Fitting a rear spoiler - effect on mpg?

[MarkDavis]

I was looking at the manufacturer instructions on fitting a manufactuer Aero spec spoiler to my Saab and noticed that the fitting instructions says that the spoiler will "....reduce aerodynamic lift at the rear by 40%"

Not, 'by up to 40%' but 'will reduce by 40%' which seems like a significant amount, even though its a discrete 'lip' type spoiler.

In F1, you hear Martin Brundle saying that F1 cars have so much downforce they could drive upside down on a ceiling. Obviously, no production car is going to have anything close to the same downforce, but a 40% increase in downforce at the rear does seem signifcant.

Not that I care about downforce because the car never feels unstable at high speed, but does anyone know what sort of effect such a spoiler might have on mpg? I'm assuming there's going to be more drag and 'weight' at the back.

 

[khevolution]

my thoughts would be there it would effect mpg but not by any great amount that you would know about

 

[Trojan]

You're getting confused between reducing lift and creating downforce, 2 separate things. Fitting a proper spoiler to your car will not create downforce, it will instead reduce the car's natural propensity to lift at the rear by making it more aerodynamic. This should in turn reduce the drag the car creates and hence actually have a positive effect on your MPG. I would guess that the difference will be negligible at normal speeds, but at higher motorway speeds it may make a difference, unless it is countered by the extra weight.

If you were fitting a downforce inducing rear wing that actually pushed the rear of the car down, it would indeed have a negative effect on your MPG.

 

[breadturbo]

here's a thought, maybe spouting crap but....

If you increase the rear downforce by 40% will it have a lifting effect on the front reducing traction? (the saab being fwd)

 

[Maccapacca]

Unless your fitting a escort cosworth size whale tale all a spoiler is going to do is help keep your rear window clean at UK road speeds that's why most spoilers pop up at 70

 

[robj20]

There is a difference between a spoiler and a wing, spoiler is to reduce air pressure behind the car this reducing drag. A wing increases downforce by acting like well a wing.

 

[[TW]Fox]

that's why most spoilers pop up at 70

By 'most' do you mean 'A very small minority'?

 

[Howard]

here's a thought, maybe spouting crap but....

If you increase the rear downforce by 40% will it have a lifting effect on the front reducing traction? (the saab being fwd)

Only if you fit a bloody Fast and Furious-style wing, i.e. http://www.nicedeals.co.uk/images/civic-coupe-boot-spoiler-2s.gif


In Mark's instance, the effect on fuel economy and drag will be barely noticeable. It's a styling thing.

I'm not saying it won't look nice, but it won't have much of an effect otherwise

 

[lordrobs]

I know that the average road car suffers a fair amount of lift at the front at speed but just how much do they experience at the rear? Assuming the answer is bugger all then a 40% reduction of bugger all will have a net result of naff all difference.

Apologies if I lost anyone with my technical terms

 

[Sin_Chase]

I would have thought a diffuser would have a far more noticeable effect to stop air ballooning in the rear bumper/back of the vehicle. I too am curious to know how lift can be generated at the rear, more specifically - How a lip would reduce lift at the rear.

 

[paradigm]

I would have thought a diffuser would have a far more noticeable effect to stop air ballooning in the rear bumper/back of the vehicle. I too am curious to know how lift can be generated at the rear, more specifically - How a lip would reduce lift at the rear.

Would it not cause the vortex of air to swirl further away from the rear of the car? Thus less lift on the car as a result?

 

[mrthingyx]

I think an American (yes, you read that correctly) car magazine took a look at thte effect of adding after market spoilers onto everyday production cars and found that all they did was eat at the cars' top speeds by a small fraction.

Reading into that, there will be an effect on MPG but it will be proportionate to the reduction in top speed.

i.e. you will notice nothing when driving at everyday speeds by fitting a spoiler to your Saab other than a reduction in your wallet size. If you're driving in situations other than those stated above, I suspect the Saab will present you with other, more significant problems than rear lift.

 

[Jonnycoupe]

I would have thought a diffuser would have a far more noticeable effect to stop air ballooning in the rear bumper/back of the vehicle. I too am curious to know how lift can be generated at the rear, more specifically - How a lip would reduce lift at the rear.

If you think of a typical saloon the airflow and then the large rear glass gives a volume that trails the car as it moves forward, the airflow tried to fill this void and hence you get a region of lower pressure, this lower pressure acting over the large tailgate is what gives you the lift. Delaying or promoting flow seperation can have a marked effect on the rear lift. The AudiTT had issue at launch with airflow attachment down over the trunk lid, with sudden side winds would break the attachment, and generate a large low pressure region applying lift to the rear end.... this clearly was an issue at the limit with even seasoned race drivers losing control.

The Teg Type R spoiler drops the cD from 0.33 to 0.32 IIRC due to the way i spoils attachment to the bumper that causes trailing vortices. Vortex of course being a low pressure region.

 

[rallyfan1986]

I read somewhere that aerodynamic devices only really have any effect when travelling over 70mph, does it specify a speed for the 40% reduction? As it could be a 40% reduction at 500mph!