Another physics-y question

[butter]

So I was coasting in my Corolla yesterday on the open freeway towards stopped traffic several hundred feet ahead -- cautiously coasting, my foot hovering millimeters from the brake, ready for people to dart into my space cushion... but nobody did. I was thus able to coast nicely to a halt. The space I had in front of me approximately matched the ungassed momentum my car had at the moment. I still had to brake a little, but not so much that I felt I wasted all the gassing I'd done the last time I used the gas pedal.

I had another Aptera-centric question at this moment: if put in the same exact situation, would the Aptera have needed more braking to stop its momentum?

I'm asking because I'm thinking about the aerodynamic qualities of the Aptera's shape and surfaces (lack of windshield wipers sitting on top of the window, etc.). If you took a Corolla and an Aptera and made them go 60 mph on a straight road with no weird weather conditions, then had both drivers release the gas pedal at the same time, which car would coast further?

I'm guessing this is a no-brainer and that it would be the Aptera, right?

Thanks for putting up with my myriad of silly questions, but I'm stillllll incredibly excited about this car and blathering like this helps temper it somewhat, esp when there's no real news right now coming from the official people...
Amy

[futura]

Just from my casual estimations, not such a no-brainer IMHO.
If by the amount of braking you mean: which car would generate more energy from braking (heat + re-gen brakes on Aptera vs heat from Corolla)?
I suspect the Corolla would require more braking energy to stop.
There's lots of factors besides drag coefficient (Cd).
The Aptera weighs about half the Corolla.
Aptera's wheels probably have less rolling inertia because of their smaller diameter.
Aptera has about 1/3 the Cd (0.11 vs about 0.3, I think) so it would not lose energy to the air as much as it slows down and that would have to be made up by increased braking (why drag chutes are used with some jets).

Anyway, I'm willing to guess the Aptera requires less "braking" than a Corolla with the hand-waving analysis given here.
Cheers.

[butter]

Well, just a little semantics-tweaking note: I was imagining just letting both cars coast, gasless and brakeless, to a halt, and seeing which one coasts further.

I assume that calculating braking energy is equivalent to calculating which one rolls further unassisted.

But yeah, I did wonder about the lightness of the Aptera being a factor in actually reducing the distance in which it'd roll, gas- and brake-free...

[KarenRei]

Aptera should coast further. The lower weight would mean less coasting distance, but the very low aerodynamic drag and rolling losses should increase it by more than that amount. And it has regenerative brakes, so it doesn't matter all that much if you have to brake ;) Unlike the regenerative brakes on hybrids, since the energy is stored in li-ions rather than NiMHs, you're not going to lose nearly as much of it (NiMHs are only 50-70% efficient, while li-ions are generally over 99% efficient).

[futura]

Ah, Ok, I did misinterpret your conditions.
That sounds more like comparing the ballistic coefficients
of the two vehicles. Excepting the rolling inertia, they would slow down faster
according to which had the lower ballistic coefficient (I'm way over my head here now). So, it's proportional to something like mass / area (with the Cd on the bottom also).
So, its Corolla's mass vs. Aptera Cd and then figuring out the area of each.

I guess I'll punt since it's Sunday afternoon and my Margarita is ready.
Cheers.

[butter]

KarenRei, I have to say, I love it when you point out details of the Aptera that are particularly attractive to me (and, I'm sure, tons of other eco-conscious people out there). I just love seeing it in print. Blah blah... caffeine high..

[futura]

Aptera should coast further. The lower weight would mean less coasting distance, but the very low aerodynamic drag and rolling losses should increase it by more than that amount.

Ok, rather than start my taxes I did a quick estimate and I think KarenRei has it about right.
Leaving out rolling losses but using ballistic coefficient only I use these numbers:
Aptera: 1400 lbs (M), 0.11 Cd and cross-sectional area (A) of 1.25 (I'll explain this later)
Corolla: 2600 lbs, 0.31 Cd and A= 1.0
BC = M/(A*Cd)
A larger BC means coasting further.
I chose the 1.25 Area for Aptera (from nowhere) it just seems like the Aptera cross-section is about 25% greater than a Corolla based on photos.

Sooo, Corolla = 8390; Aptera = 10180...Aptera rolls further.
Of course I heard S. Fambro say somewhere that just sticking your hand out the window or "exposing" the windshield wiper doubles the Cd.

So if you want to slow down faster than the Corolla just stick your hand out the window or turn the windshield wipers on! :rolleyes:

Cheers.

[3-4-me]

I think Karen Rei hit on it, but missed the bigger picture.
You're comparing an ICE with transmission(neutral gear),
and an electric with regenerative braking.
When you let off the "gas" in the Aptera, the "regen" braking puts a drag on the drive wheel similar to pressing the brake.
I'm not sure of the amount, but I'd be willing to bet the Aptera stops much sooner.

[chaosratt]

I hate to bump and old post like this, but I was trolling around and thought I had something to add:
One thing it looks like no one took into account is the transmission of the Carolla. Even in neutral, half of the transmission is still spinning, and thus, loosing energy as it needlessly moves transmission fluid around.

[LQUAN]

When it comes to braking, the Corolla has more stopping friction on its 4 wheels combined compare to Aptera on 3 wheels. The Corolla weights more and so is the frictional force. The Corolla has vacuum brake assist which has more force than Aptera's regenerative braking. I guess it depends on who is stepping on the brake on Aptera to make it stop faster than the Corolla. If brake properly, I think you can make Aptera stop faster than the Corolla. Even at 1500lbs, unless Aptera provides some sort of power assist braking, it will require you to step harder on the brake to stop it. High-end cars such as Lexus use electrical hydraulic pump for power brake. They don't use vacuum which can cause drag on engine pistons.

Regarding automatic transmission. Even at standing still, the impeller inside the torque converter is still spinning to circulate transmission fluid to create hydraulic control for certain part of the transmission - which is by the way, make your car creeps forward when not holding on to the brake. Cars with automatic transmission use more fuel when idling at a standing still compare to manual transmission. That is why it is better to put your transmission in Neutral when stopping at long red lights. I usually shift my transmission to Neutral and let my car glides to a long red light then hit brake to make it stop completely. I shift back to Drive 1 or 2 seconds before stepping on the gas. Automatic transmission shifts slower than manual transmission. Don't shift and stepping on the gas at the same time, you will wear out the clutch pack very soon! (Yes, automatic transmission has several wet clutches, compare to 1 dry clutch in manual transmission.);)