions is pure fantasy. For a start the airflow under the subframe and engine is going to be seriously disturbed. A sump guard would help this, but them the firewall would be like a brick wall to the airflow and the sills and exhaust tunnel will further slow and disturb the air flow.

By the time the air reaches the diffuser it will be slow and therefore of relatively high pressure as well as containing vortices and turbulance. This will make it very hard for the diffuser to chanel that air up and back, and even if it does the air flow will be so slow that it will produce no downforce.

 

[bradharding24]

Brad other than the exhuast recess its pretty much flat

http://carphotos.cardomain.com/ride_images/3/393/2061/25981030026_large.jpg

Here is an exige floor to compare. I think you can see the difference in a functional flat floor and the clio, which in reality is not flat.

 

[theduck]

I'm familiar with what a true flat floor looks like. Just saying as a middle ground between something like the exige and something thats not at all flat bottom, its relatively flat.

 

[Pinkorstink]

The problem is the diffuser can only work if the airflow under the car is smooth, as this accelerates the air and produces downforce. Without that speed the low pressure area cannot form, and without any low pressure the diffuser cannot channel that air upward causing an equal and opposite reaction (air goes up, car goes down.). You only have to look at the underside of that car and compare it to the flat sealed floor of an exige (or any proper sports car) to see that the lotus would indeed accelerate the airflow as it is completely flat (no sills tunnels or protrusions), whereas the protrusions from the clio would decelerate the air preventing the formation of low pressure.

The diffuser in effect just increases the surface area of the downforce produced by the floor of the car, but if the floor doesnt produce low pressure downforce, neither can the diffuser. The diffuser must work with the floor of the car.

To think that a hatchback car can produce downforce from airflow under the car with all the crevaces and protrusions is pure fantasy. For a start the airflow under the subframe and engine is going to be seriously disturbed. A sump guard would help this, but them the firewall would be like a brick wall to the airflow and the sills and exhaust tunnel will further slow and disturb the air flow.

By the time the air reaches the diffuser it will be slow and therefore of relatively high pressure as well as containing vortices and turbulance. This will make it very hard for the diffuser to chanel that air up and back, and even if it does the air flow will be so slow that it will produce no downforce.

That's cleared that up then. Silly Renault.

 

[bradharding24]

That's cleared that up then. Silly Renault.

Renault are there to make money. They are a business. Looks like their marketing department know what sells cars to 20 year old kids.

Nothing more, nothing less.

 

[Pinkorstink]

Renault are there to make money. They are a business. Looks like their marketing department know what sells cars to 20 year old kids.

Nothing more, nothing less.

And the millions they invest in R&D can be dismissed so lightly now that you have a photo of the underside of a 197? get real. Leave the hype out of it and explain, with technical facts (not your opinion) why the technology does not work.

I don't know of many 20 y/o kids who can afford a 197.

 

[Gorms]

Best thread ever never purchased mine thinkin the rear diffuser created any down force. Its pretty obviouse its all for show. Same as the front diffuser chances of that making any difference in minimal. Its crazy the figures quoted. I showed my mate some of the comments on here, he has a series 2 exige and he almost spat his drink all over my laptop

Then again im not 20 so maybe i missed something

Paul

 

[Jasey]

Renault wouldn't tell lies would they :evil:

 

[Pinkorstink]

Best thread ever never purchased mine thinkin the rear diffuser created any down force. Its pretty obviouse its all for show. Same as the front diffuser chances of that making any difference in minimal. Its crazy the figures quoted. I showed my mate some of the comments on here, he has a series 2 exige and he almost spat his drink all over my laptop

Then again im not 20 so maybe i missed something

Paul

I have no opinion either way and I think my diffuser looks gash but in order for me to dismiss it out of hand I need facts, not opinion.

 

[alanc1000]

I have no opinion either way and I think my diffuser looks gash but in order for me to dismiss it out of hand I need facts, not opinion.

exactly.

We have 197 owners who on the whole couldnt really give a toss but DO have facts from the manufacturer and have no real reason to think its ballshat.

Then we have a whole army of nervous, anxious, worried 172 and 182 owners who fear that MAYBE, just MAYBE the diffuser does do SOMETHING.

 

[bradharding24]

http://www.autoexpress.co.uk/carreviews/firstdrives/68538/renaultsport_clio_197.html

Autoexpress claim 70 kgs of downforce.

I wonder what they are smoking?

 

[alanc1000]

http://www.autoexpress.co.uk/carreviews/firstdrives/68538/renaultsport_clio_197.html

Autoexpress claim 70 kgs of downforce.

I wonder what they are smoking?

i wonder what you are smoking TBH.

 

[Ralphizzle]

Diffusers are so 2009!
This 172 owner couldn't care LESS!

It's a Renault Sport, Renault has a F1 Team... F1 tech has to trickle through.
Marketing guys ask for it, car mag editors love to write about it...

Your Z4 Coupe could use a rearwing... LOL

 

[alanc1000]

Diffusers are so 2009!
This 172 owner couldn't care LESS!

It's a Renault Sport, Renault has a F1 Team... F1 tech has to trickle through.
Marketing guys ask for it, car mag editors love to write about it...

Your Z4 Coupe could use a rearwing... LOL

nah, i dont drive fast enough, just like a diffuser, a spoiler needs speed to have any effect :rasp:

 

[Marky_]

200 trophys to have KERS button?

 

[Ralphizzle]

Then you could use some splitters and a Gurney strip. LOL

But seriously it's just a current trend in the auto industry.

 

[bradharding24]

Next people will be saying that the renaulsport boys had to increase the size of the williams rear wing over the 16v because it had more power.

Come on wake up, all this stuff is for aesthetic appeal.

 

[Gorms]

I have no opinion either way and I think my diffuser looks gash but in order for me to dismiss it out of hand I need facts, not opinion.

Thats fair enough but i do hope you dont lose sleep over this. Bigger things to worry about in the world like the front end of my 200 :dead:

Gorms

 

[Naith]

FWD cars do not need a rear spoiler, as they are useless. Ask yourself why a FWD car would need the rear wheels forced into the ground, when actually its the front ones that need the downforce.

The BTCC cars all have spoilers, but this is for aesthetics. They have even admitted this.

LOL, obviously never been on track... of course u need to plant the rear, and due to FWD engine and susp setup even more so, f***s sake.

 

[Mike_182]

It's a Clio. Not an Exige. Gravity does more than the diffuser, plus as there's no undertray to feed it then it's only going to get upset about the turbulent air its fed.

 

[Pinkorstink]

Come on wake up, all this stuff is for aesthetic appeal.

So what was the point in asking people for measurements and photographs of the diffuser if this was your basic argument?

 

[Marv-C]

Again, I can't believe what a fire I have started.

I have been thinking about all the technical posts that have been put here and wonder what you make of this:

The orginal Audi TT (without the tiny rear spoiler) lost control at high speed. Audi recified this with the small spoiler that you now see. Consider what a tiny strip has done to Audi aerodynamics and I think that the investment Renault has made in the 197 diffuser must have some practical purpose.

Personally, I don't really care how much downforce the diffuser gives. The car feel planted and however good a driver I may or may not be, I probably couldn't tell the difference.

 

[bradharding24]

Again, I can't believe what a fire I have started.

I have been thinking about all the technical posts that have been put here and wonder what you make of this:

The orginal Audi TT (without the tiny rear spoiler) lost control at high speed. Audi recified this with the small spoiler that you now see. Consider what a tiny strip has done to Audi aerodynamics and I think that the investment Renault has made in the 197 diffuser must have some practical purpose.

Personally, I don't really care how much downforce the diffuser gives. The car feel planted and however good a driver I may or may not be, I probably couldn't tell the difference.

Audi did a lot more than just add a spoiler.

 

[Marv-C]

Audi did a lot more than just add a spoiler.

Should I ask more..........

 

[_WILL_]

The problem is the diffuser can only work if the airflow under the car is smooth, as this accelerates the air and produces downforce. Without that speed the low pressure area cannot form, and without any low pressure the diffuser cannot channel that air upward causing an equal and opposite reaction (air goes up, car goes down.). You only have to look at the underside of that car and compare it to the flat sealed floor of an exige (or any proper sports car) to see that the lotus would indeed accelerate the airflow as it is completely flat (no sills tunnels or protrusions), whereas the protrusions from the clio would decelerate the air preventing the formation of low pressure.

The diffuser in effect just increases the surface area of the downforce produced by the floor of the car, but if the floor doesnt produce low pressure downforce, neither can the diffuser. The diffuser must work with the floor of the car.

To think that a hatchback car can produce downforce from airflow under the car with all the crevaces and protrusions is pure fantasy. For a start the airflow under the subframe and engine is going to be seriously disturbed. A sump guard would help this, but them the firewall would be like a brick wall to the airflow and the sills and exhaust tunnel will further slow and disturb the air flow.

By the time the air reaches the diffuser it will be slow and therefore of relatively high pressure as well as containing vortices and turbulance. This will make it very hard for the diffuser to chanel that air up and back, and even if it does the air flow will be so slow that it will produce no downforce.

Please can you explain some of these statements in more detail, I am an engineering student and have studied a reasonable amount of fluid dynamics and have used wind tunnels (only on a small scale). Some of the stuff you are saying doesn't seem to quite make sense to me? but i may be wrong

 

[Ralphizzle]

_WILL.: please run a CFD model with/without a diffuser! LOL

@ Brad Harding 24.: we have these plastic 'sumpguards' on the Phase 172!

 

[-Leigh-]

You funking idiots lol!!!

Just to clear this up, my mate drove his 197 round a corner and it felt very much on the limits, so we took the diffuser off, and he wrote it off at a slightly lower speed.

FACT. Discuss.