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Uranium

aerodynamics underneath the car

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i got inspired after seeing a +100mph TT02 run and the traxxas XO-1 to do this.

a couple of days ago, i decided to take a PS4 box and cut it up to attach to the bottom of my own TT02, as seen here:

20190626_164138.thumb.jpg.6451f71599fe561676cd8b1724e1f031.jpg

before i added those, i measured my car to go 35kmh (i know, you may laugh). after i added those plates, i measured it and it went 37kmh! i suspected a super freshly charged battery gave it the small boost, so just to make sure i tore off the plates and ran it as soon as the battery was charged. sure enough, without the plates it only went 35kmh, even from the top of the neighborhood hill. i don't have a track in close proximity, so that was really my only good place to run it.

on sunday, i decided i'd make a new set with more versatile cardstock from the back of an unused notebook, as seen here:

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this time the gaps were sealed much closer and the wheels are now super close as well. i also made a (rather shoddy) rear diffuser, in an attempt to gain downforce. i knew the cardboard seemingly affected the handling, because without it, it felt a lot more loose, but with the cardboard it felt more planted, albeit with a bit of understeer. i'm hoping the same will go with the diffuser, but i highly doubt that. my next step is to cut up a plastic lid in the same shape as the cardstock (i've yet to run the cardstock) because i'm sure that's not the most durable material, if it functions properly.

i wanted to share this with you guys, as i want to hear your thoughts. do you guys think it's just a silly thing to do? i'm confident about the plates, as seen with the (super small) top speed increase. not so sure if the diffuser will serve a noticable function aside from looking like a sore thumb. thank you!

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59 minutes ago, Juggular said:

Wow, 202 mph with a 1900kv motor?  Makes you rethink about how much power we need.  

 

 

 

I bet it's a 550 motor. I recently got a 550 motor to put in the SCT i had. It was 'only' 2670kv. With a 120amp esc it would pull wheelies and was way faster than the 4370kv 3650 sized motor i replaced. And that as on 2s. It isn't a like for like comparison, but as electricity is basically magic as far as i am concerned i don't know why. With a car that light and streamlined with more battery cells i can believe it did the job. The power will have mainly been needed to overcome drag at those speeds. 

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8th buggies usually run 1900kv motors but they also run 4S batteries. He was running 12S so 1900kv would be plenty. It was probably much bigger than 550 too, those cans are longer and wider. Higher voltage means lower amp draw and less heat, or something. Like @ThunderDragonCy said, magic.

@Uraniumthats really impressive, 5% gain from a piece of cardboard, imagine what it'll be once you're at version 56 and using carbon fibre. The only thing that threw me was I initially read it that you cut up your PS4 to use, I mean I light my cigars with $100 bills* but cutting up a PS4?! 

*I don't really, I don't remember the last time I had a cigar, or a $100 bill for that matter.

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1 hour ago, Jonathon Gillham said:

thats really impressive, 5% gain from a piece of cardboard, imagine what it'll be once you're at version 56 and using carbon fibre. The only thing that threw me was I initially read it that you cut up your PS4 to use, I mean I light my cigars with $100 bills* but cutting up a PS4?! 

ha, i wish i could afford to do that. i'd be rocking a brushless system if i could...

i'm currently on evolution 3. utilising a plastic lid for a bin that i found in the basement, and a few hours of cutting and drilling, here it is, finished at nearly 12:30 in the morning:

20190703_003706.thumb.jpg.227a07e6aed64e14ccd6174b84b17989.jpg

i decided that using cardstock was a pretty stupid idea after i ran over a puddle and the car came back to me dragging an indistinguishable mess behind it. so now i've got an actually viable material, and just because i could, i added an even larger (and most likely nonfunctioning) diffuser, since the previous one decided to fall apart.

20190703_024815.thumb.jpg.845c3b8c3007702ee6d5a9f05e06adf0.jpg

i wonder if that'll do me any wonders, considering i used more cardstock, duct tape, masking tape and the remainder of a can of polycarbonate paint. perhaps i'll generate more downforce than a brabham BT45B and match the top speed of a lancia LC2? just kidding. i hope i'll see a noticable difference.

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Isnt the point of the diffuser to generate downforce (ie better cornering). If anything this would slow you down in a straight line - right?

Smoothed undertray will reduce drag so will clearly help straight line as youve found (albeit difference you saw was pretty big).

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23 minutes ago, Ann3x said:

Isnt the point of the diffuser to generate downforce (ie better cornering). If anything this would slow you down in a straight line - right?

Smoothed undertray will reduce drag so will clearly help straight line as youve found (albeit difference you saw was pretty big).

if i'm honest, i made the diffuser just to see if it'd work. it didn't. so i'm gonna see if a cartoonishly large one will.

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On 7/3/2019 at 10:36 AM, Ann3x said:

Isnt the point of the diffuser to generate downforce (ie better cornering). If anything this would slow you down in a straight line - right?

Smoothed undertray will reduce drag so will clearly help straight line as youve found (albeit difference you saw was pretty big).

A diffuser is a form of ground effect device and ground effect creates very little aero drag.

Good work, @Uranium. Your floorpan partlially eliminates one of the major aerodynamic issues with 1:10 RC cars, namely that they are hollow underneath and therefore incapable of generating underfloor downforce like most full-size cars. Your system will inevitably suffer from its unavoidable limitations (imperfect seals between shell and floorpan, huge holes for the wheels), but it is still a major improvement.

I'm not sure how effective the diffuser will be, given that its position behind the rear wheels mean that any effect created will have been lost by the time the airflow reaches that point as the gap in the floorpan will allow the air pressure above and below to equalise. If you could find a way of filling in those gaps and giving the car proper wheel wells you would be on the way to solving a problem which RC cars have suffered from forever.

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Cut out the back of the body. You'll be astounded at how much better it is, even on a slow car. Even on 4cell NiMH 12th scale stock, the old TOJ bodies, which were phenomenal, had much more grip and speed when cut. You could even trim the spoiler quite a bit.  I'm always surprised that all of the speedy guys don't do it.

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2 hours ago, Big Jon said:

Cut out the back of the body. You'll be astounded at how much better it is, even on a slow car. Even on 4cell NiMH 12th scale stock, the old TOJ bodies, which were phenomenal, had much more grip and speed when cut. You could even trim the spoiler quite a bit.  I'm always surprised that all of the speedy guys don't do it.

There may be something in this. If your car is going to be hollow (which it has to be) then removing the rear panel would help reduce drag. However, given the state of the shell that might not be the best idea from a structural perspective. What you could do instead is drill holes in the rear panel. When I used to race 1:10 buggies on road with saloon shells BITD, there was a specific section in the rules which stated how many holes drivers were allowed to drill in the rear panels of their shells to reduce drag and how big they could be (four holes no bigger than 1cm, if I remember correctly). You are not using those rules, so you could put in as many holes as you like. But I'd start with a few placed symmetrically across the rear, then add more if it works. Don't overdo it, or it will weaken the whole shell and make it flex really badly. I don't want you to ruin your Alfa shell, but then again it does seem to have been "well used", so it might be better to experiment on this one than to buy a new one and experiment on that.

Since it is designed to create downforce, rather than reduce drag and increase speed, I'd suggest ditching the diffuser for now. It's a good idea, but of limited effectiveness and probably better suited to circuit racing than to speed runs, unless you are getting up into three-figure territory.

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If you want some inspiration in this regard, I looked for some pictures and foubd this. It shows what I am guessing to be a Thundersaloon Ford Escort Mk 2, with holes punched in the rear panel of its fibreglass body, presumably for drag reduction. Thundersaloons are actually quite similar to RC cars, in that instead of having a monocoque chassis like a normal road car, they will frequently have a purpose-built tube-frame chassis with a fibreglass shell dropped over the top of it. This means they sometimes have the same aero problems that we have been describing here, and come up with the same solutions:
1211923983_ThundersaloonEscort.jpg.882fff730f8b773f8c47c559d5dcbdfa.jpg

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