Starting to think 2010 Prius has flow separation issues. Comments?

[donee]

Hi All,

I am really starting to think the 2010 Prius has flow seperation issues, in comparison to the 2nd Gen (2004 - 2009). They lowered the rake of the front of the car significantly. The front to back dimension under the hood is much bigger.

I just do not get the coast down performance I was getting from my other car (if you have not heard, my 2006 Prius was totalled by an un-trained SUV driver - lets make SUV drivers pass special training and testing! - and I am now driving a 2010). Even though the low-speed rolling resistance seems just fine, maybe even better! Yea, the car is bigger, but its also a little heavier, and yea its got those big rear-view mirrors, but they have been aero-improved with steped down rim along the back. The 2010 has a nice difuser treatment under the rear bumper. It should be a wash.

On Boundary layers, I think allot depends on vehicle shape. An aero vehicles will have a thin boundary layer, and a bluff body car will have a turbulent boundary layer. So, I think the boundary layer can probably vary by a factor of three from car to car. The thickness of the boundary layer is dependant on the tubulence energy within the air next to the vehicle. Aero-experts does this sound right ?

[MetroMPG]

Hi donee -

I split this topic from where it started, in: http://ecomodder.com/forum/showthrea...tml#post202701

I assume since the subject of that thread was flow on the rear window, you're thinking the 2010 Prius has problems in the same area?

Easiest way to confirm this is tuft testing - and it truly is easy, because you can see the results in your rear view mirror!

I predict there is no flow separation on the car's back window (the upper, main one).

I'm not sure what you're referring to in terms of coastdown comparisons. Are you saying a 3rd gen Prius doesn't coast as far as a 2nd gen?

---

PS - I didn't read about losing your Prius - sorry to hear that.

[Frank Lee]

Perhaps there are published Cd values out there somewhere?

[RobertSmalls]

According to the Vehicle Coefficient of Drag List - EcoModder , the 2010 Prius is just slightly draggier than the 2004.

And Metro is right: tuft testing is as easy as taping 4-6" pieces of yarn to your rear hatch glass with masking tape, and looking at the results in the mirror. Check it out.

[donee]

Hi Metro,

Its not a problem compared to other cars. And its not in the same place as one would see on a 1 st Gen Prius (NHW-11).

But, I think its worth investigating whether the air is laying down on the roof of the 2010 Prius, and staying attached all the way to the rear spoiler.

Well, not completely. I am thinking the issue is primarily right above the driver's head. But, one might see evidence side-by-side with the tufts on the hatch window. BUt, I doubt the tuft movement on the 2010 hatch glass alone is enough to figure it out. Whichis what I can do here.

With my 2006 there was a hill I come down each day on the highway. As part of hypermiling, my goal was to be at the top of the hill at the right speed to drop into the 53 mph SHM mode when I restarted the engine at the base. I figured out that was 46 mph. Now, the 2010, at 47 mph, is down to 45 mph at the base of this same hill.

Maybe its alignement. But its not flush wheel covers, I have those on my 2010 now. Its not the grill flow, I have my license relocated across the grill on the 2010 too , now, and I have installed grill dams to prevent the blocked flow down in the grill cavity from going through the engine compartment. It should pool there, and spill out in the outboard direction.

I guess, untill I see some speed increase down this hill I am not going to be happy!!

[donee]

Originally Posted by RobertSmalls

According to the Vehicle Coefficient of Drag List - EcoModder , the 2010 Prius is just slightly draggier than the 2004.

And Metro is right: tuft testing is as easy as taping 4-6" pieces of yarn to your rear hatch glass with masking tape, and looking at the results in the mirror. Check it out.

Yea,

The 2006 has a Cd of .26, with a 5.83 CdA, while the 2010 has a Cd of .25 with a CdA of 5.84. Those CdAs are very approximate, using just height and width , and not a shaddow area. The 2010 has significantly larger mirrors, and wider tires (195s versus 185's).

[MetroMPG]

How do you know the frontal area was estimated from H x W? I don't see a source in that document.

EDIT: my mistake. I see the note at the top of that page.

But I believe the figures on that page are collected from multiple sources, and the H x W estimate may not apply to all of them. I'll have to ask Daox - he made that page....

[Rokeby]

It takes 3-5K miles to "wear-in" a new set of tires.
This can mean +2-3 MPGs.

Is it possible that new tires would also affect roll-down test speeds?

Could there also be contributions from tightish wheel bearings, etc?

[Clev]

Originally Posted by Rokeby

It takes 3-5K miles to "wear-in" a new set of tires.
This can mean +2-3 MPGs.

Is it possible that new tires would also affect roll-down test speeds?

Could there also be contributions from tightish wheel bearings, etc?

That was my first thought: tire toe-in, tighter bearings and fresher lubricants all around.

[donee]

Hi Rokeby and Clev,

I am thinking Toe-in too. In that the 2010 rolls and rolls below 40 mph, in comparison to the 2006. At the higher speeds, the tires toe out, and if the toe-in setting is dead on zero, this would create scuffing friction on the tires.

The tighter bearings and non-broken in tires would impact the slow speed rolling too. If the car is rolling better at slow speeds, then other than the toe-in issue, its gotta be aero-dynamics.

I have two long slow hills on one of my routes. On the really long hill, I am at 40 mph at the top, and 36 at the bottom 1 1/2 miles later. Both cars do this exactly. On the other hill I crest at 33 mph, and at the base I am doing 35 about 1/2 mile later. Again both cars do this the same.

Slow-Speed down hill coasting wise this car is very very similar to the 2006. In the flats, I think it coasts a little better than the 2006, but I guess that is kinda subjective....