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PostPosted: September 18, 2008, 10:08 am 
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Driven5 wrote:
Yarn has absolutely nothing to do with pressure. It only tells you whether the airflow is turbulent or not. Smooth airflow can still be either high or low pressure. You need some sort of pressure gauge to take readings. The most cost effective means I know of would be to build yourself a water-manometer, although I imagine that would be somewhat difficult (but not impossible) to setup for this type of experiment.

Don't you think you can infer that areas of turbulence are also likely areas of low pressure? This would certainly be cheaper and easier than manometers and probably close enough for govt work.


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PostPosted: September 18, 2008, 10:38 am 
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I'm serious about the water manometers. They're very easy to make and use. Below is a picture of one that I bought to sync motorcycle carbs, it used Mercury, but water would be more sensitive and easier for this.

A few pieces of plastic tube mounted on a small piece of wood with a water reservoir at the bottom. Then you run small flexible tubing from the top of the sensing columns to where you want to take readings. The flexible tubing can be run and held in place with duct tape or masking tape. Use a little digital camera to take a picture, some dye to make the fluid more visible.

It sounds like fun to me. Real science on a $20 budget. The simplest setup described would give pressure readings relative to the cockpit, or wherever the fluid reservoir is located, but that should be ok to start, relative readings are still useful.

If you wanted more calibration you could mount a pitot tube on your rollbar, have the only seven in your neighborhood with an airspeed indicator.


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PostPosted: September 18, 2008, 10:52 am 
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No one mentioned the Caterham 2R, which they made a couple years back to compete with in the Shell Eco Marathon. The car got 131mpg.

Image
http://www.worldcarfans.com/2060727.006 ... eve-131mpg

The biggest difference in that car probably comes from the attention to detail on the mechanical side, the tires being the most visible change. The competition rules state a 15mph minimum, so aero isn't everything at those speeds.

If someone has seen any other pictures of this car, please let us know. I'd be interested in seeing some of the details a bit closer...

EDIT: Seems like Axon, who did the eco marathon car, also offers a road version of the marathon car. http://www.axonautomotive.com/ecom.html


Last edited by hege on September 18, 2008, 11:08 am, edited 1 time in total.

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PostPosted: September 18, 2008, 11:07 am 
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Could something like the piece below be used as the cornerstone for some aerodynamic locost bodywork? :P It would help get the front wheels out of the air.


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PostPosted: September 18, 2008, 11:35 am 
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rongaudier wrote:
I think the easiest thing to do is tape tufts of yarn all over the body, and then drive around and observe what the yarn does. You would have to do this on a calm wind day, of course. I think generally the low pressure areas will have the yarn standing up or twirling whereas the high pressure areas the yarn will tend to lay flat.
Under the car? :shock:

Assphalt Kicker wrote:
so how do we test for high pressure and low pressure zones on a car?

i under stand the theory very well, just don't know how to test it in the real world term

A Google search will reveal all.

horizenjob: Drat! You beat me to it. But here's pictures of mine anyway. :cry:

Here's a few pictures of my compact water manometer. It would be very easy to make one out of some flexible clear plastic tubing and a ruler. At the pressures we are looking for you shouldn't need a very long water column.

Here's a description of one etc. I've usually heard the measurements stated in "inches of water".

http://www.engineeringtoolbox.com/u-tub ... d_611.html

To make measurements on your car you would attach a long flexible hose to one of the vertical tubes, attach the free end of the hose to a stick and while driving at some steady speed have a passenger move the end of the hose around the body work to find the high/low pressure areas.

Be careful when measuring under the car because if the hose etc gets under a tire the whole shebang could get dragged out of the car (including appendages of the passenger)! :shock:


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PostPosted: September 18, 2008, 11:39 am 
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I think once you've gone away from the open wheels in front, your no longer a seven-type car. So I focus on reducing frontal area, cleaning up the rear and underbody and then take the hit on the open wheels...

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PostPosted: September 18, 2008, 11:41 am 
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Why didn't I think of using a ruler for a stick....

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PostPosted: September 18, 2008, 11:45 am 
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horizenjob wrote:
[snip]
If you wanted more calibration you could mount a pitot tube on your rollbar, have the only seven in your neighborhood with an airspeed indicator.

I beat you by decades. :D

My 1940 Ford would bury the speedometer at 100 MPH so I got an airspeed indicator and mounted the pitot sticking out of the grill. I found out the car would go 120 MPH! Turned out the tach wasn't lying to me after all.

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PostPosted: September 18, 2008, 12:40 pm 
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Horizenjob;
Well, I'll be I had one of those that I got for MC carbs too. Got it for $3 at a grarage sall, bu then in our "Move to the country" 4 yrs ago it was broken--Peeved me to no end!!


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PostPosted: September 18, 2008, 12:56 pm 
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If the goal is to keep the traditional lotus 7 styling while maximizing aero I'd look hard at what Caterham has done with the CSR. The front fenders with the rear airfoil bump and the angled front trimlines are both going to help. The fenderw wrap further around the front than others and then are trimmed at an angle, longer on the inside and shorter on the outside. IIRC they say this helps move air around the side of the fender and wheel instead of under it causing lift.

Another measure they took to help out with aero isn't as obvious. They used smaller (shorter) front tires. The tires up front on a CSR are only 22" tall, at least an inch shorter than tires commonly fitted here in the US. I'm not sure what the "winglets" do but there is one of those on each side of the nosecone.


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PostPosted: September 18, 2008, 1:31 pm 
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This might be applicable to the rear of a Locost. It might not work properly on non-tapered rear body work since the air wouldn't be flowing towards a point behind the car as it left the body work.

http://en.wikipedia.org/wiki/Wunibald_Kamm

The Caterham 2R seems to utilizing the affect on the rear fenders and maybe the body also.

Partial quote for article;
"German Professor, Wunibald Kamm worked with aerodynamics engineer Baron Reinhard von Koenig-Fachsenfeld. They developed a design with a smooth roofline and a taper in the automobile's body that is suddenly chopped off at the rear end. This design makes the air flow act as if a full tapered "tail" was present on the vehicle."

And here's an application of the Kamm affect on a Honda CRX.

http://rides.webshots.com/photo/1173247 ... 0541QTlgqq

My CRX-HF also has that tail. Unfortunately in all the years I've been driving, I've never seen another CRX in the rain to see what the air flow pattern looks like behind the car.

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PostPosted: September 18, 2008, 2:42 pm 
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rongaudier wrote:
Don't you think you can infer that areas of turbulence are also likely areas of low pressure? This would certainly be cheaper and easier than manometers and probably close enough for govt work.
Ok, so tufts of yarn on the body tell you that a local pressure differential is causing flow separation (turbulence) in certain areas...Can you explain to me what that means for you as the vehicle designer/engineer, and what you can do with that information?...Other than the fact that it's a high drag area and reattaching the flow would reduce the overall drag on the car.



Chet...Is that vette nose yours?

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PostPosted: September 18, 2008, 4:00 pm 
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The CSR looks like they put some extrra effort into closing up the holes around the front wishbones and a vent for the radiator right after the nose.

I was thinking of putting a support bracket on the top of the front fender where the lump is on the CSR fender.

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PostPosted: September 18, 2008, 4:02 pm 
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Past efforts at reducing undercar pressure:

Sucker Car Brought Ground Effects To Auto Racing

SUCKING UP: Jackie Stewart pilots the Chaparral 2J around Watkins Glen Int’l in July 1970. (Al Robinson Photo)
By Al Robinson

NSSN Correspondent

Air is free. Horsepower costs money.
That phrase was coined in the 1960s when race-car design was much less restricted than it is today, and when race-car aerodynamics were not well understood.
The greatest innovator in sports-car racing was Jim Hall, who built his Chaparrals in Midland, Texas, with an open line to General Motors in Detroit. His 1966 CanAm series entry, the Chaparral 2E, featured the first adjustable, suspension-mounted wing. Similar wings became universal in CanAm and Formula One before being banned in 1969 after several F-1 wings collapsed at speed.
Just about the time the high wings went away, GM engineers Don Cox and Don Gates turned their attention from the air above the car to the air under it. Gates went to work for Hall in 1969 and brought the basic hardware with him that would be developed into the Chaparral 2J, the Sucker Car, which introduced ground effects to race car design.
Its racing record was unimpressive. It appeared for only four CanAm races during the 1970 season and never led a lap. But it won three pole positions by almost ridiculous margins and generated world-class levels of both interest and controversy.
The 2J wasn’t handsome. It was plain white, blunt-nosed and slab-sided. Its secret was in the rear-engine bay, which contained not only a big-block aluminum Chevy, but a modified Rockwell JLO snowmobile engine driving two exhaust fans by belts. The underside was sealed by sliding Lexan skirts, so the exhaust fans literally sucked the air out from under the car, creating far more downforce than a wing, at any speed, and without the drag penalty.
After a spring full of rumors, the car appeared at the July 1970 Watkins Glen CanAm, with world champion Jackie Stewart driving on a one-race deal. Stewart qualified third and ran there for 15 laps, but the track was breaking up on several corners and the debris sucked up damaged the fan motor and the drive belts.
The Sucker Car made three more appearances, each with Vic Elford driving. It won all three poles, each by a margin of one full second or more, but suffered fan motor trouble at Road Atlanta and Riverside and blew its main engine in race morning warm-up at Laguna Seca.
The exhaust fans were ruled to be illegal “movable aerodynamic devices” and the Chaparral 2J was never to race again.
Almost a decade later, that other great innovator, Colin Chapman, achieved ground effects legally and more simply by using underbody venturi tunnels on the Lotus 78 driven by Mario Andretti. One of the first to adopt Chapman’s system was Jim Hall on his Chaparral 2K Indy car — the last of the Chaparrals, which Johnny Rutherford drove to victory in the 1980 Indianapolis 500.


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PostPosted: September 18, 2008, 4:24 pm 
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Driven5 wrote:
<snip>


Chet...Is that vette nose yours?


No, but if it was you can bet I'd be test fitting it on my Locost. :P

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