Beyond Dismal MPG with cargo rack

I do have one picture of the MFD with full bars the whole way across, but that wouldn't really prove anything. My avatar picture is at the top of one of the passes. I'll try to find a better picture and post it later. You can read the tank-by-tank report of our trip on another site at--

http://www.priusownersgroup.com/?page_id=2840

PA P

 

This thread really hits home because I was planning on using the exact same setup (that looks like a Space Cadet, right?) on a trip to NC this July and in my mind I've been trying to think of ways to thin out our load to avoid the need for the box.

Couple more questions though:
1) Still waiting for a temp reading for your trip. Was it the same as when you were getting 52 mpg? Snowy cold temps might account for an extra 5 mpg drop.

2) The snow and reference to camping makes me wonder if there might have been significant elevation gain during your trip. Heading to the mountains maybe? Was the mileage the same both ways?

3) This one is a general aerodynamics question (ualdriver might know this). Would a fairing on the front help? This is something I've been questioning anyway. Sometimes I feel like they are just a billboard and actually act as an air dam, especially with a chunky box on top (opposed top say, bikes on top or even empty). Or do the fairings in fact help alleviate lift and benefit mileage.

 

Unfortunately I don't have any great pics of our rack set-up. Hopefully you can zoom in on the one I've attached. We bought a Thule box for this trip, but after mounting it I saw that the aerodynamics would kill the mpg. I knew there had to be a better way--

I put on the two Saris cross bars and a bike rail on each side. We packed our two tents, four sleeping bags, four pads, and four pair of hiking boots into four dry bags (the kind used for water sports). I then layed a pad (carpet anti-skid pad) directly on the roof of the car, folded a tarp to match the width between the bike rails and twice the length of the roof. I layed the tarp on the pad on the roof with the back of the tarp at the back of the roof and the front of the tarp up over the air deflector and down over the windshield. I then tucked in the four dry bags behind the air deflector and between the bike rails. Then the front of the tarp got layed back over the dry bags and pulled tight with a tightening apparatus I had arranged. Everything was then held in place by a long bungee cord stretched back and forth over the tarp. It worked great! Down side is that it was not able to be locked like a box and it took longer to pack.

PA P

 

12 mpg loss sounds really extreme. I am wondering whether other variables came into play here?

I see my pic didn't attach in my former post. It was likely too large. Here is another attempt.

PA P

 

800 miles is a small distance with respect to the life of the vehicle... less than half of one percent. Put in perspective, what's the problem?

.

 

Are you able to address my first 2 questions?

PA Prius, for my trip I have been considering a setup similar to what you describe. Basically set up the rack sans box and use the space behind the fairing and between my bike trays for overflow. My 2 concerns with this method is scratching the top of the car and inability to get the tarp tightened down enough that I'm not hearing the tarp or straps flapping around for 1000k each way. I used a Thule cargo bag for a trip several ears ago and hated it. When not completely filled taught the fabric flapping was maddening. Plus it's not watertight.

 

Exactly.

I was also concerned about the scratches, but the anti-skid pad did well.

To avoid the flapping you could make some good use of duct tape if you are going to be packing and unpacking too often. My "tightener" also served me well. I added some extra grommets to the back two edges of the tarp and then laced them together with a light rope. I then cut a piece of hardwood about 3/8" thick and 1" wide and long enough to reach between the two bike rails. I drilled a hole in each end of this hardwood and then used the bolts and quicknuts that came with the bike rails to attach it to the bike rails after passing it through the inside of the tarp where I had laced it together. I could then stretch the tarp taunt by pulling back on that wood bar and then tightening the quicknuts. Follow that?!?

PA P

 

Sorry, didn't notice it was embedded in the quote. Thanks. I'll take your word for it that your mileage was the same regardless but I have to believe a significant elevation gain along with a temp drop of 30 degrees contributed to the all around hit.

Actually I did!!!

All due respect, I thought John's comment was an attempt to show the silver lining. Although given the added info shows that he may be technically incorrect I think his intentions were to help you feel better about the situation.

At risk of also coming off negative, I don't think there is another car you could drive that would yield 28.5 MPG fully loaded with the same set up. That being said, I would probably feel let down as well.

 

problemchild,

From looking at you picture, you have the cargo box on backwards. (Yes, I know that is the way they show it being put on, I'm talking aerodynamically.) With aerodynamics, the rear of an object is more important than the front. If you put the blunt end towards the front and the tapered end towards the rear, the Cd will be lower.

 

Different mode of propulsion. Bullets have hot gasses spilling out of the back of it, filling the void. The prius has a giant vacuum behind it.

 

Sorry but my Fluid Dynamics professor would disagree with you. You are confusing supersonic with subsonic flight. The fluid dynamics change when you break the sound barrier.

For subsonic objects for optimal aerodynamics you want a rounded front that tapers very gently towards the rear. The smaller the angle of the taper and longer the length, the lower the drag coefficient. The optimal shape will be a long tear-drop shape (Think Boeing 777)
Aerospaceweb.org | Ask Us - Streamlining and Aerodynamics


For supersonic flight you want a long and conical front. Supersonic planes and rockets create thrust using rocket or jet engines. The exhaust from these engines is routed to the extreme rear of the vehicle to fill the low pressure area in the wake of the vehicle. (Think F-18)
Aerospaceweb.org | Ask Us - Rocket Nose Cones and Altitude


VLD bullets reach supersonic speed so they need long conical fronts that taper toward the rear. You can't have the rear taper be too great our you loose surface area that the rapidly expanding gases from the explosion of gun powder acts on. You can get around this problem if you use a sabot round.
Aerospaceweb.org | Ask Us - Supersonic Bullet

Below is a picture of the Oldsmobile Aerotech land-speed car that reached a tope speed of 278mph with a 2.0L quad4 engine. Notice the very long and tapered tail.

 

Yup.

 

I've always wondered the same thing myself. They clearly put the narrow end forward in all their diagrams, but the blunt end should be forward for lower drag. I'm also surprised that they don't make anything more aerodynamic.

You're comparing apples to oranges.
You still haven't mention the ambient temps, but judging by the snow it was cold. Can you try the other suggestions?
1. Move box forward so angle of attack is level.
2. Turn box around so that blunt end is forward.

I think you are blowing his comment out of proportion. Probably best to drop it at this point.

 

Do you think you might be getting a bit too upset about this?

Back to the topic at hand, you mentioned how your pickup gets 21 mpg on the highway while the Prius gets 28.5. A couple of thoughts:

-That's still a 35% improvement
-The pickup's primary role is cargo transport, while the Prius's primary role is passenger transport.
-Did the pickup also have massively non-aerodynamic objects strapped to its roof?
-Could be some measurement error in the fuel economy calculation for the pickup. The fuel economy numbers you're getting from the Prius, on the other hand, are very accurate.
-Same terrain, temperature, etc for both vehicles?