Beyond Dismal MPG with cargo rack

This from they guy who is arguing aerodynamics with an engineer that has done wind tunnel testing and worked in the aerospace industry? Anyone else notice the irony? Chill man

 

Could this be the very latest in Prius accessories for the highway driver?

 

Jeezus, what's with all the defensive hostility? I think people are only trying to offer suggestions. I understand that you are disappointed with the mileage hit but if it's that big of a problem just sell the Prius and use your 21 mpg truckasaurus from now on. Problem Child, indeed.


I'm pretty sure those examples you cited are cars that depend more on the propulsion system for high speed than aerodynamics. Poor comparison IMO. A better example might be to look at the other end of the spectrum, like solar cars.

 

How is aerodynamics off topic? You put a giant box on top of your car and complained that the fuel mileage dropped! I mentioned that if you turned your box around it would be more aerodynamic, which would lower drag, which would give you better fuel mileage. Another member has pointed out that that the box is mounted with the front higher than the rear, thus raising the angle of attack and increasing drag. If you dropped the front of the box in relation to the rear you would reduce drag and get better fuel mileage. Your loss of fuel mileage is mostly due to increase drag, if you reduce the drag you will get better mileage.

This IS the topic if you want to get better fuel mileage with your cargo box. Did you expect us to tell you a magic voodoo spell that will increase your fuel mileage?

Now to your cars:

You have pictured the Thrust SSC. It is not only the world's fastest car, it was the first vehicle to break the sound barrier on land. It is a SUPERSONIC vehicle, so it should have a pointy nose. It also is thrust driven not wheel driven.

The second car is a drag car. Yes, they go fast but the basic shape of a drag car is dictated by the very larger rear wheels. That is why most drag cars are wedge shaped. The wedge shape also creates downforce on the rear wheels which increases traction.

 

PC - Settle down and take a common sense look at the examples given:

The funny car needs downforce above the rear tires and weight stretched out in font to keep it from flipping in order to get maximim traction under aggressive acceleration. Fuel efficiency isn't a concern.

The jet powered car is propelled by the exhaust pressurizing the void behind the engines for propulsion. The twin jet engines are probably a bit less fuel efficient than your 3/4 ton diesel. Also, if you notice the narrow profile of the main body at the rear, aside from the wing used for stability, it is tapered toward the vertical centerline.

A 2.0 litre Quad 4 pushing a 4 wheeled vehicle to a speed of 278MPH requires a rather low drag considering that engine is only 1/3 larger than the one in your Prius.

Read this with a sense of humor:
Your screen name is appropriate

jhinton - Is it possible that wedge design of most boxes isn't to make it efficient, but rather to provide down force? This would give a better seal and reduce stress on the hinges and latches. I'd think that turning it around would provide a low pressure zone above the box. Any thoughts?

 

I personally loved the dicsussion on subsonic vs supersonic aerodynamics. The explained PC's comment about bullet shapes perfectly.

If I had those kind of mileage problems with that rack I would definately try a variety of configurations to see if I could improve mileage. Since the Prius has the wonderful mileage display, you could quickly determine if your modifications to mounting of the luggage carrier were effective. I would modify;

-angle of attack
-front to rear configuration
-for/aft location on the prius roof.

Looking back at your photo of the rack, it seems clear that a large volume of air is going to be slamming into the bottom of the box. You really need to move it back on the roof and also raise the rear of the box. I would try turning it around too.

As a side note, I wonder about those long Thule cargo carriers. I wonder if mpg would improve if you turned them around.

good luck.

 

It is possible. If you turned the box around it would act as a crude airfoil and would generate some lift. I doubt this would be an issue for the latches. My personal opinion is that boxes are designed with a wedge shape because most people do not know that the rear of a sub-sonic object is more important than the front. I didn't until I had my first fluid dynamics class.

I think it has everything to do with styling and little to do with engineering.

 

Get a "sports rig" trailer, put a couple of yakima boxes on it and you won't take much if any mileage hit. There is a thread somewhere on this site, perhaps in "trailer hitches" with pictures of my set up. Also I drive with a cargo basket on the hitch and take no mileage hit at all. (also on this site somewhere)

Icarus

 

This (mpg) is the main reason I'm considering a hitch-mounted cargo carrier for anything that won't fit in the car. Does anyone have one of these, or have any comments/suggestions? If so, would you please take a peak at this post?

http://priuschat.com/forums/prius-main-forum/47716-hitch-mounted-cargo-carrier.html

ThanX so much!

... Brad

 

If you think PA Prius is the only one that has been helpful you have a funny way of showing it. If I recall you implied he was lying.

From one of your earlier posts:
I often see claims of people getting massive mpg with this and that but the majority dont replicate your results.

Do you have a picture of your dash during the trip? Do you have a picture of the box you used and the mount? At 60mph with no luggage and no passengers my car is getting 53mpg so I find it reallllllllllllllllllllllllllllllly hard to believe you got 50 with 4 people, gear, and a roof rack. Heck just the roof rack by itself with no luggage and passengers in flat city driving 60mph cost me 12mpg.