Downforce, body kits

[Michael]

Pete - thanks for the advertisement, I suppose....

 

A pretty decent book on automotive aerodynamics is "Race Car Aerodynamics: Designing for Speed", by Joe Katz. Have a look at the reviews on the Amazon web page for the book, at http://www.amazon.com/exec/obidos/tg/detail/-/0837601428/qid=1060638949/sr=2-1/104-9123135-2731961?v=glance&s=books . I wholeheartedly agree with the review at the bottom of the page: the book tries really hard to cover as much as possible, but is too watered down for engineers, too long-winded for "laymen" and doesn't give enough practical advise for stock-bodied cars (sedans, coupes, etc.). But I did say that it's a pretty decent book - because books on automotive aerodynamics are rare.

 

Another choice (but out of print) is "Automotive Aerodynamics", by A. Scibor-Rylski; Halsted Press, 1984. He gives a 1970's perspective on the downforce revolution and the OEMs' sudden realization that maybe drag does matter.

 

A short and refreshingly cogent older (1965) article on downforce and spoilers is the SAE paper 650135, “Some Considerations of Automobile Lift and Drag”, by Joseph J. Cornish. I have a copy. I'm not sure about the copyright issues, but perhaps I could mail hardcopies if you're interested.

 

 

To summarize the problem of automotive aerodynamics: figuring out what the air wants to do in going around cars is MUCH HARDER than figuring out the comparable problem for airplanes! Airplanes are streamlined, with smooth walls and large length to width ratios. They don't fly very near the ground, don't have spinning wheels in flight, and modern airplanes don't have giant radiators sticking out in front of them. This is why airplane design is an established, mature science with nth decimal-point accuracy in most cases, but car aerodynamics is still largely a try-it-and-see effort. And why solutions for cars tend to look very different from solutions for airplanes. Example: rear spoilers that look like inverted airplane wings generally don't work well for sedans and hatchbacks, because of the "dirty" flow behind the passenger compartment. Neither does a big wing necessarily mean big downforce. NASCAR-type upturned spoilers do a reasonable job of downforce and slightly! reduce drag (but that depends greatly on the spoiler size and angle, of course), and they look nothing like an airplane wing.

 

Solutions found by trial-and-error by well-funded racing teams will be superior to latest-and-greatest innovations based on speculation. So there's no shame in copying a 30-year-old design like the BRE spoiler and airdam, especially if you're starting out with a stock street car and you are interested in a little firmer steering feel and less wandering at 100 mph. Whatever you do, don't slap on a radical body kit, expecting sudden improvement! Use an incremental approach - making small changes one at a time, and documenting your results.

[Michael]

By the way, Z-car aerodynamics, underbody tunnels, spoilers and airdams, explanations for the Z exhaust smell inside the cabin, and so forth, come up several times a year on HybridZ. In keeping with the "search before you post" theme that's come up recently, I thought I'd find an earlier post on aerodynamics books, paste the link, and write some smartass remark about the benefits of doing a search. So I tried using the "search" function myself, to find my own posts on aero stuff and recommendations for books, but the posts that I was trying to find didn't appear, despite some effort at keyword creativity. In the end it was easier to write a new post than to attempt looking for older ones. My conclusion is that there's something wrong with the search function. It's not the silver bullet that some people purport it to be. Perhaps all the more reason to work on a FAQ?

[Drax240z]

Some older posts were lost a year ago in the software change to the best of my knowedge. So yes the search is good, but not perfect.

[Guest thinkmonkey]

If you truly want to decrease front and rear end lift then I would focus most of your energy on keeping air out from underneath your car, smoothing out any airflow that is there, and on dealing with the airlfow problems inherit with hatchback designs. An appropriately low front airdam combined with low side skirts will stem most airflow from getting underneath there, either from the front, or by being sucked in on the sides. If you got the energy, flatten out the area between the engine bay and the front of the airdam, and around the rear suspension. Vents on top of the fenders will aleviate that notoriously high pressure area (even the mopar guys figured that one out in 1969, ala Charger Daytona). The air flowing through your radiator goes into the engine bay and usually most of it escapes through the bottom of the bay, the best solution is sealed off ducting that flows the air through the hood, with Lancer Evo type vents.

 

In the rear of the car, the more air that can escape, the better. This is why Nascar's and most race cars try to reduce the rear bumper area, or even eliminate it so that the air will either be sucked out or expand. Venturi's are an evolution of the expansion concept, and are extremely effective as long as you design it properly. Cutting out the rear valance is quick and dirty if you don't go for a full blown venturi.

 

Probably the single largest contributor to rear end lift is gentle sloping shape of the rear hatch, that creates a low pressure area. Disrupting this airflow or channeling air over the hatch should be your goal. A gen 1 supra style spoiler or somthing similar should mounted close to the top of the roof to dirty the air.

 

Lastly, remember that most of your efforts will be all for nought. Ha.

 

-David

[tannji]

would anyone happen to have a pic of a splitter on a Z? dont think I have seen it, and I would like to.

 

tannji

[Moridin]

I'd start with the front splitter and the small front under tray as said before by those who posted before me. I'd then figure out how to route the air coming in through the front on the outsides of the radiator to smooth out the flow. The trick, as I understand it, is to get the flow under the car moving quicker than the flow over the top of the car (It may be the opposite. I always forget.). If you are looking to do all this, I'd also take a shot at reducing drag too. Get rid of those drip rails, new mirrors, and maybe even a full bellypan

 

EDIT: Checked out Ron Tyler's site and found the double wishbone setup. Guess I better read everything before posting.

[JoeinCA]

I think that the rear window louvers are a great concept if aimed at reducing lift in the rear of the Z car.

 

This is what i plan to do:

I think that the first "step" of the louver should be angle up slightly, maybe 15-20*. I also think that the louver should be made out of a beefier material than the stock one.

 

I am then going to take a sheet of 18 gauge steel and cut it to fit with the contour of my airdam, the steel will go back and bolt to the crossmember and frame rails, it will follow the same contour as the stock crossmember has. the sheet will stop at the crossmember.

 

I am then going to create another pan that will go from the rear valance area all the way to the front crossmember of the rear suspension. the suspension arms themselves will have thier own metal skins on the bottom, i wil try to design it so that at high speed, the rear suspension will pull itself level, and keep the rear nice and stiff (it will want to return to the "normal" angle.

 

I think that these mods should reduce the overall pressure beneath the car, and should reduce the CD by a noticeable amount.

 

But i am keeping in mind that i should only perform one aero mod at a time....

 

I also would like to set up a cooling system/ hood similar to terry oxandale.

 

Plus numerous other little mods...

 

Joe

[Guest Want aZ]

I remember Car & Driver doing an article on improving the aerodynamics on a 240Z, Now keep in mind that this was back in the early 70's, I believe they did it in order to smooth out airflow and such, I remember that it had a full body pan, large front spoiler, and small spoiler in the rear. That is about all, somewhere in the 73 time frame if memory serves me right....

 

Damon

[Guest WanganShadow]

Actually I take that back. I never got past the words "wide body kit" in your original post.

 

Z's' date=' especially the early ones, do experience a good bit of lift at speed from all the air getting underneath the car. The steering gets very light at 100+. In that case a good air dam will keep air out of the engine bay and help keep the front end stable. I guess that is the same as downforce. A G nose looks like it would do the same thing, but I have no experience with those.

 

Try shooting Mike Kelly a PM. He once posted he added enough aerodynamic mods to make a Z stable at 160 MPH. Invited anyone who was interested to ask him how it was done.[/quote']

 

What's Mike Kelly's Forum Name? I did a search on Mike Kelly and couldn't find a post from him. thanks.

[Tim240z]

Mike Kelly = Mikelly.

 

Tim

[Guest WanganShadow]

thanks. I found it just before I checked the post. guess I should learn some new searching techniques.

[Guest rick458]

If you were to louver the hood like the MSA vented hood, would that

draft the hot air into the boundry layer and out over the car instead of under it?

seems like you could kill two birds (excess engine bay heat, under body air) with one stone, also louvering the rear valance should help with the exhaust reversion into the vehicle. or is this bad math?