TonyG

The problem is that it's mounted to a non-structural portion of the unibody.

Therein lies the issue.


TonyG

Lorax

It's all structural, that's why it's called a unibody. For every action there is a reaction and so on and so on. Even the hatch is structural, as are the inner and outer panels over the whole car. The problem is how much force it exerts flexing and how much it transfers to the rest of the chassis. If you strengthen the piece against flexing, all that force must go somewhere. The air pulls on the wing, the wing pulls on it's supports, the supports pull on the rear body, the rear body pulls on the rest of the chassis as a whole, that in turn pulls the shock mounts which applies pressure to the springs and voila, donwforce.

Lets say that we have 100% of the force the wing creates being applied to the rear body panel. For the sake of debate, lets say that in stock form, 70% of that energy is dispersed through the chassis and 30% is lost via flexing of the panel.

We can then assume that the panel needs to be more resistant to flexing. Thus reinforcement is needed. When reinforced, 100% of the energy is now being dispersed through the chassis with minimal flex.

Another way to think of it. you have using the same 70/30 figure, you have 100lb of force. According to our model the rear panel can withstand 70lb's of force before it flexes. That's not a perfect model because it isn't linear, it would have different % of flex at different amounts of pressure (there is always going to be some flex in every design, however minute it may be) but it works for us. You would then reinforce the panel to the point where it could stand 100lb of force with negligible flex, and the problem is solved.

Beside that, you could easily create a design that attaches to the rear body panel yet exerts little or zero force on it, all of it being transferred directly to a more suitable spot.

Lorax

I will also say this, having personally seen dozens of rear body panels pulled and replaced, I know the forces it takes to flex them, and the design of the 944's rear body panel is a particularly robust one in comparison to many, due to both the construction and it's design in conjunction with the rest of the body. In the design of a 944, it is a significant structural member. It is necessary that is be so when using the hatchback design. It would take a VERY significant amount of force to create even 15% flex. 100's of lb's. I would be willing to bet money on that.

HansB

This is getting into a very hairy discussion.

Let me try to put a bit of practical science into it.

When we designed our wing (TonyG likes to call it the Kokeln wing, but actually we designed it ourselves) we took a pragmatic approach: We copied a 996 Cup aerofoil, and a friend of mine which is a phd in math actually did some modeling and simulations on it.

The size and position of the wing are regulated : Not wider, higher and behind the body of the car.

Two variables remained:

1. what is the optimal height, and that came out at about the position it is in the picture some 2 pages back, and in my avatar.

2. What is the downforce, the load the support structure needs to carry.
By recollection this was about 500N at 45 m/s. (112 lbs at 100 mph). 45m/s is the speed that we carry in most of the high speed corners. Since the force of the wing increases with v^2, load at higher speeds can be easily calculated.
Top speeds on corners at Francorchamps are a bit higher, like 55 m/s (120+ mph) and then you can calculate the wing to produce 750N downforce (170 lbs)
Of course we wanted the wing not to break at top speeds, which are a bit above 65 m/s. (145mph) the downforce then is a hefty 1075N (240 lbs)

So any structure that you use to carry the wing should be able to carry quite a bit over what I guess to be the average Rennlisters weight.

So it becomes rather easy to get a seat-of-pants feel for all the structures that are presented: Do I dare to sit on it? Over and over again? And does it bend then?

That is how we came to the layout and dimensioning of the mounting HW.

Another SOP observation: I went from a bridge (968 style) wing to this (let us call it Kokeln style) wing, and it was a very noticeable difference.

Lorax

Nice to have some real numbers. Can someone tell me, when figuring net down force, you would be measuring the amount of force exerted at the mounting point, so would you then subtract parasite drag from that to get net down force? Gross force exerted is great for figuring mounting points, but for actually selecting the best height, width and placement you would need to be working with net down force.

The angle at which the force is presented to the structure is also important to determining it's strength. A rear body panel being pulled directly backwards is not as strong as if it were applied at a 45deg angle.

Not arguing with your points which are all correct, just though I'd add that.

HansB

The drag is, if IIRC from the calculation, about 10% at 100mph. Probably a bit more at higher speeds.

333pg333

Thanks guys for continuing with this. If nothing else, most of us learn something from these discussions. (Even if it's about clamshell fashion). However can we go a step backwards for a minute. I was ok to have some drilling done through the hatch as this was a spare frame with lexan installed. If we made some solid, yet not too overbearing, internal supports, we could attach the hatch and wing into place for the trackdays and remove and replace the stock glass hatch for ddriving. It may even be possible to make the internal supports modular?
Feel free to discuss other options anyway as I find it all interesting, but I just wanted to clarify my potential options.

Julian Allen

My car has the wing in the picture with the blue 968 race car (#55) shown in post 60.

This wing is made by Crawford Composites, originally for Worldwide Motorsports. It is mounted to the rear frame and cage elements: both wing support arms (available from Scott Gomes) penetrate openings in the body below the hatch and above the rear bumper cover.

The car's not available for pictures right now, but I'll post some when it is.

HansB

Then you should go with my solution, will work great for you.

https://rennlist.com/forums/6269353-post26.html

333pg333

Thanks Hans. This looks the best option I believe.

thingo

Using this logic I really think I could put my full weight on the rear body at the position where the 968 hoop spoiler is mounted, and it should be plenty strong enough for a wing if mounted properly.

porshhhh951

Nice

TonyG

Hans has some good data.

But it's not just the downforce weight. It's the continual movement of the wing relative to the car that will fatigue the relative portion of the body.


And Hans... why would you proceed to create your own wing from scratch that is a replica of the Kokeln wing? (same support design, same adjustment mechanism, same through-the-glass mounting, and the same air foil). Not that you did a bad job... the opposite. It's just that you recreated the wheel.


TonyG

Guns951

Tony, as always a pleasure to see you post. The blue car has the Under Pressure Performance (Scott Gomes) wing setup that is mounted to the frame rails via cuts in the body that channel it to the furthest point rearward in the car for maximum downforce placement. It was built at a near $200k price tag with a sister car and they both ruled Club Racing in their areas, changed hands not too long ago - very nice car. Crawford Composites had a hand in the bodywork etc.

As we all know the best place for a wing to create high downforce at high speed is as far back as it can be placed (see 9FF 996 250mph car), but most Racing Bodies state the wing must stay within 6" or so of the bodywork.

TonyG

This is not about wing placement with respect to downforce. It's about wing placement with respect to structurally acceptable mounting solutions.

The $ someone throws at a car doesn't mean its done right. I've seen many high dollar cars with stupid things done to them.

I'd like to see the pictures of inside the hatch area to see how they transferred the load. Interesting.

I have Scotts number. If I have time I'll shoot him a call.


TonyG