BMW i3 - Low Hanging Fruit (improving aerodynamics)

[aerohead]

Originally Posted by Vekke

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

I've done some temperature profiles with a hand-held infrared pyrometer,and found that,on the highway,tire temperatures will reach their equilibrium temperature in under 3-miles of driving.

[aerohead]

Originally Posted by 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 . . :/

Many automakers switched to wind tunnel testing from coast down testing,as there were as few as 7-days a year when the meteorological conditions were stable enough to actually allow the necessary parameters to be satisfied.
I've tried it,gave up,and paid to have it done at a research facility under continuous weather monitoring.Data resolution has to be at 10 measurements/second resolution,perfectly straight course,dry,wind zero,or less than,or at minimums,ten-run minimum,back-to back,immediately.All the polar-moments-of-inertia for every rotating component on the vehicle must be known,or properly estimated,using SAE protocols. Exact mass of vehicle and test driver.It's a daunting proposition even following their mathematics.
Good luck!

[aerohead]

Originally Posted by woodstock74

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:

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

This scale is too small. The airspeed required for proper Reynolds number would be beyond MACH-1.You'd be in compressible flow.It just won't work!

[Snax]

Originally Posted by aerohead

This scale is too small. The airspeed required for proper Reynolds number would be beyond MACH-1.You'd be in compressible flow.It just won't work!

Challenge accepted!

Seriously though, I didn't realize the order of magnitude was so great. Time to think 'water' perhaps.

[Snax]

Found this interesting discussion on the matter.

Seems to me like if I could generate 2 m/s worth of clean flow, it could yield applicable information. Gonna need a ladder to reach that fruit though.

[Snax]

I still don't have the model in hand however. It may be drastically off dimensionally and only worthy for conceptual exploration. Thinking it will be a fun diversion either way.

[aerohead]

Originally Posted by Snax

Challenge accepted!

Seriously though, I didn't realize the order of magnitude was so great. Time to think 'water' perhaps.

Yes,water is 833-X more dense than air.You can hit critical Reynolds number at very low velocity.
Texas Tech used to have two water tunnels.One was for 1/24th-scale models,where they just injected food coloring to image the flow.No drag measurements.
They also had a large water tank with overhead gantry-crane,providing a multi-axis strain-gauge sting,onto which an 1/8th scale model was towed along the floor of the tank,and the lift-drag-pitch-roll data transmitted through the sting to a computer.
The model of the Ford Taurus they had when I visited in 1991 cost them $68,000.Ouch!
Daimler's Mecedes-Benz has also used this technique.
Another technique is a water table,used for 2-D flow only.Blocks of wood are band-sawed into a side profile of the shape of interest,and placed onto and weighted down to the outflow table,which has laminar flow.Food coloring allows flow visualization.No measurements though.When the water gets cloudy from the coloring,you throw in a little bleach,and it clears right up.

[woodstock74]

Originally Posted by aerohead

This scale is too small. The airspeed required for proper Reynolds number would be beyond MACH-1.You'd be in compressible flow.It just won't work!

Yoshi has 50+ years in wind tunnel development. He's one of the automotive scale wind tunnel testing pioneers. He built NPTi's rolling road scale wind tunnel from nothing (starting in the very late 70s) and developed all the relevant techniques that led to multiple IMSA GTP championships for Nissan. In those days they used very small scales, 10-14%.

Yoshi's done the verification work, comparing model results to the full scale results, conducting tests in both, and found the trends are indeed translatable, even if the force absolutes are not. Remember his target was CFD; and a scale model tunnel will still beat the pants off of CFD in the amount of iterations/data it can generate.

http://www.suzukaracing.com/page6.html


The idea is to establish trends, within reasonable testing parameters. Even testing at 50% scale, we'd still need to be running at wind/road speeds much higher than we are running (or capable of) in order to match full scale Reynolds numbers. Yet we're in the tunnel day in, day out, producing tangible results for the track. To be honest, in my modest 20+ years, I'm not sure I've ever seen a scale tunnel produce proper Reynolds numbers! Yet the scale rolling road wind tunnel is still the go-to development tool because what are your options?

[Snax]

My new i3.

(Totally not fake . . )

Dimensionally I think it is actually very close in proportions except of course the underbody. I'll have to make some door windows for it, but it should be fun to play around with if nothing else.

[Snax]

Welp, I am either too discouraged or lazy to monkey with the wind tunnel idea, as I have done nada with it.

Anyway, I did finally get around to improving my mirror delete cover plates. While my previous version looked pretty good when I first completed them, the use of hot glue as the bonding material just got messy with sunshine softening it up. So all of my precision in that effort ended in misalignment, warping, and a hot glue mess to clean off. This new version however is assembled 100% with ABS glue (home made with ABS dust and acetone), and it is a better fit so has less structural stresses on it. I think the 3M tape I used to attach them will also pan out better over the long run. At the very least, they should stay in one piece if they do fly off the car.