Thank you

[JulianEdgar]

Originally Posted by ME_Andy

I poked around the site a little bit and found a reference to the original source of (one of) the templates. There's a lot of info here, just very disorganized.https://www.jstor.org/stable/44631714

It's an interesting paper - albeit now for quite old-shape cars - that I'd not seen before. But where does it describe using a template shape to do the following?

Guide the shape of extensions to cars, to guide the heights of spoilers, to assess the 'aerodynamic purity' of shapes, to determine where there will be attached and separated flow etc, etc

If you could give the page ref that would be good.

[JulianEdgar]

Originally Posted by Isaac Zachary

The thing is we can all only guess what you're trying to say sometimes. You sounded like you have no faith in shapes at all. As if an aerodynamic shape of any sort (not just the "template") has no value whatsoever.

Now it sounds like you're saying boat-tailing only works if you have it built at an optimal angle... As if a sub-optimal angle wouldn't help. After all, the template is rubbish, so following it should either not help or just make aerodynamics worse.

Is that what you mean?

Determining the optimal angle for a boat-tail (or any body extension) cannot be ascertained by looking at a template.

No predetermined shape can be used to do any of the following:

Guide the shape of extensions to cars, to guide the heights of spoilers, to assess the 'aerodynamic purity' of shapes, to determine where there will be attached and separated flow etc, etc

If it were all as easy as that, gosh that would be good.

[Isaac Zachary]

Originally Posted by JulianEdgar

Determining the optimal angle for a boat-tail (or any body extension) cannot be ascertained by looking at a template.

No predetermined shape can be used to do any of the following:

Guide the shape of extensions to cars, to guide the heights of spoilers, to assess the 'aerodynamic purity' of shapes, to determine where there will be attached and separated flow etc, etc

If it were all as easy as that, gosh that would be good.

That makes sense.

A couple questions though.

1. How can optimal angles be formulated? And can the average Joe like me figure that out with the tools we have?
2. Wouldn't a suboptimal shape, template or not, still have the possibility of providing some improvement?

[Vman455]

Originally Posted by Isaac Zachary

Now it sounds like you're saying boat-tailing only works if you have it built at an optimal angle... As if a sub-optimal angle wouldn't help. After all, the template is rubbish, so following it should either not help or just make aerodynamics worse.

Is that what you mean?

Are you satisfied with sub-optimal? Why? If it's easy enough--quite easy, in fact--to do a little testing and find out what the actual optimal angle and actual optimal shape for your car are, why on earth wouldn't you do that instead of just guessing?

I don't want to put words in Julian's mouth, but as far as I understand, that's what he's arguing. He's posted extensively on testing methods, click on his username which will take you to his page, and then for "other posts" and "threads started by."

[JulianEdgar]

Originally Posted by Isaac Zachary

That makes sense.

A couple questions though.

1. How can optimal angles be formulated? And can the average Joe like me figure that out with the tools we have?
2. Wouldn't a suboptimal shape, template or not, still have the possibility of providing some improvement?

1. By simple testing of mock-ups.

2. Yes it would, but why would you want to do that? Gains are hard enough to get without even following a completely wrong approach (copying a shape).

[JulianEdgar]

I can endlessly describe why a template-based approach is rubbish in car aero modifications - and I have done so previously - but here's one straightforward example.

1. You want flow to stay attached to the sides/top/bottom of a boat-tail extension.

2. Flow attachment is heavily dependent on boundary layer thickness - the thicker it is, the more easily separation will occur.

3. Boundary layer thickness varies, depending on the car and the surface roughness / change of shape / length of body ahead of the boat-tail.

4. So when shaping a boat-tail, how can a predetermined shape (a template) take into account the variation in boundary layer thicknesses between different cars?

Answer: it can't.

A simple low drag shape (incidentally, one of many that have been produced) has been extrapolated into purported car modification uses which are quite absurd.

Unfortunately, believing that a template can be used for all sorts of car modifications like....

Guide the shape of extensions to cars, to guide the heights of spoilers, to assess the 'aerodynamic purity' of shapes, to determine where there will be attached and separated flow etc, etc

... then has flow-on impacts on a whole range of aero understandings. That is, get the basics wrong and then so much that follows is also wrong.

[freebeard]

1. By simple testing of mock-ups.

I was thinking you were saying the best shape is no shape and the best angle is no angle, but now I can't explain the gap between a mock-up and a template.

[Isaac Zachary]

Originally Posted by Vman455

Are you satisfied with sub-optimal? Why? If it's easy enough--quite easy, in fact--to do a little testing and find out what the actual optimal angle and actual optimal shape for your car are, why on earth wouldn't you do that instead of just guessing?

The phrase: "there comes a point of diminishing returns." comes to mind. The question I'd have is how much would I benefit from the work involved. To get everything "optimal" in my mind requires not only redesigning the rear window and trunk area, but also making my own windshield and hood. At what point does optimal become OCD?

Originally Posted by JulianEdgar

1. By simple testing of mock-ups.

If all it takes is a "little testing" "simple testing" that makes sense. However, in my mind it sounds like a lot of work. I'd be tempted to see if I get good mileage with the first mockup and then leave it on permanately if it does.

I've tried testing fuel/air ratios and degrees of advancement. And even though with those it's just a small twist of an ajustment screw or a distributor it's still a lot of work. Now building not just a 3 dimentional mock up but several of them sounds a bit harder than just twisting a screw, at least for me. Or maybe I just don't understand the concept of "simple testing of mockups."

However, I'm still interested in trying a boat tail at some point as it seems to be the best way of getting better fuel mileage. So thank you, I'll keep these points in mind.

Originally Posted by JulianEdgar

2. Yes it would, but why would you want to do that? Gains are hard enough to get without even following a completely wrong approach (copying a shape).

Why? Because it's easier to just go with something predesigned. And if it produces possitive results, it stays on the car. If it doesn't it gets taken off.

But you make it sound like the level of effort isn't much different either way, template or mock-ups.

Originally Posted by JulianEdgar

I can endlessly describe why a template-based approach is rubbish in car aero modifications - and I have done so previously - but here's one straightforward example.

1. You want flow to stay attached to the sides/top/bottom of a boat-tail extension.

2. Flow attachment is heavily dependent on boundary layer thickness - the thicker it is, the more easily separation will occur.

3. Boundary layer thickness varies, depending on the car and the surface roughness / change of shape / length of body ahead of the boat-tail.

4. So when shaping a boat-tail, how can a predetermined shape (a template) take into account the variation in boundary layer thicknesses between different cars?

Answer: it can't.

A simple low drag shape (incidentally, one of many that have been produced) has been extrapolated into purported car modification uses which are quite absurd.

Unfortunately, believing that a template can be used for all sorts of car modifications like....

Guide the shape of extensions to cars, to guide the heights of spoilers, to assess the 'aerodynamic purity' of shapes, to determine where there will be attached and separated flow etc, etc

... then has flow-on impacts on a whole range of aero understandings. That is, get the basics wrong and then so much that follows is also wrong.

Fair enough.

Originally Posted by freebeard

I was thinking you were saying the best shape is no shape and the best angle is no angle, but now I can't explain the gap between a mock-up and a template.

May I point out:

Originally Posted by JulianEdgar

BBut determining its optimal angles from a template is absolute rubbish.

Originally Posted by JulianEdgar

2. Yes it would, but...

It's not that Julian disagrees with the concept. He disagrees with a one-sized-fits-all approach as being optimal.

Sounds like he can't be happy with someone trying something that's not optimal, even if it ends up giving them a small benefit.

That's what makes it hard to follow his posts, for me anyhow. It's kind of like one guy saying he's going to lean out his carburetor and another ridiculing him for not installing fuel injection and tuning each cylinder's AFR.

[Vman455]

Originally Posted by Isaac Zachary

The phrase: "there comes a point of diminishing returns." comes to mind. The question I'd have is how much would I benefit from the work involved. To get everything "optimal" in my mind requires not only redesigning the rear window and trunk area, but also making my own windshield and hood. At what point does optimal become OCD?

Well, here's an example: I have my truck now, and the first thing I want to test is an airdam below the factory front valance. I could follow the (dubious) rule of thumb here that an airdam shouldn't extend below the lowest-hanging component under the truck, and based on that assumption build a permanent airdam and hope that it's the best design. Or...I could build a simple mock-up out of cheap coroplast and tape it to the valance, extend it all the way to the road, and use throttle-stop testing to measure the change in drag as I cut several inches off to test it at different heights and find the actual airdam height that reduces drag the most on my truck. Then I'll make a permanent airdam at the same height that I know is doing what I want it to because I measured it before I invested the time, effort, and money to build it.

"Optimal design" doesn't mean anything short of reshaping an entire car isn't worth doing.

[Isaac Zachary]

Originally Posted by Vman455

Well, here's an example: I have my truck now, and the first thing I want to test is an airdam below the factory front valance. I could follow the (dubious) rule of thumb here that an airdam shouldn't extend below the lowest-hanging component under the truck, and based on that assumption build a permanent airdam and hope that it's the best design. Or...I could build a simple mock-up out of cheap coroplast and tape it to the valance, extend it all the way to the road, and use throttle-stop testing to measure the change in drag as I cut several inches off to test it at different heights and find the actual airdam height that reduces drag the most on my truck. Then I'll make a permanent airdam at the same height that I know is doing what I want it to because I measured it before I invested the time, effort, and money to build it.

"Optimal design" doesn't mean anything short of reshaping an entire car isn't worth doing.

That may work. I don't know. I have no experience with coroplast nor know where to even buy it. I've tried testing on the road, not necessarily by putting a stop under the throttle, but by holding a fixed MAP/load number on a screen and it's near impossible to get repeated results. Then comes to my mind the question of whether more effort would be better spent on figuring out if a straight up and down air dam is best or if it should come out at an angle or curve inward on the bottom, and if so, how much. And how much would I gain on a car that already has a bumper that extends down to a factory belly pan.