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i'm missing were it states a very sharp crisp transition from the FB to the diffuser angle is best ?

the car in question is far from the same layout as the 7 type car .
the car they are referring to is known as a tunnel car not a flat bottom car .

My car is currently at about 4" ride hight but if I can I will lower it a little probably close to 3" The floor stops in front of the wheels at the back of the widest section of the car. I will have to extend the floor with a flat section in front of the diffuser. The diffuser has to have <25" of expanded section.
I originally went with the curve to prevent the separation at the start of the diffuser. It does make it difficult to see where the diffuser starts and the floor ends so I made sure the end plates were <25" long and the floor must be flat within 1" so the rest can just be accounted for in the floor flatness according to the rules.

Here is a rough sketch of the footprint of my car the drawing is not to scale but the measurements are accurate. I would love to hear what angle you think would be ideal as that is what the mk2 will be.

_________________
2015 & 2016 EMod Florida State Autocross Champion
2013 & 2014 DSP Florida State Autocross Champion

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My trigonometry is a little fuzzy. I am trying to figure out the trailing edge heght (x) if I had a 12 degree angle on the diffuser.
Sin 12 = x/25
25 (0.208) = x
5.2 = x

Is that correct the diffuser is 25" deep (hypotenuse) and the angle is 12 degrees then the height should be 5.2" (opposite)

I just measured the old on and it was more like 8 so probably close to 18 degrees, as such a little steep.

_________________
2015 & 2016 EMod Florida State Autocross Champion
2013 & 2014 DSP Florida State Autocross Champion

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what i will do is either after dinner tonight of 1st thing in the morning i will run a few simulations on my program and see what i get .
i don't have a 3d scan of your car in my system i will have to use the caterham 7 i scanned some years ago .
it in no way will be 100% accurate but but should give some idea for a starting point .

That would be amazing thank you so much. If your ever at an event I'm going to I let you have a drive.

_________________
2015 & 2016 EMod Florida State Autocross Champion
2013 & 2014 DSP Florida State Autocross Champion

Scrap Metal Build Log viewtopic.php?f=35&t=14558

with the dimensions you gave me 12.8 degrees would be best for the diffuser angle .
i did the runs from zero to 150 MPH with a slight roll to the diffuser and a flat diffuser and with 3 ,4 and 5 vertical fences spaced evenly apart .
guessing your using the car more for autocrossing at speeds are not over the 100 MPH range 4 fences will work best .
as for the rolled or flat diffuser i'm seeing no difference until speeds start getting over 120 at speeds over 120 a flat diffuser will work best but the numbers are so slight hardly worth noting .


not having a real scan of the car and only playing with this for a little more then 3 hours there is allot of tweaking that still can be done . most projects we will have many 1000's of hours into simulation runs and 1/4 scale testing but i feel it's a good base line to start with .

forgot to add you stated your ride height was 3 to 4 " i did the runs at 3.5" .

Fences is a new term for me, is that;
1. 4 strakes meaning 3 channels for the air to flow though,
2. 4 fenced in channels for the air to flow through
3. 4 strakes in addition to the side strakes giving 5 channels for the air to flow though.

As there is no real benefit from the curve I will make it flat with a slight curve from the floor to the diffuser for simplicity. I picked up some material today so hopefully I will get a chance to make a lot of the rear end this weekend.

Thank you for dedicating your time to help me in this, if you have a sticker or something I could put it on the car for Nationals as free advertising to say thank you.

_________________
2015 & 2016 EMod Florida State Autocross Champion
2013 & 2014 DSP Florida State Autocross Champion

Scrap Metal Build Log viewtopic.php?f=35&t=14558

the channel or tunnel is not the fences .
the fences are the things that stick down kind of like shark fins turn up side down there are a few names for them fences seams to be most widely used.
back on page i have a diagram pointing out the fences .

4 fences will give you 3 channels or tunnels .
they tend to be called fences because like a fence they partition or divide the diffuser into parts .
you can also call them strakes it's just another term we do use strakes for upper body aerodynamics but they are one and the same things .

Well the diffuser is completed along with about half of the rear end, I got pretty close to the 12.8 degrees maybe a touch shallower but more then 12. I have not finished the strakes yet, I will add those later but the angle aluminum will provide the mounting points.
The next questions is about a spoiler, we are allowed a 10" tall spoiler across the back what would be the ideal angle of attack, how many strakes (100sq") and is it worth making it curved. The desk is only 24" deep and 45" wide so it is not going to get much smooth air flow. My understanding is it acts more like an air brake. I am thinking somewhere in the 45 to 70 degree range. Any suggestions, no need to spend much time on it just a rough guestimate will do.

_________________
2015 & 2016 EMod Florida State Autocross Champion
2013 & 2014 DSP Florida State Autocross Champion

Scrap Metal Build Log viewtopic.php?f=35&t=14558

the angle strips are a very bad idea .
look at the way i showed how to build the diffuser on page 2 4th one up from the bottom of the page .

with angle strips you will lose the efficiency of the diffuser then by adding hardware ( fasteners ) you will lose even more of it .

you want the underside of the diffuser to be as smooth as possible with no bolt heads ,rivet heads nothing to interrupt air flow . you need to make the air flow move as fast as possible even more so with a smaller diffuser like what you have .

more like this

I reused the old diffuser and trimmed it slightly as it was already the right size. I am really up against it time wise to get the body completed for Nationals in a few weeks. I actually used number 8 metal screws and will cut off the tips so they will be smooth on the air side of the diffuser. In future I am thinking about using carbon fiber for body parts but do not have time at the moment.

_________________
2015 & 2016 EMod Florida State Autocross Champion
2013 & 2014 DSP Florida State Autocross Champion

Scrap Metal Build Log viewtopic.php?f=35&t=14558