Measuring aerodynamic pressures on the road or track

After tufting, measuring aerodynamic panel pressures is the most important aero test technique you can use. With this technique you can immediately see how well spoilers, undertrays and diffusers are working, measure changes in base pressure impacting drag - and do a lot of other things. Buy my book at www.amazon.com... or Amazon in your country.

[@theshed8802]

Bought a copy of your book. Received it last week. Now that I have it in my hands, I think that it's exceptionally good value. For those who are thinking about getting the book, it's a large, thick book with lots of excellent colour photos in it. It's well written, and very informative. I've still got a lot of reading to do, along with a lot more learning. Highly recommend anyone watching this video to buy one.

[@JulianEdgar]

Thank you. Don’t forget to leave an Amazon review!

[@louisthibault555]

This is super helpful man! thank you! I may just pull the trigger on this, as a Christmas present to myself 😄

[@camposvazquez]

​@@JulianEdgarSir EJ. I own a 2nd generation Prius (2004-2009).. does your book have examples for my vehicle? My goal is to increase my vehicles fuel efficiency Thank you

[@JulianEdgar]

@camposvazquez The book isn't about covering specific vehicles and then you copy what has been done. Instead it's about developing techniques that allow *you* to modify whatever car you have. Yes, reducing drag is covered in detail.

[@camposvazquez]

@@JulianEdgar thank you

[@LochyP]

You honestly give better presentations than a lot of lecturers i've had in my degree. And you're giving it away for free! Thank you.

[@JulianEdgar]

Glad you like them!

[@intensedabberoniJG]

I have also found that a lot of KZbin presenters for a wide range of subjects tend to be far more concise and accurate than many professors I've seen through different subjects and degrees. I think it comes down to people on KZbin being truly passionate about their subject, many of them having devoted themselves to an extent that they can grapple with it on multiple levels of understanding and can profess it comprehensibly to more people than even a college lecturer. Very interesting that the knowledge we pay for is gatekept from us in a way but the knowledge we get for free has potential to be so rich.

[@Arafury57]

I had a HG Holden no tech ute years ago. I used to watch the sun visor (hand brake) wobbling in the wind. I removed it and reckoned I picked up nearly 2 mpg. It got better mileage on the highway than most cars I've had since.

[@erikredIV]

Another sweet video!

[@TheJcman72]

This is very interesting. More than the Model 3 image with absolute values, the image of the Insight with those vectors is way more useful because you get a quick and clear idea of the full effect around the car (load, lift and drag). A good thing would be to also have a single vector that would represent the full car equivalent, but also another two vectors to have an equivalent on each axle, because from your pictures we can clearly see that the front of the car can create a lift on the back of the car. At the end this is a complex system. About the color coded pressure image, the dark blue is a direct consequence of the nearby dark red that will create an aerodynamic air flap (like the deflector in place of a windshield in the Renault Spider), but what I feel is that we must consider an average for that front section to understand the effect of the front quarter of the car. It would be nice to have that same picture type of other objects like the fuselage of the Concorde, a droplet of water, or simply from a sphere or a cube. About the sensors, this is a very interesting bit, because it makes me think about those strange structures around F1 cars in pre-season testing. Although it is very easy to understand what they are doing, figuring out how they do it is far more complex. I have not yet understood how they manage to get measurements from such a thin structure.

[@JulianEdgar]

A lot more detail (including measuring techniques to show lift/downforce on each axle ) in my book - www.amazon.com/Vehicle-Aerodynamics-Modification-Development-alternative/dp/B0C87VYVL8 or Amazon inyour country

[@heitorbernardes7977]

4:07 to properly 3d print one of these pressure patches... Should I epoxi coat it before drilling the sensorring hole?

[@tomn2765]

Has there been much research into the benefit/drawback of prefilter foam being added to the front grille as a way of reducing excess airflow into the engine bay without using blanking plates?

[@JulianEdgar]

It doesn't matter what you use to stop the airflow. Stopped airflow is stopped airflow.

[@songqili8211]

Can you talk a bit about the reference pressure? Especially in a wind tunnel.

[@JulianEdgar]

I don't cover wind tunnels - I know very little about them.

[@misterbarry9653]

I bought the book and I am finding it genuinely useful. Would I be right in thinking a MAP sensor lacks the sensitivity required for aero work such as this? I have plenty of spare channels in the logger so it would be nice to log multiple pressure points simultaneously.

[@JulianEdgar]

You can use a MAP sensor, but it will likely be on the edge of its resolution - and the logger will likley be too. But specific, differential low-pressure sensors are available eg au.rs-online.com/web/p/pressure-sensors/7191080?cm_mmc=AU-PLA-DS3A-_-google-_-PLA_AU_Pmax_LocalStock_0923-_--_-&matchtype=&&gad_source=1&gclid=Cj0KCQiA4Y-sBhC6ARIsAGXF1g4HvkRnSBbHkc5qVUnbuMU4qSHk6zke3uwNDkSjEeHBj9FojPbKf0gaAhg1EALw_wcB&gclsrc=aw.ds

[@user-rs1fo2dd9b]

sir do you recommend particular specifications for magnehelic gauges? whether measures in inches of Hg vs inches of water, the range of inches of either Hg or water, etc?

[@JulianEdgar]

0-1 or 2 inches of water pressure. Doesn’t matter what units.

[@blairo15]

G'day Julian, I love your videos and have just ordered one of your books. I have a quick question for you... what's the purpose on a lot of production undertrays for a lip at the rear of the front undertray about 10 inches wide in front of each front wheel? Regards Blair Fraser

[@JulianEdgar]

Wheel deflectors to reduce drag.

[@getahanddown]

Hi Julian, I have a specific issue I can't find much info on. When sitting on high speeds (cruising 110-135km/h) my air con drain hose seems to get positive pressure and push the condensation through the system instead of dripping out. Would trying something as simple as a 90° elbow to have the opening facing rearward? A small wall infront of it to make a low pressure spot? I had thought of using a fish tank air pump to draw on it but far prefer a 'dumb' solution like a correctly placed hose over a bunch of electronics that are in the way of free moisture escape.

[@JulianEdgar]

Why guess? Just measure the pressure...

[@getahanddown]

@@JulianEdgar I would have already if the family weren't christmassing while I have time to work on the car :) Next week I'm off to the wrecker for a spare set of hoses I can butcher / T into

[@pjay3028]

Hi, thanks for all the brilliant videos. I wondered if you could help me? I have a Toyota MR2 Mk3 Roadster, this car has slats for vents in the rear engine cover above the engine. I was wondering which way the air will flow through these vents and whether it will actually flow upwards when the car is stationary and then down whilst the car moves? If I wanted to install a fan for greater cooling in the engine bay, which way would I want the fan to move the air? Any thoughts would be great, thanks.

[@JulianEdgar]

Tuft test.

[@pjay3028]

@@JulianEdgar yes, I tried that but it was inconclusive unfortunately.

[@JulianEdgar]

Ah, in that case the next step is to pressure test.

[@pjay3028]

@@JulianEdgar I'll give it a go and see, thanks

[@JulianEdgar]

It's an easy one - just tubes from each port of the gauge / manometer to the two areas you're interested in.

[@thomassavage2150]

If I plug the sensoring patch on the high pressure hole on my gauge and the pitot on the low pressure and the needle go clock wise, does it mean I have high or low pressure ? Should I always plug the pitot on the lower or higher pressure on the gauge ? I did the experiment and the first time, i placed the patch on the trunk of my car and the needle on my gauge went counterclockwise. So I switched the hoses so the needle went clockwise. But then I put the patch on top of my car and the needle went counterclockwise... so now I don't understand where to plug where and what the gauge mean...

[@JulianEdgar]

Test where you are certain there is a high pressure ie the stagnation zone at the front of the car. Then work it out from there.

[@thomassavage2150]

@@JulianEdgar ok perfect. Do I put the hole on the patch facing the direction that I drive or facing the sky ?

[@JulianEdgar]

Flat on the car body panel. Buy the book and see how it’s done in proper detail.

[@michaelbenefield8279]

What pressure range would you recommend in a magnahelic for testing aero on a street /armature race car up to about 100mph (166kph)? The only images of the cover of your book are not high res enough for me to tell, but it looks like it is a positive pressure only, with a range from 0 to 1 inches of water column. Is that a good range? The range seems a bit small to me. 1" WC is about 0.036psi. A single element wing might be 10"x50" so about 500 in sq. 1" WC differential on that wing would only be about 18 lbs. Not an impressive number for a wing that size. It looks like the next larger range is 0-10 "WC, is that too low res? Any thoughts on the numerous digital manometers? The brand name ones seem similar in cost but with more features and range than a magnahelic.

[@JulianEdgar]

I think you should buy the book. eg there are better ways of measuring wing downforce than by measuring pressures, and better to measure at 50 mph than 100 mph, and...

[@michaelbenefield8279]

@@JulianEdgarFair enough. I ordered Vehicle Aero, Test, Mod, and Dev about 20 minutes before I asked. It is my birthday present. I've also been pricing out manometers, pitot tubes, and sheet aluminum. FYI prior to my last race I put substantial effort into reducing drag. I reduced the angle of attack of my wing, blocked off lots of front end openings to the engine bay, ducted the radiator flow from right above the splitter, added diverters to direct splitter air over the front wheels, added rubber strips for clean separation on the back edges, and made vents to reduce under hood pressure. Of course there are some non-aero factors that are hard to account for, but we did see about 6 second a lap better on each driver's fastest laps, and we were seeing about 6mph faster at the end of the longest straight! 6 seconds a lap on a 2:25 typical lap is huge. The main thing I have not done is underbody work, because that work is substantially harder to do. But I am excited about doing it before the next race. Sadly this race car is not street legal, so I cannot do testing except under race conditions. BUT we have a second race car in the planning stages, and it will be street legal, at least at first. While it is still street legal I hope to develop our aero and do plenty of testing to validate, and to tune the mods. As a mechanical engineer, I appreciate your data centered vs simulation or guessing approach to car aero.

[@JulianEdgar]

Sounds good.

[@temporalmadness3756]

Julian, wich of your books would you reccomend for someone just starting on aerodynamics? In road tests, what wind condition is better?

[@JulianEdgar]

www.amazon.com/Vehicle-Aerodynamics-Modification-Development-alternative/dp/B0C87VYVL8 or Amazon in your contry. For a beginner, it will be a case of reading the chapters largely in sequence, and taking it slowly. Re testing and local wind conditions: it depends what you are testing. Usually, low wind speeds are best, but if you're testing something like added fins for stability, crosswinds are needed. Some car modifications also become more effective when there are crosswinds present eg front air curtains. Remember that in the real world, crosswinds are present most of the time you are driving.

[@namvlogschannel1199]

Hi julian i heard you mention the pressure patch in the video but unfortunately it is not available in malaysia. Is it possible to 3D print one? And how does it work? Is it just a 90degree smooth bend that only measure static pressure?

[@JulianEdgar]

Yes, covered in detail in my book.