descartesfool

Yoshi Suzuka who spent his life designing the aerodynamics of race cars recounts his aero design of the Nissan GT-R. Some good tables of data in the article for comparison.

http://www.suzukaracing.com/GTR%20pa...etoroadgtr.pdf

Just a quote: "This particular example was a large set back, but smaller set
backs were a daily occurrence, however, rarely did we see a positive
gain when switching to a new model part, it was always a step
backwards. After nearly 2000 runs, and much agony, we achieved Cd=
0.27 in drag and combined front and rear downforce."

And some people think they can just tack on some aftermarket parts to an OEM design and make the aero better!

solo-x

Careful with your final conclusion there Claude. Nissan had more variables in their equation than the average car enthusiast. Someone building a track car won't have quite the same weight applied to fuel economy or drag as Nissan did. I suspect "positive gain" for Nissan was an improvement in downforce while not impacting drag a significant amount or ruining the look of the car.

johnjw

Agreed. Thats what I got out of the article. While aero "improvements" to my racecar may help "improve" downforce, I am positive they provide ample amounts of drag.

descartesfool

The thousands of tests over 1.5 years done by Suzuka to improve the aero, which meant to lower drag to lower than 0.28 while getting equal downforce front and rear to maintain balance with his statement that they mostly went backwards with everything they tried, matches exactly what was written by an F1 aero designer in RCE who said 99% of what you try in the wind tunnel doesn't work. So how can one expect to tack on some parts and make real gains in terms of aero when the guys with the most knowledge, money and equipment to do the design say it is really, very, very difficult. It is easy to just add a wing and make more rear downforce I suppose, and lowering the car will make some improvement, but how do you get a balance of front and rear downforce without drastically increasing drag and hurting the L/D ratio? I have never tested parts in a wind tunnel, but there is a tunnel just 15 minutes from my office, and it has a rolling road from the Pininfarinna tunnel. I know one of the original scientists there, and he says the same thing. It's tough to get good aero when you try things you think will work based on years and years of experience, and they don't. I think you can do the obvious things like lowering the car, adding an airdam, a splitter and a flat undertray, with some rear wing, but how do you know the benefit? You can put your car on a dyno and for a couple of bucks find out how your power mods are working, but what do you do for your aero mods?

johnjw

I guess all I can do is rely on people who have done some sort of testing when deciding on aero products.

I bought and followed APR's directions and recommendations when it came installing my surfboard. Would be great to have a windtunnel to my disposal here in MT, but I'll have to make do with trial and error at the track. Running in the rain I can crank the turnbuckles and point my vid camera out the back window. Lol, not much tech there.

for reference http://aprperformance.com/index.php?...d=21&Itemid=44

mcolangelo2006

Amateur racers don't need to get as extreme as Nissan for a 80-90% solution for tuning their APR (or homemade) wing and other aero. Test the wing or wings (varying angle of attack, end plates, chord, airfoil section, etc.) on the car with a data acquisition system. The numbers don't lie. Do the tests back-to-back so as to minimize atmospheric variables.

Heck, famed aircraft designer Burt Rutan (who lives not far from Willow Springs, BTW) used to test scale models of his VariViggen home-built airplane by mounting them atop his station wagon and then driving down the desert highways at 120 mph! :-)

descartesfool

Have you ever done it, tested with a data logger on the street or track? Have you ever measured drag with a coast down test? Do you have ride height sensors and damper pots to try to measure downforce? It is not easy to get good data on real roads. Try to measure the same thing 3 times and see how close you get to the same answer, and then make a change to the angle of attack of your wing for example and try again to measure the effect. And then try to add downforce while not severely increasing drag. It would be nice to see some actual data logger data showing how changes to a car made improvements to Cd and CL and L/D, and the balance of CLf vs. CLr. I have never seen that.

Check out the aero design of the NSX Type R from Honda here:
http://world.honda.com/NSX/technology/t3.html

One small gain at a time, with a list of which parts do what in terms of downforce. Notice how they were not able to reduce drag on the car, as Cd stayed about the same at 0.32 vs the original. And similar to the GT-R, Honda designed the aero balance to pretty well match the staic axle loads, with 60% rear and 40% front distribution of the downforce.

descartesfool

You might also enjoy this chapter by Joseph Katz on the Aerodynamics of Race Cars:

http://www.strangeholiday.com/oops/s...304.092016.pdf

914Racer

The only thing we as amateurs can do, it test and try different things. You should always do the logical things, smooth the underbody, add the air dam, splitter, wing, etc. Those are obvious. Your tuning of those things will result in some drag (maybe), but you getting the car to feel good to the driver is all that really matters. If the driver can turn the wheels lock to lock at the end of a straightaway the aero balance is wrong. If you are not bumping the rev limiter like you were before going into a turn, its probably a safe assumption that you have increased the drag so much it slows you down. Of course if your overall lap times improve does it matter?

If I remember correctly it is around $1500 a day in the Wind tunnel in NC. That is pretty steep for most racers. You can buy a lot of practice track days for that kind of money. You will in my opinion benefit more from seat time than time spent in a wind tunnel. Most amateur racers are not good drivers so anything to help that is never money wasted. You can use those test days to test the aero improvements on your own vehicle and probably gain more speed than spending a day in the windtunnel.

solo-x

Claude, my point is that an OE car maker has many contradictory goals they must balance. Approach angle, ground clearance, overall look, cost, durability, crash worthiness, fuel economy, and a host of others. An enthusiast putting together a track car can deal with the approach angle and ground clearance, thinks a big wing and splitter look "cool", can justify the added cost, is willing to accept that the splitter isn't that durable, and will take the hit on fuel economy if the lap time is quicker.

An F1 engineer likewise has a different set of concerns. He's starting with a car that is already super light and powerful (relatively to a production based sedan) and is already highly optimized aerodynamically. The room for improvement is quite small.

So while someone tacking on some aero parts to their Civic won't see F1 levels of cfl vs. cfd, it's hardly an argument to dismiss the practice. Many IT cars have found a simple air damn improved front bite and top speed in back to back testing. I've watched autocross cars test with and without simple spoilers and see measurable improvements even at speeds under 60mph. (of course, an autocross car doesn't have quite the same drag concerns) In short, you don't need a wind tunnel to find aero improvements on a production based race car.

mcolangelo2006

You totally missed my point. I'm talking about an 80-90% solution that an amateur racer can do to get the wing design "in the ball park". I'm talking cheap, quick, and efficient using common sense and a bit of engineering judgement. I'm not talking about competing with NISMO and their 7-figure R&D budget.

Limit the number of variables, make incremental changes and run a few laps for each change to qualitatively (how does it feel to the driver) and quantitatively (performance based - were you faster at various sections on the track) to determine if the change was beneficial or not. Try to account for the effects of atmospheric changes and track conditions on your results. This is what you'll do during a typical test and tune day, just like you would for suspension settings and tire pressures.

For amateur racers on a budget, this will work fine. I don't see why you need to make it so overly complicated.

descartesfool

Nate, I don't think I dismissed the practice of adding aero mods to your car. I just wanted to highlight how hard it seems for the best in the business to make even minor let alone significant improvements to aerodynamic performance. I think looking at the articles and the numbers for the NSX-R and the GT-R make that pretty clear. You are right about OEM car requirements and limited gains available to an F1 designer, no denying that. Lap time improvements are certainly the final arbiter in what works and what doesn't, but it is important to highlight that it is very easy to make power improvements and handling balance improvements, but quite hard to improve downforce while not penalizing drag too much. I think there are a limited number of mods you can make such as lowering the car, adding a lower air dam with a splitter and an undertray, and of course some kind of wing. But beyond that, things get pretty iffy, with diffusers, fender flares, airflow through the engine compartment and coolers, underbody airflow, etc.

There are essentially no documented cases of measured aero improvements on road cars made into track cars that I know of. Many nice articles in RCE on some rather specific mods to various cars, such as a Lotus, but not much else. There are endless sources of power mods with dyno curves, but for a series of mods that improve say L/D and maintains front/rear aero balance we are on our own. It would be nice if there was more info available on what works, rather than just guessing more or less. I would like to see some magazine or other articles that give actual data on taking a stock car and improving it step by step.

914Racer

What data would you like to see? I am about to build an airdam and undertray for my CRX. Currently it is all stock. Perhaps 100mph coast down? Would you like to help get the car in the wind tunnel? That would be really cool.

descartesfool

Sure. Coast down data would be great, both in stock form and then with the airdam/undertray. Low speed and high speed data points for rolling resistance and drag, done in two directions and averaged hopefully on the same piece of road for stock and modified configurations. No wind is good too! Same tire pressures and tires, similar outside temps.

914Racer

Ok, depending on weather this weekend I will get some data. I should be able to log speed on the Hondata. I will need to go and scope the road I am thinking of using. It is a four lane highway, so I will go one west in one set of lanes and east in the other. I guess Coast down should be calculated through the Speed sensor. How do I measure rolling resistance? I assume it would be safer to run tire pressures into the 50psi range to help cut tire rolling resistance.

Will it be more accurate to coast down to a stop or just a constant change in speed. Say 100-50mph?

descartesfool

This article has a procedure and a nice spreadsheet for calculating Cd and Crr Rolling resistance from Coastdown tests.

http://www.instructables.com/id/Meas...-car/?ALLSTEPS

Spreadsheet is here linked in the article, and you can get it here:
http://www.iwilltry.org/b/wp-content...oefficient.xls

Spreadsheet uses the Solver function in Excel. Here is some info on how to load and use the Solver Add-in:
http://www.addictivetips.com/windows...solver-add-in/

To calculate Cd and Crr, click on the cell F39, then click on the Data. It should have pre-selected the two cells that need to be minimized, F41 & 42. Clikc Solve, and when it asks you, accept the Solver solution. Chart should show a curve that matches your data points fairly accurately.

914Racer

Damn Wind. Can't get a break around here. I'll keep waiting until I can get a decent day with less than 20mph winds.

descartesfool

I know, aero tests on the road or track are easier to talk about than to do. Mother nature knows no boss.

914Racer

This whole week looks like I will be dealing with 15-25mph winds. This sucks. I wanted to build my new airdam and install it this week. Oh well, guess I'll have to work on this 914 or SRF. Too bad I can't go sneak into the tunnel up in Pennsylvania.

http://www.racecar-engineering.com/a...f-laurel-hill/

914Racer

Finally got a relatively calm day to do some testing.
I tested in both directions, east and west. First test was 70-20 taken at 5 second intervals using my Hondata datalogging capability. The worksheet listed above calculated 0.341 drag coefficient and Coefficient of rolling resistance 0.0177. This was done with windows up and sunroof closed. Tire pressure is 35psi, stock body CRX Si, lowered slightly.

On the second test, I ran the car up to 100mph and coasted down to 40. Calculations returned
Drag coefficient Cd = 0.369681169
Coefficient of rolling resistance Crr = 0.010571127


I am going to get on with building an airdam and some underbody paneling to see what I can achieve. Above 100mph the car is pretty unstable, so I want to increase highspeed stability without cranking up the drag. I think a deep front airdam/splitter/undertray will not increase drag substantially.

It will probably be a couple of weeks before I finish the aero stuff. But I need it done by April 13th for a track event.

johnjw

Great! Can't wait to see some results!

liam821

Very cool link, thanks for sharing.

exgr

Claude always come up with some cool links. where do you dig all this stuff up?? :-) thanks for the reading.

descartesfool

Thanks. I get curious about some subject often because someone asks a question, and then I start digging. Success on finding good stuff depends on how you word/reword the search, and how fast you weed out the chaff. Experience I guess.