Engineers: I'd like input on two downforce measuring ideas
 

I've been playing around with different aero stuff on my 911, lately. The frustrating thing is that I have no way to get even a ballpark measurement of the 'net downforce-type effect' (reduction in lift, negative lift, whatever you'd like to call it -- I'm going to call it 'downforce') from different setups.

Of course, there are the correct ways to measure this (wind tunnels, strain gauges, load cells, laser ride-height gauges), and then there are my cheaper-than-cheap ideas. If you're a 'do it right or don't do it' type of guy, then go ahead and move on to the next thread. If not, I'd like some input on two of my latest 'cheapies.'

Here's the goal. If I'm getting 100 pounds of net downforce, I'd like some sort of measuring device that will give me a readout that will correspond somehow to putting 100 pounds of weight on my static rear wing.

One way to do this is measuring ride height. I can compare a run at a set speed with no aero to the same run at the same speed with a wing in place. I have a data logger that can take analog input and clean it up for me.

Now, it'd be about five grand to do it the right way with laser sensors. Rotary or linear potentiometers could be attached to the suspension, but the right sensors for that job cost hundreds. I'd like to do this for the cost of a burrito.

I haven't completely cracked it yet, but here's my latest try:

http://forums.pelicanparts.com/uploa...1154556007.jpg

That's a Sharp GP2D120 infrared sensor. It's used in copying machines and for robotics applications. It bounces infrared signals like a bat uses sonar, and has a measuring range of 1.5-11.5 inches. It takes 5v in and outputs an analog signal from .4v to 3v. It costs $12 for each sensor, which is more than a burrito. But I can run one in front and one in back for $25, which is a lot less than five grand.

Problem one: I wired one into my data logger a minute ago, and just did an experiment where I stepped on and stepped off the doorsill every five seconds. The sensor was mounted underneath the door, pointing at the ground. Obviously, I'm not using the full range the sensor is capable of, since ride height change when I get into or out of the car is less than a centimeter. And 160 pounds of net downforce right above the sensor is a pretty big change.

But here's the good news: on the data file from the logger, I was able to see changes in a graph showing ride height versus time that corresponded to when I put my weight onto the car.

Here's what I'd like to do, though. If it can be done pretty simply, I'd like to find a way to exaggerate the changes in ride height. Imagine using a lever off of one of the points of the suspension to move the sensor a greater distance when ride height changes. The sensor can point at the ground, or I could have it point at any flat surface that would register the change in ride height.

I don't know if I'm describing this clearly, but imagine setting up a teeter-totter with the fulcrum moved so that one kid travels a little and one kid travels a lot, and apply it to my ride height sensor and the suspension components of a 911. I need a solution for both the front and the back of the car. They don't have to be calibrated with each other, though. I can set up equations to compensate for geometry/output differences between the front and rear sensors (or rather, you smart guys will be helping me with that at a later date. ;) )

So, that's problem one: a way to exaggerate ride height changes to make both my front and rear infrared sensors more accurate/precise in their measurements.

Problem two: I thought it would be great to be able to get a much more direct measurement of the downforce effect of different rear wings and wing angles. One idea was to put some kind of electronic or mechanical scale where the rear decklid latch is, and literally measure the force pushing down on it. But this isn't the best point for measurement, since a wing that would cause more drag would show better 'downforce' because pushing back on the wing would mimic pushing down on it, since the decklid pivots on an axis where the front hinges are.

I know that strain gauges are the best way to do this. But they don't sell for burrito prices. On the internet, I found a page on model rocket guys testing their rocket engines with different devices. Some kid built a dirt-cheap test rig where he fired the rocket at a piece of metal inside a PVC tube. Underneath the metal was a veterinarian's syringe filled with water. When the rocket engine pushed the plate, it compressed the syringe, which fed water through a narrow tube to a cheap psi dial from a tire pressure gauge. The change in psi on the gauge could be correlated to engine output -- it was a simple hydraulic scale.

Veterinarian's syringes are less than a buck. Tubing is fifteen cents a foot. I've got cheap dial-type tire gauges lying around the garage. I'm solidly within a burrito budget, here.

http://forums.pelicanparts.com/uploa...1154557401.jpg

Here's my thinking. Imagine there are four 'mounting points' for a wing. They're at the top of the wing's uprights, two forward and two back. My thinking is to make the rear mounting points simply pivot (as though you could change the angle of the wing by lifting the leading edge of the wing, and having the front two mounting points be secured from letting the wing flip up, but also resting the wing's weight on the tops of two secured syringes. There's not very much travel to the syringes when they're filled with water (with air, that's not the case), so there won't be much of an angle change permitted under compression. But I could feed hydraulic pressure from two syringes into a single line that goes up to the cabin, where I could videotape both the psi dial and a GPS unit showing speed, and get a pretty good measurement of downforce from the wing and how it correlates to speed.

Here are my questions on this idea. I'd like to get the greatest range of reading on this system, as well. With my tiny non-engineer's brain, I can't see if one syringe would give me more resolution than two syringes, or if larger syringes (more water capacity in a larger-diameter syringe with a larger-diameter plunger) would give me better 'resolution' than smaller syringes. I guess I'm looking for the clearest readout of pressure from the least amount of physical travel for the syringes' plungers (since plunger travel will slightly increase my wing's angle of attack, and muddy up my results).

One or multiple? Bigger or smaller? Also will keeping the tubing shorter benefit me significantly? Any input on this one?

Thanks in advance for input. I really appreciate the patience you guys show with me.

No time to answer, but I have to say I think you are an Engineer hiding in writer's clothing. Your ability to keep coming up with creative solutions is impressive!

This is going to be a cool thread.

Jack, I thought you kicked your smack habit. I'm sad to see your back on the Heroine again.


Yeah, I agree with Chuck. You come up with some pretty ingenius solutions. I hope it works so I can do some testing myself.

What, no solutions in the JC Whitney catalog? :D

For the IR sensor, you might be better off measuring a change in suspension travel than bouncing it off the road. Not sure how road surface variation or even the speed of the car might impact the measurement.

Are you wanting to magnify the sensor signal by using the mechanical device because of a) sensor accuracy over small amounts of travel (what is the +/- accy rating) or because of your data logger input?

Disclaimer - I'm not an ME/EE but have sold a sensor or two...

Here's the simplest way i can think of:
Take some sort of shock absorber like a hood shock or deck lid shock. Any kind of hydraulic or pnuematic cylinder would do.
Carefully drill out one end to relieve the pressure without killing yourself.
Drill and tap a fitting on one end somehow and bolt it to your suspension.
Run a hose from the shock to a pressure guage in your cabin, preferably a very low pressure unit. A compound guage like one used to measure manifold vacuum/boost would be good.
You might have to do some calcumalations to determine displacement and pressure.

the more the suspension is compressed, the more pressure displayed on the guage. If you have lift the pressure would drop and the pressure would go negative (compared to atmospheric).
I would not be linear because of the compressability of air but you should be able to duplicate the readings in a static mode to determine actual force.

Hi!

hmm...after spending 5 min thinking about this...

Part 1

I can't think of a lever solution at the momment, but you might want to play with the sensitivity of the voltage measurements. If you increase the number of decimals, you increase your sensitivity to distance.

You could use a transisitor to increase the voltage differences by amplifying the output signal. As far as the details, I'm at work on a "smoke" break and can't do more at the moment.

http://en.wikipedia.org/wiki/Transistor

And this...

http://en.wikipedia.org/wiki/Image:BJT_Amplifier.png


Brad

I thought this through a bit more. A problem with measuring suspension travel/compresion is that you'll need to come up with an average over the same period of time over the same stretch of track to take out the excursions caused by normal suspension travel. That's the positive of measuring ride height directly and not messing with the suspension.

Jack,

I can help some with your first option. What you want to do is amplify the signal from the sensor output to get more resolution - that is, see a larger voltage swing for the minor distance changes you expect to detect. You can use an op-amp design to do just that. What you are after is a differential amp configuration. 1 input of the op-amp would be used to "Zero" the output for when the car is static. When the car is at speed, and theoritically your ride height drops/lifts, the output of the sensor will then be compared to the reference voltage, but amplified by the op-amp. I did a search and found some helpful information.
This linkhas some helpful information. I'm sure Warren will have a specific solution if he sees this thread.

Okay, I did some sitting and staring at both problems. The first one seems pretty straightforward. If I distrbute the measuring task over more syringes, each syringe will have to do less work toward the sum. Along the same lines, the linear travel of a larger-capacity syringe will be shorter than a smaller one. That's obvious to me, now. So, more syringes is good and larger ones are better than smaller. I'm going to rig up one on each side -- possibly one on each corner.

On the ride height sensors. I don't think amplifying the electronic signal is what I'm looking for. This may be a good illustration of my limited comprehension of this stuff, but I can amplify the signal any way I want in the software. It samples at a phenomenal rate. The problem, as I see it, is the 'resolution' of the measuring device itself. It's much better at registering one centimeter of travel than it is at registering a millimeter. I think physically exaggerating the sensor's travel will help with this -- although, again, I might just be seeing the problem wrong.

In the back of the car, I can attach a metal dowel to the bottom of the rear shock tower, where it bolts to the semi-trailing arm. Moving toward the rear of the car, I can create a fulcrum by strapping a piece of aluminum stock to one of my fixed oil lines. The aluminum will have a hole in it to function as a fulcrum. The distance from the fulcrum to the attachment point at the bottom of the shock will be about seven inches. I then have more than 14 inches of room for the dowel to continue to a point up and under the rear quarter panel, where it can either point up (to measure distance to the underside of the sheet metal) or down (to the road). Either way, it looks like I can create about a 2x exaggeration of suspension movement.

The front is more tricky, since the wheels turn. But I think I can attach a steel dowel to the traveling piece of the front strut housing, moving parallel to the strut toward the top of the strut tower. If I can hinge the dowel somehow, all I'll have to do is drill a small hole in the trunk, and have the second dowel (attached with an approx. 90-degree hinge) poke through into the truck itself. The wall of the trunk will act as a fulcrum, and the length of the dowel once it's inside the trunk (roughly parallel to the strut tower brace), will determine my level of amplification. Attached to the end of the dowel, the sensor can then measure the distance up to the trunk lid or down to the floor of the trunk.

Does that make sense?

I think I can do this with some hose clamps and steel dowels. The hinge for the front two pieces is the only part I haven't figured out, yet.

Oh, and in the back I'll have to shield the sensor from the heat of the headers.

It's all about the accuracy of the sensor. If you are measuring 1mm of travel and the sensor has accuracy of +/- 0.5mm, then you need more travel. If the sensor is quite accurate, then amplying the signal electronically is pretty much the same as using a mechanical distance multiplier, just easier to do.

The sensor itself is definitely the weak link in my system. The data logger can jump through hoops once it has the analog signal. It can eliminate the 'noise' from ordinary road bumps on a set course at a set speed at a set distance -- and basically extract the difference between one lap's input and the next. In the case of my testing wings, the 'lap' involved would be a fixed stretch of highway that I'd repeat with different wing configurations.

But then, the data logger cost $800 and is something I use every time I track the car. The sensors cost $12, and can go in the trash once I've done my testing.

Okay, according to this datasheet, the unspecified tolerance is 0.3mm. How much compression are you trying to measure? There's a nice graph on the datasheet that shows the distance/voltage correlation.

I think the differences in downforce are not going to show up very well in a suspension travel measurement. IOW, I am betting that the subtle differences in downforce are not going to cause any easily repeatable/measurable suspension ride hieght changes. I think it would be fairly easy to come up with some sort of temporary wing mnts that incorporate scales or air cylinders to monitor down force changes due to different aero setups. If you really wanted to keep it simple, maybe you could incorporate a spring under each rear mount then attach a bolt or something with a friction slip ring of some sort that would register the amount of maximum movement down against the spring after each trial run.

If you are just comparing, you really dont need to know the actual force involved, you just need to know which wing deflected the most.

Having done some interesting things with syringes (in the lab, not in my arm) I can tell you that they have a fair amt. of friction, and are not designed to make multiple movements up and down. They will fail pretty quickly. Maybe they'll last long enough tho.

Great ideas. Downforce and drag (load sensor reading at same speed/road) have to be balanced.
What about the lipstick camera and a measuring stick?

The guy who did the rocket engine tester suggested using a light application of KY Jelly as a lubricant on them. But you're right, they're less than a buck each and could be easily replaced if I needed to do more than a day's testing.

Great ideas, Jack. I like the idea of searching for a cheaper solution so we can obtain SOME data over waiting for the money and time to build a perfect system to get ALL the data.

I am not sure, but it does seem like road surface irregularities would affect your downforce measurements. I think Tim's idea of using an air tube type mount to measure pressure in the wing mount might work better. How about a bicycle pump (one of the hand types) with an adapter fitting to go to a cheap air gauge. It might leak too much, but maybe either it would work for short term measurements or you could develop a way to reset it by adding air back in from a pressure tank to reset it back to a nominal height.

You could also use hydraulics, I suppose. Maybe install a cheap brake or clutch master cylinder inline and run the hydraulic line to a gauge. Make sure it doesn't leak or your paint will suffer.

For amplifying the suspension movement, how about a bellcrank. Picture a 90 degree piece of metal that rotates about the center of the angle. Make one side longer than the other like the capitol letter L. Connect the suspension up/down motion to the end of the bottom part of the L and connect something for the sensor to bounce off of to the top of the L. The actual motion measured by the sensor will be from side-to-side rather than up and down. The amount of mechanical motion is proportional to the ratio between the two lengths.

Decent animation here (not mine)
http://www.flying-pig.co.uk/mechanisms/pages/bellcrank.html
Now that I reread this thread it sounds like this is what you are talking about for the front.

Jack, not to be a thread jacker(OK, maybe I am), but does your data acquisition system output the data live on a serial port or can it only be downloaded after the fact or perhaps both?

Did I mention I like threads like this?

Stupid question Jack, but don't you have a roof rack?? I would think it would be much easier to rig something up to put on the top of your car as a test fixture. Then a simple scale could be used to read down force, thus eliminating the whole mechanicl fulcrum problem. Even a fish scale might work and you could use one camera for force and the other for your speedometer.
On a side note ( and not to hijack this thread) has anyone looked into building their own scales for corner balancing ??

You can amplify the distance travelled with cable & pulleys. One pulley will double the distance a cable will travel, for the same movement of the load (think block & tackle) with a spring (I'd probably try surgical tubing) to take up the slack.
http://forums.pelicanparts.com/uploa...1154576068.jpg

It's capable of both. It can output live and then it also burns everything to a compact flash card. Live data would be useless for this application, I think. The differences will probably only be apparent in the averages that result from the thousands and thousands of samples.

That would be a good way to test the wing, but not a good way to test how the wing works on the car. For example, the airflow back where my wing is will be coming down at an approximately 10 degree angle, which is different from the flow over the roof. This 10 degree difference in what the angle of attack has always appeared to be and what it actually is is one of the things I'm interested in testing out. My guess is that I've always had my wings set about 10 degrees too steep.

I haven’t read most of these ideas so this may have been suggested....Why don't you rig up a mechanical bellcrank pushing / pulling a cable run through tubing (like a bike brake cable)? The bellcrank would amplify the reading for greater resolution. Easy to calibrate. You could run one from each wheel up to a panel, close to the speedometer, where the ends of the cable would move a flag or arrow in and out on a simple scale. Try to make it 2:1 to suspension travel.

I would think you could rig this up very cheep. Easy to calibrate and very accurate. You could capture readings of the panel and speedometer with a digital camera (on movie mode) for later review and analysis.

Unless you are set on an electronic idea as much for the challenge of making it work for pennies as the data you will get from it. Nothing wrong with that either.

i agree with tim. too subtle. find a real smooth road...replace your shocks with machined rods with integrated strain guages. the machined rods should be cheap...i don't know how much strain guages are.

Jack

I'm not so sure that this is the right way to do this.

You're trying to measure a change in pressure (using your dial gauge), which you will then convert into a force measurement.

For maximum accuracy, you should try to use nearly the full scale of the pressure gauge. This is because (on a hypothetical 50 psi guage) it incurs far less error to read a pressure change of 45psi (+/- 1 psi error) than it would be to measure 2psi (with the same errir) on the same gauge.

So, for a given expected force (100lbs ~ 45N), and for a given number of syringe supports to the wing (2), and for the above gauge (45psi ~ 0.310 MPa) you would chose syringes that have a cross sectional area of 0.75 cm^2 (using Pressure = Force / Area, and metric units because I can't understand imperial!)

i.e. smaller is probably better. As you've found, the volume change required to get a reading on one of these gauges is pretty small, and could be minimised by using braided hose to connect syringe to gauge, and possibly reinforcing the syringe?

Obviously you'd have to mess around with the numbers to suit your application, but I think the theory is pretty sound, and could give some really useful numbers about the DF that the wing can produce on its own.

Unless you can find a really really smooth section of road, I think this technique is going to give you the most comparable data, back to back.

Anyway, just my 2 cents worth. Good luck with the project.

Mark

*edit*

Should have made it clearer that I'm referring to a hydraulic system, using a virtually incompressable fluid. Air changes volume significantly under pressure, so you'd get too much movement at your syringe "transducer" end.

Even if your no forward movement static test worked the moving track surface would very likely scatter return bounce too much.

Quick, pulled it out of my ass in 5 min, needs fine tuning methods.
1. Make a pulley and cable system using bike brake type cable and parts. Well within the 3 tacos and a cold 6 pack cost zone This will allow you to do two things. One, to mount the sensors inboard where they are protected. Two, to "Amp" the movement as needed.
I would amp the movement rather than the signal as the signal is already in the mid range of your read/record system. To amp the movement you can use a disk pulley with the cable wound on the pulley. Mount an arm of any length you see fit on the pulley to "amp" the movement. Then aim the sensor at end of movement arm. Cable moves up and down 1 to 1 with suspension but the length of the sensor end pulley arm combo will set ratio. Disk should be sized so full scale movement will produce 90 deg rotation but wind one extra loop to cover extreme suspension movement and not snap cable. Easy to change as you see fit by moving arm in and out and easy to make. Use a small wound spring to take up slack as cable plays in and out. Mount front setups to the fender in the front on the shock tower. Mount cable to A arm at sway pick up. Rears, drill cable holes and mount setups in back above shock tower but in driver compartment. Run cable over 90 deg turning pulley and down along shocks in shock tube of frame. This will give max "unamped" movement to start.

2. Use same sensor end setup as above but wind the cable pickup points around the sway bars right in the center of front and rear bar and have one front and one rear sensor rig. This would remove single wheel travel inputs and only read overall ride hight data as the bar only rotates in the middle when the overall ride hight changes. My bet is that this would give the cleanest data overall and would be my first try for what you are looking for. Single wheel data would be interesting too but for other things.

Random notes. Do many runs over same track sector to get base line then make one change and repeat. Measure barometric pressure, ground speed and over car wind speed. Head wind will make for more down force at same track speed. Tail wind will reduce. 30 mph wind + or - can make a big dif.


Air pressure change measuring methods would be inaccurate as any temp change will skew your data. The act of moving the air in and out of a shock or piston will change the temp and pressure. Hydraulic methods would be less sensitive to temp but some would still be there. I would also expect to see a very choppy feed from a hydraulic system as there is no natural buffer for vibration from road noise unless you use soft tubing and the tubing would then add inaccuracy.

I do agree that measuring suspension height would be great, because you will see the net aero effect on the entire car - not just the wing.

But I think you will have difficulty getting meaningful data measuring suspension height changes. You've got stiff springs and they won't respond much to minor changes in downforce. If it were me, I'd like to measure changes of 25 lbs or less.

You've got 500lb? rear springs, x 2 = 1000 lbs spring rate. I think a 25 lb change might be lost in the noise.


Here is a way to measure both drag and downforce, pretty easily. It won't show total aero effect on the car, but it will show the full effect on the rear deck and wing combo.



http://forums.pelicanparts.com/uploa...1154581998.jpg


The key is that the wing uprights are attached to the ducktail with pivots. Easy to do. This allows you to put your pressure sensor (syringe) at the decklid latch yet allows you to isolate the effects of drag and downforce.

A second sensor, in this case in tension, is attached horizontally between the roofline and the wing. Thus you can measure drag too.

Key to this setup is the pivots, which allow drag and downforce to be isolated. The front pivots are of course the existing hinges. I would remove the hydraulic cylinder (decklid shock).

Note that to get good measures with the above, the pivot on the uprights needs to be directly beneath the center of pressure (COP) of the wing. The rule of thumb I've heard is the COP is 1/3 back from the leading edge. The wire must be horizontal, and strong enough to support the full effect of the drag.

Just got to find a way to mount that wire.

It could be two wires, from wingtip or upright to rear quarter window. You could still have just one horizontal syringe, and just multiply the measured drag 2X

With some further thought, it seems the drag of the ducktail will create some apparent downforce at the rear syringe. But to the extent that is a constant, you could still get meaningful data.

Is this a constant? I'm sure the wing height / angle has some effect on the ducktail drag. But I'd speculate (just intuition, nothing else) the change in ducktail drag is pretty small given the height of the wing and range of angle-of-attack you'll be playing with.

Jack/Chuck,
Thats not a good way to measure lift/drag. You already figured out the two can couple. And, The syringes would only make a measurement if the two MOVED (well, the tiny amount required to displace the tiny lever in the gauge). If the wing/lig/mounts are rigid enough there is nearly zero movement. If strain gauges were mounted to the uprights, the movement would be <<0.001" W/ the hydraulic setup, you would probably measure more slop in the hinges/mounts than actual deflection from aero forces.

How about building yourself a displacement transducer from a rotary pot? (they exist but are pricey) 1) Attach pot to car. 2) Wrap a fine string/wire around the pot shaft a few times and then to the suspension (so that susp travel 'unwinds' and turns the pot). 3) Wrap another string/wire around the pot the OPPOSITE direction and attach w/ a long spring to the car. The spring will keep the strings in tension (and the pot 'wound'). If the pot shaft is narrow enough you coul measure TINY defelections. A 1 turn pot w/ a 1" diam pulley/shaft is 3.14" of travel, a 1/8" shaft is 0.4" of travel... You would be able to power and hook directly up to your logger.

SMD

Jack,

I've got a ton of wind tunnel experience in a former life, including a bunch of work for Porsche Racing. I'm no longer in the industry (read: no access to the spendy tools and equipment), but am still a pretty big geek when it comes to testing my own projects. Hats off to you for your approaches and here is hoping I save you some time.

First off ride height is nice, but I doubt your going to see a clear correlation between height and downforce. It is there, but it is likely going to get buried in all sorts of noise. I've got some solutions if your after ride height for other reasons, but I'm cursed with fat fingers and rather not type.

As expected, the right way to do this is strain gauges or load cells. Realize that strain gauges are cheap (5 or six bucks each) but for absolute numbers you need to account for materials, side loading, temperature changes etc.. The easier way is to invest in "accurate enough" load cells which can be had for 40-50 bucks. Check ebay for "load cell" and I think you will be pleaseantly surprised. Best case a "S type" load cell, although other kinds will also work. I'll assume your data Acquisity system can deal with a load cell sans amplifier.

Reworking your uprights to incorporate a load cell should be pretty easy, giving you accurate downforce numbers without lateral load effects. Try to get the cell(s) up near the element for a short reaction arm. This gets structural effects of the support out of the picture. If you feeling fancy you can play with some cheap strain gauges at the wing/decklid mount to record drag, but this is slightly more involved.

If the up front costs are too expensive, realize that most of the digital bathroom scales at Kmart/Target have 2-4 strain gauges to yield a load cell of sorts. You'll have to play with getting a meaningful output for data acquisition, and put together a mounting arrangement, but all the parts are there. Only problem is your likely to have relative numbers between airfoil settings and not true calibrated downforce values (i.e. X lbs @ Y velocity). Probably not a big deal in the end.

Even cheaper, which I've used with good success is a hydraulic setup. Head to NAPA and snag some cheap drum brake wheel cylinders. If your really cheap, hit up your local shop or junkyard. Remove the internal spring (if they have one) fill with fluid, bleed and install a pressure sender in the outlet port. Mount as needed using some creativity and measure pressure. Decent pressure transducers can be had for cheap money. Recall that P=FA so measure the piston diameter. Unfortunately, force changes less than 30 or 40 lbs are lost due to seal friction etc. This is the 2 burrito approach your after and will likely be much more accurate than the syringe cable arrangement.

Sorry about getting long winded.

Vin

SMD, I'll respectfully disagree. The sensors need to measure force, not displacement. A bit of slop in the pivots would be irrelevant.

That said, the simple syringe is great for understanding the concept but isn't going to work in practice. It is simply not sturdy enough for the forces involved.

The syringe probably worked great for a model rocket engine producing around 5 lbs of thrust. I believe you need something that can measure full scale in the range of 200-300 lbs, for a single sensor in a given plane. That can be divided if multiple sensors work in parallel.

Vin's suggestion of a load cell is good one for the downforce sensor. You'd need something similar that works in tension for the drag sensor.

i think a cheap scale with a "telltale" on it. kinda like how the motorcycle guys put a ziptie on the shock, and see max movement. then just put the scale in the engine compartment with the various lids ajar slighty. see how hard the lid pushes down. something like a kitchen scale may work.

As for leaving the lid ajar, don't spin. Mine popped loose in a spin once. Not only did I endure lots of comments about my NASCAR style wind flap, I bent the decklid hinge and the decklid split at the seam on one side into an upper and lower section. Just be careful. Mayeb think about adding a safety wire so it can only pop up a certain amount.

How about taking two short pipes where one fit into the other. Then add a spring inside the outer pipe so that the inner pipe is pressed outwards. The principle being the same as for a decklid shock, but instead of gas you use a spring.

On the inner pipe you add an o-ring, and when the inner pipe is pressed into the outer pipe the o-ring will mark the max depression.

Then add four of these to the spoiler, like this:

http://forums.pelicanparts.com/uploa...1154627289.gif

The red lines marks the o-rings.

A rather crude method, but if you know the spring rates you can easily calculate the max force. This method obviously lets the spoiler move, but you can control the amount of movement by the choice of spring rate.

/Peter

Chuck, I see where your're coming from - we need to measure force. BUT *ALL* force measuring devices do so by displacement - however small. A strain gauge does it (you actually have to elongate the resistor), micromachined pressure sensors do it (by displacing a cantilevered beam and measuring the capacitance change), and simple air/hydraulic dial gauges do it (by displacing the bellows inside to move the spring scale).

I originally missed the second pivot in your picture. That does decouple any shift in the lid pivot from the drag 'sensor'. I assumed (wrongly) that it was rigedly mounted and thus any movement of the lid would put force on that sensor. I need to pay better attention to the pic.

It still seems far simpler to glue a couple of strain gauges to the uprights and the exact info we're after w/o any coupling (that can't be mathmatically eliminated) ;) THough I say that because Im an EE. Its also a 4 burrito solution...

SMD

SMD, yes the sensor needs to be a type that allows insignificant movement. Like a load cell, or strain gauge.

Peter, my concern with that spring-based setup is that movement great enough to be measured would allow significant change in angle of attack. Now replace your spring with strain gauges.....

- see what happens to the syringes then go back to this page for the next run(s)....


"I'll assume your data Acquisity system can deal with a load cell sans amplifier."
- see if your data logger lists accuracy, resolution, etc. and what env'l conditions are listed. Post that and folks can help you figure it all out.

If it doesn't list anything then that's a bad sign. But it can always be calibrated with some degree of labor.

Chuck's second pivot point is smart. It solves the problem I faced with the idea of a wire and only letting the decklid hinges pivot. I posted this in the 'Wingin' it' thread:

http://forums.pelicanparts.com/uploa...1154118406.jpg

It still seems to me that what Peter describes with his drawing is the simplest to implement, in a lot of ways. But the springs and a slipring will only show me a maximum reading, which could be the result of gusts or externally-generated turbulance.

If I elongate the mounting holes in the upright so that lateral movement is eliminated, but a small amount of vertical movement is possible, and then use four syringes to bear the actual weight of the wing, it seems like I could get live readings inside the car, easily corrrelated to speed.

It won't give me any data on drag. But drag is a pretty minor concern at this point.