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:

https://lh3.googleusercontent.com/7G...w2034-h1378-no

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.

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:
https://lh3.googleusercontent.com/-L...5cc9b82351.jpg

Cd
 

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.
http://i1271.photobucket.com/albums/...d2/Solar-1.jpg
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
http://i1271.photobucket.com/albums/...ntitled6-2.jpg
http://i1271.photobucket.com/albums/...d2/Front-1.jpg
http://i1271.photobucket.com/albums/...ad2/Bottom.jpg
http://i1271.photobucket.com/albums/...ohead2/602.jpg
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
http://i1271.photobucket.com/albums/...3-29BampW9.jpg

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.

http://ecomodder.com/forum/showthrea...g-cd-8402.html
http://ecomodder.com/forum/showthrea...test-2992.html

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!

Take a side and front view picture of you sitting in it and we can photoshop fairing ideas for you.

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:
https://lh3.googleusercontent.com/-x...500-Ic42/1.jpg

thank you!

here you go:
https://lh3.googleusercontent.com/-Z...2/IMG_6577.JPG

https://lh3.googleusercontent.com/-Q...2/IMG_6800.JPG

I hope you find them useful, thank you sheepdog!

my driving position is a bit upright:

https://lh3.googleusercontent.com/-A...75096372_n.jpg
https://lh3.googleusercontent.com/-Q...92053909_n.jpg

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

Are there any rules or regulations on the specifications of the vehicles? The lower angle you can sit at will provide less frontal area but it may be a compromise to steering and handling. Look at how the F-16 pilot is positioned in the cockpit. I think the seat is set around 30°. Kinda like recumbent bicycles. ;)

http://www.atomiczombie.com/plans/ma...wracer%208.jpg

hi BamZipPow,
Yes there are, want to see them? any section in particular? just tell me and i will transate the rulebook section and post it here.
i will copy the design of the recumbent bike, so i will improve the frontal area,drag and lower the center of gravity!
thankyou

I believe that is exactly what you need to do. Streamlining behind the driver will have a significant impact on drag. The lower the driver is, the easier it will be.

Are there any rules that you are required to follow that will affect what you can do with the body?

well, the overall dimensions:

https://lh3.googleusercontent.com/-X...707-Ic42/1.jpg
https://lh3.googleusercontent.com/-Y...20-Ic42/34.jpg

they aren't rules about body position.

There is a line from the top of the roll bar to the top of 'reinforcing brace to the foot'. Is that defined? Do the top contenders have open or closed wheels? Your drawing with the red fenders could be pontoon fenders, halfway in between. If the front suspension is exposed, all the members could be airfoil sectioned. With rocker A-arms the coilovers could be inside the body.

What about weight distribution (50/50 or biased to the rear)? Rear brakes only? Up on jacks how long will the wheels spin? Does high or low rotating mass have the advantage?

There's a minimum wheelbase but no maximum. A longer wheelbase would allow a recumbent position and better fineness ratio.

Hi Freebeard,

this line is not defined, is only to explain the "safe area" created by the rollbar in case of rollover.
The top contenders are the sejkora soapboxes and they are open wheeled:
http://www.honzasimunek.cz/img/mid/8...ub-sejkora.jpg
but this year they implemented a kind of front "wing" whit a curved endplates to reduce wheel drag:
http://g.denik.cz/10/30/jbc-webik-20...alerie-980.jpg

the rear suspension is already a pushrod system, now i have to upgrade the front.
i use a tail heavy car, because it has more potential energy.
the wheel now spins for 5 min.
Low rotating mass is better, lower inertia = better acceleration=better braking.
Thank you

That would be interesting. "Hey, why is that kid lying down? Hey! He is winning!"

Thanks. It looks like they use the minimum width for the roll bar, that will reduce it's drag. Could it pass tech inspection if you zip-tie an airfoil to the back of the roll bar?

http://i.imgur.com/Cdh4U9B.jpg

This is the best open front wheel I've seen. It has a narrow tire on a wide rim and a smooth cap that overhangs the rim so there is one continuous curve from the leading to the trailing edge. The inner face is also smoothed.

I see what they're doing with the endplate/cycle fender, mimicing F1 race cars. I think the rear part is more important. With a tall, narrow wheel a fender, instead of the 30s swoop your red fender shows would look more like this:

http://i.imgur.com/pfHPp.jpg

Then, just because I can, here's a picture of an electric racer from the UK.

http://i.imgur.com/oYkto.jpg

Do this with a roll bar—Short nose and tall tires. Raise the pedals and move them forward so the seat can move forward. How much contact patch do you need for the corners?

freebeard,

what do you mean by "zip-tie an airfoil to the back of the roll bar"?
the smooth wheel caps, it's a fast easy mod that i can do.
My tires have very good grip, so i reduce the contact patch to reduce rolling resistance, i run 5kg of pressure in the tires and they have a very good grip and very low friction with the road.

Just a wedge-shaped trailing edge that would give an airfoil cross-section. I said zip-tied because safety officials might want it removable for inspection.

is a good idea.
will a naca 0015 work well for the basic body shape?
https://lh3.googleusercontent.com/-4...2/IMG_6577.JPG

what will work better, a flat floor or a curved one?
the "fenders" can look like this:
https://lh3.googleusercontent.com/-S...9411159b_b.jpg
https://lh3.googleusercontent.com/-n...8b7039a02c.jpg

A narrower, less downforce, and lightless version of that.

The airfoil and curved floor would produce lift, which would unload the tires for less rolling resistance, but the cornering would suffer. Here's an example of a car that has a curved bottom and flat top, for low-drag downforce; one of Luigi Colani's Bonneville cars.

http://ecomodder.com/forum/member-fr...wu5wo1-500.jpg

There's a lot of fabrication work in those fenders, but hey — go for it.

Maybe a stupid idea. But would you like air brakes? Foils witch you use as airbrake and double up as wings for down force to increase cornering speed.

some lift isn't bad, it will unload my tires.
last sunday i cornered at a speed of 60 km/h without problem and the car only weights 70 kg, the rules allow a max weight of 170kg car for my age.
So, when the soapbox is finished i will put ballasts in it to reach the 170 kg.
if the car has some lift, it will weight more than 70 kg (actual weight) and have better grip than now, even with lift.

sorry for my English :)

i don't see why not. drag? but if you want to brake is no problem. what do you think?

Why i said it as air brakes is that it would be movable. Like in a pedal action flap. When not in use it stays flat in the wind like a neutral piece. Whit a very small frontal section. But when in use you tilt it. Creating drag to brake and increase downforce to corner even harder

Something like the veyron spoiler?

Veyron. Zonda R. P1. Like those yeah. But then simple padle action. While it is so light small flaps will do. If you can add a 40 50 kilograms of downforce it would make alot of difference. While you dont have to put on the normal brake

The body will obviously be a lot of work, but you might be able to make the suspension components more aerodynamic with simple cardboard wings-shaped covers at first and you can do runs to decide if it's worth the effort to go do more permanent suspension covers.
I also think a slightly curved belly / floor would allow for air to remain attached for longer than a flat floor, but that obviously depends on the design.
Good luck. Looks like uber fun!

+1 on lengthening the car and laying the driver down. As Freebeard pointed out that will improve the fineness ratio and allow you, with the lower helmet position, to also lower the roll bar and get it out of the wind. Incorporating a fairing into the downwind portion of the rollbar would be good, esp. if you can also have a windshield that will aerodynamically couple to the upwind side of the roll bar. Things could get very sleek at that point.

Your suspension is an aerodynamic mess. If you can fair the fore-and-aft elements together, you have only half as many wakes to contend with.

+1 on adding smoothie discs to the wheels.

Virtually any body at all is going to perform better than your bare tube chassis. If you're breaking 60kmh already, get ready for a big surprise when you add some body panels.

It sounded to me like you were describing how you had added weight to the wheels to make them coast longer. If you're running with weight in the wheels, get it out. Move the weight into the body itself, as low and as centrally located as possible. Lighter wheels accelerate better and give a much smoother ride, and keeping the weight low will allow the car to take turns better.

Hi elhigh,
i'm already working on the driver position, but i have to clear the break pedal and the steering column.
A windshield will be awesome!
Ok, so i have to streamline the suspension arms, and move the coiler to inside the cabin.
the body panels would make such a difference ? i would need to search some road to test and measure the improvements.
i don't put weight on the wheels, I'm trying to lighten them as much as i can to reduce inertia.

Some news on the project:
https://lh3.googleusercontent.com/-d...42/unnamed.jpg

Fiberglass arrived today! now time to make the molds.

about this
 

*I'd recommend a flat bottom,except for a 2.5-degree diffuser if you can do that.There is no reason to let air under the nose of the car.
*Sculpt a close-fitting fairing in front of you,to guide the airflow onto you in your safety gear.
*Your 'turtledeck' rear fairing needs to blend into your helmet,shoulders,and anything else exposed to the air.
*Fill the void between the fairings and yourself with foam padding )kinda like is done with a kayak) to prevent errant air from entering the cockpit.
*The roll bar tubing should be streamlined.
*All exposed suspension and steering links should be streamlined.
*The wheel fairings should have the 'slowest' contours that you can stand.Very slender,very long! And they could be at least as low as the bottom of the wheels outer rim.

frontal area
 

Yes,anything to minimize frontal area will pay you dividends.

airfoil to rollbar
 

The round tubing is very high drag.Attaching a fairing to it can reduce it's drag significantly,even if it's foam rubber taped on with electrical or duct tape.
The contour,3rd from the bottom is the best.
http://i1271.photobucket.com/albums/...d2/8-10-13.jpg

Hi Aerohead!
i had followed your advice of streamlining the rollbar.
fisrt i scaled the 3rd contour from the bottom and cutted it into wood:
https://lh3.googleusercontent.com/-K...2/IMG_6948.JPG
https://lh3.googleusercontent.com/-P...2/IMG_6949.JPG

next i cutted it from eps and glued it with tape:
https://lh3.googleusercontent.com/-u...2/IMG_6952.JPG
https://lh3.googleusercontent.com/-o...2/IMG_6954.JPG
https://lh3.googleusercontent.com/-j...2/IMG_6955.JPG

final result:
https://lh3.googleusercontent.com/-v...2/IMG_6960.JPG
https://lh3.googleusercontent.com/-5...2/IMG_6959.JPG

now, somehow i have to make a mold from this.

Hi,

Yes, I'm very sure that virtually any kind of bodywork on your gravracer will improve its speed. As it stands right now you have a whole bunch of messy wakes forming behind your chassis tubing; a body will produce just one wake, and anything you do to clean up the aerodynamic profile of that body will pay off in a big way in improved speed.

Adding fairing contours to the downwind side of your roll bar is a good first step and considering how much it sticks out in the wind, even that relatively small step may deliver noticeable improvements straightaway. If you have a hill that you usually test on and can count on very consistent results from one run to the next, take the racer there with the fairing in place and see how it's improved.

There is a guy on YouTube that has many videos showing how to make car parts from fiberglass and he branched into carbon fiber and Kevlar. He says that before you make a mold, to use spackle, used to fill holes in drywall, to smooth it as much as possible. You are supposed to use Bondo, or body filler, but since you are just making a mold, you might as well save money.

Or you could skip the mold and just wrap the fiberglass cloth over the packing tape, pull it taut along the trailing edge and fiberglass it. If you were making multiple copies a mold might be worth the effort.

https://lh3.googleusercontent.com/-K...2/IMG_6948.JPG

OTOH, if you have more EPS you might recut those. The 0.08 cross-section has a maximum thickness greater than the tubing diameter, and a full half-circular cutout. Your pieces have the center in contact while the tips stand proud. Or you could add wedge shaped strips to fill the gap.

thanks for the great aero ideas! keep up the good work. can't wait to see what you come up with!

best

Hi Freebeard,
i will redo those.