Figured i would share my "wing" spoiler, whatever you want to call it. free stainless steel from someone remodeling their kitchen and has 2 huge sections of this stuff on their back wall. steel riveted to the trunk with some adjustable rods on the back for support. you could stand on this thing and it wouldn't budge. we will see how much it helps (if at all) during an autocross coming up i will be testing the car at.

next up is the front splitter.

**In reply to Robbie :**

The angle i used was the best of both worlds when it comes to max downforce with the least amount of drag. For the auto cross it probably could be more aggressive but i don't plan on removing it for the drag race.

Robbie
UltimaDork
7/17/19 8:17 a.m.
How did you determine that?

**In reply to Robbie :**

I used the formula for Lift which is: L = 1/2 rho V^2 S C_L and the same formula can be used for down force by simply changing the value of C_L to negative. where S is the planform area, and rho, the density of the flow.

Also had to use a handful of other formulas to fint the lift coeffecient, etc.

then i use the drag formula which consists of a few different formulas. but mainly using D = Cd * A * .5 * r * V^2 where Cd is all of the drag variables wrapped in one term. For my wing i used just skin friction drag and nothing else. There is no way around creating drag while increasing you're angle of attack. they are directly related, greater angle of attack, greater the amount of drag. I simply just wanted a wing that would provide downforce i was happy with while creating drag i could stomach. if you look at the ends of my wing in the corners they are folded up just a hair. I did this to act as a wingtip vortices. the purpose of this is to help reduce the drag as the air comes of the end of the wing. i did not calculate this nor do i know if it will work that well. it was something i did based on pure theory and wondered if it would work. it wont hurt anything if it doesnt.

i also have a CFD modeling program at work i plan to put the wing design in and see how it flows.

In short at 45mph (assuming that is highest speed in autocross) it will create 340 Newtons of downforce. equivalent of roughly 80lbs pushing downward on the rear wheels. however at 120mph (assuming average top speed in the drag race) it will be creating almost 1800 Newtons, roughly 400lbs of force.

Robbie
UltimaDork
7/17/19 1:32 p.m.
Interesting (I'm familiar with the drag and lift force equations, but they depend heavily on having accurate coefficients - the part that is hard to measure).

How do you get the coefficients of lift and drag for a spoiler like this?

Do you consider the effect of the spoiler in with the shape of the car, or just by itself?

Robbie
UltimaDork
7/17/19 1:33 p.m.
I also can't wait to see your cfd results.

If the sheet metal is thin enough to be flexible you could disconnect the supports and let it flatten out with speed on the drag strip to reduce drag.

**In reply to Robbie :**

i consider just the decklid which is near horizontal. when i put it in CFD i use rough dimensions and angles of the car itself to accurately see how it will work.

i get the coefficient for lift from the angle of attack and camber. on a sing that is stationary the camber remains constant, just the AoA changes. using this value i plug it into the Coefficient of drag equation in which i only use skin friction drag.

my numbers are not exact by any means and definitely have a tolerance error, but it is a good rough estimate of perormance i can expect from the wing.

**In reply to NOT A TA :**

definitely not a bad idea. although it will flex down some, i doubt it will flatten completely out. but regardless even if it comes down 5 degrees it would be a substantial difference in drag.

i have considered removing the trunk all together for the drag event.

Your calculated results for drag seem a excessive. If you are really interested in the drag effects for the spoiler, do some basic coast down tests on the highway (60mph to 50mph or something similar that keeps you from getting run over) both with and without the spoiler fitted and then potentially with different angles.

Basically you are calculating the drag force based on the deceleration from the time across your speed range. I documented the math and process for the magazine several years ago and JG did some testing on a car with different tires to see the mechanical drag of them compared to each other.

This method will give you total car drag (including mechanical and aero), but should get you a decent set of numbers for the aero change. Those can then be scaled up or down with respect to V^2.

**In reply to stafford1500 **

**I will definitely give this a try and see what i come up with. **

Keep in mind you are just using the basic equation: F=ma. As long as you can make sure the units all work out, the mass of the car and deceleration (acceleration) can be collected/generated with some pretty basic tools. Mass of the car is pretty simple and does not need to be down to fractions of a pound (you will have to convert pounds to a mass unit of slugs which is ft-lb/second^2 by dividing by ~32). Deceleration is delta speed divided by delta time, speed in feet/second and time in seconds. from there it is just some basic multiplication.

Just remember to take the car out of gear for the coastdown test, otherwise you get engine braking added into the results. Also, do the coastdown in opposite directions on the same stretch of road, to account for head/tail winds. Get a few samples of the data and average them together to get a safe result. If you really want to get crazy you could convert the results to standard day.

With tha information, you can back out what the coefficient of drag is since you know your speed and the area of you spoiler. Of course it will be specific to you cars shape and height and any other peculiarity of your car, but it will generally apply to most cars.

79rex
Reader
7/18/19 8:31 p.m.
I never expected to see " hey found this free sheetmetal from a buddies kitchen lets make a spoiler out of it" to be followed by "heres all the awesome math I did to figure out how it works" Fantastic job. Ill keep this in mind when its time for me to do something similar

Well done! What kind of struts are those? If you have any extra material, you could add some vertical end plates to help channel air over the spoiler instead of letting it fall off the edges.

**In reply to maschinenbau :**

i thought about that! the struts are just generic ones from amazon. i think i paid like 24 bucks for all 4.

Just an update. i started building the model in the CFD program at work. will be a week or so roughly until i am finished with a result.