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Old 05-12-2009, 08:15 PM   #1
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Questions?? How to measure Bodyshell downforce ?


Well the tittle says it all .

I've got a Protoform Stratus 3.1 bodyshell and wondered how to measure or at least give me an idea of the downforce produced at speed .

I've tried something but wanted to know if there is another way to measure it.

I've got an air gun (the workshop kind) connected to a 8 bar compressor . I've guessed , with my biker experience by blowing the gun at my face (with my arm hodling the gun stretched ) that the air is around 80-90 km/h (equivalent helmet open while riding feeling ) .

I've got an electronic 3kg max scale and basicaly put the car on it and blew wide open on the body . Tried several position (always parallel to the ground) and basicaly got a 55 gram peak (even tried to put the air gun on the roof to shoot the rear spoiler but no luck)
Tried to measure the front downforce by putting the front end on scale and the rear end on a block and vice versa .
I got 15grams front and 45grams rear ...

So I tried the Lola 1/8 pan over my motonica p8.0r and got a much better result . A full 70 grams ... double

How could I improve the results of the measurements ? or is that it ?

A lot of people do say to feel a big diference between bodyshell so I was wondering that it should be bigger figures than this (If the downforce is 50grammes the handling aerodinamics diference should be rediculus , especialy knowing that you don't corner at 60 mph)

My first thought was that there was too much turbulences from the gun

But my putting my hands over the shell it is quite straight and directive .

Any thoughts are welcome

Ps: again pardon my poor english I'm french.
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Old 05-12-2009, 10:15 PM   #2
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Your English is just fine.

Not sure how to improve the air distribution from the gun without seeing it. How about posting a picture of the air gun. It will help others better understand what you have and how to improve it, if possible. As for measuring the down force, I believe that your results would be much more acurate if you were to use four independent scales ( one under each tire). I am not an engineer but I am trying to apply what I have read about the methods used on full scale race cars. That is my two cents worth of knowledge.
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Old 05-12-2009, 10:24 PM   #3
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Try using a leaf blower thru a box of straws that have been glued together.

That should give you a better volume of air, and the straws will act as an air straightener.

Should be able to build a small air tunnel out of it.

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Old 05-13-2009, 12:28 AM   #4
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Build up a real windtunnel using a radiator fan ?
The quality of an answer comes with the quality of the question.
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Old 05-13-2009, 01:13 AM   #5
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hold the car out the window of a car travelling at the top speed of your car??
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Old 05-13-2009, 08:04 AM   #6
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Originally Posted by Renault_Tech View Post
hold the car out the window of a car travelling at the top speed of your car??

I like the Australian approach!
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Old 05-13-2009, 08:51 AM   #7
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Lol thank you very much for the replies

I might try to make a proper tunnel , lot of work though .
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Old 05-14-2009, 06:59 PM   #8
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Rich, some years ago, we had a member who got his wife to drive the family car while he hung out the window with his pan car. He had mounted the rear wing on a pivot with a off road shock and tried different springs so that the wing would lie flat at 50mph +.
Yeah, works for me.

This is the same guy who hand built a radiator and configured a Perry pump carb adapted to run the coolant and then rand 5-6-% boat fuel through it.
I actually drove the car and found it never got any serious temp built into it.

All of this in 1/8 scale.
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