Aerodynamics - at what speed?

[alolympic]

On a road car, driving at speeds under 100mph, do aerodynamics play a significant role in lift/downforce?

It seems a flippant question, but I am currently finding myself reading Simon McBeaths 'Competition car aerodynamics' book and wondering how relevant it is to me.
So, is it worth starting to look into the aerodynamics of my car in an attempt to reduce lift, if I am never likely to be driving at high speeds?
Is it still worthwhile in the context of fast road driving?
The car in question is a 60's design, low drag but with no effort to reduce or control lift and a reputation for being unpredictable in certain circumstances.

[darcia2]

Yes Simons books are quite informative...i read simons articals in racecar enginerring and have corresponded at length with him regarding my 98 Dallara. re aero it all depends on your car..and what your going to do with it...

[JohnD]

McBeath gives a descriptive account of aero, very readable and useful, but if you can stand a few equations and charts, try Prof. Katz's "Race Car Aerodynamics" (Pub. Bentley)
Drag, for instance, is proportional to the square of velocity, so it's significant at any speed.
John

[phoenix]

Originally Posted by alolympic

On a road car, driving at speeds under 100mph, do aerodynamics play a significant role in lift/downforce?

The answer is yes. Your car will have drag and as it is an oldish design, likely a fair amount of lift at sub 100mph speeds.

Because areo forces are affected increase with the square of the speed, you will find any aero effects at 50mph are four times as great at 100mph - drag, lift - whatever.

I have driven (older) road cars that develop vague steering due to front end lift at motorway speeds, particularly noticeable into a headwind or with a crosswind. Because of the way the maths works, that means at 100mph such effects would be roughly double in magnitude compared with 70mph.

(100/70)^2 = 2.0409

You only have to stick your hand out of the window at 50mph to detect the drag and lift potential at that speed, and at 100mph it's those forces times four.

It's all a question of magnitude. Racing cyclists and certainly downhill skiers and bobsleighers pay attention to aero dynamics to minimise drag, and powered hang gliders - even light aircraft - can get into the air at quite modest speeds. The less power you have to throw away, the more important it is to reduce drag. Lift at either end of the car can cause instability, which gives the driver less control and is likely to make him drive slower than he otherwise might, so minimising lift is a good idea even if you can't add any actual downforce.

An airdam at the front of the car, if allowed, will reduce front end lift. If not, lowering the front of the car more than the rear, to give the chassis a nse-down rake, can also improve the situation. At the rear some kind of spoiler on the upper rear bodywork is required.

[forestdweller]

We've found that the presence (or absence) of a 5 ft^2 wing will significantly affect the balance of a 2100 lb. automobile at autocross speeds (< 60 mph).

[alolympic]

Thanks Guys.
The more I have read, including some interesting articles on autospeed.com, the more I realise this is important, even off the track.
A low drag design like mine has seemingly got real potential for high levels of lift so it would seem lazy to ignore. Unpredictable factors such as cross winds make it even more important.
Of course, most cars today have millions spent on testing in this area, but old cars like mine are much less a product of scientific thinking.
Just reading into the subject, it becomes fascinating, albeit quite confusing with so many variables at play but I real do quite enjoy the subject.
It seems that some simple practical tests, and some consultation with McBeath himself will be well worthwhile.
The old fashioned wool tuft test seems to be a cheap lo-fi method that will reveal some reliable results akin to those found in a wind tunnel.
I am also interested in using a magnehelic gauge to discover more about the characteristics of the car from a cooling perspective. Where should I position a NCAC duct for the intake, would bonnet vents benefit etc.
Even an idiot like me can get excited by engineering!

[Woolley]

I vaguely remember Patrick Head (I think) saying drag starts to have an effect at 10mph, downforce at 40mph and at 100mph an F1 car could comfortably drive along the roof of the Monaco tunnel without falling off.

[alolympic]

I become more and more convinced by the importance of aerodynamics, for limiting lift, then I see cars like this -
The Eagle E Type Speedster







A high performance car with very positive write ups from motoring journalists, including Evo magazine - with no mention of handling or stability issues.
A thing of beauty but surely, the shape of the car would make it prone to at least substantial front end lift?
The frontal area is high, letting lots of air pass under the car. In fact, the curve to that nose encourages a lot of air under the car. How would it not suffer front end lift?
The sides and sills are massively curved, surely making it susceptible to more air travelling below the car from the sides too.
The rear of the car is heavily curved which would no way encourage the flicking of the air off the top of the rear?
There are no aero aids at all.
It confuses me. If that shape works, why did the Audi TT notoriously need a rear spoiler to be stable?

[neiltb]

it looks like it has quite the wheelbase (maybe wider too) this will help stability. The shape (beautiful as it is) isn't likely as refined as the audi and creates a lot of turbulence to the rear of the windshield. The front does look like it should get light, maybe it has a huge great heavy lump under the bonnet.

[Woolley]

Audi TT is basically shaped like an upside-down aeroplane wing, so I imagine a spoiler is required simply to stall the air that would otherwise be creating low pressure.

Having air under the car doesn't automatically induce or prevent lift, it's the difference in pressure between the air passing over and under that does that. The E-type is pretty much a teardrop shape which is reasonably efficient in a straightline and if the air is passing over and under at the same speed then it won't generate lift.

[pickstock racing]

im building a body kit for my road/track car to direct the air where i want it more than produce lift or downforce. incorperating ducts to the brakes, front and rear, fins for the oil cooler, and fins for getting air over the carby.

[phoenix]

Adrian Newey's e-type race with the boot lid slightly open

http://www.google.co.uk/imgres?imgur...1t:429,r:3,s:0

I wonder why that is? heh heh!

And always with the hard top on, bye the way...

[Woolley]

At a guess, hard top makes for less drag but increases lift, boot lid open stalls air again to remove said lift. Result neutral downforce better straight line speed.

[alolympic]

Planning on some wool tuft testing for my car but not sure it will actually provide me with the info I require.
From what I understand, wool tuft testing will identify areas on the car that create turbulent air, and increase drag. What it can't do though is inform about lift can it?
I presume that is actually more complex to check, involving measuring ride height differences in different driving conditions? or, if I had the budget, stick the car in a wind tunnel on some corner weights.
Any other lo fi methods without driving at 140mph to see if I take off?

[greenamex2]

My kit car has lots of downforce, mostly courtesy of Mr McBeath (incredible helpful chap).

Owing to a design fault in the steering it is a little 'sticky' in places. Before adding the downforce it wasn't an issue as the car was lifting and it got better. The car would also float around a bit over 100MPH.

After the downforce you can't turn the steering wheel past a certain point once doing more than 80-90MPH. (Really must fix the steering). Unfortunately with the engine that was in the car it wouldn't go much above 110MPH with the wings but it felt totally planted (from 130MPH before, bigger engine being installed, MUCH bigger engine!).

On a very wet slippery Silverstone with the wing set flat the car spun at really low speed (50MPHish, nearly got thrown out the track day!). With the wing set properly it would take full throttle and totally destroy 4WD super saloons out of the corner, and at not at that much faster an initial corner exit speed.

[alolympic]

Resurrecting this thread as I am now in a position to do something about this. I am going to do some wool tuft testing, and also use a Magnehelic gauge, to try and measure pressure differential above and below the bonnet, to get any venting in the right place.

I have another question to help me purchase the correct sort of Magnehelic gauge though.
I will be using it with 2 tubes, 1 on the top of the bonnet, and 1 under, and then drive at different speeds.
Should I buy a gauge that has Zero in the centre? i.e. that will tell me if I have negative or positive pressure difference?
If I buy one that is just Zero to 3 inches on the other hand, I could end up having to swap the tubes around until I can measure a positive difference?

Hope you can help - most knowledgeable place for this sort of thing!

[JohnD]

"Magnahelic"! Never heard of them, so thnaks for the idea!

I'd say centre zero definitely. More versatile, you might want to measure positive as well as negative pressure.
JOhn

[Dave Brand]

Originally Posted by phoenix

Adrian Newey's e-type race with the boot lid slightly open

I wonder why that is? heh heh!

Most likely rear brake cooling - the lightweights had a brake cooling air outlet built into the boot lid.