Aero trunk lid project

[California98Civic]

Brucepick: I'll record video sometime this week at the higher speed, possibly Friday on my commute.

Originally Posted by wyatt

Is there any reason you are not going to have it go higher on your window, or further back? Both are beneficial, as the top left graph shows. You should still have good rearward visibility if you had your shelf cover the bottom two rows, and you should see a benefit.

Thanks for this question Wyatt. There are reasons. Brucepick is considering a higher shelf, which seems better from the aero perspective. I want an aero benefit with full trunk access and absolutely minimal visibility loss. And then there is the legal requirement not to extend past the bumper. I might push that limit an inch or so, but I get police attention for my mirror delete, so I'd rather not have it on this too.

Does anyone think this use of the template reasonable? (See pic). The zero line is still positioned at maximum roof camber, but the template lines are arranged to intersect the sides of the car. The template suggests a possibly effective lid/spoiler shape. Is this a stupid use of the template? I thought I saw someone reference using it this way.

[wyatt]

Originally Posted by brucepick

The reattach of flow at the trunk edge is pretty nice (in post #18 photo). But I wonder if it still happens at 60-70 mph?

Aerodynamics are basically the same above ~30 mph and below 0.7 mach. If the attachment is good at any speed in there, it should be everywhere.

[wyatt]

Originally Posted by California98Civic

Does anyone think this use of the template reasonable? (See pic). The template suggests a possibly effective lid/spoiler shape. Is this a stupid use of the template?

This should be a fine use of the template. The problem is trying to make a compound curve like that out of coroplast... If I were you, I would make the front of your spoiler square, and at the back, taper the top of it in at a 45 degree angle, and let everything else fall where it will. I had put a 4' SedanKamm on my car, and this is the method I used, and it worked great with strong attached flow on the sides.

[California98Civic]

Originally Posted by wyatt

This should be a fine use of the template. The problem is trying to make a compound curve like that out of coroplast... If I were you, I would make the front of your spoiler square, and at the back, taper the top of it in at a 45 degree angle, and let everything else fall where it will. I had put a 4' SedanKamm on my car, and this is the method I used, and it worked great with strong attached flow on the sides.

That is a good looking DIY mod. And a 6% FE gain is cool. I'm hoping for 0.5-1% FE gains with this trunk lid. Looking at your Kamm makes me feel like giving the lid up. But I want to finish what I started. I want to know methodically if it can work and how well or poorly. Are you still running your Kamm? What are the windows made of? On the curve-cut in coroplast, I hear you. I bought tin snips, curved, for the coroplast cuts. But I'm still not confident I'll get a great line. I'm patient. I'll work up a model of that section in card board first. Ultimately, I know I want to build whatever is final in durable materials (lightweight fiberglass).

[wyatt]

Originally Posted by California98Civic

That is a good looking DIY mod. And a 6% FE gain is cool. I'm hoping for 0.5-1% FE gains with this trunk lid. Looking at your Kamm makes me feel like giving the lid up. But I want to finish what I started. I want to know methodically if it can work and how well or poorly. Are you still running your Kamm? What are the windows made of? On the curve-cut in coroplast, I hear you. I bought tin snips, curved, for the coroplast cuts. But I'm still not confident I'll get a great line. I'm patient. I'll work up a model of that section in card board first. Ultimately, I know I want to build whatever is final in durable materials (lightweight fiberglass).

I think a 1% gain should be doable (but a change that small is hard to discount from being driving variation). You should be able to perform coast down testing. You just need a flat stretch of road, no traffic, and a way to take data. I have gotten quite good results with coast down testing. I can give you a template to work from that translates speed into Cd and Crr. It's just a modified one from instructables, but fits my needs much better.
The Kamm is off the car for now. I still have it, but the Duck brand duct tape started to lose it's adheasive qualities after we had a bunch of days over 105 degrees that summer. I have considered re-building it out of better materials, and making it attach in a nicer way, but I think if I do anything, it would be getting some Nashua brand foil backed duct tape (heavy duty and holds like a mother!), do a bit of cutting to make it fit a bit better, and foil tape it to my car. Maybe paint the foil, maybe get some more of the Duck brand duct tape (I got another brand of white duct tape, and it sucked! Duck brand was quite good in my opinion) and tape over the foil. A caution on the Nashua tape, it holds so well that it can be very difficult to remove, especially if you prepped the surface to any degree...
The windows are made of thin acrylic that I got at Hobby Lobby for around $4 with a 40% off coupon (they have coupons quite regularly). Plexiglass is acrylic, Lexan is Polycarbonate. I have a good amount of Lexan, and have been trying to figure out how to easily make either a more permanent kamm back, or a full out boat tail, but too many other aero projects (and other projects) take priority for the time being. The placement of the windows in the kammback was crucial! I had just enough of the acrylic to make the three small windows you see, so I planned them out well to give maximum visibility around the car. I am glad I took my time and thought it out, since the correct placement meant I could see the road behind me with the horizontal window, and I could see the traffic to the side of me with the side windows.
If you ever get to the point where you are going to build a kamm back, and want some pointers, let me know. I remember enough of how I had done the build that I may be helpful.

[Pendragon]

California98Civic,

I remember a post somewhere about picking up a salvage yard trunk lid to use for modding. If one is available inexpensively for your car, it would allow you to consider some things that you might not otherwise be willing to attempt on a lid that has good paint and so on.

I need to do a little thinking about a way to come up with a quick release for the raised section that would allow easy access to the trunk.

I am glad to see that you are continuing the project.

Cheers

[backyard engineer]

Pendragon, I think that junkyard idea is great. might be a little pricey though. would be nice to snag one from someone's on the way to the junkyard. most wrecks are on the fronts.

[backyard engineer]

hey Wyatt, was checking out your Kammback, great job! I can totally relate to the heat in AL. I'm near Charleston,SC. Have some friends in Dothan. I could use some of your pointers. Check out my project here in aero titled honda hx fastback. might be interested in kammback, fastback,or boatback, cause its got no back!

California, I'm thinking wyatt's got a good point with the plexi or lexan you would'nt have to limit your height consideration for visibility. maybe you could take your design a little further towards the kamm. maybe by leaving a sizeable gap between your mod and the back glass it would allow you to open the trunk maybe not fully but does it really need to open fully? If my car had a trunk I'd check it out for you. But I'm confident you'll figure it out with all the time and effort you've been giving it.

[California98Civic]

Originally Posted by backyard engineer

California, I'm thinking wyatt's got a good point with the plexi or lexan you would'nt have to limit your height consideration for visibility. maybe you could take your design a little further towards the kamm. maybe by leaving a sizeable gap between your mod and the back glass it would allow you to open the trunk maybe not fully but does it really need to open fully? If my car had a trunk I'd check it out for you. But I'm confident you'll figure it out with all the time and effort you've been giving it.

Thanks Pendragon, wyatt, backyardengineer... I like the lexan/plexi idea and think I'll try it on the side and underside supports for certain (greater visibility through the curved sides especially).

I'm persuaded that the Kamm is better for aero and FE (I might yet try it next). But I still see a secure attachment problem (long term) and there is still the access to trunk issue. Both should be resolvable. This car is my daily driver. I need the trunk. I can get to it from inside because I removed the rear seat, but sometimes with my daughter in the morning, racing to get her to school or something, that becomes a significant joy-killer. The lid is designed to be secured with what I'm calling "tension brackets" (for the window portion) and straps with magnets (trunklid portion). Stealthy and completely reversible in under a minute without a trace.

1% FE from the raised lid would be 2mpg at my base 50mpg freeway norm [EDIT: this is an incorrect calculation since 1% equals 0.5mpg on a 50mpg car]. An extra 24 miles per tank is almost 2 bucks at current prices. That would feel pretty good. The craft has to be in getting it just right for what it is, so I intend to do the cutting with care and use something like lexan for the sides to produce visibility. Imagine a black fiberglass, curved lid, held 3" above the trunk by clear supports. That could look fairly cool, really.

So taking what you guys have said and re-applying the template to the actual level of my lid model, I made the attached overlay quickly.

Anyone ever use a table saw to cut coroplast? I just got a table saw for father's day. (YAY!)

[Pendragon]

Originally Posted by backyard engineer

Pendragon, I think that junkyard idea is great. might be a little pricey though. would be nice to snag one from someone's on the way to the junkyard. most wrecks are on the fronts.

If you find one with ratty paint, so much the better for bargaining about price. Just sand the bad spots and shoot them with some primer to stop the rust...I won't say anything about looking like a sleeper '57 Chevy because the whole car would have to be in primer. ;-)

The trunk lid on California98Civic's car is in too nice of condition to start drilling holes and such for experimentation. I have not check prices, but if one can be found for a reasonable amount.... A hail damaged one would be perfect. "Nobody wants one of those" = cheap.

[Edit] P.S. Yes, most cars wrecks are crunched on the front end. Have you ever looked for a hood for a Z-Car?