BMW i3 - Low Hanging Fruit (improving aerodynamics)

[Snax]

Originally Posted by California98Civic

That angle is too sharp to taper effectively, and you don't want to be building some sort of boat tail extension I've heard you say. Seems like your best option is to taper the trailing Edge of the wheel well skirt a little and create a box cavity like separation point at the tail end of the car.

That matches my intuition on it. I should also point out that there are cooling vents through the liners into the wheel wells from the motor, so that might actually help with extraction - though cooling doesn't seem to be a problem as it is.

[Snax]

Here's my disc mounting system using sprinkler hose and stainless screws around the perimeter. The hose ring is rigid enough to stay in place on it's own, but cut a few mm too large is stuffed tightly into the recess.

Originally Posted by Vman455

I can't find data for the 20", but this Jalopnik article reports BMW said the 19" would be 15 lbs before production started. With the tire weights listed at tirerack.com, that would make 31 lbs front/33 lbs rear.

Just short of 36 lbs is what I got for the front wheel. Safe to bet on 38 for the rear. Could be lighter, but not much. I suspect that dropping to the 19" wheels would carry a Crr penalty though, more than cancelling out any weight reduction.

[redpoint5]

I'm enjoying this thread by the way. Thanks Snax for the photos and measurements.

[Snax]

Sadly, I think I am going to have to forgo making any claims of improvement for awhile. I was hoping to collect fuel logs that were a bit more conclusive, but they seem to be all over the place - using the same pump and station for all of the fills.

I just did a 49 mile test with the front discs installed and ended up with worse numbers. Perhaps they really do hurt the aero. Apparently it's going to take significant time to figure out.

I can't fathom any way the mirrors are not a win regardless.

Starting with a relatively clean car, I did bomb down a gravel road after logging my miles and shutting the generator down. That reveals some interesting dynamics I think.

[Snax]

I think the next thing I will do is more dust testing with the discs. I've only got 2 made at this point, so I will see what happens with both on one side as well as both on the rear. I'm really curious to see if the dust profile changes by removing them from the front and I don't expect much difference on the rear either way.

[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.

[Vman455]

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.

I would be hesitant to guess that. I came across this passage in one of the textbooks I read earlier this year and underlined it since I was surprised by it:

"The front wheels contribute the bulk (about 65%) of the total wheel drag in cars because the rear wheels are typically in the wake of the front wheels, only experiencing some of the vortices generated by the front wheel interaction with the car-body air flow, and the lower edge of the trailing stream flow from the front wheel (Fig. 5.8)" [Obidi, T.Y. Theory and Applications of Aerodynamics for Ground Vehicles, (Warrendale: SAE International, 2014), 122].

Also, recall the McLaren Speedtail, which has wheel covers on the front wheels only--something the design director said in an interview was necessary to reach their top speed target, while rear wheel covers were not.

[Snax]

Looks like a bit of both to me. Most of the dust difference is at the rear above and aft of the wheel, though there is some difference ahead of them as well. The rear bumper and hatch showed no difference I could see from side to side.

Above the rear, a clear line of dust can be seen with heavier dusting on the uncovered side. Everything else ahead of that is less obvious. In comparing these with discs only on the front, the dusting above the rear for the uncovered wheel looks about the same vs. totally uncovered IMHO.

[mpg_numbers_guy]

Originally Posted by Vman455

I would be hesitant to guess that. I came across this passage in one of the textbooks I read earlier this year and underlined it since I was surprised by it:

"The front wheels contribute the bulk (about 65%) of the total wheel drag in cars because the rear wheels are typically in the wake of the front wheels, only experiencing some of the vortices generated by the front wheel interaction with the car-body air flow, and the lower edge of the trailing stream flow from the front wheel (Fig. 5.8)" [Obidi, T.Y. Theory and Applications of Aerodynamics for Ground Vehicles, (Warrendale: SAE International, 2014), 122].

Also, recall the McLaren Speedtail, which has wheel covers on the front wheels only--something the design director said in an interview was necessary to reach their top speed target, while rear wheel covers were not.

Front wheels will contribute overall to more drag than the rear wheels yes, but I was talking about aerodynamically from the side. I probably should've been clearer. Poor wheel aero affects the attached airflow to the side of the car, but at the rear there isn't any more car length for the flow to reattach, which is why I was theorizing that rear wheel covers would make a larger difference. In terms of overall drag, including under the car and from the width of the tire, then yes, definitely more drag is caused by the fronts since the rears are in the wake of the fronts.

Interesting though about the McLaren Speedtail. Did the design director say that they were for aero, or downforce? although I can't imagine how smooth wheel covers would help downforce at all.

[aerohead]

Originally Posted by Snax

Sadly, I think I am going to have to forgo making any claims of improvement for awhile. I was hoping to collect fuel logs that were a bit more conclusive, but they seem to be all over the place - using the same pump and station for all of the fills.

I just did a 49 mile test with the front discs installed and ended up with worse numbers. Perhaps they really do hurt the aero. Apparently it's going to take significant time to figure out.

I can't fathom any way the mirrors are not a win regardless.

Starting with a relatively clean car, I did bomb down a gravel road after logging my miles and shutting the generator down. That reveals some interesting dynamics I think.

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.