# Del Sol rear box placement and car handling/stability



## ruckus (Apr 15, 2009)

The 37 Jaguar has about 200lbs over the rear axle. Body roll in corners is nill and the car handles much better than original. It is like having 1 person in the back seat. No biggie.

You probably wouldn't notice the difference either way, but further back will create more potential for "sag".


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## Duncan (Dec 8, 2008)

Hi swoozle 
I am a big fan of getting the weight as low as possible,
A possible issue could be the increased polar moment - BUT I suspect your car has a very small polar moment as standard and that it may be a bit twitchy in extremes in which case a bit more is good and will make the car less twitchy


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## TomA (Mar 26, 2009)

275lbs or so?

Sort of a tough call depending on what's left for stowage, etc., but for me it would be a pretty simple decision. I'd want the pack over the rear axle, tied into and further bracing the shock towers. The car is far stronger there than the sheet metal back in the tailpan, and the weight is likely to be better integrated into the chassis that way, keeping it as stiff and tight as possible. It will be easier on the rear suspension, too.

Just my $.02...


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## swoozle (Nov 13, 2011)

TomA said:


> ...I'd want the pack over the rear axle, tied into and further bracing the shock towers. The car is far stronger there than the sheet metal back in the tailpan, and the weight is likely to be better integrated into the chassis that way, keeping it as stiff and tight as possible. It will be easier on the rear suspension, too.
> 
> Just my $.02...


Thank you all for the feedback. This last one brings up a good question: I've always been a little skittish about cutting any of the unibody, but if I cut out a little, say 20" x 20", from the front-center of the trunk floor, the ease of mounting becomes much better. I am able to put the box pretty much centered on the axle and get better chassis integration and stronger bracing. I think I more than make up for any loss of stiffness. I'll post a photo later.

Sound like a reasonable trade-off?


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## njloof (Nov 21, 2011)

That's standard operating procedure for putting the battery tray in the back of a Miata...


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## Duncan (Dec 8, 2008)

swoozle said:


> Thank you all for the feedback. This last one brings up a good question: I've always been a little skittish about cutting any of the unibody, but if I cut out a little, say 20" x 20", from the front-center of the trunk floor, the ease of mounting becomes much better. I am able to put the box pretty much centered on the axle and get better chassis integration and stronger bracing. I think I more than make up for any loss of stiffness. I'll post a photo later.
> 
> Sound like a reasonable trade-off?


Hi if you are cutting out a bit of flat metal 
- I would weld a frame in so that any loads can go around the hole

If you cut out a piece of floor and weld a battery box in its place you will end up with a much stiffer structure
Keep it low!!


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## ruckus (Apr 15, 2009)

Do not hesitate to cut.

The flat or "wavy" bottom pan is very flimsy. I found angle aluminum provides a nice way to trim off a jigsawed hole, and is super easy to drill. After final fitment, caulk it down and use sheet metal screws every 6-10". Will be WAY stronger than before even if you use pretty small aluminum (3/4" angle).

Make sure you post some pics...


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## TomA (Mar 26, 2009)

The bottom line is that packaging considerations are the most important. If cutting a hole in the floor gives you the best packaging, that's generally the way to go.

Rant against angle iron starts here-->

I always wonder why people don't replace sheet metal with sheet metal, or make a fiberglass tub for the batteries and glue that in with 3M structural adhesive and screws or rivets to resist peel in a collision. I just won't replace 2lbs of sheet metal with 20lbs of angle iron. I also wouldn't want to compromise the crush structure of the rear clip with a solid cube of mild steel stuck in there. I'll never have a lead-acid pack, and the angle iron is overkill for lithium, so I'm not complaining about the racks in a lead sled pickup truck here. Bed frame are fine for that, but not a Li-based car. I think angle iron racks are one of the last crude science project techniques in EV conversions. Just me. Nobody has to feel that way, but not in my car.

I'm only so-so MIG welding sheet steel, but I'd still rather do that than angle iron. An afternoon at the PickAPart with a battery powered sawzall would yield all the waffled and 90 degree radiused sheet steel I'd need. I'd start on an old pickup bed, or maybe get lucky with the shapes I need in the trunk of something else. Its a lot of work for a non-body man like me, what with the metal finishing and painting, but its really the right way. Again, just me.

John Allen made his Toyota Corolla battery boxes of sheet aluminum. Good compromise.

My current plan (on the Daewoo Matiz I'll be converting once I move to Oz later this year) is to pull a fiberglass plug off the floor behind the axle to get the shape, and use that as a flange to make an S2/Kevlar fiberglass battery box that will drop, glue and rivet into the hole. If space permits, I'll sandwich foam into the box for insulation and more strength. My other battery boxes- one under the hood and one or two under the rear seat, will also be composite.

Yeah, angle iron is cheaper and easier, but the project is $15k+ so I'm not inclined to throw weight at it to save a few bucks and a few hours. Besides, my bonded and riveted-in fiberglass tubs won't leak, squeak, rust or conduct, and will make the chassis much stiffer at the lowest added weight than anything other method. 

Rant against angle iron ends here-->


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## TomA (Mar 26, 2009)

ruckus said:


> Do not hesitate to cut.
> 
> The flat or "wavy" bottom pan is very flimsy. I found angle aluminum provides a nice way to trim off a jigsawed hole, and is super easy to drill. After final fitment, caulk it down and use sheet metal screws every 6-10". Will be WAY stronger than before even if you use pretty small aluminum (3/4" angle).
> 
> Make sure you post some pics...


Better than caulk is a two part urethane automotive structural adhesive. I like 3M Scotch-Weld #8101, but there are many others.


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## ruckus (Apr 15, 2009)

TomA said:


> Better than caulk is a two part urethane automotive structural adhesive. I like 3M Scotch-Weld #8101, but there are many others.


He is right. I was just thinking to keep water out and add a bit of strength, but if you want it there FOREVER, go with the adhesive. The stuff is amazing, just be sure to wear gloves and have lots of paper towels on hand or you will be sorry.


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## swoozle (Nov 13, 2011)

TomA said:


> Yeah, angle iron is cheaper and easier, but the project is $15k+ so I'm not inclined to throw weight at it to save a few bucks and a few hours. Besides, my bonded and riveted-in fiberglass tubs won't leak, squeak, rust or conduct, and will make the chassis much stiffer at the lowest added weight than anything other method. ->


I made my boxes from aluminum for some of the same reasons that you list (obviously not the nonconductive part). I know they are very strong and lighter by far than any equivalent DIY steel effort. I don't think people realize that commonly available aluminum alloys are almost as strong as your typical angle iron alloy, MUCH less dense and very easy to work with. I cut my .040 sheet by scoring it and bending it until it fatigue cracks.

I wanted them enclosed for insulation and heating purposes and aluminum sheet is also cheaper than any plastic sheet choices I have access to.


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## swoozle (Nov 13, 2011)

Here's some aluminum versus steel specs:
(prices from Online Metals)

6061-T6


 Yield tensile strength: 28ksi
 Ult tensile: 33 ksi
 Elongation at break: 12%
 Density: .0975 lb/in3
 Price for 1" x .125" bar, 5' long: $3.00
 
A36 hot roll steel


 Yield tensile strength: 36ksi
 Ult tensile: 60ksi
 Elongation at break: 20%
 Density: .284 lb/in3
 Price for 1" x .125" bar, 5' long: $4.44
 
The steel is ~30% stronger at yield, which is what you should use for design.
Aluminum is ONE THIRD the weight (density).
Aluminum is cheaper.

I hadn't realized the trade was so one-sided. I thought steel was cheaper for sure, but no. Even on a price per strength basis, aluminum is a lot cheaper.

Addressing the galvanic issues between aluminum and car steel isn't that difficult, but welding with aluminum is. Certainly the machining and drilling part is vastly easier with aluminum.


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## TomA (Mar 26, 2009)

swoozle said:


> Here's some aluminum versus steel specs:
> 
> I hadn't realized the trade was so one-sided. I thought steel was cheaper for sure, but no. Even on a price per strength basis, aluminum is a lot cheaper.


Well, there's a lot going on here.

First, you're comparing top strength Al to mild steel. Automotive steels are better performing and even more expensive (probably not available in raw sizes retail anyway,) which is why I would go harvesting them in the junkyard.

Second, properly shaped steel can be much thinner than Al at the same strength, which is why that sheet cut out of the floor seems flimsy, but in the carefully shaped unibody it is plenty strong. 

Steel has properties beyond strength, like resistance to fatique, greater durability, and it is stiffer. Unless you can really carefully calculate and control the thickness and shape of Al panels, you're probably not going to get them lighter than sheet steel for the same performance. Even the OEMs have trouble making sheet aluminum parts very much lighter than steel.

Steel is easier to fabricate, (welding sheet aluminum is particularly difficult, especially at the thinness of autobody steel replacements,) and about cost- Al may be cheaper to buy bulk, but its expensive to fabricate if you have to buy different gas, wire and traverse a new learning curve to get there, where high strength automotive steel panels are almost free in salvage and the wire and gas are in my MIG welder now, let alone the trunk floor is ready to be patched up with nothing more than that.

So, no criticism, just ideas. Aluminum is a pretty good compromise, but to weld it takes dedicated tools, supplies and skill. Automotive sheet metal is maybe a better compromise, but requires MIG or TIG skills and tools, plus more scrounging for salvage panels. Both require fabrication and finishing time, especially steel. 

The big problem with new/bulk sheet metals is they aren't very strong or stable as flat sheet. Aluminum and steel sheet can be bead-rolled and even pounded into surface shape details to add stiffness and dimensional stability. I'd be harvesting steel from scrap cars with formed shapes and panels that had those features already stamped into them. Its still too much work, though, and I already have the MIG gear and skills.

Composite is really the easiest and cheapest to learn, especially if you have no welding skills or gas-shielded welding gear. People dismiss it because they've never tried it, and then they try to shape and weld production-quality aluminum sheet structures, which is actually much harder to do. Go figure...


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## Duncan (Dec 8, 2008)

_Composite is really the easiest and cheapest to learn, especially if you have no welding skills or gas-shielded welding gear. People dismiss it because they've never tried it, and then they try to shape and weld production-quality aluminum sheet structures, which is actually much harder to do. Go figure...

_I agree - except - certification
I was tempted to make a composite chassis but the issue is getting it accepted for road use
by somebody who knows steel but is uncertain about this plastic stuff


Instead I am using mild steel - yes its not as strong for the weight but it is nice and soft and forgiving
It's not a good choice for minimum weight but it is a good choice for a structure that will survive faults and mistakes_.

_In engineering it is impossible to have zero flaws so it is sensible to design a structure that will work with faults

A pressure vessel can be designed so a flaw will cause a leak before catastrophic failure
Me I over-design and overbuild so if one of my joints is in-perfect - no problem
Mild steel is GOOD for that


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## aeroscott (Jan 5, 2008)

With al or stainless steel you need to stay out of the fatigue range . mild steel can work closer to its ultimate strength because it has a higher fatigue range . Al has 1/3 the weight of steel /size but to stay out of fatigue it ends up being 2/3 the weight (more volume) in a structure . composites have some great advantages but not in point loading or delamination .


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## swoozle (Nov 13, 2011)

Yes, steel certainly has it's advantages. But my givens were: steel easily available for the garage enthusiast, not the semi-professional, and design by feel or at least gross ROM calculations.
For the level of over-design that the typical EVist SHOULD be doing, fatigue is not an issue. Now I realize that some of the designs demonstrated here don't meet that conservative yardstick, but they are just as likely to fatigue a poorly designed hardpoint in steel as they are in aluminum.

6061 is hardly top-shelf aluminum. Getting weaker alloys is really not significantly cheaper as far as I can tell. If we were talking 2024/7075 I'd agree with you, but those are also way more expensive than 6061.

Fabrication I would put down to preference. Personally I'd rather rivet/bond aluminum together than weld steel. That may be my familiarity with monocoque structure showing through.
I have heard (but do not know for sure) that welding aluminum really isn't that much more difficult than steel with a decent TIG or MIG setup (or even stick!) but there's learning/familiarity hump to get over, sure. Welded aluminum probably increases the fatigue risk also.
(EDIT: Oh! I forgot corrosion resistance. Again this is probably due to my familiarity with aluminum, but I'd much rather slap some corrosion preventive grease at the interfaces between aluminum and the car, leaving the rest of the Al unpainted, than run the risk of rusty steel. That stuff (steel) goes like gangbusters with just rainwater exposure once whatever protective finish is breached. Which inevitably happens. But I also live in an area where salted roads are a rare-to-never occurrence. Rainwater and aluminum such as 6061 are happy friends).

Are composites really less expensive? A good resin is pricey and carbon fiber tape or fabric is forget-about-it. Glass fabric ain't cheap neither. I like sandwich structure for lightness and stiffness but core seems expensive to come by and wet layup fabrication requires another level of skill. I'd love to be pointed at cheap sources; I've been interested in getting my hands wet in composite fab )).

To each his own. That's what makes this community so interesting and fun, hearing the reasons people have done the same thing in a dozen different ways.


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## nucleus (May 18, 2012)

*Re: Del Sol rear box construction materials*

I have experience with composites and aluminum both, and I agree than aluminum riveted (or bonded) is the best choice. It is light, strong, very corrosion resistant (depending on the alloy) and very non-toxic in comparison to composites. The resin used in composites is very toxic and very flammable. I had friend who burned his Lotus down by accident, it went up very fast, and there was an amazingly delicate shell of pure fiberglass left when it was done.

Here is some aluminum alloy info - 6061 is great stuff - from

http://www.aircraftspruce.com/catalog/mepages/aluminfo.php

1100 
This grade is commercially pure aluminum. It is soft and ductile and has excellent workability. It is ideal for applications involving intricate forming because it work hardens more slowly than other alloys. It is the most weldable of aluminum alloys, by any method. It is non heat-treatable. It has excellent resistance to corrosion and is widely used in the chemical and food processing industries. It responds well to decorative finishes which make it suitable for giftware.
2011 
This is the most free-machining of the common aluminum alloys. It also has excellent mechanical properties. Thus, it is widely used for automatic screw machine products in parts requiring extensive machining.

2014 & 2017 
The 2017 alloy combines excellent machinability and high strength with the result that it is one of the most widely used alloys for automatic screw machine work. It is a tough, ductile alloy suitable for heavy-duty structural parts. Its strength is slightly less than that of 2014.

2024 
This is one of the best known of the high strength aluminum alloys. With its high strength and excellent fatigue resistance, it is used to advantage on structures and parts where good strength-to-weight ratio is desired. It is readily machined to a high finish. It is readily formed in the annealed condition and may be subsequently heat treated. Arc or gas welding is generally not recommended, although this alloy may be spot, seam or flash welded. Since corrosion resistance is relatively low, 2024 is commonly used with an anodized finish or in clad form (“Alclad”) with a thin surface layer of high purity aluminum. Applications: aircraft structural components, aircraft fittings, hardware, truck wheels and parts for the transportation industry.

3003 
This is the most widely used of all aluminum alloys. It is essentially commercially pure aluminum with the addition of manganese which increases the strength some 20% over the 1100 grade. Thus, it has all the excellent characteristics of 1100 with higher strength. It has excellent corrosion resistance. It has excellent workability and it may be deep drawn or spun, welded or brazed. It is non heat treatable. Applications: cooking utensils, decorative trim, awnings, siding, storage tanks, chemical equipment.

5005 
This alloy is generally considered to be an improved version of 3003. It has the same general mechanical properties as 3003 but appears to stand up better in actual service. It is readily workable. It can be deep drawn or spun, welded or brazed. It has excellent corrosion resistance. It is non heat-treatable. It is well suited for anodizing and has less tendency to streak or discolor. Applications same as 3003.

5052 
This is the highest strength alloy of the more common non heat-treatable grades. Fatigue strength is higher than most aluminum alloys.In addition this grade has particularly good resistance to marine atmosphere and salt water corrosion. It has excellent workability. It may be drawn or formed into intricate shapes and its slightly greater strength in the annealed condition minimizes tearing that occurs in 1100 and 3003. Applications: Used in a wide variety of applications from aircraft components to home appliances, marine and transportation industry parts, heavy duty cooking utensils and equipment for bulk processing of food.

5083 & 5086 
For many years there has been a need for aluminum sheet and plate alloys that would offer, for high strength welded applications, several distinct benefits over such alloys as 5052 and 6061. Some of the benefits fabricators have been seeking are greater design efficiency, better welding characteristics, good forming properties, excellent resistance to corrosion and the same economy as in other non heat-treatable alloys. Metallurgical research has developed 5083 and 5086 as superior weldable alloys which fill these needs. Both alloys have virtually the same characteristics with 5083 having slightly higher mechanical properties due to the increased manganese content over 5086. Applications: unfired pressure vessels, missile containers, heavy-duty truck and trailer assemblies, boat hulls and superstructures.

6061 
This is the least expensive and most versatile of the heat-treatable aluminum alloys. It has most of the good qualities of aluminum. It offers a range of good mechanical properties and good corrosion resistance. It can be fabricated by most of the commonly used techniques. In the annealed condition it has good workability. In the T4 condition fairly severe forming operations may be accomplished. The full T6 properties may be obtained by artificial aging. It is welded by all methods and can be furnace brazed. It is available in the clad form (“Alclad”) with a thin surface layer of high purity aluminum to improve both appearance and corrosion resistance. Applications: This grade is used for a wide variety of products and applications from truck bodies and frames to screw machine parts and structural components. 6061 is used where appearance and better corrosion resistance with good strength are required.

6063 
This grade is commonly referred to as the architectural alloy. It was developed as an extrusion alloy with relatively high tensile properties, excellent finishing characteristics and a high degree of resistance to corrosion. This alloy is most often found in various interior and exterior architectural applications, such as windows, doors, store fronts and assorted trim items. It is the alloy best suited for anodizing applications - either plain or in a variety of colors.

7075 
This is one of the highest strength aluminum alloys available. Its strength-to weight ratio is excellent and it is ideally used for highly stressed parts. It may be formed in the annealed condition and subsequently heat treated. Spot or flash welding can be used, although arc and gas welding are not recommended. It is available in the clad (“Alclad”) form to improve the corrosion resistance with the over-all high strength being only moderately affected. Applications: Used where highest strength is needed.


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## fb_bf (Jul 6, 2011)

I built my rear battery box from a Pickup bed tool box. I found one on Craigs list for $65. It was about 13 inches deep and 18 to 20 inches wide. I cut the length down to what I wanted, the used aluminum angle iron, pop rivets, and caulk to attach the end to the shortened box. These boxes are light diamond plate aluminum. It cuts like butter with a metal cutting skill saw. I had lots of left over aluminum to build mounting features for my dc-dc converter, and my fuse, contactor, and shunt mounting platform. I have some pictures on my site. I felt the same way about welding and angle iron frame, then having to cover it. It seemed easier to start with a box, then build a box.


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## ruckus (Apr 15, 2009)

fb_bf said:


> I built my rear battery box from a Pickup bed tool box. I found one on Craigs list for $65. It was about 13 inches deep and 18 to 20 inches wide. I cut the length down to what I wanted, the used aluminum angle iron, pop rivets, and caulk to attach the end to the shortened box. These boxes are light diamond plate aluminum. It cuts like butter with a metal cutting skill saw. I had lots of left over aluminum to build mounting features for my dc-dc converter, and my fuse, contactor, and shunt mounting platform. I have some pictures on my site. I felt the same way about welding and angle iron frame, then having to cover it. It seemed easier to start with a box, then build a box.


What he said X2


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