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Project: ECOfamily Civic and its Aero Hitch Box / Boat Tail
In order to make the Civic a more perfect family car, it needs better aerodynamics, additional cargo space, (and an improvement of ICE efficiency by some method: P&G/EOC for now, maybe a taller transmission on the horizon). An Aero Hitch Box and other mods will improve aerodynamics and increase cargo space.
Aero Hitch Box status :
My best guess for Cd is around 0.30, but the error margin on my estimate is very wide. Crr probably around 0.12 at 45-50 PSI. (Stupid Les Schwab. I really should have upgraded.) I also have some winter tires on here, generally Nov through Apr. Lower-hanging mod to-do list:
Tail-end to-do list:
http://ecomodder.com/forum/member-ch...els-desert.jpg http://ecomodder.com/forum/member-ch...-boat-tail.jpg http://ecomodder.com/forum/member-ch...-fill-side.jpg http://ecomodder.com/forum/member-ch...ncing-rock.jpg http://ecomodder.com/forum/member-ch...l-scofield.jpg http://ecomodder.com/forum/member-ch...ch-box-car.jpg Original post: ====================================== This thread will probably mostly be about my forthcoming hitch-mounted cargo box boat-tail. Okay, not exactly the first time we've seen the general idea, but maybe the first project thread applicable to a sedan. Also I'll have some brief descriptions of other mods on our 97 family-of-four Civic, and of course results. Getting right down to it; my plan for the Aero Hitch Box (avoiding the 'copyrighted' name :D): http://ecomodder.com/forum/member-ch...7civicwbox.jpg Green - the part I'm fabricating. (Black is the hitch receiver on the car.) I'm thinking strip reinforced aluminum sheet, rivet construction. However, that method hasn't got the wife test green light yet. I could probably get it done just as fast in plywood, plus foam/bondo, and it would be more likely to pass the wife test that way, but I expect that would be heavier and less durable. I've sized the space from the bumper to the front of the box to give enough clearance for the trunk to open, plus a half-inch for good measure. I'm probably going to want to put a caster on the main steel post that it will be on to minimize damage in the event of a departure angle violation, but I probably need to experiment in my driveway. Now, I see some aerodynamic gaps here to fill. The only question is how far do I need to go to get good results? I expect this to require some testing to be sure about it. Possible Gap Fill Remedies: Opaque Half-Kammback (orange) with clear plastic side fillers (blue): http://ecomodder.com/forum/member-ch...cwboxnkamm.jpg Full-Kammback with hinged window (or louvers): http://ecomodder.com/forum/member-ch...xnliftkamm.jpg All the remedies attach to the car, so you can imagine with the aero box off the car roughly fits a Prius profile with a full Kammback. Any thoughts so far? Hoping to get this done before spring, but you never know with me. ============================================ Random background notes, if you want them:
=================================== I will be editing this post for current status and index. |
Sounds like a great project. I await more updates. :)
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Subscribed.
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troposphilic
You've covered the major bases,so you can expect the out-of-the-park home run as soon as she's on the road.
Whereas the sky is presently experiencing tropos-fear when it see's you coming,that won't be he case later. We'll induct you into the End-dependent Oil Producers Association,as your car will be producing nega-barrels of oil out of thin air as she 'barrels' down the highway once she's shaken her money-maker. And don't be surprised should you receive a membership invitation from the (F)riends of (A)erodynamic (R)ennaissance (T)echnology,whom just love to pass gas (stations) with their retrodynamic cars. Welcome to the past! I think you'll dig it.:D |
Shouldn't the top half of the hitch aero box conform to a taper from the roof line (From top down view), instead of the rear fender panel? The top half of the box should look like a miniature version of the silhouette of your car. That is, if you are trying to go for maximum aerodynamic efficiency.
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top half
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If the top portion is deleted you'll pay some in extra drag,but 'practicalities' may demand that you stray from the ideal.The missing link should create a locked-vortex of which the outer flow field will skip over. |
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What's your wife's issue with sheet alu construction ? It's visually the cleanest method of construction, lighter than wood and if you keep it galvanically separated from steel, more durable as well. Quote:
So you'll have a decent part the positive effects of a boattail even if you use just the cargo box. What you might want to fill up is the hole between the cargo box and the car, so you won't have air cross-drafting and causing drag. |
I have heard about the idea before, but I did not know that it went anywhere. Make it so, Number One! :)
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You could put the bottom contour at ~10ｰ and extend a flap all the way to the back of the gas tank/spare tire well or whatever's under there. What do you plan to do to keep the taillights visible? |
Thanks for the encouragement everyone!
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Aerodynamically I think rivet heads don't matter, but I might have to flush them just for aesthetics. For taillights, a trailer light set is the way to go. If I do a gapfill on the sides it will be clear plastic so as not to obstruct the stock headlights when the hitch box is off. |
If you look at the website, they have a wide variety of composite materials. They make 5/8" plywood with aluminum and/or plastic skins for the back doors of box vans. AlumaCorr™ is essentially pre-bonded coroplast and aluminum.
The Polymetal might survive a bead roller. But I'd settle for being able to radius an edge. My intended material was laminated redwood benderboard, or bulkheads and stringers with shrink wrap plastic skin; but this stuff would afford a higher level of finish. My local supplier is Multicraft Plastics. I'm going to get a sample the next time I'm on that side of town. |
More detailed plans
I was hoping to start fab by now, but I ended up having higher priorities this month. :(
Anyways, I did some CAD to help firm up my planning. Might as well share the aero eye-candy. ;) http://ecomodder.com/forum/member-ch...parent-cad.jpg http://ecomodder.com/forum/member-ch...g-view-cad.jpg http://ecomodder.com/forum/member-ch...lid-up-cad.jpg The lid ended up with so many compound curves that it would be very hard for me to learn how to make it - and make it look good - in aluminum. But the good news is the lid shouldn't need to take as much abuse as the rest of the box, so fiberglass should be great. I'm going to make a mold for the fiberglass with plywood and foam. The rest of the box will be aluminum sheet. The the only compound curves will be contained within 6 corners. All 6 have a 4" radius, so I'm thinking that the stump method would work well. I see one or two spots, other than the floor, that probably need some framework of one sort or another. I figure I'll want a buck, at least for a few cross-sections, as well. The 6 shrunk corners shouldn't be too ugly, even if I do them poorly, since 4 on top are covered by the lid, and the 2 on bottom might stay out of the way. This plan should pretty well cover wife certification, give or take a little bondo over the rivet heads, and it keeps my tool outlay pretty minimal as well. Not that I would really be sad if I ended up with some sweet metal shaping tools, but I'll try the stump and probably some dollies and a slapper and then go from there. Planning on a metal square tube frame floor with pink foam in the gaps and 1/4" plywood over the top. This gets bolted to the inside of the rest of the box. http://ecomodder.com/forum/member-ch...r-view-cad.jpg I've depicted circular tube cross members - I was hoping to find off-the-shelf lugs and do silver brazing, but I can't find generic lugs anywhere. If I drill through square tubes then the load on the joints is probably fine with just epoxy putty to keep them from rattling. But I might change my mind and get a welder and do everything in 1.25" square tubing. Man, that would be fun. There is also the caster shown; I still haven't decided for sure, but I think this is the way to go. This design means in a ground-strike the caster and hitch receiver will have to bear the weight of some of the vehicle. The caster here is rated at 300lbs, so it may or may not break. My experience is that the car will not strike at this departure angle if it's unloaded. So if I do this right, the weight on the caster should be no more than the load in the car, not the car itself. I've also set the bottom slope at 15.5ｰ to keep the departure angle sane, which is faster than ideal but will still provide some aero benefit. Another approach is to use a caster, hinge and chain so that the box can just bend out of the way instead of trying to support the vehicle. This would also enable lowering the bottom slope closer to the aerodynamic ideal. But space is tight between the trunk and the box, and it adds complexity, and I'm debating whether it's worth it, since I'd have to redesign everything. (Better now than later though.) EDIT: I forgot to mention; this box has about 18 cubic feet for interior volume, says my CAD software. That's gonna be great! The hitch rack / bag I've got now is 11.5 cubic feet. The Civic only has 11.9 cubic feet in the trunk. |
Looks good. Although I don't get why the lines across the top are curved when the edge fore and aft of them are straight.
What would you think if the metal frame was a complete square with maybe 2 round crossmembers? Look at the departure angle from the bottom of the caster to the back edge. Try the caster maybe 6" from the back edge. One is probably better than two so you don't get twisting forces on the tongue when you traverse a dip at an angle. I've been on that side of town twice now and forgot the Polymetal both times. Maybe I should make a special trip, but my priorities have resorted themselves this month too. |
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I've been wishing I had a CAD model of the car, because I'd like to get a closer look at how I matched the trunk, and there is also more detail on the sides of the car that I wish I could get a more careful look at. 3dcadbrowser has a nice looking 97 Civic file, and I can get it for free, but I have to wait '3-6 weeks' for my upload credit. I think the moral of this story is that if you're a CAD user you should go and upload a file now so you can have a free credit on tap when you need it. (And of course once I have a model of the car and the aero box I'm going to really wish I had CFD.) (Next best thing to the CAD file would be having it in the driveway, presumably it should be back in town later next week.) I'll be tinkering with the placement of the caster. Once I get the bottom rail formed I'll set the car in the gutter and eyeball where it would work best. I have to consider on one hand that the strike is less likely to occur with the caster farther from the car, and on the other hand that the strength of the bottom rail will be greatest if I can keep the caster closer to the car. I'd call the floor frame a trapezoid rather than a square, but yeah, that's the idea. And I may need to get the welder anyways - I'll have a better idea after I call some machine shops tomorrow on bending the main bottom rail. |
What file type do you need? NURBs or polygon?
ウェブプラモ｜CIVIC「シビック」 It doesn't say what year but it looks about right, except it's a hatchback. |
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That's an interesting business model. What's the up/down ratio? It seems like they would have to grade the submissions to keep it from filling up with cruft.
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Subscribed. I have been kicking this idea around for my hatchback for several years. I have the receiver hitch and the "stinger" (that plugs into the receiver) welded up but I have been stuck since. Perhaps you will inspire me to finish it up.
As to the caster, I have an unusually steep entrance to my driveway. I have to enter and exit at an angle to keep from rubbing. On my driveway the casters would have to be at the back corners to do any good. Also, the suspension should be working with you if you get a caster strike. You don't have to lift the weight of the whole back half of the car, hopefully only a small percentage of that as some of the load is taken off of the rear springs. |
If you get a corner strike, you'll get a *lot* of torque on the hitch.
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corners
I've laid up fiberglass compound corners inside hemispherical steel barbecue lids found at metal recycling centers.
I've also run schedule-40 PVC pipe through the table saw to create molds for the corner radii. Thermo-formed ABS plastic fish ponds have some fabulous compound forms of which the inside surfaces could serve as molds. Tractor Supply also carries cattle troughs which are thermo-formed ABS which possess both compound corners and edge radii. And children's poly tubular playground slides have compound curves almost as large as what Airstream and Argossy trailers use. Flea Markets,yard sales,junk dealers,Habitat for Humanity Resale stores can all be sources for what would otherwise be extremely expensive 3D forms.:) |
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For one thing, the caster costs 2ｰ of departure angle, resulting in 13.5ｰ instead of 15.5ｰ. For another, I want to avoid lifting the car using the hitch receiver. Class 1 should only have 200lbs tongue weight. Actually my Civic isn't rated for any towing at all, but I guess that just means I need to upgrade the rear shocks and stick to Class 1, for the purposes of a hitch box. (Anyways it hasn't stopped me so far.) The orange triangle depicts a 26ｰ wedge between the box and the hood if I locate the hinge at the point of the triangle. This pic also gives you an idea as to how I'm defining the envelope for the hitch box; I put all the pixel coordinates of the red dots into a spreadsheet and defined the scale using the known dimension specs (3.8pix/inch).http://ecomodder.com/forum/member-ch...neer-hitch.jpg Now for a demonstration: in this picture there is an 'invisible' car between the hitch box and the rear tires (actually just dimensional constraints that hold them together in the right place). The ramp is at 15.5ｰ and the driver didn't turn into it. I believe my own driveway is around 15-15.5ｰ and it's one of the worst I ever use.http://ecomodder.com/forum/member-ch...rear-tires.jpg The same, but now the ramp is at 16.5ｰ since the driver angled the car to 10ｰ. You can see the front corners of the box are under threat and there's nothing the caster can do about it. I can shorten one of the posts so the box is higher in the front. I have already taken off 0.5" and that made a big difference. But the more I do this, the more it (maybe) becomes less aerodynamic with flow detachment on the bottom. EDIT: I think this is actually not as bad as it looks since the suspension will tend to compensate.http://ecomodder.com/forum/member-ch...-wheel-now.jpg Now the driver turns to 20ｰ. No problem here. If the driver turns in much steeper the box just stays off the ground and never even hinges. EDIT: this speaks to COcyclist's concern. In most situations the center caster is adequate, at least for the hinged version.http://ecomodder.com/forum/member-ch...-departure.jpg However, sidewalks can strike the front of the box if the ramp angle is over about 15ｰ, and if the driver backs in or faces out.http://ecomodder.com/forum/member-ch...e-sidewalk.jpg The driver should avoid doing this on steep driveways that have sidewalks. This probably deserves a little more thought because it can be a big deal in business parking lots, i.e. where facing out is required. And so, here is my v2 implementation:http://ecomodder.com/forum/member-ch...ded-square.jpg
Man, what a load of details. For anyone who wants my CAD for whatever, I put it here: Index of /files Tomorrow I got to get the shop cleaned up and ready to use the new welder and planishing hammer. :D |
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<slaps head> I was going to say; the up/down ratio is 1:1.
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An anecdotal observation, when you see large RVs with extra-long tails turn into deep entrance/exit ramps the sides & corners often strike the road surface. For your extended box instead of a single center caster, the bottom corners may benefit with much lower profile sled-like runners.
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The aero box does have the advantage over RV's and such that it's not square, looking at it from the top-down view. That's why the rear corners tended not to strike in my models. It does make sense that the front corners would be more seriously threatened because the frame hinges but the front corners don't actually move out of the way, considering the pivot location. If I go for a v3 design overhaul the next level is to put in linear slides (like drawer slides) instead of a hinge, so the box translates upwards instead of pivoting. I'm still mulling it over - could be overkill. After I fab the floor frame I plan on driving around with various loads to prove it out. Having driven with a 4.5" clearance air dam in the Corolla I know where the worst ramps and dips in town are. |
Skills and tools
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Here's a brief sideline on metalworking skills and tools for this project. Sorry this is a bit of a distraction, but I found this really fascinating since it was mostly new to me, and folks who aren't in on metal shop and want to quickly gauge difficulty of the project or weigh against fiberglass may find this useful.
Fancy new tool list:
Here's a picture of a little bowl I made in 20-mil Aluminum. Looks great, and it was a breeze, only about 10 min on my first try. Step 1: shrink with the home-made 'stump' and teardrop hammer until rough shape is achieved. Step 2: planish with the pneumatic hammer to smooth out imperfections. http://ecomodder.com/forum/attachmen...1&d=1359399841 Here are some references for the curious on metal shaping: Basics and overview of sheet metal shaping by Hotrod Magazine 'Stump' shrinking video Tuck shrinking video 'Stump' shrinking results in a little bit of stretching, which I realized may not be desirable for the shape I want. This other method of tuck shrinking minimizes stretching, so I might have to drop my 'stump' and do this instead, but I'll cross that bridge when I get there. Planishing video Planishing is basically cosmetic after the shape is formed, and this can also be accomplished on a dolly with a hammer or slapper, but my planishing hammer was less than $100, dollies and additional hammers would have cost money anyways, so I consider pneumatic planishing very cost-effective. The welder actually was much harder to learn to use, partly because I practiced on zinc-plated steel strip (EDIT: DANGER, don't weld zinc / galvanized stuff - it's TOXIC, what am I doing?), which does not take a bead easily. But after an hour I tweaked the settings and got decent penetration and good bond strength in 1/8" steel, albeit with some hideously ugly beads. Most of this project will be 60-mil steel fillet joints, so I need to get more scraps and practice even more. http://ecomodder.com/forum/attachmen...1&d=1359399841 There's an argument to be made that I might have been better off with a MIG, but part of the reason I chose the flux-core was that it was a single-phase 120 V plug and quite cheap, which affect the project in other ways. I'm not really sure that extra layers of complexity for MIG actually result in a hugely easier learning curve anyways. Once you get the settings dialed in for the flux-core it goes very quickly and it's repeatable. |
That planishing tool is awesome. 10 minutes on the first try? :thumbup:
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On the other hand the demo vids I linked to were only about 2.5 min each for the stump shrink and planishing hammer. So with a little practice I should be able to get it down to 5. :turtle: (And at that rate I ought to have this whole thing done, by say... midnight? ;)) |
Welding
Time for some fab!
Welding beads shrink a bit, so I put a couple dabs around the corners to keep it as stable as possible before laying down the heavy beads. http://ecomodder.com/forum/member-ch...-rail-weld.jpg This double joint was a lot simpler than I expected. http://ecomodder.com/forum/member-ch...ouble-weld.jpg More comments on tools: A good machinist combination square would have been a big help, but a square and a little trig can do the job. Many things sold as protractors are not worth their salt. The 90 Amp flux-core welder is not a nice tool for fillet joints on 1/8" steel. It can get the job done, but it's a challenge to do it and be certain that a solid weld has been achieved, and time consuming. Something more like a 120 Amp MIG welder would save a lot of time, but not that it matters a ton for a one-off. Still a few important bits needed, but the frame is basically assembled and test fit on the hitch receiver here. http://ecomodder.com/forum/member-ch...d-test-fit.jpg |
sweet:thumbup: i really love the idea and if you have good results i will make one for my car :D
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That looks really good. I can't wait to see this finish up. I have thought about doing the same thing.
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FYI - race car with similar aero treatment
The tail reminds me of this race car:
http://photos.motoiq.com/photos/i-8T...-8T4CdQz-L.jpg The entire article here: Nerd's Eye View: The MCA Suspensions "Hammerhead" Silvia S13 Enjoy! |
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beautimous.
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Excellent project !
Have you finalized the techniques/ materials you will use for the "box" part ? What makes this especially nice is all edges radiused with no visable seems and a compound lid (as your sketch) . All possible with one-off fiberglass/epoxy and a foam core. Light weight too. You could build all the panels except the lid on a flat table and use 1/8" birch plywood for the inner skin for impact resistance and then fiberglass the outside after you glue the panels together and do the radius. I bet if you used epoxy with a filler to glue the panels together and also to glue the plywood skin you could go without a fillet in the interior.... For the lid I would check out the Teardrop Trailer guys and see how they deal with the hinges and weatherstripping etc. Attachment 12462 Just a thought if you haven't already decided on the method/ materials for the box. Also thanks for the videos on compounding sheet metal. Some of those techniques are new to me . |
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Honestly I've got a bit of analysis paralysis on the skin and lid. I think I need ribs at the midpoint on the body and around the lid. Starting to favor wood ribs, not a bad method. Still leaning toward aluminum skin for most of the box. ... and still not totally finished with the frame either. UHMW slip pad(s), rubber stop, angle reinforcements on the acute joint, test runs... I'd never heard of teardrop trailers! Those should have never fallen out of fashion. My free-time is going to slow down a bunch... Still hopeful about having this done before spring. :snail: About all I got done since last time: http://ecomodder.com/forum/member-ch...e-driveway.jpg No huge surprises. It looks like over an inch clearance, but it only takes about 100 lbs on the trunk to put the double joint on the ground. |
Design: strength, zinc, weight
Quick mind dump on some design issues that I over-analyzed:
Strength In the interest of not loosing our stuff and ruining a family trip, I put some thought into the most prominent forces the frame has to withstand, i.e. the bumpy road scenario. Self-explanatory. http://ecomodder.com/forum/member-ch...-hitch-box.jpg I came at this from a couple directions, but I think 120-wall (1/8") 1.25" square tubing has a yield strength that can be expressed as a torque of about 2,000 ft-lbs. Here is a 2-ft length of the stuff in Solidworks Simulation (formerly COSMOS), 1,000 lbf load on the end of a cantilever: http://ecomodder.com/forum/member-ch...vered-beam.jpg Of course this number needs to be derated for a number of factors; fatigue, localized stress, and rust, but I think it's pretty reasonable to expect this material to handle 600 to 800 ft-lbs for a lifetime. So assuming the box itself is under 50 lbs, considering all the margins I've thrown in, we'll probably be pretty safe if we keep the load under 150 lbs as a rule. 120-wall also seemed like an intuitive selection, but I wanted to go through this exercise because the hitch-rack I'm replacing actually uses a solid 1.25" square instead of tubing. (This one.) That seems pretty over-rated. I can't even bend the 120-wall with a sledge hammer (at least not with some wood buffers to prevent marring). The carrier I'm replacing also weighs 49 lbs including the bag. I'd like to beat that number. EDIT: I also put some thought into making the welds less critical - trying to have at least double the surface area of the tube in weld cross section. Weight The frame is about 19 lbs. There's at least 2.5 lbs I could have taken out of the side rails if I'd found just the right c-channel. I was going to use plywood for the floor, but just 1/4" of that would weigh 8 lbs. 19-mil Aluminum sheet is 2 lbs instead. 19-mil Aluminum skin would be 12.5 lbs, but I'm going to assume for now that I need 25-mil, for 16 lbs. So I'm at about 37.5 lbs, but this doesn't include a bunch of hardware and ribs. Zinc It probably isn't worth fussing much over galvanic corrosion. (I believe) there is some precedence for combining aluminum and steel in bikes and cars. On boats which are completely submerged for long periods you can get into trouble. The difference in potential is probably around 0.15 V. Galvanic corrosion - Wikipedia, the free encyclopedia (I'm hunting for a reference, but for that potential they should be fine in any environment except for submersion.) Really the only time I would see galvanic corrosion is salty wet roads where there is heavy enough precipitation to electrolytically bridge an open steel zone with an open aluminum zone. A Liquid Nails bead around the entry point for the frame into the skin, and of course painting both, should be sufficient. If I feel paranoid I might put in a zinc anode, or at least some galvanized screws, around that area as well. That, and keep rain from getting inside and puddling forever. There's definitely a strong motive for using steel for the frame but not for the skin. The hitch receiver is steel, so if I put aluminum in there it would be a zone where salty water could get trapped and cause real trouble. Steel is fine for framing but it makes a very heavy skin. |
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