Green Grand Prix modding thread
 

After 250+ posts, I think it's time to let my "3rd Gen Prius Aeromodding" thread rest. I thought I would start a catch-all thread not just for my car, but anyone else who has competed or plans to compete in the Green Grand Prix and wants a place to discuss modifications specifically for that and other fuel economy events.

The reason I'm starting a new thread for this: One thing I was not prepared for was the amount of elevation change in the Watkins Glen short course profile, something I had not researched before the GGP.

http://image.superstreetonline.com/f...vation-map.jpg

From Turn Two through the back straight, the course rises 124' at an average grade of nearly 2.5%, although it is steeper through Turns 3 and 4 than on the straight itself. Laboring the car up that grade every lap, I got to thinking--a significant aeromod (like the cargo box tail), which adds a fair amount of mass to the car, doesn't really pay for itself in an event where I have to maintain an average speed of 45mph and am speed-limited to 60mph; in other words, having more momentum or improved aerodynamic efficiency on the front straight doesn't pay off as much as it could since I can only take advantage of that up to a certain speed, for which I'm penalized anyway if I go too fast and overshoot that 45mph average. Also, while slowing down on the uphill is an option, there is a minimum 35mph speed requirement for the event. So, I can't crawl up the grade to minimize the impact of having to carry all that weight up, and I can't coast past 60mph on the front straight to recoup some of that energy expenditure by taking advantage of the increased momentum.

So, I'm re-thinking my approach for the GGP. In the next few days here, I'll be removing the tail, cargo carrier, and hitch receiver assembly (which together weigh nearly 150 lbs), and starting to gut the car to get rid of as much mass as possible. I plan on a new tail design, with light weight as a design goal; most of the aeromods on the car will stay in place (since most of them don't weigh much). Even without the tail, the Prius is sitting at Cd .21, so it's not like I'm giving up something major in that area. This is a new direction for the car; I dabbled with removing some weight a few years ago, but was never very serious about it and ended up adding a lot in the tail, sound insulation and, for a while, a big subwoofer box. I'll now be focusing more on mass reduction than new aerodynamic devices, hence this new thread, in Ecomodding Central rather than Aerodynamics.

I'm not entirely crazy, as most of my driving these days is in-town and low-speed, grocery store runs and the like. So, a drastic reduction in mass should benefit me more than it would have 4 years ago, when I was doing mostly highway miles. This is very experimental--I'm not sure what it will be like to live with a daily driver with no sound insulation, gutted interior, etc. But I'll find out! And I guess I'll find out how far I'm willing to go in the name of science, and how long before my partner refuses to ride in my car.

As of now, here's what I have taken out:
-Hatch panels: 5.2 lbs (21.6 lbs with added insulation)
-Rear door panels: 10.6 lbs (20.0 lbs with added insulation)
-Hatch door panels: 5.4 lbs (10.4 lbs with added insulation)
-Wheel covers/pizza pans: 2.5 lbs (9.4 lbs with pizza pans)
-Front swaybar and endlinks: 8.4 lbs
-Trunk mat: 5.4 lbs
-Rear speakers, miscellaneous hardware: 5.3 lbs
-External mirrors: 4.2 lbs
-Tonneau cover: 4.2 lbs
-A-pillar trim and dash speaker grills: 3.0 lbs (4.2 lbs with added insulation)
-Passenger wiper: 2.8 lbs
-Rear wiper and motor: 2.4 lbs
-Intake resonator: 2.0 lbs
-Hood insulation: 1.2 lbs
-Plastic engine cover: 1.0 lb

For a total of 102.5 lbs gone, or 63.6 lbs from OEM curb weight, and I haven't even gotten to the tail/hitch receiver or spare tire. I'll update this list as I take things out and see if I can get some pictures as I go along. I also plan on weighing the car at a truck scale later this summer (we have a CAT scale at the stop just outside town) to get an accurate weight.

Some things I've thought through already--I'll be removing the headliner and sun visors, so I've ordered a vinyl strip to go across the top of my windshield (because racecar). The pizza pan wheel covers will be replaced by light plastic or acrylic annular discs, so I have access to the lug nuts without removing them (I found this was a pain with the full covers). All safety equipment will remain in place--I won't be taking out airbags, drilling holes in bumpers, removing crash bars in the doors or anything like that. The only rule for the modified class at the GGP is that the vehicle must be street legal, so anything that violates that criterion is out.

Other than that, game on.

Reciprocating or rotating weight — Swap in Titanium valve spring retainers and the wheels and tires.

What's the plan? How about a birdcage of welded welding rod shrink-wrapped with boat wrap.

Can you fit BMW i3 19" wheels and tires?

That would be fantastic; I don't think the bolt pattern is the same, though, although otherwise I think they would fit despite being a couple inches taller.

Edit: Nope, the i3 runs 5x112 bolt circle; the Prius, 5x100.

That said, let's talk about wheel weights.

I was surprised that the pizza pans weighed that much, actually--nearly 10 lbs of rotating mass. The OEM 15" Prius wheels weigh 16.3 lbs each, and the 195/65-R15 tires are 18 lbs. The lightest available wheel on Tirerack is a forged BBS 17", at 14.3 lbs, but its cost is prohibitive, more than $2000 for a set of 4, and the weight of 17" tires would more than offset the savings in the wheels. However, there are other reasonable options between 15-15.5 lbs for around $400/set in 15" diameter. Since I want to start autocrossing this car, what I might do is pick up one of those options with some 195/50-R15 Dunlop Direzzas, which weigh the same but are a couple inches shorter than the stock tires. That would give me a set of wheels for autocross a pound lighter on each corner and with the effectively shorter gearing of a smaller diameter to boot, and leave my OEM wheels and tires for daily driving, road trips, and the GGP road rally portion.

Here is a discussion of VW wheels to make a point:
The wide 5 weights appear representative. So wheel width makes a large difference. Center Line used to advertise that they make wheels in sizes from 10 to 30", so 19x4 with 5x100 PCI is prolly available.

The low-ball approach would be used i3 rims on 5x100/112 adapter plates. That gets around hub-centric vs bolt-centric problems and introduces offset problems.

Is the Hybrid Synergy Drive sensitive to over/under on the tire circumference?

The only problem with running adapters would be the extra width, pushing the wheels to the outside of the car. Well, that and the expense--at $300 or more per wheel, and $135 per tire, i3 wheels are an expensive modification.

I don't think so; I happened to see a Jalopnik article this morning about some guys in Washington who put a Prius on 29" tires to take it off-roading (4" larger than stock) with no problems.

Updated weight list:

Added sound insulation: 48.6 lbs
Trailer hitch receiver: 28.8 lbs
Rear right seatback: 26.4 lbs
Spare tire: 25.8 lbs
Tail: 21.8 lbs
Cargo carrier: 19.6 lbs
Hatch insulation: 16.4 lbs
Rear left seatback: 13.2 lbs
Rear seat bottom: 12.2 lbs
Rear seat belts, seat brackets, hardware: 10.7 lbs
Front speakers and baffles: 10.6 lbs
Front door insulation: 9.8 lbs
Rear door insulation: 9.4 lbs
Wheel covers with pans: 9.4 lbs (2.5 lbs wheel covers alone)
Front swaybar and endlinks: 8.4 lbs
Jack and tools: 8.2 lbs
Trunk mat: 5.4 lbs
Rear speakers, miscellaneous hardware: 5.3 lbs
Hatch door insulation: 5.0 lbs
Floor mats: 4.5 lbs
External mirrors: 4.2 lbs
Cargo cover: 4.2 lbs
Front speaker crossovers and wiring: 3.2 lbs
Passenger side wiper: 2.8 lbs
Rear wiper and motor: 2.4 lbs
Intake resonator: 2.0 lbs
Sound system wiring: 1.5 lbs
A-pillar insulation: 1.2 lbs
Hood insulation: 1.2 lbs
Engine cover: 1.0 lb

Total: 323.2 lbs/140.4 lbs from OEM

Estimated curb weight now around 2900 lbs, and I still have a full interior minus the back seat.

http://ecomodder.com/forum/member-vm...505-194809.jpg

However, no sound system, and the headunit is the next thing to go.

Of course, it can't be a race car without some decoration, so I added pinstripes to the wheels.

http://ecomodder.com/forum/member-vm...505-194636.jpg

And a windshield banner.

http://ecomodder.com/forum/member-vm...505-194556.jpg

And a plug for the forum.

http://ecomodder.com/forum/member-vm...505-194833.jpg

This is great. Subscribed.

My friends have a Prius that's missing one wheel cover. I tell them it would look better if they took the other three off, but they say they like the sliver cover better. Just not enough to replace the missing one. :confused:

I was looking for a place to post that:

https://i.kinja-img.com/gawker-media...n1ideukgzk.jpg
https://jalopnik.com/this-lifted-prius-trolls-jeeps-1825777000

Ha! Regen joke.

A few more little things today. First, updated weight loss:

Added sound insulation: 48.6 lbs
Trailer hitch receiver: 28.8 lbs
Rear right seatback: 26.4 lbs
Spare tire: 25.8 lbs
Tail: 21.8 lbs
Cargo carrier: 19.6 lbs
Hatch insulation: 16.4 lbs
Rear left seatback: 13.2 lbs
Rear seat bottom: 12.2 lbs
Rear seat belts, seat brackets, hardware: 10.7 lbs
Front speakers and baffles: 10.6 lbs
Front door insulation: 9.8 lbs
Rear door insulation: 9.4 lbs
Wheel covers with pans: 9.4 lbs (2.5 lbs wheel covers alone)
Front swaybar and endlinks: 8.4 lbs
Jack and tools: 8.2 lbs
Trunk mat: 5.4 lbs
Hatch door panels: 5.4 lbs
Rear speakers, miscellaneous hardware: 5.3 lbs
Headunit: 5.2 lbs
Hatch door insulation: 5.0 lbs
Floor mats: 4.5 lbs
External mirrors: 4.2 lbs
Cargo cover: 4.2 lbs
Front speaker crossovers and wiring: 3.2 lbs
Passenger side wiper: 2.8 lbs
Rear wiper and motor: 2.4 lbs
Intake resonator: 2.0 lbs
Sound system wiring: 1.5 lbs
Vehicle Proximity Notification System: 1.5 lbs
A-pillar insulation: 1.2 lbs
Hood insulation: 1.2 lbs
Engine cover: 1.0 lb
Horns: 0.5 lb


Total: 335.8 lbs/163.6 lbs from OEM
OEM curb weight: 3042 lbs
New estimated curb weight: 2878 lbs

Second, after driving around without any speakers for a couple weeks, I removed the headunit. This took some doing, since the wiring for the steering wheel switches (that control the tripmeter and MFD display screens) runs through the receiver. After grounding the proper wire, everything works without the unit. I've got a clear acrylic panel where the radio used to be (not pictured); I like that you can see into the dash where it's missing.

http://ecomodder.com/forum/member-vm...512-211217.jpg

Third, after reading this patent, first brought to my attention by a thread here a few years ago, I thought I would experiment with conductive tape on the resin body panels. After some Google searching to make sure the tape I had on hand actually had conductive adhesive, I applied some to the car. In the patent application, one of the illustrated vehicles, in Figs. 12, 15 and 16, is actually a 3rd-gen Prius, so some of my tape is placed where those figures indicate.

http://ecomodder.com/forum/member-vm...512-211107.jpg

I also added it where a CFD pressure map shows low pressure, indicating possible flow detachment, and where the initial Japanese press on a Toyota 86 outfitted by Toyota suggested.

http://ecomodder.com/forum/member-vm...512-211139.jpg

http://ecomodder.com/forum/member-vm...512-211121.jpg

Up next, I've ordered a Magnahelic pressure gauge, which I'll use to draw a pressure map of the top side and underside of the hood before adding hood vents to evacuate high pressure and hot air from the engine bay. I'm hopeful this will improve radiator efficiency as well as improve Cd, as Hucho seems to suggest in 2.1.1 of Aerodynamics of Road Vehicles. I'm becoming more concerned with cooling efficiency with all the recent press on heat-related inverter failures in Priuses, even those that had the E0E software reflash performed (like mine). Currently, one California dealer is suing Toyota over it. So, for the time being I'm removing my grill block just in case--if Toyota ends up recalling and replacing the inverter, great; if it fails under the extended warranty, I don't want Toyota to be able to point to a grill block as a possible cause and deny coverage.

Also on my mind, I'm thinking about ways to lighten the exhaust, but still conducting research on that as the last thing I want is a loud car.

The conductive tape is a new twist. Interesting. Did you see any testing data? Online, I found copper and carbon (black) conductive tape too. But all those are more expensive than aluminium, unspurprisingly.
https://www.tedpella.com/SEMmisc_html/SEMadhes.htm

That's a good candidate for the military theme thread. They look like campaign stripes.

No data that I've found; just a lot of subjective reviews (by Japanese auto writers) with the usual hyperbole (like this one and this one). However, I've seen multiple sources claim Toyota is actually using this tape method on the inside of underbody panels on production cars; if that's the case, it must do something. I'm going to make a point of checking at the next auto show I attend.

It's also worth noting that Toyota actually sells conductive tape as a Toyota Genuine Part.

I already downloaded the decal images from that thread.:D

Good thread!
I'm bummed I couldn't make it out this year and bring home some more hardware.
How about Turn1? No joke there. I knew it was a 90, but I didn't know, if that makes any sense.
I tried to carry as much speed as I could down through the corkscrew in Turn6. At least you can generally coast down towards Turn7 and the front straight.

I'll be trimming interior weight for the GGP next year, for sure.


-ryan s.

Yeah, I hadn't even looked at a course map before this year's event. Next year, I think the best approach will be to coast through 6, 7 and the front straight, maybe even in neutral where the Prius' transmission will freewheel, and then get on the throttle and hold speed through 3, 4 and up the back. Basically make it one long pulse and glide.

Updated weight list:

Added sound insulation: 48.6 lbs
Trailer hitch receiver: 28.8 lbs
Rear right seatback: 26.4 lbs
Spare tire: 25.8 lbs
Tail: 21.8 lbs
Cargo carrier: 19.6 lbs
Hatch insulation: 16.4 lbs
Rear left seatback: 13.2 lbs
Rear seat bottom: 12.2 lbs
Rear seat belts, seat brackets, hardware: 10.7 lbs
Front speakers and baffles: 10.6 lbs
Front door insulation: 9.8 lbs
Rear door insulation: 9.4 lbs
Wheel covers with pans: 9.4 lbs (2.5 lbs wheel covers alone)
Front swaybar and endlinks: 8.4 lbs
Jack and tools: 8.2 lbs
Trunk mat: 5.4 lbs
Hatch door panels: 5.4 lbs
Rear speakers, miscellaneous hardware: 5.3 lbs
Headunit: 5.2 lbs
Hatch panels, upper and lower: 5.2 lbs
Hatch door insulation: 5.0 lbs
Lower door trim and kick panels: 4.9 lbs
B-pillar trim: 4.7 lbs
Floor mats: 4.5 lbs
External mirrors: 4.2 lbs
Cargo cover: 4.2 lbs
Front speaker crossovers and wiring: 3.2 lbs
Passenger side wiper: 2.8 lbs
Rear wiper and motor: 2.4 lbs
Intake resonator: 2.0 lbs
A-pillar trim, speaker grills and ducts: 2.0 lbs
Sound system wiring: 1.5 lbs
Vehicle Proximity Notification System: 1.5 lbs
A-pillar insulation: 1.2 lbs
Hood insulation: 1.2 lbs
Engine cover: 1.0 lb
Horns: 0.5 lb


Total: 352.6 lbs/180.4 lbs from OEM
OEM curb weight: 3042 lbs
New estimated curb weight: 2862 lbs

The windshield washer system comes out tomorrow. With only one wiper it's kind of useless anyway, and I hardly used it before.

As far as mods go, they don't seem to have a hard and fast definition of where a stick car ends and where it enters the modified class? If I end up moving there, this would be fun. I imagine I'd push the definition of stock.

The way I (and others, like goofy1) read the rules, replacements for stock parts (air filters, tires, other wear items) are fine, but as soon you do something like change tire sizes, add a different intake, grill block, alter bodywork, etc. you're in a modified class. Goofy1 had a K&N canister intake on his Mini, and that put him in Modified with no other changes to the vehicle.

This event was a blast, and will be again next year I'm sure. You should totally do it!

Hmm yeah, I'll be borderline. Depends if I stick to simply upgrading stuff as it needs replacement anyway, or if I start picking up cheap parts as I find them (body bracing, etc.), and what I end up doing for winter tires. My thought had been to run winters on the stock steelies and run something else during the summer.

I added an explanatory decal to my rear windows, to educate passers by:

http://ecomodder.com/forum/member-vm...612-202149.jpg

Maybe this will pique someone's curiosity.

Also, I tested hood pressures yesterday with my Magnahelic.

http://ecomodder.com/forum/member-vm...613-182550.jpg

The results are not what I expected. I'll put up a graphic with the numbers, but what surprised me is that--there is a negative pressure gradient (top relative to underside) across the entire hood! Although, if I had looked at this pressure map I already had stored on my computer, it wouldn't have been so surprising:

http://ecomodder.com/forum/member-vm...8118-cfd-3.jpg

Now to hunt down some vents with the proper curvature and decide on siting.

Updated weight list: The interior panels are back in; I may remove them for the competition next year, but it's significantly louder inside with them gone. Windshield washer reservoir and sprayers are now gone, for a savings of nearly 13 lbs!

Added sound insulation: 48.6 lbs
Trailer hitch receiver: 28.8 lbs
Rear right seatback: 26.4 lbs
Spare tire: 25.8 lbs
Tail: 21.8 lbs
Cargo carrier: 19.6 lbs
Hatch insulation: 16.4 lbs
Rear left seatback: 13.2 lbs
Windshield washer reservoir, pumps, nozzles, and fluid: 12.7 lbs
Rear seat bottom: 12.2 lbs
Rear seat belts, seat brackets, hardware: 10.7 lbs
Front speakers and baffles: 10.6 lbs
Front door insulation: 9.8 lbs
Rear door insulation: 9.4 lbs
Wheel covers with pans: 9.4 lbs (2.5 lbs wheel covers alone)
Front swaybar and endlinks: 8.4 lbs
Jack and tools: 8.2 lbs
Trunk mat: 5.4 lbs
Rear speakers, miscellaneous hardware: 5.3 lbs
Headunit: 5.2 lbs
Hatch door insulation: 5.0 lbs
Floor mats: 4.5 lbs
External mirrors: 4.2 lbs
Cargo cover: 4.2 lbs
Front speaker crossovers and wiring: 3.2 lbs
Passenger side wiper: 2.8 lbs
Rear wiper and motor: 2.4 lbs
Intake resonator: 2.0 lbs
Sound system wiring: 1.5 lbs
Vehicle Proximity Notification System: 1.5 lbs
A-pillar insulation: 1.2 lbs
Hood insulation: 1.2 lbs
Engine cover: 1.0 lb
Horns: 0.5 lb


Total: 338.9 lbs/170.9 lbs from OEM
OEM curb weight: 3042 lbs
New estimated curb weight: 2871 lbs

http://ecomodder.com/forum/member-vm...612-202259.jpg

Here's a graphic showing the pressure measurements across the top of the hood relative to under:

http://ecomodder.com/forum/member-vm...1230216v-1.jpg

Measurements were taken at 42 mph and averaged between one N run and one S run at each location; wind was 10 mph NW.

For vent location, the passenger side presents no difficulty; however, the driver side has the inverter at the front of the engine bay and the fuse box along the side behind the headlight, two things I would rather not get wet. A vent in the middle of each side right over that -1.3 reading would miss both of those, perhaps with an additional vent across the front just behind the radiator.

Are you running grill blocking? Because that seems like it would be relevant to these pressure readings and would be really interesting if you still had such pressure differences when grill blocks and an undertray are installed, no?

Yep, factory undertray and ~50% lower grill block.

under car venting to back?
 

I wonder if it would be possible and useful to plumb that pressure along the inside of the undertray and out somewhere toward the back, like the wheel wells.

Bumping this thread back up. GGP announced a couple weeks ago that the next running will be April 5, 2019! Although it's been cold here and I've been busy with school and work, I've gotten a a few things done, with many more on my list before April.

Completed:
-Repaired front bumper cover with fiberglass. I had a run-in with a raccoon in Indiana a couple years ago that cracked the cover and grill in several places. I had stitched it back up with zip ties, but I took the whole thing off and repaired it properly last weekend.
-New grill block. I calculated the size of the OEM grill openings (yay calculus!) and made a stealth block out of aluminum sheet to reduce it to 1/6 the radiator area, full width slot. (I'll get a picture soon). The Prius's radiator is fully ducted in front; part of the lower duct had gotten cracked as well, now repaired.
-All else as above, except the rear seats have gone back in. Since I don't have a garage to store them in, and they're quite bulky, I swapped them back in the car for the winter. Everything else is still out, including the radio. I've come to quite enjoy the silence, especially on long road trips.

Planning before April 5:
-New tail, this time a lightweight design. Details to come.
-Thermal engine blanket, to reduce warmup times.
-Thermal cabin insulation. I got this idea from reading van forums; since I camp in this car quite frequently, and will on my way to the GGP again, I'd like to add some thermal insulation both beneath the interior panels/headliner/carpet, as well as on my window blocks. That way when I hole up for the night, I can have almost a complete insulative layer wrapping the entire cabin. And since thermal insulation is light (and is better the lighter it is), it shouldn't add much mass to the car.

4 weeks of Winter Break start after my last final tomorrow, which should give me time to work on some of these.

Will you share the design before it's completed?

For some reason I forgot about this thread and assumed this was about a Pontiac Grand Prix. In which case I'd say buy full coverage insurance, and leave the keys in the ignition in a sketchy neighborhood and walk away. Those things are pigs.

Very cool though. I look forward to the results.

How much of an increase in fuel economy did you experience from your tail?

Did I ask this already... is that Lolo Pass road?

https://upload.wikimedia.org/wikiped...und_P3125.jpeg

Looks like the place I get Christmas trees and almost fishtailed my truck off a cliff.

https://ecomodder.com/forum/member-f...7-12-29-57.png

Ignore the Porsche cooling fan. What's left is full wheel spats, front and rear of all four, a rear diffuser, and the Moon disks.

https://ecomodder.com/forum/member-v...512-211121.jpg

On the boat tail have you considered cold plasma vortex de-generators? (I forget the search term at this time, in this location)

Now that this year's event has come and gone, time to turn my attention to next year. This year, this little car was the third most efficient non-plug-in overall, behind a 2000 Insight and 2010 VW Jetta 3-cylinder diesel; that's more efficient than every other Prius there except the one Prime, including two Gen 4s. I don't have any classes this summer, so I'll have a bit more time to mod while the weather is nice. The plan is:

1) Get an accurate curb weight (me + full tank of gas) on the car, at a truck scale or one of the local recycling places (we have two in town that I can potentially use)

2) Brainstorm any other weight reduction possibilities

3) Swap in the new suspension I've had sitting in my closet since last fall. They're Godspeed coilovers with 6 kg/mm front and 8 kg/mm rear springs and dampers, which will be more to my liking since the suspension is ridiculously soft for my tastes right now. I'll also be able to fine-tune the ride height; on the aftermarket Tein springs it's on now, the driver's side front sits ~10 mm lower than the passenger side for some reason. Overall it's pretty close to the optimal 0.75 body cover:tire height ratio Obidi says typically results in lowest wheel drag, so I won't be changing the ride height much.

4) More aerodynamic modifications:
-add front wheel air curtain ducts
-add front wheel strake fairings from the 2018 Ford C-Max hybrid (already ordered)
-aluminum underbody panels to replace the coroplast ones that grenaded themselves two summers ago. These will be bolted in, with a vent near the catalytic converter where the engine bay exhausts now.
-vent the rear wheel housings with convergent ducts to the rear bumper edge, and front wheel housings if I can figure out how to do that with what little real estate Toyota left there
-add hood vents
-add spoiler extension

5) Get a front photo from some distance and write a Python program to count the pixels and calculate an accurate maximum cross-sectional area, so that I can...

6) Do coast-down testing and correct for wind (good way to keep up on my physics over the summer) to calculate an accurate CD. My goal is to get under CD 0.20 with minimal extension.

I'll be attacking these in roughly that order after finals are over (week of May 13).

Explain like I'm five?

Counting pixels is tedious. I'd crop to a bounding box, reduce to black and white, do a maximal Gaussian blur and look at the greyscale value.

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.

It's easier to think of it as the simple ratio of body cover:wheel diameter, I think.

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.

But that is directly related to the cavity volume, if we're holding other conditions constant, like the body cover.

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.

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.

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


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.

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.

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.

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.

https://ecomodder.com/forum/member-v...airings-01.jpg

https://ecomodder.com/forum/member-v...airings-02.jpg

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).

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:

https://ecomodder.com/forum/member-v...314-155425.jpg

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.

https://ecomodder.com/forum/member-v...801-194755.jpg

https://ecomodder.com/forum/member-v...801-194823.jpg

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.