BMW i3 - Low Hanging Fruit (improving aerodynamics)

[aerohead]

Originally Posted by mpg_numbers_guy

You will likely notice more of a benefit with rear wheel covers than front wheel covers. I removed the front wheel covers from my Civic when I had it and left the rear ones installed, and didn't notice and discernible fuel economy loss. My theory is that rear wheel covers have a larger effect since they're near the rear of the car, and any flow disruption near the rear of the car will add more drag.

Some observations have indicated a greater drag reduction from the front skirts. I know this to be true for a 1930s Adler coupe and the 1983 Ford Probe-IV.(9% in the case of the Ford).
I do agree that if the fronts are done,then you've got the best onset flow available to the rear of the car,which amplifies the ability of the rear skirt,hence pressure drag advantage.

[Snax]

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.

I agree, however the test road is about 15 miles from my home, so hopefully at least for the direct drive-train, that should not be an issue starting from the similar ambient conditions with a cold car.

I have been aiming for consistency in how I test for that reason. I don't expect everything to jive with extended open-road efficiency.

[Snax]

Did some more 'dust-up' testing this evening to examine skirt performance and I think it's encouraging. I ended up not testing a full skirt because I just couldn't come up with an attachment method for the rear of it that didn't either rub on the tire or not reduce the wake area down lower.

I think it exceeded the benefit seen with the rear disc which is not surprising.

I also think it is interesting how the holes I left in the skirt show a pressure difference within the wheel well. Looking at the dust pattern on the wheel as well, it is clear that air is moving in toward underneath the car where they are exposed to the exterior flow. Many race cars can be seen utilizing vertical venting over the wheels to deal with this. This flow pattern has me wondering if that pressure behind the upper area of the skirt couldn't be effectively redirected back into the side airflow with side louvers or something.

And yes, I included the rear shot because it appears to be slightly less dusty on the skirted side. Whether that result would hold with both sides faired may be questionable however.

[Snax]

Latest 'dust-up' pics, full discs, mirror delete, and skirts.

Not surprising, but I'm not seeing any difference between the skirted rear with and without discs, though I think there is clear improvement between skirted and full discs.

Going to see if I can get some actual numbers in the next day. It was in the 90's today, so no chance of getting anything but full battery cooling going on until the wee hours or morning.

[hat_man]

Originally Posted by Vman455

I would hesitate to go any lower than you have already; generally, there's a "sweet spot" where CD goes down as the body is lowered, but rises again with more lowering. According to the aerodynamics textbooks I've read, this is somewhere around 0.75 body cover:1 wheel diameter.

Can you put this in layman's terms for the aerodynamically challenged folks like me in the back of the classroom? I like math, but sometimes need help with what the x's and y's represent. Wheel diameter is an easy one but body cover?

Thanks

[aerohead]

Originally Posted by hat_man

Can you put this in layman's terms for the aerodynamically challenged folks like me in the back of the classroom? I like math, but sometimes need help with what the x's and y's represent. Wheel diameter is an easy one but body cover?

Thanks

I don't understand the comment either.As to lowering,The 1970 Porsche 914.916 was tested for drag as a function of ground clearance,and its lowest drag was with the car @ zero ground clearance.
I created a graphic for this but PhotoBucket won't let me use my 770-images anymore unless I pay them extortion money.

[California98Civic]

Originally Posted by Snax

Latest 'dust-up' pics, full discs, mirror delete, and skirts.

Not surprising, but I'm not seeing any difference between the skirted rear with and without discs, though I think there is clear improvement between skirted and full discs.

Going to see if I can get some actual numbers in the next day. It was in the 90's today, so no chance of getting anything but full battery cooling going on until the wee hours or morning.

This was the "after" dust-up... was there a before that you posted? Put 'em side-by-side.

Originally Posted by aerohead

I don't understand the comment either.As to lowering,The 1970 Porsche 914.916 was tested for drag as a function of ground clearance,and its lowest drag was with the car @ zero ground clearance.
I created a graphic for this but PhotoBucket won't let me use my 770-images anymore unless I pay them extortion money.

Photobucket... for shame, low scalpers!!

[Vman455]

Originally Posted by hat_man

Can you put this in layman's terms for the aerodynamically challenged folks like me in the back of the classroom? I like math, but sometimes need help with what the x's and y's represent. Wheel diameter is an easy one but body cover?

Thanks

Originally Posted by aerohead

I don't understand the comment either.As to lowering,The 1970 Porsche 914.916 was tested for drag as a function of ground clearance,and its lowest drag was with the car @ zero ground clearance.
I created a graphic for this but PhotoBucket won't let me use my 770-images anymore unless I pay them extortion money.

Fig. 5.9

[Snax]

If I understand that correctly, it is referring to the wheel well height relative to the wheel, not ground clearance. I can't help thinking that is an oversimplification of how things interact however. Does that mean the exterior panel height, or the interior space which is larger?

Kind of a moot point for me regardless. Going any lower would severely impact ride and drive-ability. There isn't a lot of travel left and those sidewalls don't absorb much either.

On another note, I got foiled from testing more by the gods of careless contractors yesterday morning with a nail discovered in a tire. It was too warm to rely on battery cooling not to be active by the time I got that fixed.

[Snax]

Originally Posted by California98Civic

This was the "after" dust-up... was there a before that you posted? Put 'em side-by-side.

Not sure what you are asking for here unless you just missed the earlier pic.