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  #11  
Old 07-29-2008, 06:06 PM
John John is offline
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Default Re: low weight vs low drag.

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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  #12  
Old 07-29-2008, 07:32 PM
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jlsawell jlsawell is offline
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Default Re: low weight vs low drag.

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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  #13  
Old 08-04-2008, 02:27 AM
Bowser330 Bowser330 is offline
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Default Re: low weight vs low drag.

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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  #14  
Old 09-30-2008, 11:49 PM
Tommahawk Tommahawk is offline
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Cool Re: low weight vs low drag.

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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  #15  
Old 07-03-2009, 10:46 PM
Xtremek Xtremek is offline
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Default Re: low weight vs low drag.

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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  #16  
Old 07-04-2009, 05:35 AM
todayican todayican is offline
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Default Re: low weight vs low drag.

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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  #17  
Old 07-04-2009, 06:05 AM
rillip3 rillip3 is offline
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Default Re: low weight vs low drag.

Quote:
Originally Posted by todayican View Post
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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  #18  
Old 07-04-2009, 09:12 AM
tomofreno tomofreno is offline
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Default Re: low weight vs low drag.

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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  #19  
Old 07-04-2009, 05:04 PM
Xtremek Xtremek is offline
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Default Re: low weight vs low drag.

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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  #20  
Old 07-04-2009, 06:46 PM
Bowser330 Bowser330 is offline
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Default Re: low weight vs low drag.

Quote:
Originally Posted by Xtremek View Post
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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