Atuca
Supporting Member
- 1,140
- 292
- Jan 6, 2007
-
Central Valley,
California
There were less then 10 threads about rear diffuser design, and the ones I saw did not look to be anywhere as functional as I would like to make one. I have spent the last few weeks designing one specifically for the use of function in a stock body 2G DSM. This is not a universal one size fits all solution, I want to make one specifically for the 2G and have it work. Even if the function is small and even if it is nearly negligable in the end (in the sense you have to be over 100 MPH for it to have an effect), I want the design to be functional and not just be fins sticking out the rear of the car; I want it to be designed to work.
So first off, I am posting this thread in the Road Course forum because that is what I am building my race car for. The design has been based off research I have done on the Time Attack Forums, images of race cars, and conforming to NASA rules. In respect to the last point, a rear diffuser is defined as (page 28 of 2012 rule book):
http://www.nasaproracing.com/rules/time_trial_rules.pdf
As I understand, the rear diffuser cannot go past the rear axel without being considered an underbody tray, which accounts for 5 points in itself.
Taking that into account, I have designed a proto type rear diffuser kit that has 3 pannels and the rear diffuser. There is a panel that attaches to the rear sub frame to give a flat bottom to an otherwise uneven vertical mounting location for the flat diffuser, along with two rear inner fender well liner "splash/mud guard?" that attach to the stock points, but also extends down to meet the rear cross member/subframe and with its flat bottom, gives a completely sealed uniform seal for a rear diffuser to attach to. The panels block off 95% of any of the open area that used to exist north of the bottom of the 2gb bumper. Some air I guess could escape over the diffuser in front of the axel, but with the gas tank and the floor of the vehicle, air would have to be very determined to travel up and over the axel and not just follow the path of least resistance, however that is what I am here talking about.
I tried to think of a way to also go under the muffler to have 98% of the air sealed, but to be honest there just wasn't a way to do it without having to block air flow to the intire driver side of the rear diffuser. Our exhausts come out from the center of the vehicle's rear crossmember/sub frame, and in an atempt to have no bends, most aftermarket exhausts shoot from the rear of the car at the rear cross memeber 45 degrees out the back of the car. If I had a stock exhaust or a aftermarket exhaust that came out the bumper straight on (and not angled) the two 90 degree angles would effectively take the same space, blocking all the drive side of the diffuser.
When trying to base mine off race car diffusers, the exhaust in most exotic cars and other race cars either exited the car ABOVE the bottom of the bumper, or in the center of the bumper where it didn't interfere with the aerodynimcs. In our cars, thats just not possible. We'd need to make an exhaust custom to fit the rear diffuser and to make space, remove the spare tire/trunk floor with flat metal stock. Not quite your "bolt in solution".
So in an attempt to make this diffuser work with all 2G exhaust systems, I decided to just leave the diffuser flat and requiring it to be to cut it to fit the exhaust. There's a million different exhaust systems, I only want to design this once.
I am already working on a fiberglass proto-type and will show pictures once I get them sanded out and have something to show, right now I just have a few layers of fiberglass for the fender well extension and the rear sub frame panel, once I decide on the angle the flat diffuser connects to the new panels, I'll cut the panels horizontally at the right height to give the proper angle for the diffuser.
On to the questions.
Aside from the fact I am looking for design input in the most general sense of aerodynamics, I havn't discussed the question of the fin design. Should it be something that is part of the diffuser design, or something attached to a flat diffuser? If it is to be part of the diffuser design, which is fiberglass, how do I make a fiberglass mold with little fins? Would I be left to make the fins 1/2 to 3/4in thick to allow room for a roller? Or would it be better to try and create some 1/8" metal fins and attach it after the fact?
What angle from front to back does the diffuser need to be? I believe I read on the Time Attack Forums by Andrew (Gixxerdrew) himself stating 7 degrees was the perfect angle, but maybe he was talking about the fins? I can't find the thread for the life of me, but I want to make sure the flat area of the diffuser is angled right.
How many fins and what angle should the fins be at (completely straight front to back?)
The NASA rules says nothing vertical more then 5 inches can be added. Would making a panel that attaches at the stock rear fender well cover splash guard thing be considered part of that 5 inches or are they talking just about the fins and diffuser itself?
I am sure I'll have more questions but this should get the conversation rolling.
So first off, I am posting this thread in the Road Course forum because that is what I am building my race car for. The design has been based off research I have done on the Time Attack Forums, images of race cars, and conforming to NASA rules. In respect to the last point, a rear diffuser is defined as (page 28 of 2012 rule book):
http://www.nasaproracing.com/rules/time_trial_rules.pdf
Add rear diffuser (note: additional points must be assessed for any vertical panels
incorporated into a rear diffuser that are greater than five inches in height---G.8) below) +2
As I understand, the rear diffuser cannot go past the rear axel without being considered an underbody tray, which accounts for 5 points in itself.
Taking that into account, I have designed a proto type rear diffuser kit that has 3 pannels and the rear diffuser. There is a panel that attaches to the rear sub frame to give a flat bottom to an otherwise uneven vertical mounting location for the flat diffuser, along with two rear inner fender well liner "splash/mud guard?" that attach to the stock points, but also extends down to meet the rear cross member/subframe and with its flat bottom, gives a completely sealed uniform seal for a rear diffuser to attach to. The panels block off 95% of any of the open area that used to exist north of the bottom of the 2gb bumper. Some air I guess could escape over the diffuser in front of the axel, but with the gas tank and the floor of the vehicle, air would have to be very determined to travel up and over the axel and not just follow the path of least resistance, however that is what I am here talking about.
I tried to think of a way to also go under the muffler to have 98% of the air sealed, but to be honest there just wasn't a way to do it without having to block air flow to the intire driver side of the rear diffuser. Our exhausts come out from the center of the vehicle's rear crossmember/sub frame, and in an atempt to have no bends, most aftermarket exhausts shoot from the rear of the car at the rear cross memeber 45 degrees out the back of the car. If I had a stock exhaust or a aftermarket exhaust that came out the bumper straight on (and not angled) the two 90 degree angles would effectively take the same space, blocking all the drive side of the diffuser.
When trying to base mine off race car diffusers, the exhaust in most exotic cars and other race cars either exited the car ABOVE the bottom of the bumper, or in the center of the bumper where it didn't interfere with the aerodynimcs. In our cars, thats just not possible. We'd need to make an exhaust custom to fit the rear diffuser and to make space, remove the spare tire/trunk floor with flat metal stock. Not quite your "bolt in solution".
So in an attempt to make this diffuser work with all 2G exhaust systems, I decided to just leave the diffuser flat and requiring it to be to cut it to fit the exhaust. There's a million different exhaust systems, I only want to design this once.
I am already working on a fiberglass proto-type and will show pictures once I get them sanded out and have something to show, right now I just have a few layers of fiberglass for the fender well extension and the rear sub frame panel, once I decide on the angle the flat diffuser connects to the new panels, I'll cut the panels horizontally at the right height to give the proper angle for the diffuser.
On to the questions.
Aside from the fact I am looking for design input in the most general sense of aerodynamics, I havn't discussed the question of the fin design. Should it be something that is part of the diffuser design, or something attached to a flat diffuser? If it is to be part of the diffuser design, which is fiberglass, how do I make a fiberglass mold with little fins? Would I be left to make the fins 1/2 to 3/4in thick to allow room for a roller? Or would it be better to try and create some 1/8" metal fins and attach it after the fact?
What angle from front to back does the diffuser need to be? I believe I read on the Time Attack Forums by Andrew (Gixxerdrew) himself stating 7 degrees was the perfect angle, but maybe he was talking about the fins? I can't find the thread for the life of me, but I want to make sure the flat area of the diffuser is angled right.
How many fins and what angle should the fins be at (completely straight front to back?)
The NASA rules says nothing vertical more then 5 inches can be added. Would making a panel that attaches at the stock rear fender well cover splash guard thing be considered part of that 5 inches or are they talking just about the fins and diffuser itself?
I am sure I'll have more questions but this should get the conversation rolling.
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