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Old 01-10-2021, 12:59 AM   #1 (permalink)
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Stuff to not worry about

One thing I often notice is that people seem to want to make the subject of car aero modification more complex that it needs to be.

I am sure that this puts off lots of people, because they think that they need to know all of that stuff - stuff that actually doesn't matter much, if at all - in achieving good car modification results.

Here are some:

Turbulent versus laminar boundary layers

First, the boundary layer is just the 'skin' of air nearest the car's body that is flowing slower than the speed the car is moving forward. On a car, the boundary layer is basically all in the form of what is called a turbulent boundary layer. So forget laminar boundary layers as being relevant on any normal road car. (Exceptions? Maybe the first 300mm of car hood, and maybe the mirror housings.)

Don't confuse turbulent boundary layers with separated flow - the latter is where the car's body shape is no longer guiding the flow - a completely different idea.

Reynold's numbers

In effect, this is a number that allows us to compare airflow characteristics (drag / lift) of objects of different size. So, for example, if we were to use a 1:5 model in a wind tunnel, we'd have to ensure that the Reynold's Number was in the same ballpark as it would be for a full size car. (To achieve that, we need to use a proportionally higher airflow speed, ie in this case, 5 times the speed.) But if we're using our full-size car for our on-road testing, Reynold's Numbers are irrelevant - we're already dealing with the full-size model!

Fineness ratio

This relates to how long the object is compared with its greatest frontal projected cross-sectional area. It's very important on objects like tear drop shapes, because at some stage, the smaller wake created by a long tear drop starts to be more than offset by the frictional drag of airflow shearing over the object i.e. frictional drag. (The shearing coming from the different airflow speeds in the boundary layer.)

But if you're modifying your car, the fineness ratio is basically irrelevant - any tapering length you can realistically add at the back to give a smaller wake is going to be good.

Drag coefficient

If you are modifying your car, a drag coefficient (Cd) value is of very limited use. Almost, I'd suggest, relevant only in that if you're starting with a car with a high factory Cd figure, it will be easier to make improvements. That's basically it. Calculating Cd values resulting from your modifications is largely worthless - there is no really accurate way of doing so without access to a full-size wind tunnel with expert staff.

But - and here's the rub - concentrating on Cd can lead you down all sorts of rabbit holes. "That car reduced Cd by 6 per cent with [and pick your modification here]." That doesn't mean that particular modification will do the same on your car, and often nor does it even mean that the original modification reduced Cd by that percentage on that car. (So much published data is unreliable - not all tech papers are equal!)

Furthermore, applying that Cd reduction (6 per cent in this case) to your car's original Cd and saying that, for example, "I've done that modification and so I've now reduced my Cd from 0.28 to 0.2632" has got absolute knobs on it. Why? Well, there's no guarantee that the mod works on your car like it did on the other car (if it did do so even on that other car). Then there's the intellectual and engineering paucity of going from two decimal places to four places - resolution and 'accuracy' where there are actually none!

That will do for now, but I am sure I'll think of some others.

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Old 01-10-2021, 03:08 AM   #2 (permalink)
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Who is working to four decimal places?

Can you think of things that the modifier should worry about. They're standing there with a handful of test gear, and looking at an arbitrary victim.

It starts with intent obviously, but then what comes next? Rules of thumb, one from column A and one from column B, or parsing a decision tree?

Here's one design pattern: simplify and add lightness.
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Old 01-10-2021, 03:33 AM   #3 (permalink)
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Originally Posted by freebeard View Post
Who is working to four decimal places?

Can you think of things that the modifier should worry about. They're standing there with a handful of test gear, and looking at an arbitrary victim.

It starts with intent obviously, but then what comes next? Rules of thumb, one from column A and one from column B, or parsing a decision tree?

Here's one design pattern: simplify and add lightness.
Aerohead often quotes to this precision of number - to four decimal places, all with his made-up figures! Do a search and you'll see.

If you're after low drag, the easiest thing to aim at is high pressures on the rear surfaces and low pressures on the front.

But that requires actual measurement of aerodynamic pressures on real cars.

So far I have seen, only three people on this forum actually deign to do so. You, Freebeard, have never done so - as far as I know, at least. It requires doing real stuff, not just sitting behind a keyboard and postulating hypotheticals based on drawings.

It all reminds me so much of car modification of 30 years ago, when chassis dynos were rare, and people just made up engine power figures based on what they expected their engine mods to do.

To be blunt: the techniques now to exist to measure real aerodynamic stuff, and the other approaches (eg guessing from drawings) is as weak as it used to be in guessing engine power from valve sizes.
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Old 01-10-2021, 05:09 AM   #4 (permalink)
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Quote:
Originally Posted by jULIANeDGAR
So far I have seen, only three people on this forum actually deign to do so. You, Freebeard, have never done so - as far as I know, at least. It requires doing real stuff, not just sitting behind a keyboard and postulating hypotheticals based on drawings.
You don't know the half of it. The forum had 61 members in the last 24 hours. The most members online over 24 hours was 356, 12-11-2014. At the moment it's you, me, kach22i and redpoint5. Zero lurkers.

I probably put 30 miles on the Metro in the last 30 days. Half of that was a run up to Coburg to throw down a cash payment. My ROI is BTFO.

I used to enjoy the Aerodynamics subforum, trying to help newbies and sharing Aerodynamic Oddities as I found them. Maybe I'll just go back to The Lounge, since we can't talk current events or climate it's pretty peaceful there.
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Old 01-10-2021, 09:49 AM   #5 (permalink)
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You don't know the half of it. The forum had 61 members in the last 24 hours. The most members online over 24 hours was 356, 12-11-2014. At the moment it's you, me, kach22i and redpoint5. Zero lurkers.
I said ‘so far I have seen’. Are you implying that there are actually heaps of people here doing this testing?

Quote:
I probably put 30 miles on the Metro in the last 30 days. Half of that was a run up to Coburg to throw down a cash payment.
The forum is called Ecomodder. But you can choose not to do any mods, of course.

Quote:
I used to enjoy the Aerodynamics subforum, trying to help newbies and sharing Aerodynamic Oddities as I found them. Maybe I'll just go back to The Lounge, since we can't talk current events or climate it's pretty peaceful there.
I think one of the major shortcomings of this subforum has been that guessing from pictures has been equated with giving good advice. Imagine doing similarly in other areas of car modification. “Yep, I’ve seen the pic of your new springs and they look like they’ll work well.”

Last edited by JulianEdgar; 01-10-2021 at 10:00 AM..
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Old 01-11-2021, 02:46 PM   #6 (permalink)
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**Raises hand** (With regards to actually having done real testing.)

Sort of anyway. I have at least been in a wind tunnel doing actual (i.e. getting paid to do it) aerodynamic work. It was even the big GM tunnel, not a fan in my basement.

I'm definitely learning things being here. And appreciate all those who take the time to contribute.

And I thought this thread was going to be about things not worth doing to your car, like removing a wiper, or the radio antenna. Another thread for that I suppose.
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Old 01-11-2021, 03:59 PM   #7 (permalink)
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And I thought this thread was going to be about things not worth doing to your car, like removing a wiper, or the radio antenna. Another thread for that I suppose.
Typically, the smaller the modification, the less it does.

But I haven't seen too many people here recommending such mods.
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Old 01-11-2021, 04:34 PM   #8 (permalink)
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I was going to stay out of this but —Jeez Louise— Ten pages of results on

http://ecomodder.com/forum/google_se...q=wiper+delete

I for one, have done that mod myself. It saves on replacement blades as much as Cd.
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Old 01-11-2021, 04:40 PM   #9 (permalink)
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Originally Posted by freebeard View Post
I was going to stay out of this but Jeez Louise Ten pages of results on

http://ecomodder.com/forum/google_se...q=wiper+delete

I for one, have done that mod myself. It saves on replacement blades as much as Cd.
Yep, lots of silly things have been done in the past here. And taking off your windscreen wipers must be amongst the silliest I can think of.

But as I said, I haven't seen (in the year of so I have been here) any people making that suggestion. (Or maybe I did see it and thought it a joke?)
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Old 01-11-2021, 05:19 PM   #10 (permalink)
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So... are you saying you'e never pressure-tested the scenario?

It's a low investment, low penalty mod. I did it to conserve wiper blades.

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