I can't get the image of Lombard's suggestion out of my head :p

Once again Jack is doing something I've fantasized about trying!!
(BTW: my new miniVCR is TINY!, thanks Jack!)

I always imagined putting linear encoders on the suspension and measuring the changes in ride height over multiple tests. I wouldn't expect the force on the deck lid to give the whole picture since disturbing the flow over the back is going to cause some changes in other areas around the body, especially the rear fenders.

It'll take a little more calculations to convert ride height to downforce, but you could run with various carried weights to get your baseline ride-height/force chart. Do it at lower speeds to get the suspension settled out without aero coming into play.
I'd instrument both ends to get front lift numbers too.

EDIT: of course this is a MUCH bigger hassle than stuffing a load cell in the latch....

EDIT2: but oh yea... if the encoders don't get you failed in tech/safety, you can run the system on track to compare the effects among your collection of body parts!

Going back to Jack's original question, I was just thinking you'd really want to measure both hinge points at the same time as the latch to get the actual force going into the body from the tail. The hinge cells might need to be compression/tension units if the tail were levered out far enough...

I ran a crude 911 SC model through a flow program last year and saved a few of the plots. Unfortunately, these plots are based on the front of the car as opposed to the rear. Nevertheless, here are a few pressure plots:

http://forums.pelicanparts.com/uploa...1110335487.jpg

http://forums.pelicanparts.com/uploa...1110335510.jpg

Does anyone have the various wings modeled? If so, send them over and I'll slap them on the SC.

As others have stated, measuring at the bonnet latch would not give much pertinent information, other than increase/decrease pressure on the bonnet latch.
Chuck hit it. Total downforce must be measured at the tire's footprint for an accurate conclusion. In a perfect situation, measured at all four footprints simultaneously to account for weight transfer effects.

But... come on. If your goal is to determine the difference between two decklids on an otherwise-identical car, then I don't see how measuring pressure at the decklid latch would give misleading results.

It's difficult for me to imagine a less-effective wing producing more downforce at the latch, for example.

Remember, my goal isn't to replicate actual wind-tunnel research. It's to look at different tail options and see how they differ. I wouldn't pretend to be getting absolute numbers, but rather look at relative differences.

How about one of these? Linear potentiometer.

http://www.p3america.com/linear_motion_potentiometers_general_purpose.htm

thats the way.

Right Jack - strain gage at the lid

you just want an index -- don't get distracted...

How about a bicycle wheel off the back of the car with a meter that measures the compression of the vehicle to the road up to a certail speed. I think you could at least compare one ttail to another this way...

yeah, Jack (or Tyson) won't be distracted at all by trying to get a strain-gage set-up working well. :rolleyes:

Seriously, strain gages are a pain. Also, spoilers are effecting the WHOLE car.

We never had problems with them -- and that was way back when -- before they ever appeared in consumer products like scales, etc. Of course, a group of grad. students in a lab is not the same as a writer with a garage... But with a high amplification data-logger it should work easily. You can state what the pain is... and we'll see.

Actually, Jack, I bet you know people who can help you do this.

The sprins will take up some of the downforce so it is hard to measure the effect on the whole car -- and he just want to generate a figure of merit -- an index of performance -- or what have you. I don't see the need.

I am also unclear on what is meant to be doen with a bicycle wheel trailing behind the car.

And note - he doesn't need to measure displacement in the z direction. He could measure it - and then back-calculate to get the down force. But what adv. to that??

I don't think Jack is trying to measure exact downpressure (in pounds/ounces), but trying to get basic determination about which of his tails produce the most downforce (push on the rear wheels).

In theory, you could even put a piece of crushable foam (clay, or wax even) on the decklid striker and see which is crushed furthest after a 100 mph run... just basic science.

Let me re-state that I ain't no engineer. And let me also be more specific with the parameters for the experiment: I'd like to spend less than $30.

And remember: the results I'm looking for are not total downforce or reduction of lift. I'm looking for the relative differences between rear wings and tails.

To my mind, it seems like the more I can isolate the energy produced (or should I say re-directed) by the tail, the better. Using my car's 600# rear springs along with the entire mass of the (moving) car seems less precise, to me, than placing a smaller spring at the decklid latch. Or, at least, the bicycle wheel mechanism seems like a good way for me to create the need for much more precise measuring instruments of the amount of compression (or reduction in compression, probably) of those big rear suspension springs on my car.

I guess the basic question would be this: is a more effective tail or wing angle going to also produce more force on the rear decklid latch?

I'm sure a clever engineer could probably design a rear decklid that could produce counter-intuitive results, in this application, but is it likely that any of the commonly-used 911 decklids and wings would do this?

RW, just where on the hinges do these strain-gages get placed? (for example) Getting real-world useful numbers out of strain-gages means interpreting voltages and structures that they are mounted to. . ...do you know the deflection amount, (and deflection direction) of the hinge structure for say a 50lbf load?

While I think that you could figure it out, what a pain for someone who isn't an engineer.

The trailing bicycle wheel is dirt simple, by comparison. Parked, ya put the drivers weight in the front seat; note the angle of the trailing arm. Put a scale under a jack, under the rear, pump-up the jack 'til the scale reads 100lbs; note the angle of the trailing arm. Remove the jack/scale, place 100lbs 0f wieght on the ducktail;note the angle of the trailing arm. Plot the points, . . .later, rinse, repeat; as needed.

Run the car on the track; note the angles, (one of those lip-stick cams may come in handy there) and correlate.

That would be like saying 50 extra pounds at the front bumper, middle of the car, or rear bumper, would affect the handling in the same way. Location of the force will affect handling as well as the amount of force. Different tails may apply the downforce at different points of the car for more (or less) effectiveness.

Here's my stupid idea:
Remove the latch mechanism on the body
Use a bellow or rubber flask, perhaps even an inflatable camping pillow. Fasten a piece of rubber tubing to the fill valve.

Place the water filled flask between the decklid and body. Tape it in place, if need be. Run the tubing into the passenger compartment.

When BB2 is modeling on the side of the road, the water stays in the flask. When you're driving, the decklid pushes down on the flask and forces water into the tubing. You could visually watch the water rise as the speed increases and the flask is compressed.

This gives you relative measurements.
jurgen

But we're not talking about some wide-open experiment with the car's handling, here. We're simply looking at already-accepted ways of producing more rear downforce. I already know that my RS3.8 wing produces more downward push than my ducktail. But I don't know how much the difference is, or at what angle my rear wing begins to become less effective.

I take it as a given that this experiment won't answer any questions about the best way to distribute downforce front to rear. That research has already been done. It also won't answer bigger questions about what will make my car handle better. But I think it would provide useful (and admittedly limited) information about wing angles and decklid choices.

How about if there was some way to measure the speed of the air imediately behind the wing (simple wind speed device hung off the end of the wing)? The more air that is back there, the less downforce (from the diverted air). Hence different speeds would be relative to more or less downforce. Is there a flaw in my "science" here?

OK, I understand the bike wheel idea now - the angle measurement will have to be very accurate WHILE the car is at speed.

I would just put a strain gage at the end of the lever arm that the lid forms. Near the catch. He just needs an index - the ability of a tail to maybe act more towards the hinges is not a big effect. If it is, then his measurement will be off.

You can try both - or try the bike wheel first - it will be cheaper unless you have a buddy with a data logger. It will not measure the real effect of the down force as it acts on the body and will instead measure that + effect of the springs and moment arm consisting of the arm the bike wheel is mounted to.

To me, the big problem is in not measuring the drag force. Given that issue, I wouldn't worry too much about the rest.

It will be very cheap to hunt down some SAE papers and see what has been done before. You'll want to start in the 1940's and stop at say 1980. By then car manfs. had wide use of numerical methods models to run -- these things were pretty new in the 1970's.

I'm liking the trailing wheel idea, you could even run a cable to a simple mechanical dial. Sounds easier (and safer) than compromising the lid latch and should provide the qualitative info that Jack is seeking. Simple and cheap and the sensitivity is adjustable just by changing lever arm lengths.

You might be able to adapt it to the nose too!

The only question is how much ride height delta will occur on those springs. A load cell in the latch might provide orders of magnitude more resolution for making comparisons.