BMW i3 - Low Hanging Fruit (improving aerodynamics)

[Vman455]

Originally Posted by redpoint5

of course the lowest fruit is driving slower, but I understand that the question pertains to mods of the machine, not mods to the operator.

[Snax]

Could look worse.

Going to go with 10 screws I think. There's too much bowing over the curvature of the wheel to be flush with just five. (At least with the coroplast test version.)

[Snax]

I tried to get more highway data last night but the weather was not cooperating with wind at 10-20 mph from the side. No traffic, but it was tossing me around enough that I gave up on it for better conditions.

I am really looking to confirm if the spacers make that much of a difference as I know that many i3 owners install them. We should at least be aware of what effect they have on range.

It's a bit frustrating to put testing other things on hold because of it, but I can at least say that in crosswinds it makes zero difference.

I will probably have to plan a much longer trip to get good repeatable data either way.

[Snax]

I think I need to give up on trusting the built-in mi / kWh metering for any of this.

I was able to find a nice straight level section of road today with near zero traffic for 8 miles that should have provided very consistent readings against the plots, but I can't make the numbers work out in any way that makes sense. While I have been able to produce results that fall inline with the curve using published data and estimated mechanical drag at higher and lower speeds, my testing today at 55 mph with a purely inline head/tail wind of 15 mph is just way off the mark even when accounting for the higher mechanical drag going with the wind vs. against it relative to actual air speeds (40 vs 70). Trying to pad them with an error correction or baseline consumption from zero is still rather specious in terms of where results should lie.

Clearly I have to resort to alternative means. Fortunately I do have one that is relatively easy with the range extending generator and an exceptionally accurate gas gauge for it's 2 gallon tank. I should be able to preserve a set state of charge with it and just run it down the same percentage every time or just run it dry. So I guess that means this EV driver will soon be talking MPG like everybody else for testing purposes anyway.

So far this thread seems more about getting good data than anything!

[redpoint5]

Wait, you haven't been data collecting using Torque?

If you're going to take the time to carefully document your changes, which I'm very appreciative of, you should use the best tools you can. An Elm237 can be had for like $8, any Android device will work, and Torque Pro is maybe $10 (should you choose the legitimate route).

Onboard info systems tend to massage data rather than give you the raw data. Torque PIDs often give you access to the raw data.

[Snax]

I've poked around with Torque with this car and it doesn't offer anything beyond basic telemetry like road speed which didn't vary at all from what the onboard system was showing me. The i3 PIDs really just aren't documented enough yet to get anything usable from it, so raw fuel consumption seems the only accurate way to gauge it right now. There is zero information available about range extender parameters. Even that is a little tricky as sometimes the range extender run time is outside of driver control, so I have to work within the parameters they have setup - which is fine since I understand them fully and can compensate appropriately.

I did do a bit of a baseline run using it this afternoon with the same 15-17 mph head / tail winds. 46.7 miles @65 using 1.65 gallons for 28.3 MPG kinda sucks in comparison to how efficient this car and others are otherwise, but it's a start for testing things at least. The wind seems more typical than not around here, so I may just try to utilize it for future tests instead of holding out for more perfect conditions.

[mpg_numbers_guy]

Does the i3 have a built in efficiency gauge? The Torque app is horribly inaccurate when it comes to fuel economy readings. I saw as much as 20% underestimate on the highway and 20% overestimate around down with the same calibration.

[Snax]

Yes, the efficiency gauge is essentially what I have been trying to match up with expected mi/kWh directly.

I think I have figured out that the root of the variance issue however is thermal management of the battery. I just did a run in 61F vs. the 78F temperatures earlier and there was a significant difference in fuel economy, netting over 40 MPG this last time around over 79 miles at the same 65 MPH average. That is more inline with expectations for it on gas only. (And it's possible I got a short fill before my first test run as well.) Unfortunately the only way I have to control for battery thermal management is ambient air temperature during testing. Fortunately for at least 6 months of the year around here, 61F usually happens once or twice a day, but depending on the right weather still sucks and I really just need a long fast hill to coast down!

Anyway, on another note, it looks like the Torque app has had something potentially useful added to it with fuel consumption reporting. The value reported is completely wrong, but it does accumulate a long term average which settled at 17.8 for the recent test. Given the 40.8 MPG actual measured fill, I can probably safely change the displacement in the settings for a more accurate idea of the extended consumption for each trip as quick way to compare without stopping to fuel up.

I suspect it will be less vulnerable to inaccurate fuel consumption since the generator runs a fairly constant load.

Oh well, making progress even if I have yet to really test any mods!

[euromodder]

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.

Given that the A2 was 0.288 20 years ago and that the Beemer is electric, BMW have not done their homework properly.

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

Not a fan of the i3 styling
Never have been, and never will be

IMO a boattail would actually improve the looks

[euromodder]

Originally Posted by Snax

Going to go with 10 screws I think. There's too much bowing over the curvature of the wheel to be flush with just five. (At least with the coroplast test version.)

Those massive wheels are also costing range
If you ever have one off the car, please weigh it, I fear it's gonna be pretty heavy even if it looks airy.

The large diameter / small width helps, but the weight works against you