Underbody Rake and Ride Height

[ckiesz]

Hello,
I am building an unlimited class GT car for US time attacks and races such as the Silver State Classic(90 miles of closed highway, flat out). It is a tube chassis car, front engine rear drive, with 550whp, and 335mm slicks, based on a Nissan 240sx/silvia. There are no regulations for this type of car, so I am working as much aerodynamics as possible into the build.

After reading books by Simon McBeath and Joseph Katz, as well as spending endless days on the internet studying prototypes, dtm cars, alms cars, super gt cars and pretty much anything else I could find, I have come up with some ideas for my project. But two major areas remain for me to decide on and I am have trouble finding references to guide me: ride height and underbody rake.

I am building the suspension and chassis from scratch, so the drawing board is open. So far I have decided on a flat bottom with a rear tunnel starting about 2.5 feet ahead of the rear axle line. For the front I would like to incorporate a splitter leading to an under diffuser with side exits behind the front wheels, similar to a DTM car but not quite as aggressive.

My understanding is that on a flat bottomed car there needs to be a rake of a degree or two in order to create down force and keep the car from flying under most circumstances, but on ALMS GT and LMP cars there is very little or no difference in front and rear ride height. The most rake I have been able to find on current race cars seems more like 0.25-0.5 of a degree rake.

For ride height, I have been kicking around 1.5-2 inches for a safe starting point.

Would anyone be able to shed some insight on what is working for flat bottomed cars with rear diffusers or cars with front and rear diffusers? I would like to keep my error more on the safe, stable side but I need somewhere to start.
-Christian

[AU N EGL]

Hey Christian

Welcome to 10/10ths

I run a corvette C5 coupe in NASA. Have a 2" front splitter and 1" rear spoiler, no wing.

My splitter has 3" of ground clearance and car has 3/8" of body rake with a flat bottom between the axle lines.
I have front diffuser but not a rear diffuse.

my car is extreamly stable and very aerodynamic up to 160 mph. Have not gone over that yet,

Most ALMS GT cars have about 2" of front ground clearance with 1/2" to 3/8" under body rake. any lower and you may scoop up unexpected road kill.

There are some very good SAE technical white papers on aerodynamics here
http://www.sae.org/servlets/PaperEve...N_SUCCESS=TRUE

Good Luck on your project

[ckiesz]

Thanks for the reply. This info is very helpful, the 240 project has a very similar profile to a C5. Are you allowed to run any form of rear diffuser in your class?
I was lucky enough to experience the skull covered C6R's and talk to some of the crew at Laguna Seca this last season. What an impressive machine and great bunch of people, very lasting memory.
-Christian

[AU N EGL]

Originally Posted by ckiesz

Thanks for the reply. This info is very helpful, the 240 project has a very similar profile to a C5. Are you allowed to run any form of rear diffuser in your class?

Yes I can. I just have not put on a rear diffuser or wing. Most like I will when a new motor is ready, and move up a class.

[Locost47]

I'm not sure if it is out already but there will be a feature in Racecar Engineering magazine sometime soon where Simon McBeath gives a fairly in-depth set of practical guidelines for designing & optimising underbodies and diffusers. It started off as a few general "rules of thumb" but has ended up being 26 separate guidelines due to all the associated caveats and clauses. It should be pretty useful.

[AU N EGL]

Race Car aerodynamics: http://www.bentleypublishers.com/product.htm?code=gaer

Large Eddy Simulation on the Underbody Flow of the Vehicle

http://www.sae.org/servlets/productD...D=2007-01-0103

[ckiesz]

Locost47: Good to know, racecar engineering just got a new subscriber. I wonder how long it takes to get it to California.

AU N EGL: Thanks for the great links for the tech articles. I have the Race Car aerodynamics book and the newest book by Simon McBeath, any other auto aero books I should look into?
-Christian

[AU N EGL]

WE get racecar engineering about a month behind the Europeans.

[Goran Malmberg]

Originally Posted by ckiesz

Hello,
My understanding is that on a flat bottomed car there needs to be a rake of a degree or two in order to create down force and keep the car from flying under most circumstances, but on ALMS GT and LMP cars there is very little or no difference in front and rear ride height. The most rake I have been able to find on current race cars seems more like 0.25-0.5 of a degree rake.
Christian

How did you find out those rake numbers?
Regards
Goran Malmberg

[ckiesz]

One of the two above mention books loosely stated "only a degree or two" in regards to flat bottom cars. It was not stated as fact.
The 0-0.5" measurement was an assumption by me from estimating in pictures and guessing while looking at a few of the cars at the track.
I am referring to only the flat portion most parallel to the ground, and not the overall car with rear diffusser.
I really have no idea what the actual rake measurements for safety vs. best aerodynamics are on a GT or Prototype. I know this is a very loaded question, with estimated in-race pitch and height changes coming into the equation(spring rates, roll, anti dive/squat).

I would love to hear from the builders and designers.
-Christian

[Goran Malmberg]

Originally Posted by ckiesz

1
The 0-0.5" measurement was an assumption by me from estimating in pictures and guessing while looking at a few of the cars at the track.
I am referring to only the flat portion most parallel to the ground, and not the overall car with rear diffusser.
2
I really have no idea what the actual rake measurements for safety vs. best aerodynamics are on a GT or Prototype. -Christian

1
Thanks, thats what I wanted to know.
2
Well, I am not a Le Mans prototype engineer so I can only give my personal opinion.

If the car has a parallell to ground under body there will be zero downforce,
provided the car does only has a flat underside.

I should not use the word " best aerodynamics" as different aero balance for the car is used.
If we just look at the flat bottom effect 0,9dgr makes for a 1,5" rake over a 100" distance.
This makes for about 4" of water at 100 mph i the middle of the wheelbase.
But, the LMP cars have both frot and rear difussers, that change the picture
of the underbody rake use.
Regards
Goran Malmberg

[ckiesz]

I'd also be interested in your experience with cars with rear tunnels / diffusers. My plan is to incorporate a rear tunnel starting about 2.5 feet ahead of the rear axle line.
A front under diffuser with side exit is also in the works for this car if I can come up with a design I am confident in.
If you are willing to divulge information, I would love to hear about your findings on your Corvette project. I am a fabricator by trade and the pictures on your web site got my heart racing.
Thank you,
Christian