# low weight vs low drag.



## Zer0 (May 9, 2008)

IMHO, I would go for low weight first, although depending on your range/battery requirements, you may need a vehicle with a higher Gross Vehicle Weight Rating.

Especially with a commuter car that will see low speeds most of the time, drag isn't much of an issue. Do ever hear those fuel-saver tips that say something to the effect of "keeping speeds at or below 50 or 60 mph [depends on the ad or who you talk to] can significantly increase fuel mileage" The way I understand it, this is because that is the approximate speed that drag really starts to affect how much power a vehicle must produce to maintain or increase speed. As a general rule, every time your speed doubles, the amount of drag triples (I don't know if this helps, just thought I'd throw it in)

On the other hand, weight is always present no matter what you do. I could be wrong here, but I think it would be easier to make some choice aero mods (underbelly pan, close up most or all of the grill opening) than to try to shave off 30% of the car's weight (the approximate difference of the two cars you mentioned).

Hopefully others can chime in and give their $.02 and point you somewhere else if I've steered you wrong.


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## ww321q (Mar 28, 2008)

Weight is a problem during acceleration or on a grade . Once at speed on level ground , the weight effects your current draw less . The curb weight gives you a starting point , but what you really need to know is the weight with all the ICE parts removed . Look to see if one is a cast iron block and the other an aluminum block ? Check the EV album and you may be able to tell which would have a better finish weight . J.W.


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## judebert (Apr 16, 2008)

It depends largely on how fast you intend to go.

Weight is a problem in the energy calculations: F=ma. The more m, the more force required to achieve a particular a. Of course, after acceleration, more weight means more momentum, so you coast farther. As J.W. says, hill climbing is affected by weight, too, but that's because gravity is providing its own acceleration, downward.

Aero drag is dependent on the _square_ of the velocity: F = Cd * {p + 0.5rv^2} (Coefficient of drag times {pressure + half the density of the air times the velocity squared}). You probably can't modify the air pressure or density, so the only factors anywhere near your control are the Cd and the v. 

Notice that at low speeds, drag is not a big deal. But as you double the speed, the drag _quadruples_. 

So, if you're driving around town, mostly 35 or less, or traveling a lot of hills, you want a light car. If you're mostly driving the highway, pay attention to your aerodynamics.

If you're drag racing, you need both.


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## Zer0 (May 9, 2008)

judebert said:


> ...But as you double the speed, the drag _quadruples_.
> ...


oops, my mistake. I was just pulling that from memory, I didn't have the math to back it up. Thanks for correcting me.


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## judebert (Apr 16, 2008)

Sorry, Zer0, I hadn't even seen your post! I was just adding emphasis. 

I've gotta slow down and read the whole thread.


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## TX_Dj (Jul 25, 2008)

judebert said:


> But as you double the speed, the drag _quadruples_.


Exactly... but keep in mind this isn't just affecting drag, it also affects other formulas which count on speed. I.E. at double the speed, you have 4x the impact force, at double the speed, it takes 4x the power dissipation to stop, etc.

Another interesting point- which I think you went into briefly, judebert (I suffer the same issue, didn't read the whole post, just skimmed over it) is that a heavier vehicle at speed stores more kinetic energy (joules) than a lighter vehicle at the same speed. You have to put more energy to get there, but once they're at speed the power draw is similar if you don't factor in drag and other items...

Also, it was mentioned that weight affects acceleration (as I somewhat infer above) but also taking off on a grade.

If it was my choice, I would pick the car with the lower drag, because any EV will be heavy unless you're Daddy Warbucks and can afford lighter battery tech, and when you really need the benefit (distance @ speed) the drag comes into play. Of course I don't recommend exceeding GVWR either, so the question then becomes which vehicle has the best drag coefficient with the more suitable GVWR for the design goal's requirement on the battery system?


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## Mad_Scientist (Jun 10, 2008)

Sorry if this is too obvious, but it may be worth it to look for a car with both. If a two seater will do, there is the Honda CRX, Mazda RX-7 (some years are much lighter than others), and Toyota MR-2. If memory serves, all three are light and aerodynamic, but with the Mazda RX-7, you have to watch the model years. Some aren't all that light.

One thing I haven't done is to look at the weight minus the ICE and other parts.


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## TX_Dj (Jul 25, 2008)

Also, on the MR2, if memory serves, you can modify the center tunnel to hold some batteries too, if you're going with sealed cells. Every little bit of space helps in the range and performance game.


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## manic_monkey (Jun 24, 2008)

Mad_Scientist said:


> Sorry if this is too obvious, but it may be worth it to look for a car with both. If a two seater will do, there is the Honda CRX, Mazda RX-7 (some years are much lighter than others), and Toyota MR-2. If memory serves, all three are light and aerodynamic, but with the Mazda RX-7, you have to watch the model years. Some aren't all that light.
> 
> One thing I haven't done is to look at the weight minus the ICE and other parts.


of course, it would be great to have both. i love honda CRX's, they always looked great. problem is that the british climate didnt treat them well, so good examples are costly. Not a fan of the early rx-7's but the 92 onwards model is a fantastic car, bit lardy though at 1300kg+. mr2's are for hairdressers....

been nice to hear peoples opinions on the subject though. but im still leaning towards vehicle weight beeing more important for an EV. mainly because I dont believe many people use them as highway cruisers where drag would be a bigger factor, and most are for tootling around at mixed speeds, where acceleration and weight would be key.


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## John (Sep 11, 2007)

Another thread on low rolling resistance tyres quoted this article

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=29

_Quote_ "During stop-and-go city driving, it's estimated that overcoming inertia is responsible for about 35% of the vehicle's resistance. Driveline friction is about 45%; air drag is about 5% and tire rolling resistance is about 15%.

Overcoming inertia no longer plays an appreciable role in the vehicle's resistance during steady speed highway driving. For those conditions it is estimated that driveline friction is about 15%; air drag is about 60% and tire rolling resistance represent about 25%."

Of course the weight of the vehicle also affects the rolling resistance and drive line friction so the heavier car will consume more energy just rolling down the road. The short answer the relative importance of weight and aero depends on your mission but weight features heavily in both highway and city driving.


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## jlsawell (Apr 4, 2008)

When I was a kid I read a book about motorcycle racing in the UK. I think it was by Robert Westall. The crusty old timer is talking to the newbie with his brand new racebike and says something along the lines of: 

"think light, not speed"

lesson: get rid of excess weight like the electric starter. It may save time initially, but it's excess weight you don't need on the track.

Now the point may not apply exactly to EVs, but the lesson is still valid.

I'm planning to go as light as possible, then look at drag. Of course, keeping to the speed limit will not only reduce your drag, it will extend range as well. Damn those lead batteries for being heavy!!!

Not to mention all joining hands and praying for cheap lithiums


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## Bowser330 (Jun 15, 2008)

I like the topic and I like how we are getting a lot of different perspectives. right on.

my thoughts are...weight is more important because, like others have said, the most amp draw comes from acceleration, not sustaining a (nicely geared) cruising speed. Cut down on the amp draw and you extend range.


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## Tommahawk (Sep 28, 2008)

Weight is king, and for this reason only its easier to improve your cars aerodynamics than to make it weight less.

Generally speaking you are building on an existing model, most modern cars have decent drag coefficients, they tend to fail not on design but on requirements such as side mirrors and air holes causing drag.

Generally speaking cars are getting heavier, carbon fibre offer 1/5 saving on weight of car, so a 1000 kilogram car would weight 800kg if it was built from cfibre but you are talking approx 1,000 per panel.

So while cars got more aerodynamic over time they got heavier instead of lighter. Here is a list of improvement ev owners can make to their cars to improve drag.

*Here are things that can be done to improve your vehicle's aerodynamics:* 

 Lower the car - Lowering the car reduces the effective frontal area, increasing efficiency. Note that this only works up to a certain point. There will be an ideal ride height for each car. According to this article, 2.7" ground clearance is a good minimum hieight to shoot for. According to Mercedes, "Lowering the ride height at speed results in a 3-percent improvement in drag."
 Remove that wing - Many "sports" cars have a non-functional wing on the back. Removing it will improve the fuel economy. The exceptions are the small rear fairings that are designed to detach the airflow from a rounded trunk.
 Clean up the underside of the car. - Installation of a "body pan", while a labor intensive operation, will provide a significant improvement in mileage. More...
If a body pan is not practical, an air dam will redirect air that would normally pile up under the car causing drag. Not as good as a body pan, but better than nothing. Should be combined with side fairings. 
 Fair the wheel wells. - Yeah, this looks funny, but completely covering the rear wheel well will help improve efficiency. While the front wheel can not easily be completely faired due to clearances needed for turning, a partial fairing can be made. In addition, fairings can be added in front and behind the tires to help transition the air around these large appendages.
 Clean up the front of the car. Basically the smoother the better. If the car has a large air intake under the bumper, it may not need that opening above the bumper (they are often just styling cues). An aerodynamic plastic or composite panel can be built to cover the opening. 
Remove the side view mirrors and instead use a remote camera system.
Replace large whip antennas with smaller powered antennas. 
Vehicles with steep windshields can benefit from a hood fairing to help smooth the transition of air between the hood and windshield. 
A small "tail cone" can be affixed the the rear bumper to help transition the air from under the car. 
Side fairings can be used to clean up the lower half of the body between the tires. More...
 ​


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## Xtremek (Jun 13, 2009)

Ok, here's the thoughts of this shade tree engineer. It would depend on where I'm going to use the car the most. If all I did was city driving, weight would be the the most important. But for me, the commute is mostly freeway, so I'd go for the best aero package and improve that. From there I'd strip everything not needed. I like some of my creature comforts, but I'd lose all but the driver's seat and the one seatbelt. Interior courtesy lights, sound deading would go. I have a half hour commute, so I'd keep the radio, but only 2 speakers. How about carbonfibre body panels(there are aftermarket panels for the tuner guys). Almost anything that is available for performance is likely to be light(cross-drilled rotors, aluminum/mag wheels, aluminum calipers, suspension pieces). Just to deaden the road noise a little(long commute, creature comfort)bedliner just the floorboards. I'd pitch the power assist and go to a manual brake system and a manual rack(some thinking, vehicle comparision and fabbing to be done properly). There's a lot of ways to lose the weight and still have a decent daily driver. If you wanted to go crazy, how about ditching the stock dash for something handbuilt out of aluminum or carbonfibre. Weight and drag are both evil, and shaving them can be fun and inventive.


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## todayican (Jul 31, 2008)

A topic I see rarely in this debate is "ground up build" or "start with existing car" with a ground up build, you can use a space frame ala locost, a booming community of lotus 7 clones. and adapt the chassis to carry batteries low and where you want them for the most part, almost always lighter AND better with aero.

Whats the consensus, just how big or small is the desire to start from the gorund up?


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## rillip3 (Jun 19, 2009)

todayican said:


> A topic I see rarely in this debate is "ground up build" or "start with existing car" with a ground up build, you can use a space frame ala locost, a booming community of lotus 7 clones. and adapt the chassis to carry batteries low and where you want them for the most part, almost always lighter AND better with aero.
> 
> Whats the consensus, just how big or small is the desire to start from the gorund up?


There are some folks doing this. But a lot of folks don't have the engineering skills to do everything on their own. A lot of us are taking 6 mo. - 1 year to finish these projects, some longer. Most people want to wait longer while they work on the frame. Liscencing and registration can also be more tedious, depening on your local laws, for a self-made vehicle.

I think ground-up is a great idea, a great way to get low weight and drag, if you have the time and dedication to do it.


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## tomofreno (Mar 3, 2009)

Rolling resistance force is proportional to mass so more mass, more force required to maintain a given constant speed, and more energy/mile required. As said, drag force is proportional to velocity squared. It is usually about the same magnitude as the rolling resistance force at 50 to 60 mph, at the lower end for larger drag coefficient and vehicle cross sectional area. It is typically about half the drag force magnitude at around 35 mph, so still quite significant. The energy consumed in accelerating a vehicle to moderate speed is usually fairly small. The power (energy/time) is large, but the time is short, so the energy is typically less than that consumed in traveling a few miles at 50 mph. It only becomes a significant factor on range for very frequent stop/start driving (bus or delivery vehicle). 

Tom


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## Xtremek (Jun 13, 2009)

A complete ground up would be a daunting task. To calculate Cogs(centers of gravity, both static and instantaneous, total, front and rear) alone is a big task. Then to figure out front and rear rollcenters, Ackerman angles, bumpsteer, pick-up/pivot points, camber curves, sway bar dimensions, spring rates, and shock valving. That's just to get the front suspension done correctly. Then you should do FEA on all the parts you are going to build. I've done the FSAE(Formula SAE) competition and it's a hell of a lot of work for a team of 4-6 people to do in a year. It usually takes 2-3 people about 4-6 months just to design the chassis and suspension using a CAD package like Catia or UG(student versions are about $1500-$3000 a computer). Most of the safety standards/structural part dimensions are already dicated by SAE. And those cars aren't really worried about inclement weather, and don't care about being road legal, creature comforts, or Federal Motor Vehicle Safety Standards. To design a car properly would be a huge undertaking. Using a kit car wouldn't be a bad idea, but they have their issues as well. Anyway, those are some of the lessons I've learned from my past. BTW, I'm working on a Formula Hybrid(SAE/IEEE collegiate design comp) and it's taken me most of the summer(although part of that is managing the team) just to get the design of the EV side of things done properly, and I'm stil taking short cuts.


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## Bowser330 (Jun 15, 2008)

Xtremek said:


> A complete ground up would be a daunting task. To calculate Cogs(centers of gravity, both static and instantaneous, total, front and rear) alone is a big task. Then to figure out front and rear rollcenters, Ackerman angles, bumpsteer, pick-up/pivot points, camber curves, sway bar dimensions, spring rates, and shock valving. That's just to get the front suspension done correctly. Then you should do FEA on all the parts you are going to build. I've done the FSAE(Formula SAE) competition and it's a hell of a lot of work for a team of 4-6 people to do in a year. It usually takes 2-3 people about 4-6 months just to design the chassis and suspension using a CAD package like Catia or UG(student versions are about $1500-$3000 a computer). Most of the safety standards/structural part dimensions are already dicated by SAE. And those cars aren't really worried about inclement weather, and don't care about being road legal, creature comforts, or Federal Motor Vehicle Safety Standards. To design a car properly would be a huge undertaking. Using a kit car wouldn't be a bad idea, but they have their issues as well. Anyway, those are some of the lessons I've learned from my past. BTW, I'm working on a Formula Hybrid(SAE/IEEE collegiate design comp) and it's taken me most of the summer(although part of that is managing the team) just to get the design of the EV side of things done properly, and I'm stil taking short cuts.


As you mentioned, kit cars can offer exactly the lightweight chassis we need...however the kit car bodies aren't some of the most aerodynamic...example the cobra, probably the most replicated car in the world...being a convertible and have a larger, bulbous, front surface area, I don't think it will hack it for maximum efficiency...

What we need is someone to come up with a body style that is very aerodynamic that would fit on a cobra kit car chassis...

Another thing you could do is take an existing car and strip it down to the tub and put the kit on top, like the lambo replicas do with the pontiac fiero..


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## todayican (Jul 31, 2008)

Ive built 3 vehicles so far from scratch, and have learned many things from lots and lots of mistakes 

There are some "middle paths" that can be taken, like the use of a pre engineered front end, common on ebay most of them follow the design of the ford mustang, just search "mustang ifs"

also, personally, I am a fan of the reverse trike, better aero, lighter and a whole lot easier to register.

there is an active community of scratch builders over at locostusa.com for the not faint of heart as well ;-)


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## sunworksco (Sep 8, 2008)

Some of the kit-cars out there offer some very lightweight aluminum GM style suspensions and can be converted to custom machined Ducati 1098/GM Siamese hubs for using BST carbon fiber wheels weighing 5lbs. each.The wheels will accept car or motorcycle tires.Lathes are very resonable now , used or new.You can machine some of the parts.For anyone considering building a scratch-built ev,you might look at the reverse-trike car designs.Like the comments here,it qualifies for a motorcycle license (K.I.S.S.) and takes you places and gets you parking spots that cars can't compare with (Diamond lanes).These can be built out of aluminum with minimal fiberglass body panels and as light as 600lbs. (single place) with batteries included.Google up Lotus aluminum chassis bonding/Egot rivets for the chassis fabrication and you will find that you don't even need any welding skills since the strongest chassis is bonded with epoxy and rivets.This technique has made it possible for Lotus to fabricate one-off cars on the cheap.They stretched their Lotus/Elise chassis for the Tesla electric car for battery storage space.Also take a look at the latest in lightweight aluminum panels from www.alulight.com. These aluminum panels make the best battery bulkheads and floor panels.They are nearly crush-proof,float on water light,electro-magnetic radiation shielded,heat resistant and can be bonded to aluminum extrusions.


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## TomA (Mar 26, 2009)

Wow, sunworksco, that's a lot to take in...

First, what kit car suspension components in aluminum are you referring to? I would like to have a look at them as they may save me having to fab CrMo arms for my vehicle.

Second, my study of the Lotus bonded aluminum chassis indicates this is beyond the home builder. Most of it is extruded, so those bits are out. Then, it is bonded with a cured 200 degree centigrade adhesive process. No oven that size in my house. Also, the rivets go in with a special high speed tool. I could use other rivets, but it is getting pretty tricky and pricey now to do this thing. Lastly, that Alulight foam sheet doesn't seem to be available in the US at any price, and I'm thinking its really expensive wherever its available. Am I missing something here, or is this technology really suitable for a home builder?

Third, my reverse trike single seater vehicle will be far lighter than 600lbs- my target is only 400lbs including batteries. My studies show only composites, (carbon and kevlar cloths matrixed in epoxy over foam cores) with or without small chromoly subframes, are the way for me to get there and still be able to build it myself without going broke on components or tools. Again, am I missing something? Is there a better way?

And please do show me those aluminum kit car suspension arms. That will be really cool.

Thanks,

TomA


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## Duncan (Dec 8, 2008)

Hi Tom

Warning Composites and aluminium do not go together well

Gluing aluminium is the sort of thing that gives engineers grey hairs
It is possible but it uses processes that are not home friendly

If you decide to use aluminium inserts in your carbon/glass fiber you need to get them properly treated or the joint will fail due to humidity after a short time (two years?)


Other comment, 
It is nearly impossible to make carbon fiber parts as strong and light as S glass using a wet lay-up 

If you are using pre-pregs ignore this comment


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## sunworksco (Sep 8, 2008)

Alcoa Aluminum has spent $millions$ on developing this bonding method and it has been proven in the Lotus chassis.Since the aluminum epoxy fabrication was published , 3M has developed 2-part epoxies that do not need oven-curing.You also have the option to use a powder-coating oven.These are usually made from large ship containers.The epoxy is for shear strength ,only.The mating surfaces do not need anodising as well.The mating surfaces can be acid wash prepped,cleaned in distilled water,dried then epoxied together.Yes , Lotus has custom aluminum extrusions but anyone can use standard square tubing and aluminum sheet.The Ejot rivets can be driven into place with a simple air or electric drill.Lotus has developed an assembly line robot for this.The chassis can be built with a 10,000lb. torsional rigidity.The fiberglass body would be non-stressed.Alulight panels are availible in the US but you can use regular aluminum sheet material.Alulight 4'x8'x1/4" panel with 1mm aluminum sheets on each side of the aluminum foam is around $800.00.It is pricey but coming down in price.I would recommened using this for the battery bulkhead and the main floor of the chassis.It is super strong and heat resistant.I'm a home builder and very confident that I will be using this technology with ease.You can still keep your target weight with this technology by using the aluminum chassis and fiberglass for the body.
The suspension shown here can be modified and down-sized for any size vehicle.


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## TomA (Mar 26, 2009)

Thanks for the info. Looks like these pictures came from Cutting Edge Replicas. Nice, and probably pricey. I like the bellcrank low-profile front end. Asking these guys to fab up something custom for me is, no doubt, too expensive.

I appreciate your preference for the bonded aluminum. Its an interesting material and has the gee-whiz factor to go with the high cost. Its just too exotic for my skills, budget and desire. When you start fabbing up your monocoque, please post pics for us to follow along. Sounds like a great project!

And Duncan, yes, I'm aware of the hard point problem with composites. My brother-in-law has a LongEZ and a bunch of RC helicopters, and is a good resource for composite aircraft construction techniques. All my inserts would have been alondine coated, but I'm still not happy about them. I'm also aware of the electrolysis problem between CrMo steels and Al alloys, and for that reason I will most likely delete the aluminum altogether from the chassis.

At this point, I'm playing a weight, space and packaging game. I'm using CrMo and composite because both are relatively cheap and very workable at home. Low weight is the trickiest part of the game. Low drag is almost completely controllable by shapes and surfaces, but no less tricky, really. Its a four dimensional (the fourth being cost) puzzle that would be familiar to anyone scratchbuilding a vehicle of any kind.

Fun, yes? I'll keep y'all posted...

TomA


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## sunworksco (Sep 8, 2008)

Type III anodising will not corrode in fiberglass.The price of aluminum has hit the market bottom and is very affordable now.The nice thing about Cutting Edge is their suspension is fully engineered.
I will post images when the chassis is built.
Thanks for the exchange of ideas.I think your build with aeronautical design is the best way to go.I really like the Aptera trike-car.


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## TomA (Mar 26, 2009)

sunworksco said:


> Type III anodising will not corrode in fiberglass.


Hard anodizing? Wow...

That's what I love about you, sunworksco. Every time there's an obstacle in the path, you have the most exotic, difficult and expensive answer for it. If you are taking that approach to your vehicle, when finished it will be worthy of any OEM, and truly a thing to behold.

One thing I've learned about "fully engineered" off-the-shelf hot rod products, though: they usually aren't. Just because someone can CAD/CAM billet parts doesn't mean they are properly tested, durable or even soundly designed. When I see the track mule Cutting Edge Replicas uses to wring out these suspension bits, and the pile of previous designs and broken bits, then I'll believe it. 

Even small volume OEMs have tons of downstream design engineering issues, many of which never get corrected- how about the rear wheel bearings on the Subaru SVX, for one small example: undersized, they last only about 10k miles, and there was never an upgraded design in many years of production.

That's kind of what is so awe-inspiring about your thinking. Everything you suggest, from bonding aluminum panels to hard anodizing parts, is a slow and expensive trip up a steep learning curve to master an advanced construction technique and material. That's really something to see for a guy like me- I'm looking for the easiest, cheapest way to get everything done within my performance envelope and tight budget. That doesn't make everything easy or cheap, but it does mean I don't think about solutions the same way you do.

Happy New Year, man, and stay in the deep end of the pool. Its nice to know you're there...

TomA


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## sunworksco (Sep 8, 2008)

Hi Tom,
Your RT build sounds like a very good one.Will it be fwd or rwd?Is the wheelbase longer?
I will try to attach the Lotus aluminum epoxy bonding sheet.
It is and old article and the bonding is much simplified now.


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## Matthijs (Jun 19, 2009)

*Aerodynamics: a weighty matter* Allert Jacobs added 88 lbs / 40 kg worth of aerodynamic modifications to his bike. He admits he didn’t try particularly hard to fabricate light parts, so his Honda now weighs 43% more (!) than it did when he bought it.People who aren’t familiar with the subject of aerodynamics often predict (mistakenly) that the extra weight of aero mods will hurt fuel economy more than reducing drag will help.
Obviously that’s not the case here. Aerodynamic improvements trump weight.
The exception may be for vehicles that spend the _majority _of their time in heavily congested urban traffic at _very low _average speeds. Clearly that’s not Allert’s situation. In fact he figures the added weight on the Honda is a benefit because “it will make it more stable in side wind situations.”


http://ecomodder.com/blog/diy-aero-fairings-honda-125cc-motorcycle-214-mpg/


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## sunworksco (Sep 8, 2008)

I like the write-up.Very good design.When you keep the total drive-away weight under 600lbs. , you can use a very small propulsion drive system.
I'm considering using 9mm thick,with 1mm aluminum sheets on both sides, Alulight aluminum foam panels to build my monocoqe body.The material is almost bomb-proof and strongest,lightest metal.The military is testing it in military vehicles because the sheets can take a bomb blast.
Check out how aerodynamic this slab-sided body is.
This double-steering Heinzmann double-powered bike is way cool,too!
Here is his blogsite : http://rohorn.blogspot.com/


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## Tom Sines (Mar 24, 2010)

Hi, I think you misunderstood, the $100,000 I spoke of was to develop the EVette prototype. You can go to youtube, under evelectric, and see a channel 25 interview, where I mention the EVette could be factory built for $20,000, maybe less. Your just a little confused.
Tom Sines


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## Guest (May 20, 2010)

I am shaving some more weight from my car by making a fiberglass hood and trunk lid for starters and then eventually the front and rear pans. I am using aluminum for bracing that will be sandwiched between layers of fiber glass. I read earlier that gluing aluminum can cause corrosion. Will I have this problem with fiberglass? If so would aluminum primer be good enough treatment?


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## TomA (Mar 26, 2009)

notmrwizard said:


> I am shaving some more weight from my car by making a fiberglass hood and trunk lid for starters and then eventually the front and rear pans. I am using aluminum for bracing that will be sandwiched between layers of fiber glass. I read earlier that gluing aluminum can cause corrosion. Will I have this problem with fiberglass? If so would aluminum primer be good enough treatment?


Most of the aviation guys who embed aluminum in composite anodize, or more commonly, alodine coat it first. That's a corrosion-proof chemical change to the surface of the metal. Primer is much less effective, and adds weight. If you are going to screw anything into the aluminum, like steel bolts, it will corrode from electrolysis and not exposure to corrosive agents. 

Is there a good reason to use aluminum for bracing? Hard points, yes, but aluminum probably isn't going to add any stiffness as a purely bracing member in a composite part, and its pretty heavy. Why wouldn't you use shaped foam between the glass layers like most everyone else? 

Just wondering,

TomA


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## Duncan (Dec 8, 2008)

Hi Notmrwizard,

Using aluminium and fiberglass is tricky - 
I may be out of date but the treatment of the aluminium parts so that they can be successfully bonded to fiberglass is tricky
In the textbooks I have it is recommended to be outsourced to the aviation guys who can do it in batches with good control

The failure mode is that the bond fails with time due to corrosion at the interface so a perfectly good bond may fail in a years time


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## sunworksco (Sep 8, 2008)

The great thing about bonding aluminum is that it is practical and affordable for an individual or a company like Lotus to use.
Regards,
John


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## Guest (May 22, 2010)

Thanks. I haven't done much with fiber glass since I built my own surfboard in the 60's. There is a place that builds boats about ten miles from me. I think I will go over there and offer to sweep the floor or something in return for some pointers.


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## TomA (Mar 26, 2009)

notmrwizard said:


> Thanks. I haven't done much with fiber glass since I built my own surfboard in the 60's.


That is a great start. You'll be surprised both how much has changed (if you get the newest non-blushing epoxies and kevlar or S2 glass in advanced weaves,) and how much is the same, like getting out the air bubbles and making the surface smooth without wiping it too dry.


> There is a place that builds boats about ten miles from me. I think I will go over there and offer to sweep the floor or something in return for some pointers.


That's a great idea, but keep in mind they most likely aren't doing what you need to know about. They are probably spraying pounds and pounds of chopped mat into gelcoated molds reinforced by very thick layers of glass matt, or if they are up to speed on current boat building, they're vacuum bagging and heat curing. You won't be. Moreover, boats are still largely made with resins and very heavy fiber structures that just aren't appropriate for lightweight vehicle purposes.

You need to know about hand layups of high-strength epoxy/cloth composites. If they are into that, great, you can learn something. Otherwise, look at the aerospace side of epoxy composites, particularly the home builders and the EAA. They are making exactly what you want out of the material, as were Hobie Alter and the dudes 50 years ago...

TomA


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## Guest (May 22, 2010)

Yes they do use choppers to spray hulls but the also do carbon fibber and I can only guess what else. They do a lot of racing so I am bound to pick up some knowledge. I’ve already made a mold of the trunk and made one skin but I don’t like it so I will try again.


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