How hatchback and wagon rear spoilers work

[@ronaldderooij1774]

I have a Mitsubishi Mirage (Space Star) with rear spoiler. My son and I discussed the rear spoiler's function. We did not agree. Now I know we both were right.

[@SoloRenegade]

This is what makes you so good at this. you can summarize an idea so simply.

[@RwP223]

6:57 "To get both...a reduction in drag and a reduction in lift" I thought you were going to finish that by saying "get an undertray" ;)

[@JulianEdgar]

Yes!

[@kivanccelik51]

wait, does an undertray actually reduce drag? I knew it didn't create drag but I didn't know it prevented it

[@channelmachinebroke9638]

@@kivanccelik51 less components hitting the wind on the underside

[@kalui96]

The visualized wake at 3:18 is pretty awesome

[@russellmoore1533]

A photo of the car without the spoiler would have been good for comparison

[@JulianEdgar]

Sure - it's in the book.

[@channelmachinebroke9638]

1:20 I know about the tear drop shape being the most aerodynamic shape, but not why. When you described wake specifically as “pull the car backwards” it puts the pieces together, I understand now.

[@JulianEdgar]

Yes, the next idea to take on-board is the thrust developed by the teardrop shape's curved front surfaces...

[@TC-V8]

My mates Lancia Delta Integrale Evo had a two position rear spoiler - now i know why! Looked much better in the 'up' position!

[@JulianEdgar]

I didn't know this car (none sold here) but that is very cool.

[@MITSCG]

Fantastic explanation

[@robertcook4215]

Looking forward to the book! In a previous video you answered a question I have always had on the Toyota Eagle GTP car. I'm interested on what I will be able to apply to my car in the future.

[@jonbradley4789]

Well done. Nice narration and great use of visuals.

[@JulianEdgar]

Glad you liked it!

[@michaelblacktree]

Excellent video!

[@cupofmozzarella]

Julian you could consider creating a community about vehicle aero/vehicle dynamics. That way you can engage more directly with interested people like me (I have recently been building a new track car and have been getting more interested in aero), and therefore sell more books. Just an idea, let me know if you'd be interested :)

[@JulianEdgar]

Like a discussion group, you mean? Terrible idea! I have had a lot of experience of web discussion groups. Basically, those who are really interested in what I have to say will buy my books; those who want quick fixes / superficialities / to share misconceptions will participate in a web discussion group. It's extremely interesting to note that very few people who have bought my books contact me - they're too busy using the book content to do their successful modifications. In contrast, if I receive an emailed question that shows a basic misunderstanding (eg the writer wants rule of thumb aero advice), then every single time, the person hasn't got my book on the topic. There are literally no exceptions to that!

[@cupofmozzarella]

@@JulianEdgar I meant it more like a general interest group for people who want to modify their car in the right way and connect with each other. The book can be perfectly placed there as you're already attracting the correct target audience. Like a Facebook group but actually useful and geared towards teaching from the start.

[@LilBeckferdanddehmoh]

I love the hat back videos❤

[@lau_taro0037]

Hello Mr. Julian, I wonder if it were possible to reduce both lift and drag by adding a wing atop of an extended spoiler. It would surely look overdesigned, but in practice, I believe it would result in the best outcome, right?

[@JulianEdgar]

Sti Subaru of the past basically did that, yes.

[@highmolecularweightRDX]

What about double spoilers like you see on the Golf R, AMG A35, Veloster N, etc. Does the top spoiler dominate the aerodynamics and cause downforce, while the wave-reducing lip does nothing? Or does it split the air and do both? If I've learned anything from you, these are mounted too low to act like a wing; so their mechanism of action isn't like a wing, creating a low-pressure area below itself. But that would mean they allow for wake reducing effect with the lower spoiler.

[@JulianEdgar]

Most are just styling. Only wing/spoiler combination I have seen on a road car that is likely to work is the STi Subaru Impreza ie the wing is about 300mm above the spoiler.

[@highmolecularweightRDX]

@@JulianEdgar So just look at the top spoiler? Since not enough air goes through the gap to make a difference.

[@JulianEdgar]

Yes I think so.

[@Serris412]

Could you explain the relevance of those long extended spoilers seen on drag race cars? See them a lot, and I can guess at what they do, but I'm not totally sure. Drag racing aero and suspension is something I'd love to know more about

[@JulianEdgar]

Do you have an example pic?

[@onlyifusayplz5407]

I imagine for a lot of squareback cars whose backs are somewhat sloped, a roof extension will often also provide a better separation edge than the "bare" hatch would. So even a "flat" roof spoiler that doesn't affect the size of the wake might still reduce drag somewhat by providing "cleaner" flow separation. Am I correct in thinking that?

[@JulianEdgar]

Yes and no. Most square back cars of the last 20 or so years have a square edge to the hatch metal work anyway.

[@las3k91]

I heard that the spoiler on the Fiat Abarth 500 reduces the Cx coefficient from 0.40 to 0.32. Aerodynamics is completely non-instinctive - shapes that look streamlined are not necessarily streamlined and vice versa

[@JulianEdgar]

The chance of a rear spoiler reducing drag by 20 per cent is basically zero - check the credibility of your source. And once you know what to look for, shapes that are streamlined look that way.

[@Surestick88]

I don't know about the amount it reduces drag but I did read the designer didn't want any spoiler to keep it true to the original 500 but it provided enough of a fuel efficiency improvement that they had to keep it so I'd guess it does have a big effect on the 500.

[@JulianEdgar]

Even a tiny effect on drag (eg even a few percent) will have an impact on fuel efficiency.

[@RootGinger]

One thing I’ve always wondered since watching your videos is about cars that have spoilers with holes. Ford use these a lot on the ST models and I’ve seen other manufacturers now using them. It appears the holes allow the air to flow through reducing drag, while the rest of the wing reduces lift. Is this a trick to get both simultaneously.

[@JulianEdgar]

It’s to keep the rear window cleaner.

[@heroinwithease]

@@JulianEdgar funny you say that but on my focus it didn't one bit but hey ho ford for ya

[@JulianEdgar]

You would only find out if you blocked the holes and compared the rear window soiling.

[@dmain6735]

Do drag reduction spoilers work the same way along the sides of the vehicle? Would extensions that slope 'inwards' to the centreline be effective on a slab sided van for instance.

[@JulianEdgar]

Yes but you'd need to check the flow stays attached, and not to make the angle too acute. In general vehicle design it's called boat-tailing - covered in my book.

[@jonathanang8740]

Hello... Got a question here that so far hasn't got an answer yet. How effective would a reverse delta wing or fwd swept wing be for aero dynamics?

[@JulianEdgar]

I've already answered.

[@user-td8ic1hj9w]

what a bout the big spoiler on the mercedes amg hatchback? is it too big?

[@JulianEdgar]

Too big for what?

[@user-td8ic1hj9w]

@@JulianEdgar Sorry I was too vague. I was discussing with my dad about spoilers on hatchbacks (which is why I found your video). And he said they were basically useless and mostly for show on hatchbacks and particularly the one on the mercedes was too big for the car and would not help with down-force at all. He argued that spoilers would probably only work on very fast sports cars and I just thought it very unlikely that engineers making the car would add one just for show. Anyway I think I will just show him the video haha thanks!

[@JulianEdgar]

I think he is wrong on each of those points.

[@ELJ666]

Can a rear wing do the same thing depending on angle of attack

[@JulianEdgar]

A proper wing is not a spoiler and they work in different ways.

[@ELJ666]

@@JulianEdgar what about eBay or Amazon wings since they're mainly for looks

[@JulianEdgar]

You can use one as a spoiler but it's not as effective because of the gap beneath it.

[@michaelblacktree]

If you mount it like the lip spoilers shown in this video, then maybe? The problem is that the wing won't be contoured to match your car. So for example, if the wing is straight but your car is curved across the top, it won't fit very well.

[@kevinhohne7889]

What about wagons where the upper part of the rear lid is already angled upwards, compared to the shape of the roof? (Opel / Vauxhall Astra J Sports Tourer). There won't be a chance to further reduce drag by adding a spoiler, right? But i'm going to add a undertray, so should fix the largest part of the drag that can be reduced in a more or less legal range

[@JulianEdgar]

Angled 'downwards', I think you mean? Yes, a good undertray will reduce drag - how much it does so depends on what the car looks like underneath to start with.

[@SpeedyHoon]

This aerodynamics explanation is grossly oversimplified. The spoiler's angle of attack versus the surrounding airflow is most important. An upward-angled spoiler can generate lift, and the same with a downward angle can generate downforce or reduce wake depending on where it is positioned. The 1991 Ford Escort Cosworth had a flat rear spoiler that was aero-tested to create downforce.

[@JulianEdgar]

Obviously, the spoiler's angle of attack versus airflow is important, but the angle of airflow on a squareback car (the topic of this video) is set by the angle of the roof. An upward angled spoiler on a squareback car cannot develop lift - and if you have evidence of one that does, please cite it. Ditto a downwards angle spoiler on a squareback car cannot develop downforce. The Cosworth Escort is (1) not a squareback car, and (2) doesn't use a spoiler - it's a wing.

[@SpeedyHoon]

​@@JulianEdgar Do some research on external windscreen sunvisors available as an optional extra on most Holden or Ford cars between 1960 and 1980s in Australia. They were dropped in the mid 80s because the sunvisors generate lift even though they are angled upwards. There are also several cars (including Commodores VT-VZ wagons) featuring roof flaps above the rear window directing air downward which generates a net decrease in lift due to interaction with the car's wake. Without it, water flows up the rear window rather than down (inverting the wake's rotation). Imprezas also featured them, but for different reasons, to increase flow downward onto the rear wing (common in F1 too).

[@JulianEdgar]

A front windscreen sun visor is zero like a squareback rear spoiler, and an airflow deflector to direct airflow is again nothing like a normal squareback rear roof extension spoiler. The points made in the video, together with the self explanatory images, are completely valid. I’ll leave it there.

[@nealesmith1873]

They should make the bottom of the car curved to reduce lift and the top just flat. Does not seem easy to do, but maybe some genius engineer could figure it out.