Horsewidower
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| 01-25-2010 12:31 PM |
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Not yet a fab thread, but hopefully soon.
Our rear wing was manufactured by G-stream, a company that no longer exists as their investor fled when things went to hell. The IR and fabrication is now done by Tiger Racing...Paul Brown. http://www.g-stream.com/index.html
Anyhow, the wing is supposed to provide a maximum of 600 lbs of downforce at 100mph. I need to fabricate a splitter to balance the wing.
I always read Simon McBeath's columns in Racecar Engineering, which provides some guidance. And recently I purchased Katz's Racecar Aerodynamics, which is unfortunately pretty light on the issue.
Can anyone point me in the direction of a decent article, DIY, how-to etc on designing an efficient splitter?
Bob Holmes
Grabaldi Racing |
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sticky667
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west covina
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| 01-25-2010 01:03 PM |
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pm'd you
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Horsewidower
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| 01-28-2010 11:43 AM |
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Nobody else??
Do you guys usually balance your aerodynamics by getting as much as you can out of your splitters and canards and then trim the rear wing to match?
Our proposed test method is to balance the car with no aero and then rebalance after the aero is added. Anyone want to share their experience?
Bob |
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smartbomb
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| 01-28-2010 03:53 PM |
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I will give you the quick Mike opinions on splitter design. These are rough rules of thumb which are better than gussing if you have no wind tunnel or CFD. Closer than 4" to the ground and you run the risk of making it pitch sensitive although it may work better. 4" is the point of diminishing returns in lowness, sure it can make more downforce lower than this but pitch sensitivity is lame.
The splitter will work much better with a belly pan. It should go back as far as easy. to do at least make it go past the center of the wheels, longer is better. If you can vent the radiator air out the top of the hood, even better.
A splitter much longer than 4" is the point of diminishing returns as well. It will make more downforce but less than you might think. The splitter should seal well to the body, leakage can reduce its effectiveness by 20 or more percent. If the front of the car is square with the splitter it will increase stagnation and the splitter will be more effective. If the car has a round profile in plane view like a 350z, then fences to force stagnation instead of side flow may help improve splitter effectiveness.
Rule of thumb, if you can make air back up on top (stagnate) of the splitter, it will work better. |
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spdracerut
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| 01-29-2010 06:27 AM |
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I just read an article in Racecar Engineering last night; front splitter on an old school Mini. That thing had a cD of like 0.57 or something! They tested it out in a few lengths up to 150mm (rule limitation) and it increased front downforce up to that length. By adding front, it did REDUCE rear. So you'll have to work on the balance.
One of the key points though was having the smooth underbelly extend as far under the car as possible which also helped reduce drag. |
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smartbomb
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| 01-29-2010 08:17 AM |
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Posted By Horsewidower on 01-28-2010 01:43 PM
Nobody else??
Do you guys usually balance your aerodynamics by getting as much as you can out of your splitters and canards and then trim the rear wing to match?
Our proposed test method is to balance the car with no aero and then rebalance after the aero is added. Anyone want to share their experience?
Bob
I think this is reasonable because aero gets knocked off in the heat of battle sometimes. |
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Horsewidower
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| 01-29-2010 08:19 AM |
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I went over to the local Barnes and Noble and they were sold out of RCE. Bummer. I guess I'll have to buy it on Zinio.
Everything I've read to date leads me to believe that you have diminishing returns once the splitter is extended more than 4". That matches your statements. I had intended to make the splitter adjustable, from 2" to 4". At least to start, I'll make the splitter and belly pan two separate parts.
In American Iron the front pan cannot extend past the centerline of the front axle
In ST2 it doesn't appear that there is any limitation, I could belly pan the entire bottom of the car.
Mike: The 4" from ground plane issue is troubling. Without a splitter our car's air dam is within 3"of the ground. Although it is easy to change ride height. One of the interesting discussions in Katz's book is about creating a venturi in the front to speed the air and lower pressure. The unfortunate issue with the book is that it is long on theory and about Zero on application. I just ordered McBeath's latest book and I hope it will spend more time on application.
I think I'll make the prototypes out of marine plywood, although you can get NASCAR Tegris take off splitters for not a lot of money. Then you just refab them to fit. Tegris is some tough stuff.
Bob |
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smartbomb
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| 01-29-2010 08:22 AM |
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Bob, I think 3" will work just fine. If you make the leading edge of the splitter have an upward cant at the bottom or have the center have a W like thing, this reduces pitch sensitivity. So now when you see those features on DTM, and BTCC cars you know why.
The Dog III splitter has a slight upward cant for this reason. |
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smartbomb
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| 01-29-2010 10:43 AM |
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Bob here is a pic of what I was talking about. The center section allows the splitter to keep working even if the car pitches or rolls. Also note the upward facing rounded leading edge. This also makes the splitter less sensitive. This stuff is good for sedan based cars because they are not as stiff as true race cars. Also look at the hood vents. |
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Horsewidower
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| 01-29-2010 11:10 AM |
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Thanks Mike. The picture is very helpful.
I'm not sure my fab skills are up to that. I may have to work up to it. :>) |
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smartbomb
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| 01-29-2010 02:36 PM |
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I would just make a flat splitter for now and see if you do have a problem. The downforce could drop quickly under roll or if you compress the suspension but being a race car, I assume its probably stiffly sprung. I think it would be an issue if the splitter is any lower but 3" is probably ok I think. |
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Horsewidower
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| 02-04-2010 01:42 PM |
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My copy of Simon McBeath's Competition Car Aerodynamics arrived today. More good reading.
It's becoming very clear to me that we spend far too much time worrying about rear wings and not nearly enough time on what it takes to balance that downforce on the front of the car. If, as advertised, my wing can generate 600 lbs of downforce at the rear and I needed to create 600 lbs of downforce at the front, and was successful, it would be a hell of an advantage. But the trick is how to generate the downforce at the front.
Wings are easy; splitters, front diffusers, vortex generators etc, not so much.
I hope you guys don't mind if I muse my way through this.
Bob |
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smartbomb
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| 02-08-2010 01:59 PM |
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Is that book any good? My aero PHD buddy doesn't think too much of his stuff in Racecar Engineering. My advice is common sense one semester of fluids stuff. One thing is that the rear wing usualy won't work as well as advertised because its flow field is affected by the body.
The Dog III is my first car with a lot of aero and I am having a hard time getting enough front downforce. The Dog II is my first car with a rear diffuser and it works pretty good considering that my wing shape isn't ideal. The Dog 1 has a wing and splitter and that works pretty good and was my first experience with aero.
Since the Dog 1 doesn't have much power, adding the aero didnt help lap times at first very much and reduced my straight line speed. It did however make the car really easy to drive, especially the high speed stuff. Turns like turn 9 at Willow Springs, Turn one at Streets of Willow and Talladega at Buttonwillow are yawners now when they used to be pretty hairy.
I did the splitter first on that car and it made the car very loose so I had to add the rear wing. It had too much angle of attack and I flattened it out (had to fab new mounts) and ran the wing about flat. Wow, didn't loose any rear stick and suddenly I was about 1.5 seconds a lap faster. I went from being one of the slower of the 5 or so fast guys in the class to one who was always in contention to lead. |
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Horsewidower
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| 02-08-2010 09:06 PM |
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Mike: My basic take on books is that if I only come away with one or two things that are applicable, it was still the cheapest way to pick up the knowledge.
McBeath's book is a much easier read than Katz's. In fact, I quit reading Katz about 1/2 way through and switched to McBeath, now with a better "layman's" understanding, the Katz book has become more digestible. I can see why your PHD bud might be less than impressed with McBeath's attempt to bring aero understanding to the masses, but as one of the mass, I appreciate it.
McBeath's method of testing a car's aero balance, even if it was only a couple of paragraphs, was worth the price of the book. If you don't mind waiting for me to read it a second time, I find that the second read helps to pick up the things missed in the first, I'll lend it to you. But only if you promise to send it back!!
Bob Holmes
Grabaldi Racing. |
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smartbomb
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| 02-08-2010 10:06 PM |
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I will just buy it, but than you for the offer. |
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jere
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| 02-09-2010 09:13 PM |
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I have the Simon McBeath book it was a good intro on the subject of basic aerodynamics (knowing nothing to start with), but it left a lot of real world questions. |
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Horsewidower
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| 02-10-2010 10:05 AM |
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Which one, there are two. |
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jere
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| 02-11-2010 05:58 AM |
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I have the second one I think.The one that has all the CFD stuff in it. |
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Horsewidower
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| 02-11-2010 10:20 AM |
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That is the better of his two books. The first one wasn't worth the read.
I have yet to find a decent "applications" book. I think we are all looking for a plug and play menu that will get us x downforce at y mph. It just doesn't work that way. Every install must be tailored to the application. |
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smartbomb
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| 02-12-2010 08:03 PM |
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CFD for dummies? |
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