Guys,

I've been reading a little about aerodynamics lately and was thinking about how one would go about practically measuring drag and lift.

I was thinking: for a given AFR ratio, at a given speed, and controlling everything else as much as possible (gear ratio, tyre, tyre pressure, weather), could you
get a measure of the energy required from the injector pulse width required?

That would give you something of a measurement of how much power/energy is required to move the car at a certain speed.

For lift (as in negative), I was thinking you could just have a linear pot (which I hopefully have access to) between the suspension members to measure the height of the car. Controlling the bit of ground you went over and plotting the values against one another should give an indication any additional compression of the springs caused by downforce. Knowing the spring rate then lets you calculate the effective downforce at either end quite readily.

Any thoughts?

Dave

_________________
USA Daily: 2014 Nissan GT-R, very minor modifications for the track
USA Project: 1978 280Z, minor suspension upgrades, VK56DE conversion in progress. SOLD
AUS Race Car: 1973 240Z, L28ET, Autronic, GT35R. SOLD
AUS Project: 1972 1600, 3200km old S15 SR20DET, ground up rebuild. SOLD

Hi Dave,

Coast-down tests are normally used for drag measurement. Very simple & cheap. Not much to go wrong. Need somewhere that's not windy though.

For downforce, I remember reading that in the 60's people ran a bit of steel rod attached to the front suspension, up through the body work. The driver could simply watch the rod go up & down. Rod had marks on it in set increments, so a dimension could be obtained.

These days, you could probably avoid cutting holes in bodywork, and use a video camera (with a lipstick camera, perhaps?), and use the same basic technique.

So that's a similar thing to what you had in mind with the linear pot.

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Recommended trading: datto-510

I recall reading an article in zoom where they tried using the AFM voltage to measure drag at a particular speed. The coast down method sounds like a good one. If it was an alteration that you could easily take on and off the vehicle you could do say 10 tests, 5 with the mod and 5 without, and alternate the tests to try to average out noise like wind conditions.

Coast-down tests sound good, does anyone have more details? or links? I am really only interested in racing speed though, so I would probably modify them to suit. They are also a little less useful, though more practical, as they are taking an average across a number of speeds. Controlling the speed would be important I think.

Other things that I'm not sure how to test, how would I measure drag for different pitches, ie, accelerating and braking?
I suppose the main issue with pitch sensitivity is downforce, and I can get that from the linear pot. Might be trickier given the shorter time things will happen in. I imagine if I'm standing on the brakes at speed it won't take too long (time wise) to drop down a lot. Would have to make sure I get the driving procedure as repeatable as possible, markers on the road at which I start braking/accel and the like.

Re the camera/rod, great idea but it sounds expensive/dangerous for my camera, and doesn't really take into account bumps in the road. Atleast with a linear pot I will be able to get a number of runs on a given stretch, average them to get a baseline, then do the same for different aero configs and compare the plots, and maybe average the difference between the two to quantify how its working. Just seems much easier than watching videos 100s of times.

Thanks guys,

Dave

_________________
USA Daily: 2014 Nissan GT-R, very minor modifications for the track
USA Project: 1978 280Z, minor suspension upgrades, VK56DE conversion in progress. SOLD
AUS Race Car: 1973 240Z, L28ET, Autronic, GT35R. SOLD
AUS Project: 1972 1600, 3200km old S15 SR20DET, ground up rebuild. SOLD

Why not grab yourself a webcam off ebay? They can be had for virtually nothing. Then you could write yourself a python script to analyse the video for you. Saves some fabrication for the linear pot, interfacing circuits, a datalogger etc etc.

Pics can be quite useful in regard to visualising lift. Zeds without an effective front spoiler definately lift substantially at the front, the S30 is very bad for that.

Then there was the rear spoiler test carried out in the US which showed that an angled flat aluminium plate mounted at the rear edge of the hatch both reduced lift and drag.

The coast down test for drag sounds good, just need somewhere to do it. For lift I'm sure that there are simple ways to measure it eg by using bent wire pieces attached to the suspension and body that will change in length when pulled.

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Richard... http://www.youtube.com/watch?v=rqM9_Wq-TKc

Bob,

Pot, interface and data logger I hopefully am loaning from a mate. So the linear pot is really the easiest to set up.

Some other things I've been having ideas about, and where a camera would be useful, is things like testing the use of vortex generators at the top of the rear hatch to delay boundary layer separation. I think if you tape some string at intervals down the match you should be able to visualise where the flow separates easily enough?

Dave

_________________
USA Daily: 2014 Nissan GT-R, very minor modifications for the track
USA Project: 1978 280Z, minor suspension upgrades, VK56DE conversion in progress. SOLD
AUS Race Car: 1973 240Z, L28ET, Autronic, GT35R. SOLD
AUS Project: 1972 1600, 3200km old S15 SR20DET, ground up rebuild. SOLD

Bob,

Pot, interface and data logger I hopefully am loaning from a mate. So the linear pot is really the easiest to set up.

Some other things I've been having ideas about, and where a camera would be useful, is things like testing the use of vortex generators at the top of the rear hatch to delay boundary layer separation. I think if you tape some string at intervals down the match you should be able to visualise where the flow separates easily enough?

Dave

_________________
USA Daily: 2014 Nissan GT-R, very minor modifications for the track
USA Project: 1978 280Z, minor suspension upgrades, VK56DE conversion in progress. SOLD
AUS Race Car: 1973 240Z, L28ET, Autronic, GT35R. SOLD
AUS Project: 1972 1600, 3200km old S15 SR20DET, ground up rebuild. SOLD

Dave,

Don't have any links, sorry. It must've been close to ten years since I was looking into this stuff.

I vaguely remember that the different speeds thing isn't the big issue you'd first think. I seem to recall that when you work it back you get a Cd figure. Speed + Cd + frontal area -> drag.

I'm not sure what kinda suspension you're running, but if you had C/O's, you could dial in the pitch that way.

The camera itself isn't actually tucked into the wheel well. When I did some spanner-turning for a formula holden team, we had to mount the camera for the onboard TV coverage. The only place for the camera was in the nosecone (seemed a bit venerable to me!), and then a little wuzzy lipstick camera was placed on the roll hoop. Whilst I haven't looked into it, I thought those little bubby cameras didn't cost that much these days? Bob's suggestion of a webcam isn't too bad.

You're right that the camera doesn't account for bumps in the road, but I can't see how a pot would either?

If you want to get carried away, you could wind tunnel test a platic model kit. Not real-world, of course, but you can do a lot in a small time, and for small money. Making 50 different sets of bodywork in 1:1 scale is gonna be costly! When I was in high school, I made a small wind tunnel and tested different ideas I had for sprint kart bodywork. It's not as hard as you'd think.

_________________
Recommended trading: datto-510

Dave,

Are you trying to convert your negative lift into positive?
What is the reasoning behind wanting to know what your car is doing in terms of drag and lift?
(stupid question, but I have to ask as it might give me some hints to which direction you are heading!)

_________________
1972 Datsun 1600, S14 SR20DET Engineered (204rwkW @ 17psi.)
viewtopic.php?t=6579
#SR20Datsun @SR20Datsun

I want negative lift, as much as I can get at both ends. Then I have another method to tune the car's balance in high speed corners, Lucas Heights springs to mind.

Dave

_________________
USA Daily: 2014 Nissan GT-R, very minor modifications for the track
USA Project: 1978 280Z, minor suspension upgrades, VK56DE conversion in progress. SOLD
AUS Race Car: 1973 240Z, L28ET, Autronic, GT35R. SOLD
AUS Project: 1972 1600, 3200km old S15 SR20DET, ground up rebuild. SOLD

Dave,

One suggestion I recommend (and shoot it down in flames if you wish) is to put an underbody onto the Z. This cleans up the parisitic drag underneath the car and will slightly lower your drag co-efficient. (but only by a couple of percent...) In addition it would lower your positive lift slightly. (that's if you have positive lift at the moment.)

Putting a front air dam on the front and an underbody tray would be a pretty good method of getting more negative lift, but this increase in frontal area would give you greater drag. Don't forget also you can make up a rear diffuser. I'd try all

I did a thesis on this in my final year of Mech Eng, but I did it for a 1600!!! not a Z! I guess the difference is like a brick and a bullet respectively! (HAHAHhahahaha )

_________________
1972 Datsun 1600, S14 SR20DET Engineered (204rwkW @ 17psi.)
viewtopic.php?t=6579
#SR20Datsun @SR20Datsun

I assume you mean Lukey Heights?

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Classic Auto Fabrications:
http://www.ozdat.com/forum/viewtopic.php?f=38&t=31659

http://www.lescollinsracing.com/

Actually a front air dam will decrease frontal area at speed because it will hold the front of a Z down. It will also reduce drag by reducing the amount of air flowing under the car while an undertray will reduce drag because it smooths out air flow underneath and through the engine bay.

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Richard... http://www.youtube.com/watch?v=rqM9_Wq-TKc

260DET,

I don't understand what you mean.

My logic is bolting on an air dam increases the frontal area no matter what. Even if the ride height changes, the frontal area is still the same. Unless you are talking about the pitch of the car? But if you are talking about the pitch of the car, the faster you drive, the more pitch is induced (due to greater downforce) and therefore the cross sectional area increases... Therefore the drag would increase...

Reducing the amount of flow underneath the Z means the flow has to go around the car or above the car instead. This effectively increases drag as you are forcing air to go somewhere it naturally would like to go but is forced to go somewhere else. So by putting a front air dam on, you are shifting the point at which the air swaps from going under the car, to going over and around. This effectively adds drag, but would provide negative lift or downforce.

To go fast in a straight line, you want no lift either positive or negative. (maybe slightly negative). To go fast around a corner, you want heaps of downforce at the expense of drag. Basically because you slow down to go around a corner (well some of us do!) Drag is proportional (as Beef said earlier) to area, and the square of the speed you are going at.

_________________
1972 Datsun 1600, S14 SR20DET Engineered (204rwkW @ 17psi.)
viewtopic.php?t=6579
#SR20Datsun @SR20Datsun