10% = 5%

[JulianEdgar]

Originally Posted by JulianEdgar

I am happy to discuss the relationship between aerodynamic drag changes and fuel economy changes - and they don't appear to match either Aerohead's rule of thumb or the ARC one (for which we don't know what 'highway' speed is).

Before you can even start to describe such a relationship accurately, you need to know:

1. The starting Cd (a car with a Cd of 0.35 will have a higher aerodynamic contribution to overall resistance than a car with a Cd of 0.25).

2. If at a constant speed, what that speed is (aero drag goes up at the square of the speed; rolling resistance does not), so if the term 'highway' is used, we need to know what highway speed is.

3. If it's in a drive cycle test, what is that drive cycle?

The nearest I can find to a rule of thumb for modern cars is this, from the Bosch Automotive Handbook (8th ed):

On an average production vehicle, 10 per cent reductions in weight, drag and rolling resistance result in fuel consumption reductions of roughly 6 per cent, 3 per cent and 2 per cent, respectively.

Note the 'roughly'.

Motor Vehicle Fuel Economy (Stone) describes a drag coefficient reduction from 0.45 to 0.35 (22 per cent) giving the following constant speed changes:

50 km/h - 6.7 per cent
80 km/h - 15.2 per cent
120 km/h - 20 per cent
160 km/h - 22.5 per cent

Aerodynamic of Road Vehicles (5th edition) does not even try to use any rule of thumb, suggesting it is too closely tied to speed and vehicle type (ie it depends on what percentage contribution to overall resistance is made up of aero drag - my points (1) and (2) above.

Road Vehicle Aerodynamic Drag (Barnard) gives no rule of thumb.

People love simplistic rules of thumb, but the real world of car engineering is nothing like that - in any area: suspension, engine management... or aerodynamics.

Without extensive qualifications and caveats, the title of the thread (10% = 5%) is misleading rubbish.

I might add, I've written pretty well all of that here before. Aerohead ignores it, waits a few months, then again posts the same misleading stuff.

People pick it up just as before, and so away it all goes again.

I honestly don't understand why people don't say: "The 10%=5% rule was debunked last time, so why are you posting it again?"

(Or, at its most generous: "Everyone should note that the source Aerohead is quoting is 23 years old, and car drag coefficients have changed a lot since then. Also, it applied only at a fixed 55 mph. So the heading of this thread is really deceptive and shouldn't be applied to drag changes you make to your own car.")

Instead - ME Andy says: The info he posted this time seems 100% reliable

It's very odd.

[deluxx]

Originally Posted by JulianEdgar

At least we're talking about aerodynamics. What contribution have you made?

(Oh yes, except for sending an abusive PM?)

What contributions have you made except selling a youtube channel and books?
Why are you here on ecomodder.com and not on Julian.God.Of.Wind.Edgar.com??

Obviously here to upset people for attention to try and make people think you are the god of wind so you sell more books and get more video plays...

[JulianEdgar]

Originally Posted by deluxx

What contributions have you made except selling a youtube channel and books?
Why are you here on ecomodder.com and not on Julian.God.Of.Wind.Edgar.com??

Obviously here to upset people for attention to try and make people think you are the god of wind so you sell more books and get more video plays...

If I wanted to sell more books I'd never create any angst, would I? I'd just agree with all that people say. (Your logic that upsetting people will sell more books is strange to say the very least.)

And you didn't actually answer the question: where are your useful, interesting and informed posts on a car aerodynamics? I've not seen any.

[deluxx]

I don't have anything useful to add, its why I'm here to learn.
Imagine that a sub-human who doesn't know everything about the ****ing wind.
I just enjoy reading about cars and actually driving them in the real world, not making ****ing internet battles and calling names and pointing fingers.

[freebeard]

I'd just agree with all that people say.

I'd settle for not claiming that people slavishly follow along.

[JulianEdgar]

Originally Posted by freebeard

I'd settle for not claiming that people slavishly follow along.

You always say that, but the evidence is heavily against you.

In this thread, where are others questioning Arrowhead's "10%=5%" rule? Not you! (Unless you think your post about rolling resistance variations on highway pavement is somehow a huge rebuttal.)

Incidentally, I value kach22i's post, despite it contradicting what I have said. Why? Because it shows that there's probably evidence that at some 'highway' speed that relationship, as a rule of thumb, is probably OK for current cars. But we don't know what that highway speed is! (And I also learned something from his link re hybrids - very interesting for BEV cars too.)

[RedDevil]

Originally Posted by JulianEdgar

... Arrowhead ...

I've learned that those who insult others are seldom right.
Changing usernames is the ultimate ad hominem. Please behave.

I've been involved in a game of thought, Go. As an organizer of game events and chairman of our little club I've encountered a pair of very strong players with nasty habits. They were strong enough to beat most other players, but would insult the other players for moves they thought were inferior.
Funnily they never questioned moves of players that were stronger than them. If you see someone do something you wouldn't do that would be the ultimate chance to learn something new?
People grow weary of the abuse. For an organizer it is instrumental to kick them out, even if that lowers the average strength of the club, because otherwise there won't be a club before too long.

Aerodynamics are hard. I am sure nobody has a complete grasp of it, neither Aerohead nor you. I accept anything I read on aerodynamics as opinion, and only use it as pointers for trying to understand what's going on. That way I'm not easily misled by anything you two post if it happens to be wrong.
The template is just that - a template. I think I understand the reasons behind it, but I'm aware of its limitations and the availability of other options. In fact, there where other tested designs deviate from the template that is an opportunity to learn.

My point is that a difference in opinion can be a great starting point for further learning. If we keep things civil of course.

[Ecky]

Originally Posted by JulianEdgar

For someone who claims to be an engineer, your maths seems very poor.

Originally Posted by JulianEdgar

...you are ...

Exactly as people in other forms of social media distribute and promote conspiracy theories (etc) by linking to them or distributing them (and then claim, "Oh it wasn't an endorsement, I just linked to it!".)

Despicable.

Originally Posted by JulianEdgar

I think you are the number one source of misinformation on this subforum. You achieve that by a number of mechanisms.

Originally Posted by JulianEdgar

Here in Australia - with our apparently much more blunt approach - he would have been sent on his way many years ago.

All I know is that now is it much safer to assume his posted information is wrong (or misleading) than it is right.

Originally Posted by JulianEdgar

At least we're talking about aerodynamics. What contribution have you made?

Originally Posted by JulianEdgar

where are your useful, interesting and informed posts on a car aerodynamics? I've not seen any.

Originally Posted by JulianEdgar

In this thread, where are others questioning Arrowhead's "10%=5%" rule? Not you!

Julian, I doubt anyone here doubts your knowledge and talents. I would ask (from a place of keeping the forums a healthy community) that you please be a little more mindful about what constitutes fact, and what constitutes a personal attack.

[JulianEdgar]

Originally Posted by Ecky

Julian, I doubt anyone here doubts your knowledge and talents. I would ask (from a place of keeping the forums a healthy community) that you please be a little more mindful about what constitutes fact, and what constitutes a personal attack.

I think in fact you need to be mindful of what constitutes a personal attack, or in the vernacular of arguments, an ad hominem.

ad hominem: an argument directed against a person rather than the position they are maintaining

Each of the quotes you have listed that I wrote in response to Aerohead is against the positions that he puts forward here (and where in your quotes they appear not to be, it's due to your editing).

If I said Aerohead was a moron, a fool, an idiot, smelt of old rags, was whacky (etc, etc) than they would be personal attacks. But none of my comments are like that: they are comments on what Aerohead puts forward here, viz, a large amount of misleading and incorrect rubbish.

As for your other quotes, how about the context of writing what was being directed at me?

[JulianEdgar]

Originally Posted by RedDevil

I've learned that those who insult others are seldom right.
Changing usernames is the ultimate ad hominem. Please behave.

It wasn't intentional: 'Arrowhead' is what my spell check changes Aerohead to! Usually I pick it up and change it, but obviously in this case I missed it.