Aerodynamic or Brick?

[FrenchTVR]

Ok fellas, what's the drag factor (CD) of a Wedge?

Just a curiosity thing really, I've read that they aren't as aerodynamic as they look, but just how do they compare with other cars?

[FHCNICK]

i cant quantify it but i'm pretty sure they are brick like...........but who cares

[jmorgan]

My brick started bricking it at around 120

[chassis 33]

from www.carfolio.com/specifications/models/?man=6380

1980 Tasmin (FHC) 0.370
1983 350i 0.370


Which isnt bad for its day, indeed even 20years on the Audi TT coupe is 0.32. The two best production cars for drag are the Audi A2 and the Vauxhall Calibra belive it or not (atleast they were during my uni days) down around the 0.250 marker, if you look at both cars closely there are alsorts of aerodynamic enhancing features (ever wondered what the little fins on the Audi's rear light cluster did...ever notice them infact?

I would imagine drag would increase on cars like the SEAC or any with rear wing elements, sadly its a fact of life that lift (or in a cars case, downforce) is proportional to drag (except if you use a gurney strip, that give you a little extra for almost free)

If you wanted to help things with the top down, add a mesh screen like on TT's or MGF's etc etc they help pop the recirculation bubble caused by the airflow becoming turbulent as it runs off the top edge of the windscreen. I could probably did out some MIRA or SAE papers on the subject if you were that interested.

Regards
Iain

[badredwedge]

[quote=FrenchTVR]Ok fellas, what's the drag factor (CD) of a Wedge?

Thats a good question back in the 80s / 90s it was a big thing, fuel saving etc etc for shopping carts and rep prams, but when you have a V8 under the front or even a Henry V6 who gives a monkeys.

[Number 7]

chassis 33 said:

1980 Tasmin (FHC) 0.370 1983 350i 0.370 Which isnt bad for its day, indeed even 20years on the Audi TT coupe is 0.32.

This is not another case of TVR "massaging" their numbers is it?

chassis 33 said:

If you wanted to help things with the top down, add a mesh screen like on TT's or MGF's etc etc

And I thought they were just for keeping the occupants syrups in place


Whatever the figure, you probably have to add at least 25% for roof down motoring anyway.

7.

[grahamw48]

How about a net curtain, or would that flap about too much ?

Examples of the benefits of good aerodynamics:

Cortina Mk4/5 (2litre) Top speed = 105 mph.

Sierra Mk1 (same engine/gearing) Top speed = 115 mph.

Lotus Seven Twincam. Top speed = 110 mph.

Lotus Elan Sprint(same engine/gearing) Top speed= 120mph

[chassis 33]

Number 7 said:

This is not another case of TVR "massaging" their numbers is it?

I doubt they are manufacturers figures, because i doubt that until relatively recently TVR had the cash to put a car in a wind tunnel to test it.

Regards
Iain

[350zwelgje]

The cd factor is just a small parameter in the total equasion to calculate drag. Very important is the maximum cross-section of the car in the direction that it is moving, because this has to be taken to the power of 2! And to give an example, formula 1 cars have something like cd 0,5 (mainly due to the tires). But a very small maximum cross-section, thus less drag compared to a 'slippery' A2. It is always the same when marketing grabs some information to sell cars, the essence is often gone (but it sells).
Forgot what the whole formula was, as it is too long ago I took an exam in airodynamics. So you can add another old wedger to the pack.

Rob

[chassis 33]

350zwelgje said:

The cd factor is just a small parameter in the total equasion to calculate drag. Very important is the maximum cross-section of the car in the direction that it is moving, because this has to be taken to the power of 2!

Yes, if you plot cd versus yaw angle the graph can almost asymptoe when the airflow is blowing side on. Remember hte early sierras, loverly and streamlined with a pure head wind, but an ant farting at the side of the road would send the car across to the other carrigeway. As you mention the cd figure is meaningless interms of drag without knowing the frontal area, a better but by no means complete estimate of the effect of drag is found by multipling the frontal area by Cd figure, but Jo Public has it in its head that Cd is all that matters.

350zwelgje said:

And to give an example, formula 1 cars have something like cd 0,5 (mainly due to the tires). But a very small maximum cross-section, thus less drag compared to a 'slippery' A2.

Tyres account for approx half of the drag and lift (yes lift, the wings fight against it)generated by an open wheeled Formula car, the next biggest is suprisingly the cooling system.

The difference between an F1 car and an Audi A2 is the F1 car generates a fair amount of downforce from the drag it creates; the A2 still produces lift, as per virtually every road car.
eg 1994 BMW M3 lift coef. at front axle 0.214, at rear axle 0.126, even in race trim with spoilers etc the M3 still produced lift 0.099 and 0.006 respectively


A good book on vehicle aerodynamics is "Road Vehicle Aerodynamic Design" by R.H. Barnard, published by Longman in 1996. Also if anyone is REALLY interested i have an SAE paper on the stability of Peter Dumbreck's Merc CLR from Le Mans 1999 (the one that flipped and sumersaulted).

Still awake down at the pottom of this post???

Regards
Iain

[GreenV8S]

CdA is a far more useful figure to work with, and if you're doing any sort of experiments it's what you will naturally end up with.

[Gerry Attrick]

chassis 33 said:

(except if you use a gurney strip, that give you a little extra for almost free)

What's a gurney strip? Is it like a Fowler Flap or is it a piece of sticking plaster to hold your face into funny shapes?

[350matt]

I have to say I think the Wedge might produce a bit of downforce as well, as mine definitly seems more stable at high speed (3 figures obviously on a private test track hem hem) than it does at 60 / 70Mph

Amusingly if you raise the headlamps the wind noise reduces, don't know what you can read into that aerodynamically

Matt

[jmorgan]

350matt said:

Amusingly if you raise the headlamps the wind noise reduces, don't know what you can read into that aerodynamically Matt

Not just me then.

[wedg1e]

No, I'd noticed that as well!

[350zwelgje]

350matt said:

I have to say I think the Wedge might produce a bit of downforce as well, as mine definitly seems more stable at high speed (3 figures obviously on a private test track hem hem) than it does at 60 / 70Mph Matt

Do you have a spoiler perhaps? As mine ('87 350i) feels stable at all speeds (without a spoiler), and it is not more stable at high speeds. To add: Live 'close' to Germany in Belgium, so legal to test on the road.....). Even at 200 km/hour open is no issue (the looks you get are priceless). Cam shaft is on it's way out, so think it is difficult to reach that speed now (another job added to the to-done-list).



Rob

[350matt]

No spoiler or other body addenda, mabywe something in my suspension set-up perhaps. I still maintain it tracks truer and feels 'more' stable at 3 figure than 2 figure speeds.
Its not 'unstable' at any speed mabye its time to borrow the works logger and set up a couple of linear pots on the suspension

Matt

[GreenV8S]

Did this on the V8S and found there was a small but measurable lift (roughly proprtional V squared as you might expect). I would be surprised if a wedge was *very* different, but the windscreen is at a lower angle which would reduce drag and lift, and if you managed to get a significant nose-down rake I expect you would be able to reduce the lift to a negligeable amount.

The amount of lift we're talking about is very small in absolute figures, but I think it is the front/rear difference which matters more than the amount. It would only take a few % difference front/rear to affect the stability. If you manage to get more lift at the front than the rear then in theory it would become more stable (in yaw) the faster you go, right up to the point at 300 mph or so where it flipped end over end ...