Hi All,

does anyone understand this 'black art'.

Basically looking to understand the benefits/impact of splitters/diffusers/proper wings. :confused:

Cheers for any help

Its all to do with downforce I think.

The clio has a rear diffuser to increase downforce

There was an F1 car in the late 70s with a fan that sucked air from under the car to increase downforcde but it got banned

Aerodynamics is how slippery a car is through the air. Fitting diffusers/wings/etc breaks up this slipperyness which can help to slow the car down (eg Bugatti air brake) or help to increase downforce and increase grip. Advantage of being slippery are reduced mpg, tyre wear and better acceleration/top speed. Downside is trade off this grip and keeping the car planted.

However saying all that I'm not sure you'd notice a huge difference at road speeds - Prodrive claimed that one of their rear spoilers improved downforce significantly at 100+mph IIRC....

Chris

On high performance racing cars, you have what is known as a 'Venturi tunnel' This forces air to pass under the car at high speed so it exits the rear of the car at low pressure creating suction, this is like an aeroplane wing in reverse as it forces the rear of the vehicle downwards like a spoiler.

As for downforce - do you recall the Koenigsegg that crashed on top gear? the stig lost the rear end of the car at high speed because it had no spoiler;

No spoiler = no downforce,

No downforce = reduced tyre grip = crash.

Barryboy spoilers do not provide downforce, they are merely fitted to make the car look crap

Barryboy spoilers do not provide downforce, they are merely fitted to make the car look crap

Indeed. Think about it...in order to gain a significant improvement in grip from downforce, a downward force equivalent to 10's of kg's needs to be applied...now I would like to see what happened if you stood on such a barry boy's front splitter, screwed on with self tappers and filler!

As for making a car more slippery, the less imperfections, dimples, proturions etc the better. Thats why the Honda Insight has the rear wheel arches covered, it significantly reduces drag to increase fuel economy.

The key point of splitters etc is that the shape of the body of any car is an approximation to an aerofoil, and so will produce a certain amount of lift at speeds, which will reduce traction (as per the crashed Koenigsegg) There are several ways of countering this, all of which will be familiar, but work in slightly different ways:

Wings, like at the front and back of F1 cars and the back of proper rally cars are upside-down aerofoils, and so produce DOWNWARDS lift ('downforce'), pushing the car into the road at speed;

Spoilers, OTOH, work to disrupt the airflow, usually at the trailing edge of the car, thus producing turbulence and 'spoiling' the car's inherent upwards lift;

Diffusers and splitters, AIUI, work on a similar principle to wings by going some way to countering the tendency of the car's body to produce upwards lift, and;

Miscellaneous things like the little sharks' teeth on Evos work by managing airflow to enhance the effect of wings etc.

Most 'bodykit' wings aren't viable aerofoils, so won't do anything to improve downforce. Any protusion (wing or otherwise) will, as has already been said, produce drag, which will act to slow the car down. This is why the wing on the Veyron has to be retracted to achieve ultimate top speed (at the cost of poorer traction), and is also possibly why the Koenigsegg didn't have a wing at the time TG tested it since IIRC, it was the fastest road car in the world at the time (in a straight line), which presumably would have been prejudiced by fitting a wing...

There was an F1 car in the late 70s with a fan that sucked air from under the car to increase downforcde but it got banned

It was a Brabham, and it wasn't banned. If Brabham hadn't withdrawn it voluntarily, it would have been banned at season end though.

But can an F1 car drive on the roof of a tunnel without falling off?

Some people say it can due to sheer speed and the downforce created.

Was that like with the John Player Special Lotus 79, in that it was dangerous in case of failure, rather than being an unfair advantage? I gather the Tyrell 6-wheeler never took off because it wasn't as good in practice as it should have been in theory, so was it perhaps like that???

But can an F1 car drive on the roof of a tunnel without falling off?

Some people say it can due to sheer speed and the downforce created.

They can in principle, but I'd rather not try it.

But can an F1 car drive on the roof of a tunnel without falling off?

Some people say it can due to sheer speed and the downforce created.

There was a thing about that on the telly, I think. Provided the downforce created at speed is greater than the weight of the car, then yes, an F1 car could drive on the roof of a tunnel. A Mini Cooper won't however, if you fancy a go at the Italian Job!

Oh, and you need to be careful that traction isn't too reliant on downforce, as demonstrated by that road-going F1 car thing that burned Jason Plato, and can't do slow corners for toffee...

Was that like with the John Player Special Lotus 79, in that it was dangerous in case of failure, rather than being an unfair advantage? I gather the Tyrell 6-wheeler never took off because it wasn't as good in practice as it should have been in theory, so was it perhaps like that???

Lotus 79 - was that the "twin chassis" car? If so I think the problem was that no-one else agreed with Colin Chapman that the word "chassis" is both singular and plural.

As for the Tyrell P34, the problem was that Goodyear never kept the tyre technology for its special fronts up with the normal fronts and with the rears, so it developed a tyre-based handling imbalance as time went on. The the FIA banned 6-wheel cars just as March and Williams were about to enter cars using dual rears (effectively 6 front tyres).

Lotus 79 - was that the "twin chassis" car? If so I think the problem was that no-one else agreed with Colin Chapman that the word "chassis" is both singular and plural.

The 79 was the first of the rubber skirt ones (fnarr), although a quick Google for the 88 shows me that the 88's twin-chassis was a different way of attempting to exploit the ground effect, and yet more Wikipedia-ing has shown that it didn't always work!

In which case it took several years for sliding or flexible skirts to be "banned", by means of a ride-height rule, so we got the dual-rate springs, hydraulic ride-height selection etc.

does anyone understand this 'black art'.

I understand it well enough to know that you can't at all predict the effect of aerodynamic conversions without detailed testing (or simulation if you happen to have a Finite Element Analysis aerodynamics software suite at your disposal). About all you can reliably figure is that things like protruding door handles and radio aerials worsen aerodynamics but realistically, on any car going at road speeds, you'll struggle to notice much difference from dealing with that stuff.

But can an F1 car drive on the roof of a tunnel without falling off?

Some people say it can due to sheer speed and the downforce created.

Theoretically, but if you think about it the grip wouldn't be huge as you wouldn't have the normal weight of the car (plus the downforce at speed) to force the tyres into the road.

The greater the weight on the tyre the more the grip:

An F1 car doing 150mph (assuming 150 is quick enough!) the right way up:

force on the tyres = (weight of car) + (downforce) - (aerofoil effect of the car)

An F1 car doing 150mph upside down :

force on the tyres = (downforce) - (weight of car) - (aerofoil effect of the car)

A much much lower figure. I suspect in reality it'd skid all over the place, not to mention trying to operate the pedals upside down with gravity trying to pull your legs away from the floor.

There was a thing about that on the telly, I think. Provided the downforce created at speed is greater than the weight of the car, then yes, an F1 car could drive on the roof of a tunnel.

Surely that would be upforce it generates? :P:rofl:

Chris

Don't worry it won't be 'Barry Boy'.

I'm improving the esprit and I have organised a proper rear aerofoil made by a racing car company. It's designed for GT style cars and thus should provide some downnforce at speed.

What I'm looking at is creating a diffuser for the rear and also deciding on a splitter at the front or to go with a rubber bib spoiler which would reduce the air under the car.

What I was worried about was upsetting the handling balance so just trying to get a better understanding of the subject before I start the cardboard mock-ups

Cheers

In which case there was a homolgation version of the Esprit V8 (same base shell as yours) done by Lotus, so a bit of net searching might help locate well made and specifically designed parts.

It was GT1. I do have plenty of photo's of that one. Trouble is it's a tad extreme for road use.

My main issue was what effect each item would have on the car.

Obviously the wing is straightforward. I just wasn't sure what the diffuser and splitter really does.

Cheers

The splitter moves the "stagnation point" which determines whether air goes over/round the car or under it down, reducing lift and drag, particularly at the front end.

A proper diffuser (as per the Esprit GT1) expands air at the back, and creates actual downforce there.

I have no idea what a wing or spoiler for a specific application does, without seeing CFD or tunnel data.

Excellent, Thanks for that.

I am basically looking at making a slightly smaller version of the GT1 style rear diffueser (I've managed to get a good pic of how it is made). However my problem is then the front end. I assume I can't really do one without the other i.e. too much rear downforce could cause handling problems.

The GT1 front splitter is ace but I would rip it off in seconds on some of the roads near me. So would a rubber bib have a similar effect to a splitter, i.e. reduce the air going under the car??

Cheers

An olde-fashioned "airdam" you mean. It does pretty much the same thing as a splitter; just not as well.

As for a smaller diffuser, shortening an existing design could have some pretty unpredictable effects in terms of the effect on total downforce, and even on when/if it stalls out. Also, do not underestimate the loading on an effective diffuser; they're not made from carbon fibre on kevlar cores just to be light or look pretty!

I was planning on making the diffuser in aluminium in the first instance. If it works then I might get it made in carbon (Depends on the weight difference really).

I assume that a proper splitter would need to be as near the road surface as possible though??

Cheers

I don't think so - 'ground effect' or the venturi design of F1 cars depends on a very low ride height to work properly, but a splitter can induce a low pressure region at the front of the car (producing less lift, if not actual downforce) merely by changing the shape/profile of the front of the car. At least I think that's right. (Waits to get shot down) Nice pictures - MGF aerodynamics

Has anyone found any benefits (other than aesthetic) to the Cupra splitter on a Fabia or Octavia? Don't laugh