[manic_monkey]

when it comes to EV efficiency, low weight and drag are key, but which is king?

obviously there needs to be a balance, but which do you think has a greater impact on the range and performance of an ev? if you was selecting a donor car, would you go for the lightest you can find, or for the one with the lowest drag coefficient?

for example, an opel calibra has a CD of 0.26, but a weight of ~1200kg
where as a 1980's vw golf (mk2) has a CD of 0.36 and a weight ~850kg

which would be likely to give the greater range?

[Zer0]

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.

[ww321q]

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.

[judebert]

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.

[Zer0]

Originally Posted by judebert

...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.

[judebert]

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

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

[TX_Dj]

Originally Posted by judebert

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?

[Mad_Scientist]

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.

[TX_Dj]

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.

[manic_monkey]

Originally Posted by Mad_Scientist

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.