BMW i3 - Low Hanging Fruit (improving aerodynamics)

[Vman455]

PDF of a conference presentation, but if you want the published paper by one of the Tata engineers on the project with follow-up in a French moving-ground wind tunnel, go here for the Google Book, p. 145.

[aerohead]

Originally Posted by Snax

I recently watched a video where it was mentioned that the cd for the i3 is rather high at 0.30. Looking for some low hanging fruit to improve upon that.

Obviously the mirrors are an easy target. One owner has already done a delete and camera retrofit to that end and it is probably going to be my first step.

I may go so far as to put fairings over the rear wheel openings as well.

That said, a boat tail per-se is out. The car already looks funky enough as it is.

Got any other suggestions likely to produce a measurable result?

The car looks pretty optimized already.Without a box-cavity or boat-tail,I don't know how you could ever achieve any significant drag reduction.That enormous wake behind the car is its major source of drag.

[aerohead]

Originally Posted by Snax

Agreed. I don't think there is much to be gained there, although I am also looking at adding a front splitter. The front bumper profile rounds under relatively smoothly, but that would reduce some of the pressure and lift I get under the front end at higher speeds - and hopefully clean up aero overall.


It is a REx, so it does need to breathe a bit, but I wonder if just adding an exhaust louver over that rear most opening flat with the rest of the covering would help. Supposedly the REx version has a 0.01 cd penalty, and I can't figure any other place that could be coming from aside from that opening.

Looks like it's already got an integrated splitter.Any more,and you'll begin to induce drag,through downforce.I found this to be the case on my truck at the wind tunnel.
Lift should never be an issue at legal posted speeds.

[Snax]

Originally Posted by aerohead

Looks like it's already got an integrated splitter.Any more,and you'll begin to induce drag,through downforce.I found this to be the case on my truck at the wind tunnel.
Lift should never be an issue at legal posted speeds.

I think it's one of those things that could potentially provide an improvement, but like you say, go the other way.

Approach angle is definitely a limitation with the car from the factory, so the front end profile may have been truncated for everyday functionality at the expense of aero. It won't cost much to try it out either way though. I do race in hillclimbs with speeds approaching 80 mph, so a little downforce would not be a problem in the front with the it's tendency to understeer at higher speeds. I'm hoping I can come up with a quick system to remove and replace it if that is the case.

[Snax]

Tire Inflation Pressure Testing:

Figuring this to be one of the quickest and easiest things to modify, I did some quick low speed testing at two different tire pressures, and long story short; there isn't much 'fruit' there.

Over an admittedly short test course of straight back country road of about 1 mile in length, using the on-board power consumption data at a speed of 36 mph on OEM Bridgestone Ecopia EP 500 tires, and climate control off, this is what I got:

Pressures F / R, 50 / 50 psi, Directions 1 / 2 in mi/kWh
7.2 and 5.7
7.1 and 5.7

Pressures F / R 33 / 40 psi (BMW Spec), Directions 1 / 2 in mi/kWh
7.3 and 5.6
7.3 and 5.5

So the only thing I think I have really demonstrated here is that the wind changed.

I know that empirically there has to be some difference there, as I could feel a clear change in ride firmness, but over that relatively short distance it does not seem to be significant and certainly does not show in the numbers. Frankly, getting more conclusive data is not going to be very easy given the high dependence of EVs on temperature conditions as well as difficulty in finding roadways to conduct a bunch of low speed test runs. I'll give it another shot at some point though, hopefully with a much longer test track and a slower speed.

[California98Civic]

Originally Posted by Snax

... I did some quick low speed testing at two different tire pressures, and long story short; there isn't much 'fruit' there....

You will want to consult MetroMPG's test:
https://ecomodder.com/forum/showthre...html#post30483

And Mythbusters TV testing results (confirmed MetroMPG's results):
https://ecomodder.com/forum/showthre...tml#post235681

You might save yourself some frustration by aggressively using the search function on the EM page (upper left corner). There are lots of discussions about issues you are raising. Some really good testing too.

A key image from MetroMPG's test results report:

[Snax]

My next round of testing will definitely include coast-down measurements. I think there is just way too much aerodynamic effect happening at 36 mph to get good measurements over any kind of short term / distance.

I still don't expect dramatic results there, as the 20" wheels and low profile tires don't give much resistance at even low pressures. Just gonna have to find out though!

[Snax]

Not really happy with this data set as there were significant deviations in distance that seemed anomalous. I used a GPS app, but my sense of where I was stopping each time seemed to match up with what it was producing for distance.

I didn't go any lower than 30 psi because the BMW spec is 33f /40r and it was already complaining about 35 /35. Regardless, it seems pretty clear that exceeding the maximum spec of 50 psi for the tires is likely not productive. Much over 45 may or may not be of measurable benefit. Dropping below 40 however seems to have a clear penalty based upon the averages.

Even so, I have decided to keep pressures at 45 / 50 for now as I like the way the car feels there. Lower pressures just feel mushy.

[Snax]

Just a quick calculation on what a mirror delete could potentially net me:

Assuming a roughly 1 square foot reduction in frontal area, and a Cd drop of 0.01, the aerodynamic improvement could approach 7%. Without the Cd drop, it is nearly 4%. Here's to hoping reality matches theory . .

I've already installed one of these mirrors inside the driver door and I like it so far. It has a very firm but easy to change adjustment setup, but more importantly to me is that it is actually glass with minimal distortion. (Which I can only regrettably confirm since the 2nd one arrived broken!)

I can certainly see a much wider area with it if not directly below the rear quarter panel, but I am not concerned with that blind spot with the rear view camera covering a lot of what I will lose while backing up. I think it fits in relatively well too without all of the complication and expense of doing outboard cameras.

[Snax]

I don't believe that actual efficiency will improve by anywhere near that much BTW, as this model is for aerodynamics only. It does not account for other inefficiencies which are far more difficult to change such as electrical losses to the motor and other mechanical factors.