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Old 08-12-2008, 01:14 PM
Motorsport_E Motorsport_E is offline
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Talking One-Off Tube Frame Chassis (MR2 drivetrian) EV Build Looking for Advice

This post turned out to be pretty long and I really appreciate any advice as you'll likely spend a bit of time reading, so thanks in advanced!

Your skill level with auto mechanics and fabrication

I have a significant amount of fabrication experience working with carbon fiber, plastic, fiberglass, metals, cnc machines, assorted power tools, and others technically unrelated to automotive. Basically I know my way around making something you can't find on the local harware shelves. Also, I've always been into cars and have ripped apart my fair share of them... just not all of them went back together in the end.

The range you are hoping to get (how many miles/charge)

I'm interested in a 144 volt system with a hope for a range of 40-50 miles and a top speed of 90 mph. These are the number quoted with kits I've seen; however I'd never be against a bit more distance, haha. For now I'm content with a 25 mile per charge goal but still want freeway capabilities. While 90 mph would be nice when/if needed for passing purposes this technically only means 65 to 75.

I work 10 miles away from home (just under 20 there and back) with average roads (45 mph speed limit) under minimal traffic but there is a short distance on the freeway (one exit). If achieved, this would be ideal as it would recoup some of the build cost fairly quick, but I'd still want the potential for greater range later.

What level of performance you are hoping to get

Besides what's meantioned above I'd like to be able to get up and go with the other cars on the road without holding up traffic.

How much money you are willing to put into your project

I want to build it right, so the budget is more of a money/time combination. The longer it takes, the more I'll have to spend as I save. However, I'd like to start seeing it come together within a reasonable amount of time and money but I'm not holding on to a number as I'd likely shoot past it in no time.

What parts you've already considered, if any.

*AC motor with regen braking
*Solar panels (a nice thought but who knows)
*I've looked into many of the fixes for brakes, ac, sterring, ect. and feel relatively confident these can be handled during the build once I get a better feel for the actual effects of each
*Batteries are a blank sheet and all will be considered when I buy but suggestions are welcomed; LiFePO4, Flooded, AGM's, Ect.

While my build has taken inspiration from many sources and uses the MR2 drivetrain, in most other regard it's a one off vehicle. I know many would suggest using another vehicle for ease of registration and other legal reasons, but this is something I've always wanted to build and I'm double checking every rule in this process. I'm looking forward to getting started with my build but I want it built right the first time and while I don't usually practice the "measure twice, cut once" rule I'm trying to this time, so thanks for the help!

Last edited by Motorsport_E; 08-26-2008 at 04:23 AM.
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Old 08-12-2008, 04:33 PM
john818 john818 is offline
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Default Re: One-Off Tube Frame Chassis (MR2 drivetrian) EV Build Looking for Advice

Hey! I'm thinking about doing a scratch-built, tube frame, mid-engine car, too! I don't have your fabrication or mechanical skills, so I'm going to try to keep it as simple as possible. Actually, I'm also considering a Reverse Trike (2 front, 1 rear wheel).

Sorry, being a noob myself, I can't help much with EV advice, but if you haven't already, you might want to check out http://www.locostusa.com/. For those who don't know, a "Locost" is an inexpensive Lotus 7 replica built from scratch by DIYers. There's a lot of great info there but not much on EVs and probably not much applicable to someone converting from an ICE.

I think the MR2 is a great choice! Don't know the drivetrain differences between the 3 generations, but they were all pretty good. I'm still considering donors.

Good luck with your build!
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Old 08-12-2008, 10:23 PM
Motorsport_E Motorsport_E is offline
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Default Re: One-Off Tube Frame Chassis (MR2 drivetrian) EV Build Looking for Advice

Actually, a locust was one of the influences; I gained inspiration for this build after I read "Build Your Own Sports Car for as Little as 250 and Race It!" by Ron Champion.

Good luck on your build; I had previously considered a reverse trike but decided I wanted to go all out and build a true car. Although, I may still build a reverse trike once this one is finished as a fun little track toy. I suggest getting the Ron Champion book, or atleast checking it out at a local library, as it's very useful. A good way to stay safe when building your chassis is to use stock measurments for all mounting points, then it becomes almost as easy as a simple bolt on for parts from your donor car.

Last edited by Motorsport_E; 08-26-2008 at 04:36 AM.
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Old 08-13-2008, 01:39 AM
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Default Re: One-Off Tube Frame Chassis (MR2 drivetrian) EV Build Looking for Advice

With a spartan, lightweight tube frame design you should be able to get decent performance and decent range, even using relatively low cost components.

I am not familair with the kit / diy sports cars chassis you mention but you want light, low, and aerodynamic. You have the luxury of a purpose built chassis to fit the batteries in one or two easy access, low and centered areas. Build in enough room to allow insulation to enclose them unless you live in a very warm climate, or you don't plan to drive in cold weather.

As for batteries, I am strongly in favor of flooded lead despite its disadvantages of 1) a bit of maintenance and 2) its reduced power density (watts per pound) vs. AGM. Reasoning is they are the cheapest, most forgiving lead acid and are available at any battery retailer. Flooded lead generally has superior cycle life to AGM and gel as well. Flooded lead also has superior energy density (watt-hours per pound) vs. AGM which means less weight for the same range. Furthermore golf batteries are purpose-designed traction batteries, designed for moving a vehicle. Many AGMs and such may be deep cycle but still aren't really designed for the high power drain of an EV. Whatever you do with batteries though, pay the money for a good brand. Don't buy them from wal-mart.

For what you are talking about (a chassis weighing as a guess 1000lbs with mechanicals installed) you could put 12 or 14 12v golf cart traction batteries in (144-168v). Your battery weight to curb weight ratio would be around 50% which is excellent and should easily get you that 40 to 50 miles range, especially at 45mph.

I'd use an 8" DC motor which would get you pretty decent performance given the light weight of the car even with the common and low cost battery form factor.

144v and an 8" DC will easily get you a top speed north of 80 in a light, small car with typical sized car tires and a 4:1 or so overall drive ratio.

AC will get you regen but you will be paying an additional 5-10K for the privelege. What else could you improve on the car for the same $ (or much less) to make up the 10% or so difference in range? But if you have the budget, go for it, AFTER budgeting more or better batteries, better or lighter other components, low RR tires, etc.

6V and 200Ah at 20h rate sounds like your standard golf cart battery, but maybe in a sealed form for the ones you are looking at. You'd need nearly a literal ton of these to get 144v and that might actually be overkill in a tube frame car unless you really think the added range will be useful to you. 20H rate means the 200AH was calculated by pulling 10 amps for 20 hours. Due to the Peukert effect (google it) lead acid batteries don't have the same effective capacity at different rates of discharge. In an EV that 200AH lead acid battery will be lucky to give you a useful 100AH of capacity.

As a data point, my 1985 MR2 conversion does about 50 miles at 55mph on the highway under good conditions. It has 1250lbs of batteries (126v of 6v, 200AH flooded batteries). Due to its weight (3400lbs) it is not fast 0-60 but I have had it over 70mph. Knock 1000 pounds off that for your lighter tube frame and somewhat smaller but higher voltage battery, use a higher-revving (at expense of some torque) 8" motor instead of my 9", and use a Z1K controller for good off-the-line torque and it should be pretty sprightly.

Even if you went for flooded lead to start with, you could try a set of AGMs at a later date. The car would be a screamer then when coupled with the Z1K, but you would sacrifice some range.

The best place for solar is on your house.

Last edited by madderscience; 08-13-2008 at 01:50 AM.
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Old 08-13-2008, 07:16 AM
Motorsport_E Motorsport_E is offline
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Default Re: One-Off Tube Frame Chassis (MR2 drivetrian) EV Build Looking for Advice

I've considering flooded for range but I'd really like to save the maintance issues and go AGM or LiFePO4 (depite the cost) since I'll still have my hands full once it's up and running. I'll still need to get the brakes, steering, lights, ect. taken care of so I can go in for an inspection to make my "home-build" vehicle street legal. If I went with flooded, what would need to be changed if/when I move up to AGM's? What is an ideal Flooded battery (brand/specs) for my build? What is an ideal AGM battery (brand/specs) out there? What price range should I expet for either batteries?

Volt vs. Weight vs. AH... how do you choose wih so many options? Sure, I could get 24-28 of the 6 volt AGM batteries with 300 AH but it would add a literal ton to my car... but what could be gained , if anything? Sure, with these I'd be lookin at 3000-3500 lbs, but that is a common weight amoung alot of the EV's I've seen. Or, of the same brand (Lifeline) I could have a mere 12-14 of the 12 volt AGM batteries with 100 AH, but only add half the weight. Where do I hold the most value when choosing a battery; volts, weight, AH, or other?

Last edited by Motorsport_E; 08-26-2008 at 04:38 AM.
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Old 08-13-2008, 08:02 AM
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Default Re: One-Off Tube Frame Chassis (MR2 drivetrian) EV Build Looking for Advice

These sound like fun builds. I'm jealous

Ya know, a VW setup on the back of a kit car wold kinda nice. I don't know about you, but one of the reasons that I steered away from a kit car is that they're all 2-door sports cars. I want a hatch back type car or perhaps a sporty wagon, so I went with a Civic hatch. However, one could build a car with a very flat drive train in the back of the car. This could yield a layout as sporty as a Porsche Cayenne or as pedestrian as a VW Squareback. Unfortunately for a guy like me, the car's body would need to be fabricated.

I'm just thinking out loud here.
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Old 08-13-2008, 12:24 PM
john818 john818 is offline
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Default Re: One-Off Tube Frame Chassis (MR2 drivetrian) EV Build Looking for Advice

Quote:
Originally Posted by Motorsport_E View Post
I suggest getting the Ron Champion book, or atleast checking it out at a local library, as it's very useful. A good way to stay safe when building your chassis is to use stock measurments for all mounting points, then it becomes almost as easy as a simple bolt on for parts from your donor car.
I don't have "The Book", but I have Keith Tanner's book. His is a Miata based build on a pre-made frame. Tanner's is one of the cars in the Car & Driver Locost article. I also have a book on CD, but I don't remember the title or author. I haven't even taken that one out of the package yet. You're right, though, I should at least check out Champion's book.

I'm not sure what I'm going to end up doing, but it will be small and relatively light. I'm leaning toward a single seater, but I might go with a tight tandem back seat. Because of this and the mid-engine configuration, I figured I'd end up pretty far from a Book frame. I was thinking about having pods on both sides of the car for batteries. With single or tandem seating, there should be plenty of room for them to not stick out beyond the wheels.

Quote:
Originally Posted by madderscience View Post
I am not familair with the kit / diy sports cars chassis you mention but you want light, low, and aerodynamic. You have the luxury of a purpose built chassis to fit the batteries in one or two easy access, low and centered areas. Build in enough room to allow insulation to enclose them unless you live in a very warm climate, or you don't plan to drive in cold weather.

As for batteries, I am strongly in favor of flooded lead despite its disadvantages of ....
Lotus 7 type cars are all light and low, but aerodynamic they are not. I think CD is usually around 0.7! People have built more aerodynamic versions, but most stick closer to the original design. Their light weight gives them great handling and acceleration, but the shape gives them low top speeds. My use will be similar to Motorsport_E's, not much highway, so I'm hoping light weight will be the bigger factor in range. My target is 1000-1200 lbs including batteries.

I'm leaning toward AGM, too. As I understand it, they make better performance batteries at the expense of range. My range requirements aren't too high, so I'm hoping that with AGMs I can get better performance/pound vs flooded. Is my thinking correct?

Oh, as far as insulation goes, I don't have much of a problem with cold, but I'm getting a little worried about heat. How hard are 100*F days on batteries? That kind of heat is not uncommon where I am, and my battery boxes would be pretty exposed.

Quote:
Originally Posted by xrotaryguy View Post
These sound like fun builds. I'm jealous

Ya know, a VW setup on the back of a kit car wold kinda nice.
I was considering VW. I don't know if they can be easily flipped to mid-engine, but if not, a rear engine configuration with a light (vs ICE) electric motor would be okay, too. I figure a high performance version might hold up to the torque I'm hoping for, but I wasn't sure if they were too pricey.

Some people build rotary Locosts. I saw one a few years ago at SevenStock in Irvine. Online somewhere, I think I even saw a 20B build! Can you imagine a 20B in a 1400lb car?
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Old 08-13-2008, 02:28 PM
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Default Re: One-Off Tube Frame Chassis (MR2 drivetrian) EV Build Looking for Advice

Cd by itself is meaningless. CdA (Cd multiplied by frontal surface area A) is what matters. A lotus 7 might have a bad Cd as many open cars do, but it is tiny so CdA is still quite good. Plus, if you custom building things anyhow you could probably find a way to add additional fairings or whatever to improve on that.

A guy in our local EV club in seattle is building a "formula V" style tube frame EV, with specs similar to what I was proposing in my earlier email. It isn't done yet but the frame is about there, and it looks pretty darn good.

The wide range of lead acid is confusing, but it boils down to this: Go Flooded if you want range, go AGM if you want performance. Flooded has longer cycle life and is cheaper but you have to top off the water every 1000 miles or 30-40 charges. (easy and cheap) AGM has less maintance and twice the power density of flooded so you want that for tire screeching performance or if the batteries will be inaccessible. Insulate and heat the batteries if you will drive in the cold. Stay away from exotic or weird form factors. Best to stick with common sizes and well known locally avaliable brands. Trojan or Interstate for flooded, and Optima and Exide orbital for AGMs seem to the the current favorites, at least in the Seattle EV crowd. Lead acid batteries are a consumable and will need to be replaced every 3-5 years or 15-30K miles depending on various factors, so they should be an affordable and an available type.

Good Luck.
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Old 08-13-2008, 07:14 PM
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Default Re: One-Off Tube Frame Chassis (MR2 drivetrian) EV Build Looking for Advice

Porsche and VW transaxles can be flipped without too much trouble. I know of a few people who did this back in the 80's when bus transaxles were the hot ticket. The MT900 uses a Porsche ZF transaxle that is turned upside down and backwards.

I must have missed 7 stock the year that the 20b locost was there. Bummer.
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Old 08-14-2008, 12:28 AM
Motorsport_E Motorsport_E is offline
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Default Re: One-Off Tube Frame Chassis (MR2 drivetrian) EV Build Looking for Advice

I may consider other transaxles later but I'm using a number of parts from the MR2 at this stage. One car with all the parts needed is saving me alot of trips the the scrap yards.

Thanks for clearing up the battery confusion; despite all my reading and searches I was still basically lost in all of the details. I figure for this first build I'll go with the AGM's as it will save me some time later when it's needed to make this legal in the eyes of the DMV, besides a little performance would be nice.

Last edited by Motorsport_E; 08-14-2008 at 10:06 AM.
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