Guessing

[JulianEdgar]

Originally Posted by jakobnev

If you don't guess at all, you won't know which test to perform.

Yes that’s quite true. But it’s very different suggesting some different approaches to test, compared with guessing that something will or will not work - and then giving that guess as advice to follow.

[aerohead]

Originally Posted by JulianEdgar

In another thread I wrote: "To be honest, I think the huge underlying issue - that I am afraid you, Freebeard are a strong part of - is that speculation has become largely the culture of this group. To be blunt, posters just guessing."(And I wasn't picking on Freebeard, but as a prolific poster he's a person who does a lot of guessing.)

What I meant by that is this.

Car aerodynamics is a very complex area. This is particularly so because changing one area of the car is likely to impact airflow behaviour in completely different areas. It is not like altering (say) suspension, where a change in suspension spring rates won't affect the engine's air/fuel ratio.

While I mention this idea in my Veloce book, after reading it, both Dr Thomas Wolf of Porsche and Rob Palin (ex Tesla) suggested to me that this point needs to be far more greatly emphasized. That is, the 'element reductionism' approach, where different aerodynamic elements of car can be viewed separately, is not a good idea. For example, Rob suggested to me that altering the rear separation height may impact the height of the front stagnation zone!

So any modification that makes a major change in airflow at one location (and if it isn't a major change, it is unlikely to do anything measurable) is likely to cause changes elsewhere.

Furthermore, the impact of a specific modification is very hard to assess based only on its description.

So - a simple example - a front air dam may increase or reduce drag. It depends on the existing undercar aero and the height of the air dam.

A rear spoiler may increase or reduce drag - on a car with attached flow, it depends on the increase in pressures of angled rear panels versus the increase in the size of the wake.

Boat-tail extensions may increase or reduce drag - it depends on the force vectors (direction and magnitude of the forces) as the airflow wraps around the curved panels versus the reduction in wake size versus the disruption in trailing vortices.

Obviously in that context, numerical / shape rules of thumb are, to put it mildly, highly problematic.

That's why the literature has no agreement on the best angle for rear diffusers, no agreement on the best angle for boat-tailing, no agreement on best ride height for low drag, no agreement on the ultimate low drag shape for a road car, no agreement on the best design for low drag wheels - and so on. (Those who pretend there is agreement simply haven't read widely and recently.)

Now does that mean that no advice can ever be offered?

No it does not.

For example, I think you can be pretty confident in saying that, on a car with a rough underside, a full undertray will reduce drag and lift. If you are building a car from scratch, any of the five(!) differently-shaped low drag templates in Hucho (2nd ed) would be good starting points. Wheels with flat faces are very likely to have lower drag than those with many exposed spokes.

But - and here's the key point - no-one can say with certainty what an outcome is until the modification has been tested. Without trialling modifications and testing them, you are - to a much greater degree than in other car modification - blundering around in the dark. But instead of saying that, on this group it's far more likely that someone will ask about a modification, and people will simply leap in with guesses - guesses that apparently are treated as quite credible.

But my observation is that those guesses are often based on rules of thumb, misquoted or selective use of references (eg quoted angles), irrelevant historic parallels (that aren't) and so on.

And let's look at that point about misusing examples from the past. When I have pointed out that an old reference may be useless as a guide to today's modification, it's been demanded of me: "Well, what in fluid dynamics has changed since that book was written?" The answer is: "Nothing."

But it's the wrong question.

Our engine designs don't look like those of the 1930s and 1940s because our understanding of combustion chamber design, tuned length intake systems, variable camshaft systems (etc, etc) has changed. As have our requirements of engines. We don't take the combustion chamber design of a Merlin and say that all modern engines should have their combustion chamber designs based on it - and if current engines aren't doing that, they're wrong. The combustion behaviour of fuels hasn't changed since the Merlin, but our understandings and requirements have.

On my YouTube channel people write in all the time asking for advice. So for example I run a video testing rear wing and spoiler combinations on the back of a Subaru Impreza for lift/downforce. Someone writes: "Is it the same with the DC5 Integra?". How would I know? How would anyone know who hasn't done testing (or seen manufacturer's testing)? How could such a question even be asked - or expect to be answered?

So the next time someone here asks for advice on a car aero modification, stand by - unfortunately - for the contribution of a lot of guesses.

' the vehicle aerodynamicist must refer to a large amount of detail resulting from earlier development work. His success depends on his ability to transfer these results to his own problem and to combine results originating from many earlier developments to a consistent solution.
It is the intention of the present book to introduce the vehicle engineer to this approach.' Wolf- Heinrich Hucho, page one of Preface, 2nd-Edition.

[aerohead]

Originally Posted by JulianEdgar

If someone has done testing on a particular car and can demonstrate the positive results, fantastic!

But if we were to take the advice of what worked on the Beetle and apply it to a Prius - and you see stuff like that here all the time, just not with those models - then it is not much better than guessing.

That is my point!

Excepting of course, if Aerohead demonstrates it.

[JulianEdgar]

Originally Posted by aerohead

Excepting of course, if Aerohead demonstrates it.

You very often take the example of what works on one car and imply / state that it is applicable to a completely different car shape.

You're probably the worst person here for doing the 'it worked on a Beetle so let's try it on a Prius' type of advice.

[aerohead]

Originally Posted by JulianEdgar

You very often take the example of what works on one car and imply / state that it is applicable to a completely different car shape.

You're probably the worst person here for doing the 'it worked on a Beetle so let's try it on a Prius' type of advice.

* It goes back to the fluid mechanics ground rules Hucho wrote about :
' the essential experimental results, presented as ground rules of fluid mechanics and brought to general validity wherever possible;'
Hucho, page one of he Preface
* fundamentals are fundamentals.
* it's the whole point of his book!
* and I promise to continue whenever something dovetails into what's already established in the public domain.

[freebeard]

Originally Posted by aerohead

Excepting of course, if Aerohead demonstrates it.

Now he's got you doing it!

[JulianEdgar]

Originally Posted by freebeard

Now he's got you doing it!

Yep, I think you lost that battle... Freebeard!

Proper names are proper names.

[freebeard]

People call an octothorpe a hash, doesn't make them right. aerohead hasn't changed his handle yet, has he?

You haven't used my proper name yet. Please don't if you can find it, that's considered Doxing on Internet. And that's bad.

Thought.co seems reputable enough in their domain. On Internet, I'd follow the W3C Internationalization Working Group:

BY THE WAY
A note on sorting
.....
The treatment of small words such as "von", "de", and "van" brings additional complexity to sorting. Sometimes the prefixes are significant, other times they are not.

I couldn't find the classic article on designing input fields for names. They can start with apostrophes and High-ASCII characters.

The handle that a properly named individual is asked for when they sign up to EcoModder doesn't have to be capitalized. That is the nature of a handle or avatar, as it should be. Two of the three legs Internet stands on are anonymity and authentication (the third is cryptocurrency).

You had to provide identifying information to Google to sign up here. You didn't have to use your proper name and you shouldn't disparage people who choose not to.

Anyways, nobody is going to stop you being a b*tthole about it.

[JulianEdgar]

Originally Posted by freebeard

You didn't have to use your proper name and you shouldn't disparage people who choose not to.

If I wanted to disparage people because of their handles I'd call Aerohead "Airhead" (as someone keeps suggesting I should do) and I'd probably call you "Firebeard"!

Capitalising a name is not disparagement, except in your world.

[freebeard]

Knock yourself out, but namecalling does not show respect. At least you're calling it a handle instead of a Proper Name.

Oh to be a fly on the wall when you come across a Marxist-feminist who demands that you use their pronouns. Compelled speech.