CAUTION: throttle-stop testing

[freebeard]

American History
vs
The multi-disciplinary background doesn't 'hurt,' in the context of how one experiences the world,

Well, I guess they have some limitations for an undergraduate degree. Like... Space Weather.

[redpoint5]

Originally Posted by aerohead

If you were actually interested in the subject you would have had ample time to acquaint yourself with the particulars, study the formulas, do the math, and discover for yourself the utter folly of the technique.
I led you to the water. It's up to you whether or not to 'drink'.
Since you're no doubt aware that, I've made transcription errors in the past, a reading of the actual particulars on the part of all interested parties makes it possible to maintain the fidelity of the data. Method in the madness.

You're mind reading. The fact that I clicked on the thread based on the title suggests an interest. My reason for not spending 2,000 hours researching a very niche topic might have something other than complete lack of interest as an explanation.

It's an insult to proclaim someone is wrong, and provide no basis for the claim other than suggesting people learn everything in the universe to arrive at the level of enlightenment you have achieved.

[redpoint5]

Originally Posted by aerohead

If you were actually interested in the subject you would have had ample time to acquaint yourself with the particulars, study the formulas, do the math, and discover for yourself the utter folly of the technique.
I led you to the water. It's up to you whether or not to 'drink'.
Since you're no doubt aware that, I've made transcription errors in the past, a reading of the actual particulars on the part of all interested parties makes it possible to maintain the fidelity of the data. Method in the madness.

You're mind reading. The fact that I clicked on the thread based on the title suggests an interest. My reason for not spending 2,000 hours researching a very niche topic might have something other than complete lack of interest as an explanation.

It's an insult to proclaim someone is wrong, and provide no basis for the claim other than suggesting people learn everything in the universe to arrive at the level of enlightenment you have achieved.

See how I provided specific criticism rather than say "aerohead is wrong, go study psychology and sociology"? It's comprehensive within seconds to minutes rather than an unpersuasive appeal to invest 2,000 hours into some niche subject.

Any concept that cannot be simply explained by someone demonstrates their lack of understanding of that concept, and nothing more.

[Vman455]

The obvious (and valid) criticism of throttle-stop testing hasn't even been brought up in this thread, despite all the ink spilled. It is: can we be sure that a constant throttle opening really produces a constant torque on a real vehicle? For large changes in RPM where the engine might be in a different part of its torque band, probably not (which is why this technique can't be used to calculate percent change of large changes in drag). For small changes in RPM where the engine is in a narrow part of its torque band, probably. Keep everything else as consistent as possible and test a known change in drag to see if it works on your car.

Before I even attempted this, I wired in a display showing throttle position sensor voltage so I could watch it during testing and ensure the throttle was kept at the exact same opening each run. I then used a wood block with through-bolts that rest on the floorboard and a wood screw on top where the pedal rests so I could adjust the stop. Only then did I go out on the road to try it, and it still took several tries before I got consistent enough that the tests showed a change in drag with windows down.

You need a good, flat road with light/no traffic. Identify a test section using a road sign, light pole, or mailbox as a marker. Enter the test section at the same speed every time and, when you pass the marker, press the throttle to the stop. Once the vehicle has accelerated to its new top speed and settled there, take the GPS speed reading at another marker, using the same one each time. Do this and you should get consistent results; verify using windows up/down, which on most cars increases drag 6-12%. If the results you get are way outside that range, try to figure out what's going wrong.

To ensure that the tests still worked at different throttle openings, I did windows up/down tests on my truck at three different throttle positions. Finding that these were consistent, then (and only then) I tried measuring drag changes of other modifications: grill block (reduced drag), mirrors removed (reduced drag), large air dam (reduced drag), wheel covers (front reduced drag, rear did not), and large spoiler (increased drag).

The maximum change I've measured was 91 kph/95 kph for a drag-reducing change and 93 kph/88 kph for a drag-increasing change. If you're seeing something like 90 kph/100 kph, that large a change in RPM might put the engine in a different part of its torque band; if I saw that on a test I wouldn't bother trying to calculate a percent change in drag.

[Phase]

i tried throttle stop testing on a flat empty highway in texas gojng 90 mph. windows up and down with my ioniq. speed didnt change at all

btw thats all 4 windows. not just the front windows ( no idea if people mean all 4 windows or just front windows when they say windows up versus down)

[aerohead]

I'll keep this simple to begin with, and we'll get into the weeds later.
1) In 1973, Professor Edward F. Obert, University of Wisconsin, and author of my textbook, Internal Combustion Engines and Air Pollution, essentially debunked any notion of a so-called 'throttle-stop testing technique ( which will be referred to from here on out as the 'technique' ) by describing SAE standard practice for road-testing, explaining how vehicle quanta are officially obtained, and providing the formulas which 'define' values of interest, associated with the 'technique.'
2) The first 'elephant in the room' is thermal equilibrium testing, of which all SAE is conducted, and no mention, nor requirement for it is broached within the methodology laid out for the 'technique.'
3) It has been scientifically demonstrated that, testing results, obtained under conditions of thermal nonequilibrium can produce results which are off by up to 67%, with ZERO modifications to the test vehicle.
4) Thermal nonequilibrium testing introduces seven easily identifiable unknowns into test results.
5) In order to thermally stabilize all pertinent parameters of a motor vehicle under road test requires a minimum of 30-miles of driving at 50-mph immediately before testing begins.
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From SAE Paper 780613, the second elephant in the room involves:
6) Testing from a 'cold-start', depending on ambient temperature, which can introduce a data discrepancy of of 27% at a distance of 3.6-miles.
3% discrepancy @ 21-to-32 miles distance.
14% overall average.
With ZERO alteration of the test vehicle.
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Also from SAE 780613:
7) The ambient temperature of the 'short' cold-start test can corrupt the results by 53%-to-36% depending on OSA temperature.
8) Engine oil, coolant, transmission fluid, and differential lube do not reach equilibrium viscosity until the 30-miles @ 50-mph is achieved.
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9) SAE Paper 780596 explains improvements obtained from SAE SE rated, lower viscosity engine oils, and GL-5 transmission and differential lubes, however, they cannot compensate for the lion's share of losses from 'cold-start' conditions.
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10) Hucho et al's SAE Paper 760185 essentially debunks, by default, any notion of a 'technique,'
11) OEM 'torque' ( measured both at thermal equilibrium and 'best-torque' ignition spark advance ) is structurally defeated by a 'stopped' throttle.
12) 'Best-torque' is a function of bmep, which is governed by BSFC, and as defined as bmep= power-times 1224,divided by displacement times engine rpm, once a vehicle changes speed, the consequent change in engine rpm, by default, violates 'best-torque', by definition. Hucho et al. provide no caveats/ conditions for 'exceptions.'
13) The very act of reducing drag, lowers engine load, altering all the 'momentary load points' which previously DEFINED the engine map.
14) 'The new load point normally is associated with a higher BSFC.' Hucho et al..
15) Torque is not 'constant.' It falls because bmep fell, as BSFC climbed.
16) Hucho et al. demontrated that 'potential fuel economy improvement' from a Cd reduction could be compromised by 50%, by not changing the gearing to move the engine back to the original 'load' at any specific rpm. ( And bear in mind that this is all recorded at thermal equilibrium and best-torque ignition timing, which is defeated when the throttle is 'stopped.')
17) No 'correction factors' can be applied to the 'technique's' partial-load tests, as atmospheric pressure is no longer controlling or limiting engine output.
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18) ' automotive engineers prefer the specific fuel consumption as a parameter to indicate efficiency.' Professor Obert. page-94.
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19) SAE J1082a, Road Test Procedure, SAE Standard Practice, and Gino Sovran et al.'s SAE Paper 810184 & SAE Paper 830304 expand on Hucho et al.'s SAE Paper 760185. Again, no caveats, no conditions, no exceptions.
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20 ) In his PREFACE to his 1987, Aerodynamics of Road Vehicles, 2nd-Edition, Wolf-Heinrich Hucho admonishes: '(T)he vehicle aerodynamicist must refer to a large amount of detail resulting from earlier development work.'
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21) We know that Mr. Edgar has been in possession of Hucho's 2nd-Edition, which would make Hucho's 1976 research available, as Fig. 3, from 1976, reappears as Figure 3.7. page-90, in the 1987, Aerodynamics of Road Vehicles.
22) Why Mr. Edgar so vigorously defends the 'technique', while having been in possession of official SAE testing evidence which is counter-factual to his thesis of 'constant-torque' escapes me.
I'll be happy to 'go deeper'. Please limit questions to 'technical', 'quantifiable'
aspects of the 'technique.' ( nothing subjective, anecdotal, etc..)

[Phase]

Then what’s the best way to test?

[freebeard]

A Templated tow vehicle, 100ft cable, and a spring scale.

[Vman455]

Originally Posted by Phase

Then what’s the best way to test?

Throttle-stop testing is just one method for measuring drag changes--a very good one, but one that may not work on all cars (e.g. electronic throttle controls). Other methods include measuring fuel economy over long distances/repeated long trips, and coastdown testing. You have to be careful with coastdowns; you can find my attempt to improve coastdown testing on the road here.

But drag changes are just one aspect of aerodynamic measurement. Tuft testing and pressure measurement (of both body panels and heat exchanger flows) will tell you a lot. Ride height sensors will show you whether your car is creating lift or downforce. If you buy Julian's book, you'll see new techniques for measuring lift and trailing vortex strength/direction.

Approach aerodynamic modification like an investigation: Your job is to figure out what happens to airflow as you change the shape of your car. Same as if you were diagnosing a fault code to figure out what's wrong with an engine.

For example, I bought a cheap Hellcat spoiler on Amazon to test. I didn't just use one method because that wouldn't give me a complete picture of what this spoiler does on my car. I taped it on and measured pressures on the rear window and hatch; I tufted the window and spoiler and videorecorded it; I did several coastdown tests and ran a statistical test to analyze the data. Even having done all that, I still don't have a complete picture, as I don't know what that spoiler does to the trailing vortices, for instance (so I'm planning on finding out soon).



Unfortunately, what you'll find on this site and a lot of others is a belief that guessing is better. It isn't.

[Phase]

Originally Posted by Vman455

Throttle-stop testing is just one method for measuring drag changes--a very good one, but one that may not work on all cars (e.g. electronic throttle controls). Other methods include measuring fuel economy over long distances/repeated long trips, and coastdown testing. You have to be careful with coastdowns; you can find my attempt to improve coastdown testing on the road here.

But drag changes are just one aspect of aerodynamic measurement. Tuft testing and pressure measurement (of both body panels and heat exchanger flows) will tell you a lot. Ride height sensors will show you whether your car is creating lift or downforce. If you buy Julian's book, you'll see new techniques for measuring lift and trailing vortex strength/direction.

Approach aerodynamic modification like an investigation: Your job is to figure out what happens to airflow as you change the shape of your car. Same as if you were diagnosing a fault code to figure out what's wrong with an engine.

For example, I bought a cheap Hellcat spoiler on Amazon to test. I didn't just use one method because that wouldn't give me a complete picture of what this spoiler does on my car. I taped it on and measured pressures on the rear window and hatch; I tufted the window and spoiler and videorecorded it; I did several coastdown tests and ran a statistical test to analyze the data. Even having done all that, I still don't have a complete picture, as I don't know what that spoiler does to the trailing vortices, for instance (so I'm planning on finding out soon).



Unfortunately, what you'll find on this site and a lot of others is a belief that guessing is better. It isn't.

I follow your website. Yeah testing is hard. Usually the best way that I’ve been “ testing drag” is “ feel of my car”. I drive so much that I can tell if something is off. Like you know how you can feel that difference in driving power on a wet road versus smooth dry concrete? I can kind of feel the diff with aero drag. I can feel how my car drives with and without my side mirrors on since I put them back on any time I go somewhere for an oil change or tire rotation and balance and so on. I know it’s not scientific, but it’s like muscle memory.

For example, people here are telling me to start my “ boat tail” at the end of my spoiler, and not to follow the roof line of my hatch because it doesn’t “ match the template “

But I’m assuming I can just extend the roofline and then add a spoiler for pressure recovery and maintain attached flow and tuft test it. Obviously I’d be able to “ feel” a diff with a boat tail keeping attached flow even if it’s not the template. I mean a Tesla model 3 has a pretty steep rear roof but a little spoiler and keeps attached flow