Mk1 aerodynamics

[Nick Read]

Ive just been pondering this one for a while. Im no fluid engineer or anything but Ive got a rough idea of the aerodynamics of your average road car and Ive been wondering - what purpose does any kind of tailgate spoiler serve at all?

Im not talking about a sticking up spoiler like say a Clio Maxi - because this seems clearly to cause a high pressure area to build up in front of the spoiler and thus helps to cancel out the firstly the low pressure from the speeding airflow over the roof and secondly the flow detachment which causes a vacuum effect over the tailgate as the car moves forward. Large wings or spoilers above or behind the top of the tailgate I can also understand ie. like the hatch-based WRC cars have as they harness the air flowing over and around them to create downforce or frontal high pressure areas which are then transferred through struts into the rear of the car.

The kind which seems useless to me is the small wedge at the top of the tailgate - as seen on the valver and Williams, and basically every single mildly sporting hatch ever since as well. Its not a wing and causes no downforce through airflow above/below/around it as it only has an upper surface in the airflow. It cant be a spoiler either as for it to create a spoiler effect, impeding or damming the air to create a small area of high pressure, it must surely be in the airflow which I dont think it can be? Surely the air flows roughly horizontally across the roof and then when it reaches the rear edge of the roof, carries on clear of the tailgate before finishing off in a whirl of turbulence behind the car and leaving a low pressure are over the whole tailgate? Surely the valver/ Williams spoiler would only be any use at all if the airflow really did follow the line of the tailgate smoothly without any disturbance. And if it did this, then there would be no need for a spoiler on the tailgate because it would actually ruin the smooth airflow off the back of the car?

I can understand why a saloon would have a wedge shaped spoiler at the rear of its boot, but I just cannot see why a hatchback should need a similarly shaped spoiler at the top of the tailgate. And dont get me started on midline spoilers as seen on dodgy old Fiestas and Scooby wagons.

Somebody tell me what Im missing here!

 

[telford_mike]

Good question. I think if they really helped then the economy-oriented models would have them too, as theyd undoubtedly assist fuel efficiency, and thats what sells vehicles in that category. I think its a bit like alloy wheels - they look nice but dont offer any particular advantage.

 

[SteveJJH]

Hello

Id guess its mainly a cheap cosmetic tool to seperate the "sporty" models from the rest of the range. I could believe that it would make a small difference to the flow over the rear screen though. if the air flows smoothly over the surface of the rear of the car the body would be working like a wing, air travelling over the top would be forced to travel faster than air travelling underneath, i think you know the theory. diturbing the flow would stop the air that has passed over the top of the car from rejoining the air that has passed underneath, reducing the effect.

Agreed, the one on the williams and valver is titchy though, surely the effect would be minor

 

[edde]

Its more of something to look sporty than have any noticible difference.

Youd have to have the car in a wind tunnel to see any gains and sicne there are so many other variables it would be a waste.

However BWM Mini was tested round the ring a while back with an adjustable spoiler for height and angle etc to see what worked best so maybe there are some advantages.

 

[Nick Read]

Hmmmm...yes so maybe the spoiler would be to disturb the smooth passage of air and stop it causing an overall lift effect over the whole car...in other words, to actually promote flow detachment over the rear...

Anyway, where you been all this time Steve?

 

[rockport]

Quote: Originally posted by Nick Read on 30 March 2004


Anyway, where you been all this time Steve?

How cheeky is that steve? says the man thats almost been forgoten about, not seen or heard for months on end!

 

[SteveJJH]

true ive not been on here much over winter. ive been spending a lot of time trying to get a motorbike license sorted out and spending a lot of money keeping the clio going. plus i also got bollocked at work for spending too much time on here.

 

[unofficialPete]

Well Ive got a 1.4 Sport complete with 16v spoiler, doubt very much is does much but it sure as hell makes the rear of the car 10 times better!

 

[Fred@BTM]

Why does the 182 have an option of Cup spoilers?????

Answer = cos it looks a bit better!!!

no advantage at all (apart from giving the owner the idea, "oooh i have spoilers!!! i can go faster!!!")

 

[BenR]

Quote: Originally posted by Nick Read on 30 March 2004


Hmmmm...yes so maybe the spoiler would be to disturb the smooth passage of air and stop it causing an overall lift effect over the whole car...in other words, to actually promote flow detachment over the rear...

Anyway, where you been all this time Steve?

Bang on the button.

The roof of the clio is curved and tapered towards the rear, fliud likes to follow a surface, and when it comes to the tailgate, to an extent it will turn before detatching in a mass of turbulance. The trailing edge of the spoiler is too abrupt and the angle too severe that seperation is the only result.

And due to the natural laws of energy, any decrease or increase in either velocity or pressure directly affects the other. So by discontinuing the line of the chord, the vehicle no longer acts as a wing.

And the spoiler cannot be called either a spoiler, or a wing as it neither creates a high presure top side, or a low pressure underside. Its merely an aid to reduce overall lift.

Even in F1 most effort is put on reducing lift rather than creating negative lift.

 

[Nick Read]

Very interesting Ben, that pretty much confirms what I was thinking. Especially about it not being a spoiler either. So am I right in supposing that the Willy spoiler which is basically bigger than the valver one, because it extends further back and out from the tailgate glass, is slightly more effective at its job ie. promoting separation and reducing overall lift?

 

[BenR]

could well be, but id err towards the marketing dept having more input on the larger one, as i see it being no more efficient at anything.

 

[Nick Read]

Hmmmm, yes thats likely but then again, most people wouldnt even notice the difference between the two unless you actually see a WIlliams parked next to a valver and theres only a cm or so in it. Just I seem to recall something in a Williams marketing brochure ages ago that boasts of the slightly improved aerodynamics and specifically mentioned the larger spoiler. Ill see if I can dig it out. Even if it is true though, the fact that they used it on the 1.4 Maxim later on kind of ruins the effect LOL

 

[BenR]

i guess, but i doubt any clio has seen wind tunnel time