Rear Wing For a Hatchback

[VT-R]

I originally bought and read Competition Car Aerodynamics: A Practical Handbook by Simon McBeath with the intention of getting a few pointers on undercar aerodynamics, but I'm now reading it for the third time and I can't help, but think i'd be doing myself a great diservice by not fitting a rear wing.

The main problem I can see is that the regs (see below) state that the wing most not protrude above the roof line of the car, and it can only be half the vertical height of the original rear window. So I was thinking of something similar to the item pictured below, but I'm unsure how effective it will be mounted so close to the trailing edge of the roof.

Would this be worth the effort, or am I more likely to be laughed out of the paddock with my homebrew aerodynamic efforts?


Cheers

Ed






"Rear spoilers are permitted within the overall plan periphery of the engine cover/luggage compartment lid. ‘A spoiler is a raised surface of opaque material integral with the rear deck with no gaps or openings in the surface, front or rear, and not exceeding in height half the vertical height of the original rear window fitted to the vehicle, measured at the centre of the original window’."

"Rear aerofoils, maximum length 20% of wheelbase. Maximum width must not exceed the width of the car measured across the rear wheel arches. Overhang must not extend beyond the original bodywork by more than 40cm. Height must not exceed the maximum height of the roof. For measurement purposes all measurements relating shall include wing end plates."

[tristancliffe]

If you can afford to waste a bit of money, then I'd say try it.

There is no exact right or wrong answer, because it depends not only on the aerodynamics of YOUR car on a particular day, but also whether you want downforce, or need straight line speed, and where you tend to lose out to your competitors. If the car is understeering then you don't want a rear wing on it's own.

Buy a wing you think is right. Put it on, and try it. Then take it off and try it (do it in this order, because you'll get better at the track on the day). If you are quicker the second time around, refit it, and see if you are slower. If you are still quicker then YOU are the best area of improvement, and what you do to the car isn't that important. Yet.

I've been there - fitted gizmo xzy to car, only to find that each session I go a bit quicker regardless of it's installation.

[SidewaysFeltham]

If you are prepared to experiment, a cheap way to work out the two most critical dynamics is to find a nice guy with a rolling road and a portable high volume high speed cooling fan.

Arrange experimental fixings so you can move the wing vertically and also laterally to increase the distance from the rear screen.

Then fix vortex indicators (simply three inch bits of thin string: white parcel string is ideal) secired by a tiny strip of masking tape all over the rear of the roof and the top surface of the wing and the top 50% of the rear screen.

Stand the fan right up so its main airflow gallops over the roof.

Undersirable eddies and vortices will be recognised by the string twirling, instead of standing straight back in sympathy with the primary airflow.

Also try this before you fit the wing, to assess what vortices exist already.

This was a tip given to me by a scientist from RAE Farnborough whom I met at Silverstone, when I was experimenting with the FII wings.

Since I already had a fan as part of my rolling road equipment my life was easy in this regard.

Most of these cooling fans provide serious volume of air moving at circa 70 MPH from memory.

The other way is to inject smoke just after the fan as this will also illustrate eddies.

If you can find any pics of the BMW works 3000 CSL European Touring Car Championship winning cars from the early 70s onwards, these will show how many little ally deflectors and spoilers the BMW engineers used to try and solve the airflow problems.

It aint simple and it aint logical!

The core danger is that you make your car worse and even dangerous: as well as slower.

Both the Capri 3 litre and the E Type Jag created an air cushion beneath the front at speeds of 90 mph +: side winds on motorways, e.g. were exciting, to say the least.

John Whitmore gave a lecture back in the mid 60s which I attended where he recounted how he and Jack Sears were road testing an early protoype GT40: and it took off and turned turtle over a humpback bridge!

Back to the drawing board.

[Locost47]

"Rear aerofoils, maximum length 20% of wheelbase." Flippin' heck, that's huge for that type of car!

You're right that the restrictions on where you can locate the wing very strongly limit the benefits that can be gained from it. Having the wing so close to the tailgate and not protruding above it means the drag penalty from generating any meaningful level of downforce is huge - proportionally much higher than for a saloon or coupe body shape.

With front wheel drive though the normal preference is for maximum downforce on the front and then almost a token gesture on the rear. I've worked on several Touring Cars with 75-85% of their downforce on the front, and at least two where there was only close to zero lift at the rear, rather than any meaningful downforce. This is compared to GTs and single seaters where they normally aim to match the weight distribution, so typically somewhere between 35-50% front balance. The reason for this is that cornering speeds are normally lower for the type of racer which will have front wheel drive, so the influence of the aero is proportionally less and a bigger bias is less of an issue.

For your car i'd think a spoiler would be more efficient than a wing, and focus on maximising the front-end bite instead. If you're not exactly power limited, however, then you'll get maximum effect from that wing in the picture if you can mount it as high up and as far backwards as possible within the regulations, to try and separate it from the car's own flowfield as much as you can (it being the interference between the two which causes the additional drag).

[VT-R]

Originally Posted by Locost47

"Rear aerofoils, maximum length 20% of wheelbase." Flippin' heck, that's huge for that type of car!

Exactly what I thought. For my car that would be something ironing board size!

Sorry should of said the car is RWD

Thanks for the great replies though guys!

I think you've confirmed what I thought, the position of the wing isn't ideal and moving it further aft would perhaps be better and will reduce the interference from the trailing edge of the roof. As with most things though I think its going to be a case of just trying things out and seeing what works, if nothing else I'm sure there'll be some degree of
psychological advantage

Can anyone suggest any good suppliers of off the shelf wings?

[SidewaysFeltham]

BAE Systems?

[SidewaysFeltham]

Here is an excellent pic which shows just how far back the read aerofoil is mounted: how they needed, despite this, to mount an extra flow spoiler on the roof; and how apparently negative to good aerodynamics and CD the front air dam is.

The BMW engineers spent lots of time working on these aspects, at the time.

And that's now 35 years ago!

http://www.racingsportscars.com/etcc...-07-22-011.jpg

[Locost47]

Simon McBeath sells wings of different shapes & sizes, as well as textbooks. His webpage is here:

http://www.sm-designs.ndo.co.uk/wingshophome.html

Simon's a great guy with an incredible knowledge of the practical implementation of aero on track cars. I'm sure he'll have something suitable.

Sorry i mistook the car to be a front-driver. From the rear track in the pic i should really have guessed that it wasn't!

[Notso Swift]

Great thread, thanks SidewaysFeltham and Locost
Love the home wind tunnel dyno fan thing!

[Nero]

Go and buy Joseph Katz's book Racecar Aerodynamics...it is twice the book and will give you a much better idea of whole vehicle aerodynamics.
...do you have enough power to afford the added drag?