heinrich

The forix site was a fun read, thanks.

Hammerhead

Good Morning.

"Hammerhead is not the only one talking with the backing of facts, his facts are derived from one website. And that website doesn't qualify whether it is giving total or net numbers. It seems logical to me that if technical specifications list downforce as 6,000lbs, then it is total downforce."

I checked with the creator of that website (Mike Fuller, who is a racecar engineer, formerly with Jim Downey Mazda Kudzu racing in the IMSA and ALMS series). As indicated before, I asked him about the downforce ratings on the website. He replied: that yes, a 2000# car producing 3000# of downforce at 100-mph would weigh 5000#. (Refer to my post on page 3). Actually he stated that my statement was correct and I stated basically what is listed above. (nice sentence, huh?)

I believe that the two different sides here are correct. For example, IF you remove one of these vehicles from the ground, the net effect is lift; this is obvious since the upper part of the car is longer that the bottom (air on top has to move faster than the air on the bottom, thus creating a low pressure area on top). However, when the car is on the ground, it is NOT acting a gigantic lift-producing device, because of the introduction of the road surface in close proximity to the bottom of the car. The net effect when these cars when in contact with the ground is downforce. Think of a vacuum cleaner nozzle and a flat surface, the closer the nozzle is to the surface, the stronger the effect. Once you move the nozzle a certain distance from the flat surface, there is no longer any force pulling the nozzle towards the flat surface.

The reason why race cars are designed to produce that much downforce is to improve cornering and braking at high speed, NOT SPECIFICALLY to keep the car on the ground. If you removed the wings and underbody tray, these cars still would not fly off without the front of the car separating from the road first. There still would be a suction effect from the road and bottom of the car (much less since the delete of the underbody tray).

Anyway, the net effect of a GTP car at speed on a flat surface is downforce (NOT zero lift, ie: just enough to maintain static weight). The net effect of a GTP car at speed and separated from a flat surface is lift (more than enough to send the car flying over trees).

Obviously, none of us are aerodynamicists and we're not getting anywhere arguing this. I have asked an engineer who worked on one these race teams and that's good enough for me right now. He confirmed what I have learned through the years of watching IMSA GTP/ALMS/F1 on TV, reading about these cars in magazines (Racecar Engineering, Road and Track, Motorsport, etc.), and reading technical papers and articles on various websites.

I sent PM's to two members to discuss how we could improve the aerodynamics of our 928's. I did this to avoid nonproductive inputs. I respect the opinions of the members of this site, however some inputs are counterproductive. For example, I've seen folks bring up an idea and then the first inputs are "that'll never work." This type of input would be productive, IF the author had tried the idea before and it didn't work. However, in most cases, the author just "thinks" it won't work. This is the attitude of the folks who think synthetic motor oil is bad, that reducing backpressure in your exhaust is bad, and that nitrous, without exception, will cause your engine to explode into a thousand pieces.

Instead of arguing semantics, I think we should discuss design ideas, probable failures, materials, and then test results. I believe that Greg Gray is already in the process of making his underbody parts. What I would like to learn is 1) where to obtain carbon fiber and resin material and 2) how to work these materials. I am under the assumption that you have to "bake" carbon fiber to cure it.

heinrich

I run a 4-foot smooth unperforated underbody tray, I've done so for almost 3 years, and the result is very satisfactory to me in many ways, though I have never had lift issues. My '85 is now owned by a local guy and when I loaned him a bellypan (stock little one) for it, he was amazed at the stability he gained up around 140mph at the track. My next escapade will be a 6-or 8-foot pan of the same nature as my current one.

heinrich

Btw Hammer, I don't consider it arguing I consider it discussion. And I for one enjoy it. I'm not here to win arguments, I'm here to put forth my thoughts on interesting things and to read others'. We don't have to be rocket scientists, we can learn a bit by talking, and I've learnt much from this thread.

slate blue

Hi Hammerhead, about the carbon fibre, from my research and this included talking with a fellow who has made a car from carbon. Albeit a VW dune buggy. There are different resins available that cure in different ways, some need the high temp you speak about and are often cured in an autoclave. Others behave more like fibreglass. To do this type of carbon work well you will need a vaccuum bag. There is a small article on doing your own carbon fibre on the carcraft website.

The carcraft website has many great articles and its free too.

Hammerhead

-Heinrich

You're correct this is a discussion and I guess I got defensive. I have learned much as well. I just wanted to get back to design and development for our vehicles. I don't think I enjoy debating as much as you, even though I think I have the lengthiest posts.

Jim bailey - 928 International

For information about composites including carbon fiber look for information about home built airplanes ..... those guys risks there lives based on how well they measure the resin !.......... Aircraft Spruce has a very informative catalogue and SELLS carbon fiber , kevlar, ceramic , glass of various flavors . The "real" carbon fiber is called "prepreg" and has the resins already bonded to the fiber BUT it must be baked (autoclave) to react and set (the Carrera GT chassis pressure of 6-8 Bar , 300 Degrees F for up to eight hours). It also must be refrigerated until used . So 99.9 % of all the "carbon fiber" being bolted on from the aftermarket is plain old fiberglass or polyester resins mixed in a bucket and slathered on to some carbon with a brush . The better pieces might get bagged and the vacuum used to remove some of the excess resin BUT not very often ! Carbon fiber is strong and light BUT also very brittle and easy damaged by IMPACT .

Hammerhead

-Jim/Greg: Thanks for the info.

The material I was original going to use was 29 or 26 gauge, Grade 80, Galvanized steel sheet--I have a source in Houston (steel building company). I believe Pro Technik in Houston used aluminum or steel for a flat underbody on one of their 911's. Steel would be much more durable, but not as stiff as carbon fiber.

One of my main concerns is how to keep air from getting in between the underbody panel and the actual body of the car. Also, if the exhaust is covered by the panel, will this be an area of concern?

Nathan Valles

I have also wondered about that problem. I think that if you can keep the front lip fairly tight, or continue the engine pan, you have a good starting place. Also if the right and left sides of the belly pan were bent into a "T" shape on it's side, then the fin extending down would help to control air to keep it moving straight under the car while the top fin could be trimmed to the floor. If you are still worried about air between the floor and the bellypan, you can install a couple of NASA type air ducts to remove excess air.

2V4V

Hammerhead,

I think you might find aluminum a far bit easier to work and it has much better corrosion resistance than galv steel (easier to drill, cut, etc).

Also, as soon as you drill the galv steel to mount it, you'll have a corrosion point.

If you enclose the exhaust (between the cover and the underbody) you will need to wrap the entire length of the exhaust with thermal wrap (to keep in as much heat as possible) ensure it's 100% leak-free (carbon monoxide leaking into cabin = bad) and insulate the hell out of your floorboard. I would also be very (very, very,very) concerned about raising the temps of the fuel and brake lines which (I presume) would be enclosed under the bellypan.

In short, I would say without doing a LOT of engineering, it would probably be bad to enclose the exhaust - it can be done - but it would require some serious engineering time, which I doubt would prove a great return on investment.

Just a coupla thoughts,

Greg

drnick

just an update on this topic, having installed an undertray from the front spoiler through to just beyond the engine and also a rear wing.

the undertray did improve higher speed stability, it was noticeable on driving.

the rear wing has made a HUGE difference! its dark outside but otherwise id take some pics, ot wont be to everyones taste, but it really works!!

PorKen

Cool!

Fastest928

If you flat bottom the 928, which we did, be careful of the change in pressure center. At high speed, it is a significant factor in handling and as e found out, can be quite dangerous! When we had it wrong, we were unable to run over 165 with considerable oversteer!

Also, wind is not your friend.....!

Also, be prepared to deal with a lot of heat reflected and directed upwards....floor boards get mightly hot.

Good luck with your experiments.

Regards,
Marc
DEVEK

drnick

at this stage the undertray only extends as far as the stock s4 type combined trays. as far as oversteer goes - get a bigger wing!

this is the biggest wing i could find without either having something custom built or importing the 928gt.com wing in carbon fibre.

drnick

another pic