Aero mods - new grill block and wheel covers

[PaleMelanesian]

With the angled-back upper grill block, I'm aiming for some of the benefit listed here.
Basjoos confirms that a low nose helps.
I believe this is from Hucho's definitive book on car aerodynamics.

[99LeCouch]

Any early results to report?

[PaleMelanesian]

I'm not really testing it specifically. And this time of year, the weather overpowers any other factors.

I don't know if it's helping. On paper, and from others' tests, it should. I'm mainly banking on that.

Oh, and that whole "placebo" thing? It's very powerful.

[Daox]

Looks good Andrew.

[jimepting]

I think that there are a number of modest aero mods which each provide some small improvement. As we all know the list includes grill blocks, fender skirts, wheel covers, various underbelly covers, removal of mirrors, and others. As as an engineer, I'm always interested in the actual documented improvement, assuming that the effects are measurable. I have added grill block, underbelly and fender skirts. I only had a chance so far to try to measure the effect of the fender skirts, which seemed to indicate .7-.8 MPG on the SGII.

I'd be highly interest in any measurements that others have made on any of the aero mods. Perhaps with some careful measurements, we could actually figure out which mods are worth doing, from an aero viewpoint. Obviously, the results depend somewhat on the car, but any data at all is better than the guessing we seem to have engaged. Or, am I wrong - is there some true knowledge of the effect of aero mods?

[drimportracing]

Palemelanesian,
Great mods, looks like you could use some fender skirts too.

I should talk. I'm such a slacker. I promised myself I would do a proper grill block by spring and I've yet to finish it. Maybe I can stay inspired long enough to complete it. Same with the smooth covers, I lost my two front SS moon discs a month after I bought them, $43 gone, not very economical. Thinking about the kind that screw into the rim, though that has me wondering about the drilling.

Jimepting,
I'll offer my car as a test subject, I'm more backyard than scientific as you already know. Where are you calling home this week? - Dale

[donee]

Hi All,

I have .010" thick aluminum flashing on my Prius trim rings. This is not only for aero-dynamics but rusty rotor mitigation. The Prius rarely heats up its brakes in normal operation, and not heating up the brakes, it does not cook the moisture out of the grain in the metal. Additionally, slashes from other cars running through pot-holes on the road do not wet down the rotors - a safety benefit.

The Prius trim ring is dished inward, so one can stretch flashing around the OD and get a perfectly flush cap. The trim rings easly come off and on for maintenance. There is a groove in the ID of the wheel, which an expanding ring spring pushes portions of the trim ring into.

The first thing I noticed when I put these on, was how much quieter the car was above 35 mph. There was a great drop in the rushing noise from the left front, while in the drivers seat. And if you have ever been in a Prius, its already one of the quietest highway cars one can drive.

I did some coasting testing, , and found that with the flush hub caps, the Prius would maintain the top speed (with .1 mph acording to Mycanscan) after coasting down a hill for several seconds. Versus peaking and droping .5 mph or more after a second without the flat hubcaps. That has gotta be worth something.

If your familiar with the knife edge of pedal control one needs for SHM highway driving in the Prius, my subjective feeling is that the flat hub caps makes keeping this level of control much easier. And that is very beneficial in morning rush hour traffic.

As to the strength of the flashing, having had the flush hubcaps on more than a year (late December '07), they have held up acceptably. I did not paint them, just used some wax. The surface does have some salt corrosion effect. But its not terrible. The salt makes a starburst pattern on the aluminum. Around the edges there are some outward pushed small (1/16 inch diameter) dents. This was from pebble sized stones that get trapped between the metal cover and the trim ring. From the outside in, there is only one pronounced dent on one cap. This was actually a accidentaly caused dent. I showed them to a Mechanical Engineer at work, and he commented that the metal was too-thin, and would not last. He then reached up with his foot to toggle the metal back and forth to see how tight it was wraped on the trim ring, but it was evening and dark, and he mis-judged the distance, and kicked a dent into one of the caps. Over the time these have been on the car, that is the only dent (at a distance of a few yards) one can see. The flashing gives a little when something hits it, and that avoids the dent.

I have heard splashes bouncing off them. As the car is much quieter at speed, I can hear the oil-canning noise if something hits the flashing. But this is a very rare occurance. Yearly brake dust cleaning inside the cap is more than enough.

On Prius Chat, one other poster here in Chicagland has done this as well. I see people looking at them all the time. With the color of my car, and the silvery metal hubcaps like on aircraft, it does give the car an "Air Force" kinda look.

Unless one is hypermiling, its probably not a good thing to do on standard cars, due to great heat the brakes give off. But the Prius brakes are not much above human body temperature after a 25 mile drive home on the undulating (max ridge line to valley altitude change of about 400 feet around here) river valley terrain here, even without hypermiling. So no problem.

Here is a link: http://priuschat.com/forums/prius-mo...rim-rings.html

[PaleMelanesian]

They survived a highway trip intact. Max speed 65 mph with a 25 mph headwind.

I still can't say if they made any improvements. Never drove that route before for comparison.

I haven't noticed any noise reduction, but that's because my tires on the pavement were already the loudest thing on the car.

[99LeCouch]

I noticed a small improvement with my version of aero wheel covers. The fiancee maintained ~31 mpg at 68 mph on cruise over some hilly terrain. Previous best without wheel covers was about 30.

Small difference to be sure. Get enough small differences together, and it's one big difference!

[PaleMelanesian]

Nice! thanks for that report.