Rounded corners vs. Straight edges on kammback

[aerohead]

If the unit is rear-hinged and forward-latched it could rise and overrun the backlight header as the trunklid is raised.Plan-taper wouldn't interfere at all with clearance.A single soft-faced roller could engage the outer surface of the car.For no more than what it is, it shouldn't weigh very much.You'd need to be mindful of wind and gusty conditions when it's up,but that can be engineered for.
A mod like this gave Hucho about a 32% drag reduction on the 1974 Scirocco.

[ChazInMT]

Originally Posted by drmiller100

There are plenty of studies which show IN GENERAL the smaller the back of the car, the shorter the trailing vortices, the less drag there is.

Show me 1 study that says this.

I have pointed out a study in this thread that shows the opposite of this, prove why his numbers are wrong.

I have asked repeatedly for you to give us some reference regarding the things you say, you have yet to do so. In your entire post #41 you say many things which you believe to be fact, but do not supply any drawings, or links to other discussions, or a link to a paper, nothing. We are calling you out and saying you're wrong, it is not up to us to prove why you are wrong at that point. It is up to you to show why you are indeed right. This is another principle in the discussion of science and engineering; you must provide facts to back up the things you say when asked. In this way, we can talk about the facts and keep the personal out of it.

If you say, "That's just the way it is." or "I heard this and this from a guy once.", it is difficult to understand why what you say is anything other than a bad opinion. We really, Really, REALLY, want to deal with facts here. Opinions stated as fact are not welcome.

[drmiller100]

Originally Posted by ChazInMT

FYI 20° max angle is based on curving your way down to it, not going from 0° to 20° in 1 inch. You need to follow the template if at all possible.

FYI "Filling the wake" is not the ultimate goal. Releasing the air off the back of the car with as little disturbance as possible is the goal.

FYI There is plenty of information out there for you to find that shows radius corners are better than straight. Keep in mind, a 2 inch radius tilted at 10° turns into a 12 inch, or more, radius in the direction of flow, where it really matters.

back at you. got any studies to back your points????

the study you showed pointed out cutting it hard to 18.77 caused no turbulence at the transition point and proves my point.

The study you provided showed negligible turublence with square corners, contradicting your comments, and proving my point.

If we are not trying to fill in the wake, then why bother with a kammback? We could throw on 3 inch transition wings ,and call it done. But "everyone" (including me) seems to think a boattail leads to reduced wind resistance. I think you are missing the point to all this.

Again, I challenge you to give hard descriptive instructions or recommendations for Neon to build his project.

Sniping at my posts with vague descriptions of salt water does none of the DOERS any good.

If you don't like 20 degrees, and you want some theoretical curve, describe how to build the curve. Give specific direcitons how to follow the curve and build the curve for the Neon, and for my suburban, and for my trike, and for my race car, and for semi trailers, and for pickups.

I'm not going to argue and say the "correct" curve is not better than chopping it to 20, but chopping it to 20 with a small radius area is pretty easy to implement, easy to describe, and effective, and I don't know how to BUILD the ultimate correct curve for his Neon.

Do you????????

If you have a better way to DO, then speak up.

[MTrenk]

Don't fret miller! This is where my experience will come in handy. Not only will I come up with an effective Kammback, which (time permitting) I shall test in a basic CFD flow simulation, but I will also come up with a process to build it in a structurally sound, useful way that will retain the original function of the car's trunk.
#I_<3_Engineering

[Daox]

Thats enough of the arguing here. Any further arguing will be deleted. Useful discussion is of course encouraged. I'm pretty sure the information has been posted and the OP can figure out what to do from here. If not he can inquire further. No need to keep rehashing it.

Just to clarify, we here at EcoModder ascribe to proven facts and testing, we always have. If you think you have a good idea, tell us about it and test it. Until then it is only an idea. Not that we don't love ideas, that is what we're all here for, and we love discussing them day in and day out. But, if we didn't base our mods on facts and testing we would all be running hydrogen generators with acetone in our fuel and getting worse mileage than without any mods just because someone said it worked.

[2000neon]

To guide this back on topic, I did some shopping today

I got the aluminum flat bar, rivets, coroplast, hinges and adhesive foam tape for between the kamm frame and the car, a total of $68.

I like Aerohead's (Phil right?) idea of hinging it at the back. It still is a possibility, but so far I am still leaning towards hinging it at the front, that way I can open the trunk without the weight of the kamm on top of it, which I think should make it a little easier. Then at the rear of it, I am leaning towards using a hood pin style system from the trunk lid to the kamm frame. It will centre the kamm, keep it secured at the back and still allow very quick and easy access.

As far as the actual build goes, I am going to add the details and pictures to my existing thread, 2000 neon aero mods. I can't link to it right now, but I will do that shortly.

Thank you all for your help. I have to get to work soon and work every day, so it may be a few days before I can start building, but its coming soon!

[ChazInMT]

Steve, I have thought about doing this on my Honda, and what I figured the easiest thing to do was build the kamm so it is "Pre-Stressed", with the front of trunk/bottom of rear glass area being held up, while the top of glass & back of trunk are touching. Then put in some sort of hook brackets that grab the front edge of the trunk lid, put 2 straps to the center area of the kamm off to each side a bit, and when you tension the strap, it puts force on the front and back, and cinches down tight. Depending on what you build yours with, the gap could be ¼” to 1”. I never got mine started but I was thinking ⅜” would do. I was going to be very rigid.

It wouldn't move because it is locked in by the angles. I wanted to do this so my contact points wouldn't be on the paint. It would be the top of rear glass and tail lights. The only thing I have not figured out is how to do the strap for sure. I was going to make mine with a wood frame and cover it with styrene insulation board. That way it would be rigid and I could shape the foam easily into a nice radius. I planned on putting a piece of lexan in the rear upright area to see out of. I was going to make that about 9" on my car. It would have been a little above the template, but that is preferable for optimization to being below it.

A little sketch of what I mean.




My idea for the Honda.

[ecomodded]

Curved shapes remove mass from the car, the curve will provide a shorter distance between the corners by shaving them off.
Less Surface area = less drag plus with the corners nicked off the flow is less disturbed.

[2000neon]

Hey Chaz, I was considering something similar to your idea. On a nice car, I think that is the way to go, and as long as it is built rigid enough I don't see any problems with it. Other than trunk access.

My car is already very rough, scratched, rusted, etc etc. So for me, drilling into to the car really isnt an issue. As I said, trunk access has to be very easy and quick, so trying to reach in and loosen straps, and then have to out them back on, tighten them up again ks just too much of a hassle. In the right application and use of the car, you're idea is awesome, and probably a lot better than mine, especially for no damage/ Paint scratches and being removable.

I am going to try and make this as good as possible, so the least amount of disturbed flow is one of the main goals. I will be building the kamm with rounded corners on the top to side transition, and aiming for 15'. I'm excited to start building it, and I hope that my planning will pay off for me, with the build and mpg numbers.

[drmiller100]

on the curve shown in the pic of the honda, it looks like the template is below the plan for the rear deck.

does that mean you are guaranteeing separation? does that negate the usefulness of the kammback?