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Old 04-15-2019, 10:54 PM   #31 (permalink)
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Old 04-16-2019, 12:31 AM   #32 (permalink)
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Okay, so 3/4ths the tire height minus the ground clearance, over 1/2 the tire height, I guess. Thanks.

Except the text is talking about the wheel well radius, not the tire radius called out in the illustration. According to this a close fitting cycle fender would have more drag than a looser fitting one?

A tight-fitting Beemer-style lip would isolate the wheelwell more than an open 4x4 wheelwell — that's different than the cavity volume.
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Old 04-16-2019, 06:25 PM   #33 (permalink)
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Quote:
Originally Posted by freebeard View Post
Okay, so 3/4ths the tire height minus the ground clearance, over 1/2 the tire height, I guess. Thanks.
It's easier to think of it as the simple ratio of body cover:wheel diameter, I think.

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Except the text is talking about the wheel well radius, not the tire radius called out in the illustration. According to this a close fitting cycle fender would have more drag than a looser fitting one?
I don't think a cycle wheel in a fender is analogous. For one thing, it doesn't have a vehicle body around it. Second, as Hucho notes, in zero-yaw conditions the windstream at the front wheels is approaching at an angle of ~15 degrees, where no such shear would be present for the bike wheel.

The last few sentences of that paragraph I take to mean "...increased [wheel] radius and reduced [wheel housing] clearance," based on what's written before it.

We should also note that Hucho, citing an earlier study by Cogotti, indicates a completely linear relationship between wheel housing volume reduction and wheel drag. Interesting.

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A tight-fitting Beemer-style lip would isolate the wheelwell more than an open 4x4 wheelwell that's different than the cavity volume.
But that is directly related to the cavity volume, if we're holding other conditions constant, like the body cover.
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Old 04-16-2019, 08:00 PM   #34 (permalink)
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I was trying to make it more complicated.

Re the cycle fender I was trying to mentally separate the skin friction of the moving tire and the general turbulence of a cavity normal to the air flow.

An interesting case might be the SAAB prototype, with it's downward-facing opening with an internal cycle fender.
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Old 06-01-2019, 05:38 PM   #35 (permalink)
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Update, Summer 2019:

I have just a couple aerodynamic projects planned for now: first, adding the front wheel strake fairings from a 2018 C-max hybrid and second, air curtain ducts from a 2018 Honda Clarity (will be easier to retrofit than the Hyundai Ioniq ducts I have on hand).

But the big project I'm working on right now is accurate coast down testing, to see if I can calculate drag coefficient with some precision. I've downloaded and read several papers on the subject, relevant chapters in two textbooks, and have worked through most of the math. Last week, I downloaded a topographical map and identified several stretches of road that are candidates for a test site with minimal/no elevation change. I'll drive out and check them out in person this week. I'm also about to order a Kestrel 2500NV ("night vision," since I'll probably end up doing this at night when winds are calmer and traffic is lighter) wind meter and wind vane (made for the 4000 series, so I'll have to modify it to fit the 2500). That meter will measure windspeed to 0.1 m/s accuracy, as well as temperature, barometric pressure, and wind direction with the vane mount. Using that data, I'll be able to calculate the actual wind speed of the car.

For the overall force calculation and rolling resistance force component, I'll be using the corrected mass of the car. Chapter 14 of Aerodynamics of Road Vehicles suggests a formula for calculating this using the inertia of the driven wheels and driveline and non-driven wheels and their respective radii. I looked up the weights of the stock wheels on Priuschat, the tires I have on the car (thanks Tirerack!), and averaged the listed weights for various replacement front and rear rotors, wheel hubs, and the front axles and used the dimensions of all these to calculate the inertias. The overall correction factor results in an effective mass of 102.19% of measured mass. Next, I'll use the scale at one of the two recycling centers in town here to measure the mass with me, the wind meter/tripod, and a full tank of gas.

I'm going to write all this up like a lab report when I'm done so if others are interested they can read it.
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Old 06-02-2019, 12:36 AM   #36 (permalink)
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Where will the vane be? In the center of the hood or held out the driver's window?

My favorite candidate for coast-down testing (on the Left coast ) is the Van Duser Corridor, https://en.wikipedia.org/wiki/H._B._...cenic_Corridor


Quote:
H. B. Van Duzer Forest State Scenic Corridor is a 12-mile (19 km) scenic driving route along Route 18 in Lincoln, Tillamook, and Polk counties in the U.S. state of Oregon that passes through a forested corridor.
It cuts through a forest of 80-100ft tall trees. It's just a narrow slot with trees right up to the ditches on both sides.
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Old 06-02-2019, 06:58 PM   #37 (permalink)
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Quote:
Originally Posted by freebeard View Post
Where will the vane be? In the center of the hood or held out the driver's window?
Wind will be measured and recorded before each pair of runs; in the future I might experiment with vehicle-mounted anemometers, but not now. I've looked at stormchaser equipment, which seems to be the primary market for vehicle-mounted systems, and all of them stick up into the flow field around the vehicle.
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Old 06-02-2019, 09:20 PM   #38 (permalink)
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I'm starting to like the idea of a miniature weathervane hood ornament. On my Superbeetle it would be on the emblem just ahead of the windshield. It would be like a semi-permanent tuft test.
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Old 07-23-2019, 07:59 PM   #39 (permalink)
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It's been a while since I've actually done any aeromods to this car. But today, that changed--I finally added the front wheel strake fairings (from a 2018 Ford C-Max hybrid) I've had laying around for several months now. I added a flat piece of plastic to close off the channel in the factory undertray at the front and attached the fairing to it; the Gorilla tape is just to seal edges, everything is attached with extra Toyota OEM plastic fasteners I had left over from adding a JDM panel to the front underbody panel to close up two of the larger holes.





Because of the way the undertray is shaped, they sit a little lower and slightly further inboard than they do on the Ford, about even with the lower edge of the enlarged strakes. We'll see what effect all this has on drag when I do the coastdown testing in the next few weeks (I'm going to try and finish the front wheel arch air curtains before I do that).
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Old 08-01-2019, 10:41 PM   #40 (permalink)
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Earlier this spring, I noticed on an Insight that Honda had added some devices under the rear bumper cover when they refreshed it for 2012:



I found them in a parts catalog--they're called "rear air splitters"--and ordered a pair. The Prius' bumper cover is slightly too short, but the locating pin at the end of the splitter still fits in a hole I drilled in the cover, so the whole thing is stable.





This will be the last mod I get to before I do coast down testing; my anemometer came in today and the wind direction vane should be here tomorrow.

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