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Discussion Starter · #1 ·
This is the only forum here I could think of that someone may have an answer, before I go spending time and effort experimenting on something that might not work.

Have any of you gravity racers built a DYI Wind tunnel ? If so....
Do they work?
What's the best design ?
 

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I dont know of anyone who has but personally think the effect of aerodynamics would be negligible over what friction (and rolling resistance) takes away from the constant velocity of gravity.

Meaning reducing friction to and improving the rolling resistance of a casting would be more effective in improving speed and distance.
 

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I understand the reasoning behind your theory and agree it will have a bearing on the speed, but still dont believe the effective change in speed will be enough to measure a difference with the current tools we have available. Though there are electronic timing gate systems out there that measures (and shows) to the thousandth of a second now. And with a computer based system you may even be able to set up a time to even more higher decimal points.

This link has some lab experiments you can set up with Hot Wheels. I dont remember seeing one on aerodynamic differences before but there maybe a new one added by now. (It has been awhile since I looked thru them.)


Empirically though, how would you account for the added (or reduced) weight of the aerodynamic modifications and its effect on the acceleration speed of gravity to the new mass measure?


On the other hand you can build a wind tunnel for under a $100 bucks....

 

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The Serendipitous got me thinking about the tunnel. I just thought it would be cool to see how the air flowed over it and if smoke would show any down force patterns on the design of the body. Then, of course, that got me thinking of other cars that I've done that would be kind'a neat to see the aerodynamic flows over their bodies. Not saying any would ever see a track because most of mine are made NOT to roll.

Here's a question. Since gravity is the "power source" of a gravity racer. Would you not want a gravity racer to have lift instead of down-force? Seems to me the weight would remain constant. But if some lift were generated the car wouldn't be pushed into the track as much and would go faster. If not only by a fraction. Valtteri Bottas can testify to what a difference of .001 second can somtimes make ...

Only experience I've ever had with gravity racing were racing MOY's down the driveway when I was a kid. Three generations of Cub Scout Pinewood Derby's and driving a homemade gravity cart with a #2 washtub as a seat; down the road that leads up to Chimney Rock Mt Park. In Chimney Rock, NC. That was back in the early 80's when they still had the Chimney Rock Hill Climb. The washtub race was an intermission race during lunch break. It was nspired by a streaker in the early '70's, going down the mountain in a red, Radio Flyer wagon. Don't know where he started from on the mountain. But the road rash he got when he wiped out at turn 5 only added insult to injury when he got arrested for indecent exposure. The washtub cart is a planter now. No.... It didn't make it down the hill. It got up to about 40 mph then shook and bounced itself to a stop. Yes...... I did think I was going to die. This was called "The Mosquito."
Grass Plant Grass family Soil Tree
 

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I would have loved to see the Mosquito fly! Not so much the streaker.

My gravity racing experience started with a Big Wheel and a sloped drive way. I even learned how to hit the right apex and drift it onto the 90 degree side walk - eventually.

I have been thinking about your quest though and the different body shapes I have in my current race box of castings. The top six are a Way 2 Fast, Cadillac ATS V, a School bus, 2 Ford F-150s, and a Corvette Grand Sport.

The ATS is the closest to being an aerodynamic body shape in design, has a larger rear wing and flat bottom and diffuser out back. The front splitter normally drags or catches on seams and transition curves for castings that have them but this one was very surprising and in comparison to the other 2 same ATS castings I have tested rather remarkable in performance. It is also the lightest by weight. If I remember correctly out of those 6 cars.

I have run more than 3500 different cars over the past 8 years now of all brands and unless you have the wheel set up and good axles in the chassis - even weight is a none factor to being a winning casting.

And when you send them to a different track - that all changes.
 

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Discussion Starter · #7 ·
Yeah. The wheels always seemed to be a mitigating factor as well, during a Pinewood Derby. The next factor was to not scrape something during the transition. The biggest factor weight played was to put it as far to the rear of the car as possible. Some cars took that to the extreme and would actually pull a wheelie at the transition point which was very detrimental to their performance for the rest of the track. I did however notice some aerodynamics come into play with this size of car. The thin wedges always seemed to perform better than the blocks of wood with wheels.
 

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I only did one Pinewood Derby event and was beat by one of those flying wedges. Later I found out they had drilled out and weighted the rear end. My lighter car was making a come back on the flat but ran out of distance as theirs was slowing down.
 

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I have also been watching rebroadcasts of the Red Bull Soapbox races on Fox Sports. I think you can find them on youtube as well. Same theories and techniques with the front wheels failing as the primary cause of their DNFs. 🤙
 

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Here's my take on 1/64th scale aerodynamics...Yes, while the resistance of static air on a body in motion is a factor on any scale, it will become less influential when the average frontal area is already pretty small(eg. 1/64th). Also, consider the fact that real world benefits of maximized aerodynamic designs are typically evident over long distances/periods of time. That said, I do factor in obvious positive aero qualities/frontal area when I'm qualifying potential entries. Of coarse, roll-ability and track-ability are still "king", but I have opted for minimal frontal area when all other factors have been equal, especially when racing on long(40ft+) courses.
 

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Well yeah, as I understand it, at 1/64 with a max scale speed of about 250m/hr is real world about 4m/hr, a small breeze. If one where to apply a point system(my interpretation of someone else's research) to the factors affecting speed, avoiding extremes and luck, Total weight does not significantly affect velocity among the experienced builders, Weight placement is worth 3 points, favoring the back side of car center for drag racing(single lane tracks), and forward of center for fat tracks(road racing). Wheel friction gets one point(don't let them rub on the body and use graphite). And then there is the aerodynamic drag which is also suppose to be worth one point. Of course one should take a look at the things that affect drag.
For the testing of effects on your car you need a fan and an accurate scale. A scale that measures in milligrams (I have one on the way from Amazon for $25) must be protected for the breeze. To measure drag, tie a string to the rear of the car pointed nose down on a slight slope(for tension in the string) run the string over a pulley down to a weight on the scale. The less change in weight with fan means less drag. To measure down force, place car on platform that rests on but protects scale from direct breeze getting the tare in breeze first.
So, I have to yet to set up to evaluate the benefits but will post (maybe in a few months) when I have some data.
 

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Sounds like 2 interesting experiments. Be sure to let us know about your results and any performance improvements.

I also favor the ideal of weight not being a higher asset once a die casts terminal velocity is meet. The other factors being as you mention friction and aero drag.

I also think wheel alignment plays a more important asset in performance over weight. It also effects friction and aero drag as well.
 

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Yes, Wheels and Axles.

For drag racing they say three wheels (lift one front for less drag) and adjust the steering to slightly favor one side so that the car isn't losing Energy bouncing from side to side. For road racing I am going to try lifting one rear (with straight steer) and a slight toe. Been thinking about Caster but haven't yet devised any simple solution.
Axles must also be critical. The Wheel Wobble of Death is quite serious in the real world. Harmonic vibrations by loose axles must cost the cars Kinetic Energy as well as make the cars more unstable. Watching the other guy go off the cliff though is quite enjoyable.

So I am new to the sport. Been watching 3D and Chaos, what great fun. Just bought my first car a week ago and now already have 25 more. Am collecting my tools and supplies for mods and will soon set up a test track. Hopefully we will meet on a track in a few months.
 

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This is the only forum here I could think of that someone may have an answer, before I go spending time and effort experimenting on something that might not work.

Have any of you gravity racers built a DYI Wind tunnel ? If so....
Do they work?
What's the best design ?
I don't think aerodynamics has much affect as the car is traveling relatively slowly and is relatively small. The biggest affect is friction and movement. Friction from the car but also movement in the track. Remember the first law of energy conservation - energy cannot be created or destroyed. But how do you measure the energy available and the energy used, or more properly, converted?

The guys at Cook's Projects have developed a spreadsheet that will analyze the energy used by a car on a particular track. Using this data, you can quickly reveal how mods have improved, or reduced, the performance of a car or track. It can also be used to review your car collection to find the most efficient car in the group.
 

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Aerodynamics still has the same effect - only at the 3" scale.

To me it would be more to race against the casting that beat me previously and see if it wins this time! 🤙
 

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Yes, friction, wheels, suspension and control of vibrations do rule the car's performance. Scale speed on some tracks is over 250mph which is a real speed of about 4mph. A 64 scale race car at 50 to 100 grams give the car a scale weight somewhere around 5 tons so the shape, surface area and lift of the car must have an incredibly small effect however I just can't help myself without checking out those effects. Sooooooo.... I have two of the Hotwheels Lamborghini Geneno. Same casting, same mold marks, both recent purchases. Ran them on my little test track and they are equal performers. One I will leave alone and the other I have begun to clean up. It will take a lot of filing and Dremel to open up the vents and I will add back weight to keep them equal. We shall see if I get a car length out of it I will be ecstatic. However to be honest half of my motivation is just to make the car look better. I will post pictures in either case.
 

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This is the only forum here I could think of that someone may have an answer, before I go spending time and effort experimenting on something that might not work.

Have any of you gravity racers built a DYI Wind tunnel ? If so....
Do they work?
What's the best design ?
There is an other forum for diecast car racing. It is called redline derby. Search it up on Google
 
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