Steve Rea

We are developing a car now that we hope to run in the 25 Hours of Thunderhill this year, in the Super Unlimited class. As it's unlimited, we basically have no restrictions on our aerodynamics.

I am interested in hearing ideas on aerodynamics for our campaign.

These are our current considerations:

front - splitter, whiskers, hood fences, vented hood and possibly vented fenders on top

rear - diffuser, unsure about wing (if diffuser will be sufficient without the drag of the wing?)

I am no expert on diffusers, or underbody aerodynamics, yet from what I've read, it sounds like these are the most effective, with the least drag (compared to a wing with aggressive angle of attack).

The fun part is that we have no rule limitations, so I'm open to suggestions.

Thanks,

Steve

Car Specs:
1987 944 turbo chassis
Chevy all aluminum block LS-1 V8
about 350hp
about 2200-2300lbs
currently 18x8 & 18x11 wheels with slicks
4 corner coilovers with torsion delete
spring rates 600-800 range (we'll be testing)

Cory M

I guess it just depends how much you really want to get into the aero side of things, you'll probably have to do a lot of testing (preferably at T-hill) to see if the the changes you are making are significantly lowering your lap times. I would concentrate on keeping the air from getting under the car with a really low splitter and some new rockers, probably put a smooth undertray on the car and add a diffuser. You can experiment with different rear wings and dive planes, maybe run with the windows up if you can. You just don't want to go too crazy and add a bunch of drag relative to the downforce you're creating. I would look at touring cars and DTM cars (see lots of pics at speedarena.com).

For lots of aero analysis on Lemans prototypes see this site:

http://www.mulsannescorner.com/

Click on the cars listed on the left, then on the pics for details. The site also has a bookstore section, the Race Car Aerodynamics book by Katz is good but reads more like a fluids textbook, Competition Car Downforce is a bit simpler.

If you want some advice from the D-Sports racer group you should post your question in the Aerodynamics section of this forum: http://p081.ezboard.com/bdsrforum or go to dsportsracer.net
There are some knowledgeable racers there who may be willing to help you. You could also post at the formula SAE site (http://fsae.com/groupee/forums), there are a bunch of eager young engineers who will probably overanalyze the problem for you.

bruinbro

How fast do you think you will be running on the straights and how much downforce are looking to get with the suspension you have? These are the key questions to address for your aero design. Playing games with the underbody to get downforce is probably not in the cards given your spec weight, but cleaning up the underbody is a definite plus. Also, keep in mind that aero design is a systems thing, where the goal is to get around the track in question in the shortest time. The aero must be integrated with te rest of the cars components and capabilities to reach that goal. For instance, a car with a high weight to power ration will benifit more from lower drag than downforce, while a high hp car can make use of as much downforce as you can generate. If T-hill is the only track you ar going to run, then design the car aero for what you percieve is important. If a variety of tracks are going to be run, then compromise and/or variabiltity/adaptability of the aero components is in order.

How big is your budget? Is wind tunnel testing an option? Can you get enough track time to test various configurations? Do you have the money to invest in data acquisition equipment and someone that can do something with the data? How good is the driver? Will he/she be able to make the most of aero improvements and be able to notice the difference? Is a "sexy" look important (sponsors?)?Is there a successful car that you can emulate and then tweak? Or do you want to "do your own thing"? Are you looking to win, or just have the experience? These are some of the questions you need to answer for yourself in order to direct your aero effort.

Bruinbro

JackOlsen

The best tactical strategy, usually, is to look at the winners in your class and copy what they're doing.

I'm still trying to put together a decent way to measure changes in ride height, so I can test some low-tech pieces I threw together. Diffusers are a real trick with a rear engine design -- there are issues with the complicated airflow, cooling needs, and engine access.

I'm an unabashed junk-yard racer.

First, keep the air from getting under the car:





Then, make the bottom of the car as flat as possible:



Then, play around with managing the airflow under the car, creating a low pressure zone underneath the rear:

Skip Wolfe

Jack - love the rear diffuser - did you do you own fab work? Did it make a noticable difference.

As far as the 944, has any tried the adjustable 968 Turbo wing. Seems like it would be a good compromise between gaining some adjustable downforce with a minimal drag penalty. Just don't know if it is high enough to get clean air to make much of a difference. I know Chris Cervelli used on on his GT1 951 from quite a while and was wickedly fast but haven't seen to many other people use it.

macnewma

I'm sure you have already thought about this, but you will want to make sure your aerodynamic downforce is balanced to your liking from front to rear. It probably wouldn't be difficult to add a bunch of downforce, but if you get an extremely effective rear wing and a so-so front splitter, it might not work well.

Just something to think about. I think the suggestion of testing to the best of your abilities is important.

Rich Sandor

Jack Olsen is my hero. One question though - why do you have an air dam instead of a splitter? Is it just easier to make, or easier to play with how much air gets under the car? I would think that a proper, protruding splitter would be more efficent at letting less air under the car as opposed to a simple air dam.

Steve Rea, are you using the 944 in your sig as the race car? I have a few pretty cool 3d models that I am working on which will showcase what I will be doing to the bottom of my 951 to make it as slick as possible.

JackOlsen

If you saw my fabrication up close, I'd lose all traces of heroism.

The dam is below the spoiler portion of the aluminum piece because the two versions I made with the splitter down low were both torn away in 'offs' at the track. The dam design bends in, but doesn't tear my bumper away when things gets dirty.

Steve Rea

Cory and Bruin - Yes, testing is certainly on the schedule, though Thunderhill is, I believe, about 10 hours from Los Angeles, where I'm at, so not too practical. We will be out there Oct and Nov, but it will be too late for in depth aero tests. We plan on hitting Buttonwillow, which is probably the closest track to Thunderhill around here (or so I understand).

The drivers will be strong (not pro), but excellent racers in the POC. Significant differences in car handling will be noticed and we will be able to do some comparative lap time testing ahead of time. Hopefully we can get the car in the ballpark here at Buttonwillow, then fine tune it in the last 2 months of Oct/nov at T-Hill before the Dec 3-4 race.

Objective of 25 Hours - If we can run the car trouble-free for 25 hours we should have a good chance at doing well. This year's goal is to be strongly reliable, then taking our experience and the extra year to continue to develop the car, to be highly competetive in next year's 25 Hours. Last year's #2 place finisher, a 911sc I believe (don't know the mod's), only stopped for gas, tires, and swapping front brake pads twice.

Budget - Hmm, since we're putting all our resources to getting the car as reliable as possible, which when you add the disposable race costs, makes a healthy expense, we are not budgetting a lot for enhanced performance this year. This year's success will come from a trouble-free race. So...bang for the buck aerodynamics fit the bill....hahaha, yep, no wind tunnel testing :-) .

Rich - Nope, that 944 was my spec 944, and I will be using an '87 951 chassis. I'd love to see your designs.

Max - Yep, to attain front downforce we have many options: splitter, whiskers, hood fences, venting hood, venting top of fenders

Jack - Cool work on the underbody aero. What kind of results did you notice? Do you have any close comparisons from before and after? laptimes, etc?

Cory M

pics

Cory M

more pics

carreracup21

Jack, that is very clever. I've never seen anyone put a rear diffuser on a 911. In fact, that was always the knock on the 911 design compared with other mid or front engine cars.. that you could not really do an effective rear diffuser, but you seem to have proven that wrong. Do you think cooling will be a problem ? Can't wait to hear how it works on the track.

JackOlsen

I've taken it out once, but I'm still trying to configure a better way to test the effects on front and rear ride height. I've got a datalogger, and my goal is to find an affordable way to do ride height sensors, so that I can 'wind tunnel test' it without having to pay for a wind tunnel.

The good news from my initial test was that both engine oil temps and cylinder head temps were fine with the pieces in place. There was a lot more cabin heat, because the normal exit flow of air from the heating system was obstructed, somewhat. But that can be tackled with simple ducts (and wouldn't be an inssue with a watercooled car).

The bad news from my initial test was that my lap times didn't change dramatically. But that may have been because of additional push caused by the read diffuser working -- I wasn't able to tell, and I put off more testing until I could first do detailed ride-height testing so that I could balance my front and rear aero a little better.

carreracup21

Hmm, I would guess a nice big front lip and some rake added might help. It will probably take a good bit of testing to totally optimize the set-up. Seems like it should work though.