The Most Aerodynamic Shape?

I'm no expert but more weight will give you more horizontal push but mean more inertia. If you can put an air scoop on the front, you might cause a reduction of pressure so the car sits heavier giving you more push without the inertia gain.

In a vacuum, all objects will fall, or roll down a ramp at the same speed, ignoring friction of rolling.In the real world, the ratio of friction to weight becomes a factor.The fricton factors are wind resistance,friction between the wheels and axles, and surface and the compression friction of the wheels against the pavement.Maximum weight,maximum hardness of tires,minimum contact area of tires with pavement,minimum frontal area to reduce drag,lowest possible center of gravity for stability.

Use a sleeve bearing for the wheel axles,with dry graphite lube,bevel the tires to minimize contact with the track,use hard rubber tires,if allowed.Try to imitate the head of a fish with the front of the car to minimize drag.Routing parallel slots lengthways along the top and side will prevent the air from becoming turbulent and reduce drag.

That is about it.Good luck.

HTRN

Depends on the shape of the ramp. An object will roll down a parabolic-shaped ramp (similar to a slippery-dip) faster than down a straight-line ramp.

Have you already searched CR4?

Let us not forget this hot-potato discussion... Homework?

Thanks guys, The wheels / tyres we are using currently are 20" by 1 1/8th bicycle stock, as slick as we can find and inflated to 120psi. since the photos we have put discs either side of the wheels to enclose the spokes in hope of reducing drag.

As drivers we are all about 5' 10" - 6' 0 and 75- 80kg's

we were talking of ducting air from the front to the centre of the back to try to reduce the low pressure point behind us. Would this be better than streamlining the back to as thin a point as possible? - i.e tall boy bomb shape?

Thanks again

Toby

The threads I've seen before as highlighted are very helpful and point me in a certain direction. The difference as I see it on this thread is the aerodynamic shape has to contain a person, be of certain size and weight and work at a specifc speed of 0-80ish mph.

Race car and Dragsters designs generate downforce to help them stick to the road. In our case would this downward force waste energy? We won't be cornering fast and doubt we will go so fast as to take off. I thought the aim was to disturb the air as little as possible?

Many car companies have spent serious money building soapbox racers and have come up with very different looking machines, so I guess they are working to different principles, or at least priorities. I was just hoping for as many ideas as possible.

thanks

Toby

Downforce trades coefficient of drag for traction. The more downforce you create the higher the drag.

Downforce is great for high speed stability or high speed cornering where aerodynamics create enough downforce to artificially create greater contact pressure between tires and road surface.

Unless you have traction problems, need increased grip for high speed turns, or need increased stability at high speeds, wings will just be a drag. It will be like carrying mini parachutes out the back of the vehicle and slow you down.

What are your competitors doing, what have previous teams done, and why do they do it? You need to understand these questions better and most of your questions will be answered.

It does look like you have turns and such in the course. Do you have lateral traction problems in turns? In other words, is your speed limited by the lateral grip of the tires?

If yes, can you employ active aerodynamics such as a moveable wing? If you could tuck a wing into a drag neutral position when going straight and deploy the wing when cornering you may get an advantage. Only testing can determine if that is true. Even in the neutral position there will be drag, so the downforce advantage must be enough to overcome the loss of the drag.

Even if the answer is yes that you can use active aerodynamics the next question is how do you deploy them when needed. That increases driver load because it is one more thing they must think about and use.

Looked at the pictures, that's all. Read the thread, and a lot of a prior thread about aerodynamic shapes. I thought about airplanes, bicycles, influence of gravity power for race route grade, required manuverablility combined with the necessary study of what shapes have been winning. Scale models and route tunnel tests or access to Boeing Design Studios...And my main recommendation regardless would be cowling and canopies to reduce drag.

Bicycle racers want light bicycles. So I'd push light over heavy since the passenger driver is likely heavy enough.

Of naturally occurring materials I wonder if you could build a frame from thin or split bamboo, and cover the whole thing wheels and all with 6 or 4 mill plastic taped together for just the race? Is that legal?

Spheres are for me, hard to build, but if round is better than long you could consider extending the wheels to carry the sphere control, like a drag racer design extended fore and aft.

Building with Yew or Cedar may have advantages as concerns weight strength and flexibility. (I only know of them as bow building wood.)

P.S. My brother the bicycle racer, told me it wasn't so much drag that caused bicycle racers to shave their legs, but it made the crash wounds easier to clean, and less likely to get infected.

You Devil!

I don't know if this allowed.

Me neither, but: Once in a while I have the occasion to sail this fast beauty. The sailboat has one V mast of 150 feet and everything is controlled by hydraulics. The keel can swing from SB to Port and when doing that in manoeuvers we make a lot of speed gain. This keel has been developed in a laboratory tank (is still 8 feet high) and had the best gliding properties (costs also twice the price of my Moody 40) Maybe this gave me the idea? Check the shape as it is moving to the right.

National Aeronautic Civil Administration cowling research contributed quite a bit to superior aircraft. While initially far as soapbox derby cars I had a much cruder image of the materials, I feel alright about my concepts after looking at the vid of the winning design.

I imagine that simply turning a boat over would illustrate how its shape would transfer to a shape good for a fast gravity powered vehicle, if not a motorcar. Possibly a mercury filled ballast bladder would shift weight where it did the most good?

Let us suspend the soapbox derby rules now and think of best design in an upside down gravity world, and move from there to something practical for getting around town.

I didn't build soap box racers but I have "consulted" on some pinewood derby wins. 1) the speed is ultimately governed by the distance the weight falls. So if the weight is more to the aft of the racer it will fall further than a simlar racer with the weight forward. That extra .3 meter or so might be the difference if all else is equal. 2) as was mentioned by others more hands on than me, rolling resistance is a major negative factor. in our pinewood straight aways only racers we won whenever we ran on 3 wheels, with one wheel not touching. Alignment was critical, but our entries were "guaranteed" to have 25% less rolling resistance than cars that appeared to be "better made. HA! Milo

Point of interest on the 3 wheel thing --- FIA has long had this rule for automobile landspeed record attempts that all four wheels must be continuously in contact with the ground during record attempts to the extent that one of their record certification officials is concerned only with that issue. Still the rule is hard to enforce for extreme speeds on non paved surfaces. It becomes a non-issue at speeds over 300km/hr where vehicle control is compromised anytime one wheel loses contact and creates an assymetry in the forces acting on the vehicle. This makes a functioning suspension that keeps all 4 wheels on the racing surface of critical importance.

With respect to aerodynamic shape there has been much written and not everyone agrees to what is best. But remember that aero drag = drag coefficient, a function of body geometry times frontal area. Frontal area is the real enemy often trivialized. And in the types of vehicles under discussion here the major contributor to frontal area is the crossection of the driver's body. If the competition rules put a low limit on frontal area then build to that limit and find a driver that fits. If not, go for the skinniest driver you can find and keep the driver's body as close to horizontal as you can get.

With respect to shape remember that Reynolds number counts here. The air is pretty much the same for this argument; but substantial differences in Reynolds number are important as to what aerodynamic trick works and what doesn't. What helps a 25 foot long landspeed racer at 300 - 400 km/hr may be meaningless for a soapbox racer.

Ed Weldon

But remember that aero drag = drag coefficient, a function of body geometry times frontal area.

This could be misleading, but only because of punctuation. Better would be:

• But remember that aero drag = drag coefficient (a function of body geometry) times frontal area.

By some calculations, my old SM had a Cd x frontal area (in feet) value of 6.55. The GM EV1 had a value of about 3.95. ( I think the advantage may actually be more strongly in favor of the GM, because the areas in these calculations are not accurate, but just width times height -- and I think the GM was rounder, viewed from the front.

I just had a random thought: The (Maserati four-cam, triple two-throat Weber - powered) SM did 0-60 in about 8 seconds. So did the GM EV1, accompanied by only a little whirring noise.

I assumed it was a downhill, gravity powered machine, that was why I suggested the air dam in the front, so as to pull the car down harder on the slope without increasing it's mass.

Judging by some of the answers, I'm not sure we are all answering the same question.

Unless you are operating in a vacuum, an air dam will certainly slow the vehicle down, not "pull it downhill".

I think your last statement is correct.

Aerodymanically speaking, I would guess at your speed, the Reynold's number is low, so a tear drop is the "most efficient" shape. However, air drag is probably not your biggest concern.

Let the racer hang as low to the ground as is permitted or possible, depending on road conditions.

Put a streamline fairing over the axles for streamlining. Round tube has mor air drag than a streamline shape.

If you can, balance the racer so you only run on three wheels. This was previously suggested, and is advised for Boy Scout pinewood derby racers.

Run your tyres as thin and as hard as is possible or permitted.

Ried

The driver intimated that he stopped accelerating, suggesting drag IS the problem.

If the driver can feel the wind in his face, he is the problem. Remember the man-powered cycling record of around 100 MPH (?) was done behind a wall pulled by a truck, along a railway track.

Try pedalling a bicycle against the wind, first sitting upright, and then crouching, and you have the gist of the problem.

Error: I said before that dimples stop drag vortices. In fact they initiate small vortices, and prevent larger ones (which create more drag). So any suggestions that create a rougher body surface, or initiate minor vortices, will help a little.

So any suggestions that create a rougher body surface, or initiate minor vortices, will help a little.

This is virtually always untrue. At the Reynolds numbers involved at this speed and scale, smooth surfaces are better. This has been demonstrated in thousands of wind tunnel tests, and is seen in application on aircraft, land speed racers, sailboat hulls and keels, etc., etc.

This is an interesting article on boundary layer turbulence, I've seen similar reports from NASA relating to drag reduction on ships & aircraft. The principle described has been banned in yacht racing because it gave an unfair advantage.

"I said before that dimples stop drag vortices. In fact they initiate small vortices, and prevent larger ones (which create more drag). So any suggestions that create a rougher body surface, or initiate minor vortices, will help a little."

Sharks have rougher body surfaces, more in the style of pimples. Whether that would assist the racer or not is conjecture. Another idea would be to have an extended stinger tail, lengthening the aft structure to 'point' and 'stream off' vortices.

so... the driver of the vehicle should have pimples and dimples... perhaps a bit of chicken pox...

I really like your vehicle. I just finished rendering a design.. it could perhaps be morphed into something... see what you think...

otherwise, best wishes.

Chris

the Greenbird must be pretty close.

Chris

"Make body smooth and punch small dimple marks all over,..."

The dimples has to do with ball flight stability, not COD (coefficient of drag).

You never see this employed on aircraft nor auto racing.

Sharks have a rough skin, like sandpaper. They are the fastest swimmers in the sea.

Sandpaper is too fine for air at low speeds, need something coarser to cover the vehicle body. At aircraft speeds, would be ideal.

Tradition has stopped many ideas. There is also the problem of putting dimples in an incredibly thin aluminium skin, without compromising strength.

the fastest creatures in the sea are:

Sailfish
Bluefin Tuna

Tradition has stopped many ideas.

In this case however, tradition has little to do with it. Extensive tests have been done by the military and others regarding the optimum surface for low aerodynamic drag. You can look through the numerous wind tunnel test results in Abbott and Doenhoff, and find that the drag is higher in rough condition (in which sandpaper is attached to the wing surface) than in the smooth condition. In one study, canoes with dimpled surfaces generated about 6% higher drag than those without.

This article discusses golf ball dimples and why they do not reduce drag on streamlined shapes.

As I seem to remember, the dimples on a golf ball are to help the ball maintain a straighter flight, not for drag reduction.

A faster travelling ball will fly straighter when under the influence of side winds (in somewhat the same ways that a bullet travels straighter than a falling leaf, or that a small plane is a handful to land in a crosswind that a jet handles easily). But otherwise, the dimples only have a direct effect on drag. The drag of a dimpled ball is generally considered about half that of a smooth ball. If you don't believe the article I linked previously, try this one. Golf ball flight dynamics are described pretty well in this one

Given that golf balls are placed on the tee in random orientations, and given that the ball has no onboard navigation system, what mechanism, caused by dimpling, would enable the ball to counter sidewinds, inadvertently applied spins, etc?

This car had one of the lowest drag coefficients.

It has a shape which could be a guide. You can reduce the front area by an optimal position of the driver (see F1) but you can also reduce the aerodynamic drag and resistances (frictions) connected to bearings and roll:

- reduce mass since mass= weight and the only part where adherence is important is the curve since the the mobile has NO engine it is moved by gravity, a higher mass increases the active force but also the friction and friction growth can be proportional higher thus reducing the net active force.

-reduce friction: bearings (ball bearings have the lowest internal friction) , ventilation in the wheels (spikes have a higher fan effect thus higher friction), rolling on the road (a softer tire will have for same load a higher contact area and a higher eccentricity of the contact force thus a higher resistance to roll).

- is it compulsory to have 4 wheels ? can you think about 3 (reduction 25% of wheels friction) ? The side stability problem can be solved by a adapted cinematic.

You can test the friction by the angle at which the box starts to roll on a slope. This will be determined by the total friction (driver should be in).

"This car had one of the lowest drag coefficients." This Citroen ID or DS was a revelation of the sixties, compared with the competition that was building different trendy cars. Aerodynamics were then a new topic. I loved it.

Looks to me like the drivers head/helmet is the biggest problem: are you allowed a windscreen/canopy/cockpit.

1. Look to the shape of fast birds - these are nearly perfect although Re-number in birds flight is somewhat lower than at your maximum speed.

2. Look to the wings of medium speed aircraft, there are small "fences" of laminar to turbulence inducing disturbing the airflow obstacles.

These are used because the boundary layer may detach from the hull and then give rise to an apparently much higher cross-section.

Look with some fume generator where this happens and place some pins there.

On passenger aircraft-wings those disturbers that I saw often had 2 to 4 cm height, half that as width and oriented with the 1x2cm area facing the streamlines. Typical distance was 10cm.

Sucking in from front will not help unless you feed this at the side to help to prevent under-pressure there. Location unknown but experiments will be necessary.

RHABE

You might like to look at some of the entries for the soapbox race at the Goodwood Festival of Speed. Some of the motor manufacturers entered & took it all far too seriously, Lotus was the winner. It looks to me as though you need to keep the centre of gravity low.

It would be more useful if you could post a link to or post all the "rules" for the vehicle construction.

Otherwise we are just spinning our wheels offering ideas and thoughts that are not valid due to rule violations that we don't know about.

The most Aerodynamic shape for a Soapbox? hmmmm, I'm no Aerodynamicist, but I do know my airplanes pretty good.

I'd think that the shape of a 0.50 Cal. bullet is pretty good, almost optimal for an aerodynamical shape. That shape was good enough to be used on the Bell X-1 that Chuck Yeager flew back in 1947 to break the sound barrier. Problems is, what's good to break the sound barrier probably isn't equally as good for a 60+ MPH soapbox racer because of the highly different speeds and relative Reynold Numbers.

Come to think about it, you may want to investigate modern sports car shapes (like Lotus, Ferrari, Bugatti, etc.) as well as Formula F1 or Indy race car shapes.

Hey might be worth a try!

Good luck with your endeavor!

very informative. thank you. ga

Try making models, say out of balsa, and letting them drop in a vertical column of water, say about 3 feet tall. Hollow out the models and add lead shot, so they are all the same weight. They should preferably be accelerating most or all the way, say taking 5 to 10 seconds to hit the bottom (experiment!). Better still, have two columns, and compare two models in the same run.

Make a sphere, two tear-drops (one rough skin, one smooth skin), and one .45 shell, all same diameter, all same weight. Same length for tear-drops and shell.

They should hit the bottom in the following order:

rough tear-drop

smooth tear-drop

sphere

.45 shell

Consider positioning the driver with his head forward and laying down. Then design around that. It may reduce your frontal area considerably.

Two more remarks:

wheels as small as possible

COG as low as possible to reduce distance between wheels without loss of side stability.

The first condition will allow the second

Thanks people - thats a lot of posts in a very short time.

they're slightly different for each race (depending on location and organisation) but only very slightly.

Traction on corners is a bit of an issue, but we put this down to the very skinny very high pressure tyres we use - the tyre/road contact area is tiny but we thought its better to sacrifice this in the corners for lower rolling resistance in the longer straights. The bottom of the soapbox is about 1" 1/2 off the road and its only 1/2" ply with us lying on it. the centre of gravity is as low as we can get it. It feels very stable at all speeds.

The next one will be totally enclosed to help lower drag. The car that beat us was totally enclosed and we were right on him until the speeds got faster where he pulled away which would suggest an aerodynamic advantage. The bloke who designed it knows what he is doing - it was all carbon fibre and beautifully built - he works in the aero industry and does work for Aston Martin. To come second to him in a battered old roof box wasnt too bad!

We did beat a car built by Bently though (to be fair to them I think they were under strict instructions not to break it)

The Trucks in the U.K. have a foil on the front corners to force air back down the sides (I guess) can this be an advantage in our situation or are we better off keeping the whole thing smooth curves and teardrop shaped with as little disturbance to the air as possible as as we were originally thinking?

thanks

Toby

Don't have time to read through the rules before I rush off to a meeting but could you address the cornering issues by having some camber on the wheels?

Sorry, forgot to log in. the above post was obviosly by me.

Toby

I glanced through the rules, and see no note of driver weight. Also saw a note recommended substantial protective clothing and helmet.

Use the team's heaviest driver, wearing steel toe boots, lead underwear, chain-mail, and armored suit!

awesome again...ga.

Theoretically, what would be the effect of injecting compressed air from a nozzle just in front of the vehicle, so that the resulting jet of high speed air is closest to the skin of the vehicle. Would that not decrease drag, because the environmental air has only the rapidly moving jet of air to relate to? just wondering... _{^{(I'm not saying it would work or it would be allowed in soap box racing, if it did.)}}

Chris

Very good both of your comments! GA to both

I have to give them tomorrow I do not for which reason but I cannot give them today

Bumble, you've earned my GA vote today for two very awesome and highly educational postings!

I retract my posting entirely, basically because I was just guessing what shapes and vehicles would be aerodynamic. It turns out the a 0.50 cal. bullet, exotic sports cars and racing cars are not slippery in the air....I do stand corrected and have learned something new today.

Thank you MoronicBumble!

This is a well thought out post, and offers many excellent ideas.

Could you explain further the no waves in a single fluid (medium) idea?

Thanks.

Milo

I found the Cx of the car type Sm as in below picture Cx=0.33 according to WIKI

It is in fact one of lowest.

I am sure that if you put together all counsels you will have a fair chance to win.

A last remark, if the body will generate too much under pressure between car and ground the force will increase the resistance to roll. Since you do not use the adherence to move the body the contact to ground has only to offer the required side stability in curves.

I used to own an SM, as well as a DS21. Both quite clean for their times, and great fun to drive, with a very high weirdness factor.

The GM EV1, a quasi-production car, however, was much sleaker at .19

I see the driver has to travel feet first: can you get him/her in a luge like posture:-

Of course you have to enclose them in a shell obeying Moronic Bumble's other suggestions.

the other aspect of this question that I don't think has been mentioned (mighta missed it) is that more is less. the most aerodynamic shape, ultimately, is Nothing. The closer you get to nothingness, the less drag you experience. What this suggests is that the shape with the smallest total surface area is going to experience less drag. Obviously there are other factors, as a flat plate face on will have a drastically different drag than the same plate edge on.. but I think it is a factor anyway.

Chris

I think the pic I just posted is a bit large !!! However we race with Toby in the uk a lot and would be interested on peoples thoughts about our cart,

Cheers,

Andy.( Centa Bavaria )

how did you get such a large image?

http://www.google.co.uk/imgres?imgurl=http://farm5.static.flickr.com/4080/4860214390_12086f7497_z.jpg&imgrefurl=http://www.flickr.com/photos/aalmediascotland/4860214390/&usg=__6dKs4Niq_S_yEt7oH9DwOVPSGc4=&h=426&w=640&sz=125&hl=en&start=0&zoom=1&tbnid=Enk4j2SQZQsqbM:&tbnh=149&tbnw=213&prev=/images%3Fq%3Dcenta%2Bbavaria%26hl%3Den%26sa%3DG%26biw%3D1020%26bih%3D597%26gbv%3D2%26tbs%3Disch:1&itbs=1&iact=hc&vpx=137&vpy=88&dur=3190&hovh=183&hovw=275&tx=150&ty=96&ei=6sjnTPWZKtuShAexrMG2DA&oei=6sjnTPWZKtuShAexrMG2DA&esq=1&page=1&ndsp=12&ved=1t:429,r:0,s:0

KingKay: I was crew chief for the winning car at the US Soapbox derby unlimited division this year. The division is called Ultimate Speed. This was my 7th year in Soapbox and my third year in the unlimited division. This division is extemely competitive. The top teams spend thousands of dollars and spend 100s of hours building and testing. Our team has developed 3 types of custom wheels out of both urethane and rubber. I'm sure i could be of help to your team if you are still looking for help. if so E-mail me at ultimatespeed@live.com Mark