BMW i3 - Low Hanging Fruit (improving aerodynamics)

[Vekke]

Originally Posted by aerohead

https://ecomodder.com/forum/showthre...r-37641-2.html

Technically,at 49-miles,you'd just be warmed up enough to 'begin' testing.It takes that much distance for all the lubricants and temperature-dependent rolling components to reach their ambient-condition stabilization.Without pre-warming,there's so much thermally-related 'noise',you'll never be able to 'see' the 'signal' you're looking for.

Also remember that road temperature effects a lot to the rolling resistance of the tire. So you should also pay attention to the outside temps if you are doing testing in different days. Warmup time goes also for the tires.

OEM wheel weigths can be found here: https://www.fuchsfelge.com/en/wheel-...ll/bmw-i3.html

[Snax]

I am about to give up on tracking actual efficiency with this thing on any short term kind of basis. It is simply proving too inconsistent to be indicative of anything at this point.

I could be wrong, but my gut tells me that the addition of discs front and rear, as well as half skirts on the rear have reduced drag, but my numbers simply don't reflect that even when testing in conditions that are virtually identical to unmodified test conditions. There must be something different, but not sure what - or perhaps even the possibility that discs and skirt actually hurt drag. Could that be??

I've decided that the only thing that is going to produce any usable short term A-B-A data is the coast down test. Eliminating drivetrain variables will be the only way to see what is actually happening with each mod unless I want to spend countless hours driving around for little other reason.

I'm gonna need a different test road . . :/

[redpoint5]

Yeah, I was going to say coast down would be your best bet.

Driving at high speed would be the best way to observe aero differences if you could find a stretch to do 90.

[Snax]

Originally Posted by redpoint5

. . if you could find a stretch to do 90.

The *problem*.

I tried to scope out some roads this afternoon, but none of them will work for the daytime with the amount of traffic on them. At night, doing 90 on those same roads just feels suicidal. LOL.

One of those times where living in the middle of nowhere would be beneficial . .

[California98Civic]

Originally Posted by Snax

The *problem*.

I tried to scope out some roads this afternoon, but none of them will work for the daytime with the amount of traffic on them. At night, doing 90 on those same roads just feels suicidal. LOL.

One of those times where living in the middle of nowhere would be beneficial . .

This is why I went to El Mirage, and may go again this fall. I don't think 90 mph is necessary. The further you roll the more difficulty in avoiding irregularities of ground & wind. The faster your start, the harder a clean and precise start at a marker. 40 mph was enough for nearly 0.66 miles coasting. My total course was atleast a mile.

[Snax]

Originally Posted by California98Civic

This is why I went to El Mirage, and may go again this fall. I don't think 90 mph is necessary. The further you roll the more difficulty in avoiding irregularities of ground & wind. The faster your start, the harder a clean and precise start at a marker. 40 mph was enough for nearly 0.66 miles coasting. My total course was atleast a mile.

I've started thinking that this is really the only practical legal and safe way to do any real testing with my car that doesn't depend upon long term data. That means testing is likely to be more of a destination / event activity for affirming beneficial changes.

I did order a 1/32 scale model of my car to monkey around with and depending upon how accurately sized, I may build a wind tunnel for it for giggles and grins if nothing else.

[woodstock74]

Originally Posted by Snax

I've started thinking that this is really the only practical legal and safe way to do any real testing with my car that doesn't depend upon long term data. That means testing is likely to be more of a destination / event activity for affirming beneficial changes.

I did order a 1/32 scale model of my car to monkey around with and depending upon how accurately sized, I may build a wind tunnel for it for giggles and grins if nothing else.

Noted aerodynamicist Yoshi Suzuka (father of Nissan's IMSA GTP and All-Japan race cars) was at one time attempting to market a desktop wind tunnel. He did a lot of work sourcing scales and developing the product such that it produced repeatable results.

At one point he had a number of videos showcasing the wind tunnel, but they don't appear to be up any more. However, he still has his functioning prototype and that can be seen in a few of his videos as he investigates certain road car aero features:

https://www.youtube.com/watch?v=aD5gjfFRyyc

https://www.youtube.com/watch?v=1V_A1rsLsfs

https://www.youtube.com/watch?v=uVbaBXGy-x0

So, if you have in the region of $100,000, you might have something to do with your 1/32 scale models!

[redpoint5]

I'd say scale models and a wind tunnel is no longer in the realm of low hanging fruit, but I'm even more interested now.

Maybe a brick of straws could be used to produce the requisite laminar flow in the mini windtunnel? Seems like a difficult yet fun project.

[Snax]

Originally Posted by woodstock74

So, if you have in the region of $100,000, you might have something to do with your 1/32 scale models!

I'm thinking cost is less of an issue than potential inaccuracy of the design. Very small changes make a huge difference there, but that may be less important than overall generalized effect - even if it does not end up accurate in real world use. If nothing else, something for my teenager to lean on for a science project at some point perhaps . .

I did score a nice recycling yard find today however of a 4x8 1/8" sheet of black plastic. Not sure it's ABS, as it seems more flexible than that, but it seems a good mix of rigidity and flexibility to conform to simple curves and mild complex ones.

At the very least, I think it would be a decent upgrade for my wheel discs in being solid and less susceptible to cosmetic damage, along with being thinner than the 4 mm stuff I have now. It should work well around the lower suspension arms too.

[woodstock74]

Originally Posted by Snax

I'm thinking cost is less of an issue than potential inaccuracy of the design. Very small changes make a huge difference there, but that may be less important than overall generalized effect - even if it does not end up accurate in real world use. If nothing else, something for my teenager to lean on for a science project at some point perhaps . .

I did score a nice recycling yard find today however of a 4x8 1/8" sheet of black plastic. Not sure it's ABS, as it seems more flexible than that, but it seems a good mix of rigidity and flexibility to conform to simple curves and mild complex ones.

At the very least, I think it would be a decent upgrade for my wheel discs in being solid and less susceptible to cosmetic damage, along with being thinner than the 4 mm stuff I have now. It should work well around the lower suspension arms too.

Yes, that small of a scale tunnel would purely be used to establish trends and not absolutes, and would be compared against CFD, not larger scale tunnels. It would also have an advantage over CFD in the amount of data you could generate over a given time period. CFD is still struggling there as the models are getting more complex, but the solve time is staying about the same.