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Old 07-24-2015, 02:50 PM   #1 (permalink)
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Making a more efficient, aerodynamic soapbox gravity race car

Hello,
My name is Joan, i'm from barcelona, Spain.
My project is to improve the aerodynamics of my soapbox, gravity racer or carretons (which it is the name in Catalan), As you may know , these vehicles are powered only by gravity, and one of the factors which limit its speed is the aerodynamic drag, and many of the ideas explained in this forum apply to my project.

so my goals are:
-determine what the actual cd is
-design and improved streamlined body
-build the new aero body
-determine the new cd
-calculate the improvement, and test how effective is the improvement

the actual soap box:


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Old 07-24-2015, 04:14 PM   #2 (permalink)
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This looks like you are intending to travel at high speeds for long distances, rather than a typical soap box derby. How far is your race? About how fast do you go?

I actually just got done building a soap box derby car for my 6 year old son (and he got first place in his class!), so I am very interested in this topic. However, I'm not sure how you would calculate the Cd of a tubular chassis like that with any degree of accuracy. I would guess empirical testing would be required.
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Old 07-24-2015, 04:43 PM   #3 (permalink)
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Hi Darcane!
Well, in spain we do the soapbox racing in public streets ( closed during the event) and the track is pretty long, watch this video:
---https://youtu.be/nTqPZygdRRU---
My soapbox is unfinished yet, the max speed that i reached is about 60 km/h ( about 40 mph), but the top level ones reach 95 kmh (near 62 mph).

i'm not sure too about how to calculate the cd, it exists some empirical method?
some advice?

some drawings of the body shape:

Last edited by orengomotors; 02-03-2017 at 02:23 PM..
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Old 07-24-2015, 06:26 PM   #4 (permalink)
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Cd

Quote:
Originally Posted by orengomotors View Post
Hi Darcane!
Well, in spain we do the soapbox racing in public streets ( closed during the event) and the track is pretty long, watch this video:
---https://youtu.be/nTqPZygdRRU---
My soapbox is unfinished yet, the max speed that i reached is about 60 km/h ( about 40 mph), but the top level ones reach 95 kmh (near 62 mph).

i'm not sure too about how to calculate the cd, it exists some empirical method?
some advice?

some drawings of the body shape:
The lowest drag bodies are fully-enclosed,including the wheels.Even with a larger frontal area they have low overall drag.
Here is a Honda Dream solar racer of the 1990s with Cd 0.10.

For your race car,for safety's sake,and handling,you may need to make some compromises to aerodynamic drag.These really low drag shapes aren't intended for speeding around curves.
This template embodies low drag architecture




Seems like you'll want the car as low to the ground as you can get it,to keep the center of gravity low.
Shrink-wrapping the body around you and the chassis will reduce frontal area which is good drag-wise.
If you take the wheels outside the body the drag might degrade to Cd 0.15 or so.A lot depends on the width of your tires.And your tires may be the only thing that keeps you on the track without torque-vectoring of a powered car.
The wheels can protrude into blisters if your concerned about frontal area
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Old 07-24-2015, 07:45 PM   #5 (permalink)
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Thanks for the video. Very cool.

My son raced on a public street as well, but the distance was only 100 yards or so (~100m). As he gets older, he can do the longer course, but it's still not very far or fast. Peak speeds for the faster kids is only about 28mph (45km/h).

For testing, I was envisioning actually taking it to a hill and timing how long it takes to travel between two specific points. Then you will have a quantifiable measurement for any change you make (like a new aerodynamic body). Basically, this would be a variation of the coast down tests that a lot of people here use.

Coast down test methods and determining cD?
How do you perform a coast down test
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Old 07-24-2015, 09:31 PM   #6 (permalink)
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Orengo, welcome to the forums! You will find some very knowledgeable and helpful people on here, but I do not know if anyone is more of either than Aerohead; he is extemely respected around here, so his advice is top-notch!
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Old 07-25-2015, 11:52 AM   #7 (permalink)
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Take a side and front view picture of you sitting in it and we can photoshop fairing ideas for you.
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Old 07-25-2015, 05:03 PM   #8 (permalink)
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hi Aerohead,
I think I can not adapt the soapbox to that solar car design, the shape is a lot different.

what do you think about this:


thank you!
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Old 07-25-2015, 05:08 PM   #9 (permalink)
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Quote:
Originally Posted by sheepdog 44 View Post
Take a side and front view picture of you sitting in it and we can photoshop fairing ideas for you.
here you go:




I hope you find them useful, thank you sheepdog!
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Old 07-27-2015, 08:56 AM   #10 (permalink)
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my driving position is a bit upright:




I can change the seat to be more horitzontal, and so minimize the frontal area and drag. what you think?

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