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Old 05-04-2020, 06:05 PM   #9 (permalink)
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Originally Posted by M_a_t_t View Post
When replying I wrote a paragraph filled with questions. I think it would be easier to list them.

1.You said "most of the aerodynamic modifications that I see people undertaking do very little, especially those that attempt to decrease drag." Are you suggesting the mods themselves are ineffective or that our current testing system is not good enough to show small improvements and therefore unable to tell if they are actually helping?

2. Do you think these smaller mods would show up using the throttle stop method and could you give some insight into when you think mods start getting too small to make a measurable difference?

3. Would a higher test speed change the threshold of mods too small to test? E.g your experiments have been around 105 kph (65 mph) if we tested at ~125 kph (78 mph) would smaller changes be more evident?

4. Can you link those papers? I have been trying to get a small collection of reading to do this summer on aerodynamics (when I have the extra cash to purchase some of the texts anyway). If it seems I am avoiding the other methods of testing it is because I haven't completely grasped how to interpret results from them. This is not a direct request for you, I just haven't done my part to sit down and read up on anything yet to be able to ask informed questions about them.

5. I do have 3 questions for the magnehelic gauge. The "ideal" car would have very little pressure change over the surface of the body correct? So with the magnehelic gauge you check for high and low pressure zones and try to reduce the size of these zones. I think thats how it works right? Is there a limit to effective hose length? For example, if I have the gauge towards the front of my van and ran a hose (~15-20 ft or 5 meters) to the top edge of the rear would the hose act as a restriction to the changes or is that effect really tiny and therefore wont matter? The hose running along the body line wouldn't be enough to change the results of the test correct? Like if I had a hole in the roof above the driver seat and ran the hose down the length of the roof rather than just poking out the top.
1. Probably a bit of both. For example, moving from a good standard wheel to one with a full cover I doubt does anything at all, and if it did in fact do anything, it would be so small as to be next to unmeasurable. I also see lots of modifications that I am sure do nothing. (Unfortunately what I have found with discussion groups is that if you attempt to call these out, people get very upset. Notably, there is never any evidence that these mods work. I've stopped looking through build threads because I find it so frustrating.) To keep it simple, I think the physical size of the modification usually correlates pretty well with its likely impact - positive or negative.

2. I am still exploring the throttle stop method but I think that mods that makes less than a calculated 1-2 per cent difference in drag won't be measurable. My latest test session showed that pretty well I think. Taking the rear spats off the Honda didn't give any real measured change (only within margin of error), whereas opening the windows is always measurable, with a greater difference occurring in gusty crosswinds. To put that another way, if I can measure a change that is anything like as great as that caused by opening the windows, I know I am measuring something real. (That was the case with the Edgarwits, so they are very exciting to me.)

I have been measuring coarse body aero pressures since 2000, body panel pressures since 2018 and lift/downforce also since 2018. I think I have a good feel for these measurements in that I can quickly see if things are going in the right direction or not - as the case may be! What I have found with these measurements, and it looks the same with throttle stop method, is that it's best to make large changes and see what happens. If they appear to work, you then finesse them. Fiddling around with initially tiny changes often means you spend a lot of time and get nowhere.

3. Yes, as I have said in the videos, test at the highest legal speed. I have done some testing at 140 km/h and I did notice that the throttle-stop top speed wasn't held as consistently as at lower speeds - there was say a 1-2 km/h variation around a mean. But the mean was still obvious to the eye. Of course, from an aero perspective, top speed without a throttle stop is best of all, but then there is a much greater likelihood of the engine's power output varying as the engine will be developing full power eg on a turbo car intake air temperature rising. (I did do just one top speed run, and noticed that the speed at the end of my straight (still rising, but I don't have a longer straight) was 190 km/h, down 10 over what I achieved with the rear wing set to provide less downforce. That's when I started thinking about the throttle stop method... so I wouldn't be arrested.)

4. For coastdown testing: SAE 950626 - ABCD ľAn Improved Coast Down Test and Analysis Method and SAE 940420 - A Detailed Drag Study Using the Coastdown Method. I think after you read these papers you will probably discount doing it without their equipment and computer modeling. Measuring of body pressures with the surface measurement sensor, and measuring downforce, are as far as I know covered only in my book - I've not read coverage anywhere else on how an amateur can do these.

5. Actually you want to maximise low pressures on the front of the car (creating thrust) and decrease low pressures on the rear of the car (reducing drag). But even that isn't accurate, because you need to take into account the vector (direction) those forces are acting in. (Page 15 of my book.) But in fact you don't even use a Magnehelic gauge like that much anyway! There are basically two uses of the Magnehelic gauge:

(a) Fairly coarse measurements eg finding the right place for hood vents, measuring pressure drop across a radiator core (eg to see if an undertray is reducing radiator flow), making cooling ducts that work, etc. These measurements are easy and don't require a reference pressure reservoir or any sensor.

(b) Measuring very small pressures eg on body panels above and below the car. These require the use of a surface pressure puck (I made my own) and a reference pressure reservoir. You need to be quick and careful when doing these measurements. You can use these when assessing if an undertray is working at developing a low pressure (reducing lift), see if a rear spoiler is increasing pressures on rear panels, see if average wake pressures are being changed, etc.

It doesn't matter in terms of your pressure readings how long the hose is. As long as it has no leaks, it can be (say) 5 metres long without issues. You may find the reading is more damped (changes more slowly) - that's all. When doing coarse readings, the presence of the hose doesn't make any difference. In fine readings, I always use a very small diameter hose near the surface pressure measuring puck, and try to position it directly upstream or downstream of the puck so that the blockage factor is kept as low as possible.
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